Basic Characteristics and Evolution of Sanjiang Tethys Archipelagic Arc-Basin System

Abstract

The rise of plate tectonics theory in the 1960s contributed to a revival of the long-neglected idea of continental drift, and revolutionized Earth sciences by shifting people’s perception that continents were fixed to a belief that they were in a constant state of motion.

The rise of plate tectonics theory in the 1960s contributed to a revival of the long-neglected idea of continental drift, and revolutionized Earth sciences by shifting people’s perception that continents were fixed to a belief that they were in a constant state of motion. The extensive practice of plate tectonics in the world in the past 40 years has proved to be a highly successful theory. It features a simple principle that involves a wide range and conforms to scientific laws, as well as great compatibility, explanatory power and foresight. The charm of plate tectonics: At the level of earth view, the theory was changed to the active theory which saw the significant drift and transformation of the ocean and land on the earth surface from the original fixed theory which believed that the position of the continent was unchangeable; at the level of geo-view, the theory was changed to the holistic, systematic and process geology which is integrated from space to depth and from ocean to continent from the original inclination to the theory of uniform change or sudden change, power curve model, and the theory of vertical movement or horizontal movement and rotary movement; at the level of methodology, both the method of induction focused on observation and facts and the theory of causation focused on deduction and hypothesis should be paid attention to. Most geologists explain geological event groups and their processes in the overall framework of plate tectonic evolution from the continent to the ocean, the method of “observing the present to study the past” to the method of “observing the past to study the present”, and the global plate tectonics, completely changing the geoscience concepts. There is no doubt that there are traces of oceanic geological evolution on the continent, that is, the geological records of oceanic lithosphere evolution, development, destruction and transformation into continental orogenic belt in the plate tectonics model can be found and reshaped on the continent. The landing of plate tectonics does not announce the end of this theory, but indicates the beginning of a new journey.

There are obvious differences in lithosphere, thickness, genesis and age between continental zone and oceanic zone. In geoscience research, scholars often question whether it is reasonable to adopt plate tectonics originated from marine geology to know, explain and understand continental geology and dynamic processes. However, the research shows that because the most complete geological records of the earth evolution, the marine sediment records from at least Proterozoic Eon, and the geological event group records of ocean-land interaction and transformation can be found on the continent, the plate tectonics theory has been released successfully, and many marine geologists, together with scientists who stick to continental geology theory, launch the Global Lithosphere Comparison Program and Continental Dynamics Program in all continents and orogenic belts around the world after participating in the geological and geophysical surveys of the submarine sediments in the Mediterranean, Atlantic Ocean, Pacific Ocean and Indian Ocean. The practice of earth science shows that after some limitations have been corrected, the classical plate tectonics theory still has great scientific values and is favored by geosciences. The emerging of plate tectonics initiated the upsurge of research on island arc and back-arc-basin. The study on the island arc and back-arc-basin from the perspective of plate tectonics changed the traditional tectonic pattern of trough-platform structure, and revolutionized the entire earth science system. Therefore, the tectonic evolution of continental margin and the formation and development of continental lithosphere are not only the basic problems of contemporary geological research and the frontier subject of geotectology research, but also the introductory guide for the landing of plate tectonics.

2.1 Basic Characteristics and Main Arguments of Archipelagic Arc-Basin System Tectonics

2.1.1 Proposal of Multi-Arc-Basin-Terrane (MABT) Tectonics

The establishment of sea floor spreading and plate tectonics theory and the landing of plate tectonics ushered in the golden age of Tethys geology, especially the research of Qinghai-Tibet Plateau geology and continental geology. For nearly 30 years, a series of works on the geological evolution of Qinghai-Tibet Plateau have consistently incorporated this research into the general framework of evolution of Tethys Ocean (Fig. 2.1), but there are different opinions on the specific evolution models. To sum up, there are mainly “scissors-stretching”, “conveyor belt” and “accordion” models (Huang and Chen 1987). All models are based on the formation of Pangaea and Tethys being a bay in the Panthalassa Ocean, the splitting of Gondwana and the accretion of the Asian Continent. Most geologists believe that there are only five suture zones in Qinghai-Tibet Plateau, namely Yarlung Zangbo River, Bangong Lake-Nujiang River, Jinsha River, Kunlun Mountain and Qilian Mountain suture zones, and five accretionary terranes separated from Gondwana. They believe that these suture zones are ocean basins destroyed one after another, and all of them suffered subduction destruction pointing to the continent and dipping northward.

Fig. 2.1

Sketch of geological structure of east Tethys

Since the 1980s, when the researchers studied the geological and tectonic evolution of Sanjiang (Jinsha River, Lancang River and Nujiang River) Region in Southwest China, they found that the ocean basins restored from Jinsha River Zone and Ganzi-Litang Ophiolite Melange Zone are only about 1000 km wide, and the adjacent island arcs and land arcs indicate that the ocean crust subducted westward and southward (Li et al. 1991; Mo et al. 1993). The main body of the Bangong Lake-Nujiang River Junction Zone also subducted westward and southward. The study of paleogeography and paleostructure once compared it with the arc-basin system in Southeast Asia, pointing out that the Paleozoic Era-Mesozoic Era Tethys had a paleogeographic pattern of islands and seas. Some scholars put forward the idea that the Sanjiang Paleo-Tethys in Southwest China is a multi-island ocean or a multi-island sea (Liu et al. 1993) discovered that 90% of the orogenic belts were formed by the reduction of back-arc-basins and the arc-land collision through the observation, research and analysis of tectonic evolution of the major orogenic belts in the world. This discovery is undoubtedly of great significance to the study of continental geology and continental orogenic belts. Based on the detailed anatomy of the tectonic facies in the major orogenic belts of Qinghai-Tibet Plateau, the hypothesis of a multi-island tectonic model was formally put forward.

The author has discussed the meaning and evolution of Tethys originally put forward by, the spatio-temporal structure of Tethys, the spatio-temporal framework and evolution relationship of three landmass groups in Laurasia, Gondwana and Pan-China, and studied many ophiolitic melange zones and various types of island arcs and basin systems in East Tethys with Qinghai-Tibet Plateau as the main body. It is found that the early so-called scissors-stretching, conveyor belt and accordion models cannot give a relatively reasonable explanation to the spatial configuration of many ophiolite melange zones and various island arcs and basin systems in Qinghai-Tibet Plateau (1997). The spatial configuration of the arc-basin systems in South Asia and on the west coast of the Pacific Ocean shows that the continental accretion of the southwest Pacific Ocean is completed by the complex of back-arc-basin reduction and island arc orogeny, instead of the continental accretion breaking away from Gondwana and drifting northward. Based on the analysis of more than 20 ophiolite melange zones and related island arcs and basins in the arc-basin system in Southeast Asia and the west coast of the Pacific Ocean and the Qinghai-Tibet Plateau (including the Sanjiang area), the author puts forward an MABT model to explain the formation and evolution of major orogenic belts in Tethys and the Asian Continent (Pan et al. 1997).

2.1.2 Definition of Multi-Arc-Basin-Terrane (MABT) Tectonics

The MABT tectonics at the continental margin (Pan et al. 1997, 2003, 2012): the combination of the frontal arc and a series of island arcs, volcanic arcs, blocks and the corresponding back-arc ocean basins, inter-arc-basins or marginal sea basins formed by the subduction of ocean lithosphere on the ancient continental margin, which as a whole shows a specific composition, structure, function, space distribution and time evolution features in time–space domain between continental lithosphere and ocean lithosphere. The identification and in-depth study of the MABT tectonics at the continental margin can not only dissect the material composition, structure and evolution history of the orogenic belt, but also have important enlightenment for analyzing the formation of the Precambrian continental craton basement. Southeast Asia and the west coast of the Pacific Ocean, which are restricted by the subduction of the Indian Ocean and Pacific Plate, are the most typical areas of Cenozoic MABT tectonics.

The formation and evolution of MABT tectonics at the continental margin is an important symbol of the tectonic system transformation from ocean lithosphere to continental lithosphere. The formation of orogenic belt at the continental margin is driven by the reduction of back-arc or inter-arc ocean basin and island arc marginal sea basin, and the continental margin accretion is produced by a series of archipelagic orogeny such as arc-arc collision and arc-land collision. The development of back-arc foreland basin and peripheral foreland basin is an important symbol of basin-range transition. The evolution of the MABT tectonics at the continental margin formed the orogenic system, the destruction of the Tethys Ocean and the splicing of the MABT on both sides of the continental margin into the orogenic system formed mage-suture zones, and finally formed a giant orogenic system (tectonic domain).

2.1.3 Basic Characteristics of Multi-Arc-Basin-Terrane (MABT) Tectonics

According to the analogy analysis of comparative geology, the MABT at the continental margin during ocean-land transition is the continental margin of the Western Pacific Ocean, which concentrates more than 75% of the marginal sea basins in the world, and its formation is closely related to the (super) subduction zone of the Western Pacific Ocean, the most spectacular subduction zone activity in the current global tectonics. Among them, the formation of the MABT tectonics in Southeast Asia is also restricted by the northward subduction of the Indian Ocean Plate, which occupies an important position in global tectonics. It plays a very important role in the comparative study of continental geological history, especially in the formation and evolution of orogenic belts at the continental margin (Pan et al. 1997). Based on the analysis of the material composition, structure, tectonic characteristics and evolution history of the MABT in Southeast Asia, the basic characteristics of the MABT can be summarized as follows.

2.1.3.1 Specific Space–time Structure and Material Composition

The Asian Continental Margin is composed of the Indonesian Island Arc controlled by the one-way subduction of the Indian Ocean and a series of arc-basin systems (Hsü 1994). The Java Trench indicates the subduction of the Indian Ocean Plate, and the Indonesian Island Arc is the frontal arc of the MABT in Southeast Asia. Behind the frontal arc of Indonesia, there are more than ten back-arc-basins, micro-landmasses and island arcs, as well as countless shoals and hundreds of island reefs. Some back-arc-basins have spreading oceanic ridges, oceanic crusts, oceanic islands and seamounts, while others are only marginal seas (Fig. 2.2). The South China Sea Basin is the largest back-arc-basin, with a length of 2600 km from north to south and a width of 1300 km from east to west. Its marginal zone has different tectonics. The basin has a passive margin in the north, sheared margin in the west and an active continental margin in the east. Nansha Islands in the south are located on the residual arc separated from Palawan Trough and are dominated by neritic carbonate deposits. The Izu-Ogasawara-Mariana Intra-oceanic Arc is a frontal arc related to the subduction of Western Pacific Ocean Plate, and there are a series of back-arc-basins, ridges and island arc systems in the west.

Fig. 2.2

Archipelagic arc-basin system tectonics and space–time structure in Southeast Asia

Except for Southeast Asia, the Andes Active Margin in Western South America is the continental volcanic-magmatic arc. On the outside of the arc, there is a deep-sea trench, and there is an accretionary wedge on the inner wall of the trench. Some places are fore-arc-basin deposits, which are developed on some landmasses split by extensional structures. When the continental part behind the magmatic arc turns into island land and shallow sea forms islands and back-arc-basins, Andes Margin turns into island arc margin, and the MABT tectonics begins to develop.

2.1.3.2 Three Types of Basement of Island Arcs or Frontal Arcs

In the island arc or frontal arc of the MABT, the basement and composition of different members may be different. Generally speaking, there are at least three different types of basements in the island arc or frontal arc. For example, the frontal arc in Indonesia, the Sumatra Volcanic Arc in the west member, the West Java Volcanic Arc and their back-arc-basins were formed on the continental crust basement; the volcanic arcs of Central Java and East Java in the middle member were formed on the Mesozoic accretionary wedge complex; Sumba Island and Flores Island in the eastern member were formed on the oceanic volcanic arcs and the Neogene back-arc oceanic crust.

2.1.3.3 Three Types of Back-Arc-Basins

A back-arc-basin refers to the marginal sea basin on the continent side of the island arc, which is distributed in many margins of the ocean in the world, and the Western Pacific Ocean Margin is the most typical one. According to the research results of Asia and Southeast Asia, at least rift basin, marginal sea basin and inter-arc-basin can be found according to the development time, tectonic position and evolution characteristics.

• (1) Back-arc rift basin: It is behind the volcanic arc, the basement is a stretched and thinned continental crust, and is generally less than 30 km thick, with low negative Bouguer anomaly, mainly neritic or transitional sediment, and “bimodal” volcanic rocks with high heat flow value, such as South Sumatra Basin.

• (2) Marginal sea basin: It is located between the active continental margin and the marginal arc, beneath which are the sea floor spreading oceanic crust and marginal oceanic crust. The crust is about 20 km thick, and moderate and deep earthquakes occur, with high heat flow value, such as the South China Sea Basin.

• (3) Inter-arc (rift) basin: It is often between the residual arc and the frontal arc, with an oceanic crust about 10 km thick below it, and the upper mantle upwarping is obvious. When the inter-arc-basin expands to a certain extent, it will form an intra-arc rift basin with high positive Bouguer anomaly and extremely high heat flow value, such as the Philippine Sea Basin.

The causes of the back-arc-basins are usually explained by the models of back-arc rifting and sea floor spreading, and the process of back-arc spreading is undoubtedly related to the subduction of the ocean lithosphere.

2.1.3.4 Short Life of Back-Arc-Basins

Compared with the main ocean, the life of the marginal sea basin, back-arc-basin or back-arc ocean basin in the MABT at the continental margin is short, usually only tens of millions of years. The spreading of the back-arc-basin stopped, and then the back-arc-basin was transformed into a residual back-arc-basin. The residual back-arc-basin and back-arc-basin have similar basement, which is usually a new oceanic crust expanded from the sea floor or a thin continental crust. The Caspian Sea and the Black Sea are residual back-arc-basins, while Junggar and Qaidam in China are respectively related to Paleo-Asian Ocean and Paleo-Tethys.

2.1.3.5 Three Different Types of Space–Time Evolution

• (1) Back-arc-basins or back-arc ocean basins from the frontal arc inward were formed earlier one by one. For example, the frontal arc of Izu-Ogasawara-Mariana mainly developed Eocene-Miocene volcanic rocks with Pliocene biohermal limestone. Frontal arc inward (from east to west): Mariana Trough is a spreading back-arc-basin. Karig speculated that it was spread in the Pliocene (3 Ma), and its adjacent West Mariana Ridge was formed since 11 Ma; In the west, the Parece Vela Back-arc-Basin was formed by intra-arc spreading, in which there are typical oceanic basalts. Pillow lava, diabase, gabbro and peridotite have been found in Parece Vela Rift, the sea floor spreading occurred in the Oligocene–Miocene, the Kyushu-Palau Ridge in the west is the residual volcanic arc in Oligocene, and the Western Philippines Basin in the west was formed in the Eocene. It can be predicted that the backward subduction of the lithosphere plate in the Western Pacific Ocean will lead to the continuous eastward advancement of the arc-basin system in the future, and the split of the arc will separate the new intra-arc rift—back-arc ocean basin by residual arc or new ridge. The ultimate destruction of the Pacific Ocean is not caused by the continental collision but by arc-arc or arc-land collision.

• (2) The back-arc-basin with the frontal arc inward was formed earlier one by one. For example, the basin groups after the Indonesia-Timor Frontal Arc (to the north) become new in turn. Banda Sea spread 700 km from east to west, with the estimated sea floor age of about 60 Ma. The northward Sulawesi Basin was formed in the Middle Eocene (50–43.5 Ma). The spreading time of the Sulu Sea floor is from Late Eocene to Oligocene; the first sea floor spreading of the North South China Sea Basin was in Oligocene-Early Miocene (32–21 Ma), and the second one was in Early-Middle Miocene (volcanic rock age 15–10 Ma).

• (3) The marginal sea basins parallel to the frontal arc were formed in roughly the same period. For example, Sumatra Basin, Northwest Java Basin, Northeast Java Basin were all formed in 50 Ma, while Japan Sea, East China Sea and South China Sea and other basins were all formed in Oligocene–Miocene.

2.1.3.6 There Are Three Different Types of Collision Orogeny

The present geological and geomorphological features of frontal arcs in Indonesia and its MABT in North Southeast Asia were caused by the interaction of three lithospheric plates: Neogene Philippine Sea Plate (NNW movement, 10 cm/a), Asian Plate (SE movement, about 0.4 cm/a) and Indian Ocean Plate (NNE movement, 7 cm/a). A series of geological event groups (Fig. 2.3) occurred during the formation of MABT in Southeast Asia, such as subduction, accretion, volcanic arc formation, back-arc thrust, strike-slip fracture, back-arc spreading, separation and formation of micro-landmasses, arc-arc collision, arc—micro-landmass collision, arc-land-collision, and the formation of foreland fold thrust zone, obduction and uplift of mountain chain zones. The prominent manifestation is the orogeny restricted by three different dynamic mechanisms:

Fig. 2.3

Orogeny during the formation of archipelagic arc-basin system in Southeast Asia

• (1) Island arc orogeny restricted by oceanic lithosphere subduction: different island arc orogeny styles may be shown due to different subduction convergence directions. For example, Sumatra (volcanic arc) orogeny restricted by the oblique subduction of Indian Ocean Plate and Java (volcanic arc) orogeny restricted by the forward subduction of the Indian Ocean Plate. The subduction caused accretion complexes and fore-arc-basins. The volcanic arc is located on the pre-Paleogene continental crust basement, with the Barisan Right-lateral Strike-slip Fracture Zone developed, and the Tertiary back-arc-basin developed at the north side of the volcanic arc.

• (2) Collision orogeny restricted by the subsidence of the back-arc ocean basin: different orogenic styles caused by arc-arc collision or arc-land collision. Maluku arc-arc collision orogeny is the only example of arc-arc collision orogeny in the global tectonics at present. The Maluku Back-arc Ocean Basin is subducted to the west by the Sangihe Volcanic Arc and to the east by the Halmahera Arc. Their fore-arc zones collided, the subduction complex formed between the two arcs with opposite polarities, and Talaud Ridge was formed. Their fore-arc zones thrust backward on the attached arcs. The Sulawesi arc-land collision orogeny was caused by the collision between two micro-landmasses (Buton-Tukangbesi and Banggai-Sula) separated from the Australian Continental Plate, and the eastern part of the Sulawesi Volcanic-Magmatic Arc. The collision resulted in ophiolite thrusting upward on the micro-landmasses, and the metamorphic zone in Middle Sulawesi thrusting westward on the volcanic-magmatic arc, resulting in the formation of foreland thrust fold zone in Tertiary.

• (3) Collision orogeny restricted by continental craton subduction: for example, Banda land-arc collision orogeny is caused by the subduction of the passive margin of Northern Australia under Banda Volcanic Arc and its fore-deep complex in Timor Trough. In the southern deformation zone of Timor Island, Australia, the Permian-Pliocene strata were folded and imbricated, and the accretionary complex and ophiolite in Timor Trough thrust upward on the foreland. The fore-arc zone of Timor arc is 100 km wide. It spread westward to the east of Sumba Island, with a 400 km wide fore-arc zone. The collision just started recently. It spread eastward in the northeast of Timor Island, with a 40 km wide fore-arc zone, and then spread northward. The volcanic arc and the nearly disappeared fore-arc zone overthrust northward on the submarine oceanic crust of Banda Sea, and Banda Sea Floor subducted southward into the Australian Continental Margin.

The Melanesia land-arc collision orogeny in New Guinea resulted from the subduction of the Australian Continent under the Tertiary Volcanic Arc and the collision with the oceanic island arc which is considered as New Guinea, and the ophiolite melange zone was obducted and uplifted into the central mountain zone. A foreland fold thrust zone was formed in the northern margin of Australia. The volcanic island arc thrust northward on the Caroline Crust. The transverse strike-slip shear zone in Northern New Guinea was caused by the westward movement of the Caroline Plate at a rate of 12.5 cm/a relative to the Australian Plate.

The study of the MABT tectonics and their evolution characteristics in Southeast Asia is of great enlightenment for researchers to study the continental geology, especially the formation and evolution of Qinghai-Tibet Plateau with complex orogenic belts (especially Sanjiang Orogenic Belt).

2.1.4 Main Arguments of Multi-Arc-Basin-Terrane (MABT) Tectonics

2.1.4.1 Spatial–temporal Pattern of Coexistence of Three Landmasses and Three Oceans

In the history of global ocean-land evolution, there have been more than three spatial–temporal patterns of ocean-land distribution where landmasses and oceans coexist at least since late Neoproterozoic. Since late Precambrian, the world can be divided into Laurasia Landmass Group, Gondwana Landmass Group and Pan-Huaxia Landmass Group (North China, Yangtze, South China, Indo-China landmasses and other landmasses) (see Fig. 2.2), which have their own systems and unique geological evolution. At least in the Paleozoic, there were Paleo-Atlantic Ocean, Paleo-Asian Ocean (including the Ural Ocean) and Tethys Ocean all over the world. Since Mesozoic, there has been the present spatial and temporal distribution pattern of oceans and continents.

2.1.4.2 Long Period of Ocean Evolution

The Atlantic Ocean, Indian Ocean and Pacific Ocean are in different tectonic evolution stages of the ocean lithosphere. Currently, the Pacific Ocean has a history of spreading at least 200 Ma. Since the mid-ocean ridge has dived under the North American Continent and transformed into the San Andres strike-slip fracture, there is no sea floor spreading in the North Pacific Ocean. In the Western Pacific Ocean, it shows the evolution of the MABT in East Asia and Southeast Asia. According to GPS measurement, the distance between Shanghai and San Francisco is shortened by 5 cm every year, thus the Pacific Ocean will be destroyed after 400 Ma. In Tethys tectonic domain, the spreading basins restored by many ophiolite zones have only been “opened and closed” for several tens to millions of years. The period of tens to millions of years is at most the lifetime of marginal sea basins or back-arc-basins. Therefore, there is a long evolution process for an ocean from its occurrence, spreading to its destruction. For example, the Tethys Ocean has a lifetime of at least 600 Ma from its occurrence, development to its destruction (from Late Precambrian to Eocene).

2.1.4.3 Similar Original Scale of the Tethys Ocean and the Present Pacific Ocean

When the Tethys was evolved out of the “Pangaea and Panthalassa Ocean”, the prototype of the Tethys Orogenic Belt, including the Qinghai-Tibet Plateau and its adjacent areas, was restored to a paleo-ocean, instead of a vast Pangaea Bay or shallow Tethys Sea, which led to the quantitative problem of the width of this primitive ocean. If we consider the formation and evolution of the global ocean lithosphere, the Paleo-Tethys Ocean is a link of the global ocean-land transition and evolution historical chain in Phanerozoic. The Tethys Ocean existed at a time when there was no Pacific Ocean. The opening, formation and evolution of the Pacific Ocean is exactly the shrinking and destruction of the Tethys Ocean, i.e., the Tethys Ocean, which was destroyed and closed at the end of Mesozoic, was originally as large as the Pacific Ocean today. The research results of Gondwana Continental Group show that the India Landmass was a part of Gondwana Continent in the early Mesozoic, and it began to split and advance northward from the end of Jurassic and the beginning of Cretaceous. Currently, the distance between the India Landmass and the Antarctic Pole is more than 12,000 km. The width of the Tethys Ocean disappeared along the north–south direction, not less than 12,000 km. The great shortening of the Tethys Himalayas and Qinghai-Tibet Plateau along the north–south direction generally only reflects the relationship between Laurasia and Gondwana Continental Group, while Pan-Cathaysian Continent Group wedged westward between the two continents to form a series of mountain chains (Hengduan Mountain and Helan Mountain, etc.) spreading in south-north direction, which also showed great compression in east–west direction. The spreading of the Pacific Ocean is the sum of the shortening range of the Pan-Huaxia Continent and the shortening range of the nearly north–south orogenic belts to realize the balanced compensation of the horizontal movement of the lithosphere; otherwise, the formation of the East Tethys MABT orogenic belts cannot be explained.

2.1.4.4 Continental Margin Volcanic-Magmatic Arc and Multi-Arc-Basin-Terrane (MABT) Formed by the Two-Way Subduction of Ocean Lithosphere

The two-way subduction of the lithosphere of the Pacific Ocean formed the Cordillera MABT and collision orogenic belt on its east side in Late Mesozoic, and the Cenozoic Andes continental margin arc. The evolution of the Paleo-Asian Ocean formed the MABT of Kazakhstan-Altai-Greater Khingan and a series of arc-arc, arc-land collision orogenic belts from the southwest margin to the southeast margin of Siberian Traps, while the continental margin volcanic-magmatic arc was the main structure in Tianshan and North China Margin (including the early back-arc-basin subduction complex zone). This asymmetric evolution of the oceanic lithosphere also shows the same characteristics in the evolution of Tethys Oceanic lithosphere. The asymmetric destruction of the North Pacific Ocean is characterized by the destruction of the old oceanic crust, the emergence of the new oceanic floor, and then the spreading of the mid-ocean ridge. When the continent crosses and buries the spreading ridge, the oceanic crust will destruct in reverse order, that is, it will destruct from the new oceanic crust to the old oceanic crust. This feature is very important for understanding the occurrence of ophiolite in the same ancient suture zone in the continental orogenic belt at different times.

2.1.4.5 Existence of Ocean Basins Indicated by the Remnant Arc

The existence of remnant arc indicates the existence of ocean basin, and the MABT is the symbol of the evolution of ocean lithosphere from occurrence and development to shrinkage and reduction. Many ophiolite melange zones composed of ophiolite and subduction complex have been found in Qinghai-Tibet Plateau. Most of the “trinity” ophiolites are “small ocean basin”, back-arc-basin and island arc marginal sea type. On the north side of Tethys Ocean, there are the Early Paleozoic Qinling-Qilian-Kunlun MABT and the Late Paleozoic-Triassic Qiangtang-Sanjiang MABT, as well as the Gangdise MABT on the south side of Mesozoic Tethys Ocean. The existence of MABT indicates the existence, reduction and transformation of ocean lithosphere. The lithosphere of Tethys Ocean experienced a long-term continuous and complex evolution from occurrence, development to shrinkage and destruction at least from Paleozoic to Mesozoic. The Paleo-Tethys was the inheritance and development of the Proto-Tethys, and the Mesozoic Tethys did not reopen after the destruction of the Paleo-Tethys Ocean. Some Tethys Oceanic crusts could be merged by the subsequent Indian Ocean. Bangong Lake-Shuanghu Lake-Nujiang River-Changning-Menglian Mage-suture Zone should be essentially a relic of the destruction of Tethys Ocean.

2.1.4.6 Action Mode of the Transformation of Oceanic-Continental Lithosphere Tectonic Systems

The transformation of oceanic-continental lithosphere tectonic systems occurred through the island arc orogeny of oceanic lithosphere subduction, back-arc-basin reduction, arc-arc collision, and arc-land collision. According to the geological characteristics of Qinghai-Tibet Plateau, a series of marginal basins, island arc-basin systems and sediments and other rock assemblages on the western margin of the Pan-Cathaysian Continent Group in the reduction of Tethys Ocean have been continuously involved in orogeny since Paleozoic, and finally become a part of the Pan-Huaxia Continent. The lithosphere of the Tethys Ocean has been formed as an island arc orogenic belt from northeast to southwest, and the shrinkage of the back-arc-basin of the ancient marginal arc was not caused by “soft collision” or “collision without orogeny” during the horizontal tectonic movement of the lithosphere transformed into the continent of Qinghai-Tibet Plateau, but was caused by the island arc orogeny of arc-arc collision and arc-land collision. The transformation of back-arc foreland basin and continental margin basin into peripheral foreland basin is the geological record and important symbol of basin-mountain transformation. The mechanism of the ocean floor renewal, shrinkage and destruction is unknown yet, but it is obvious that it is important to enlighten how to identify and understand the formation and evolution of paleo-oceans in continental geological research after the transformation of oceanic lithosphere into continental lithosphere through island arc orogeny.

2.1.4.7 Qinghai-Tibet Plateau Crust Mainly Composed of Remnant Arc Orogenic Belts

The Qinghai-Tibet Plateau is composed of crustal materials formed and modified by complex and diverse geological processes, with the tectonic characteristics of a mosaic of strips and blocks, which is now manifested as a series of island arc orogenic belts of different scales, types and periods formed at different times, and generally manifested a complex tectonic domain composed of remnant arcs of different sizes and their back-arc, inter-arc or small ocean basin remnants and collision zones. The research shows that the tectonic framework of Qinghai-Tibet Plateau is bounded by the ophiolitic melange zone in the southern margin of Kunlun and the Bangong Lake-Nujiang River Ophiolitic Melange Zone, which can be divided into three tectonic regions: the Early Paleozoic Qinling-Qilian-Kunlun MABT in the northern plateau, the Late Paleozoic-Triassic Qiangtang-Sanjiang MABT in the central and eastern plateau, and the Mesozoic Himalayan-Gangdise MABT in the southern plateau. It is the formation and evolution of the three MABTs that restrict the Cenozoic tectonic deformation history and stress state of Qinghai-Tibet Plateau and its adjacent areas, and control the uplift of Qinghai-Tibet Plateau. The composition of the MABT is the essential reason for the uplift of Qinghai-Tibet Plateau.

2.1.4.8 Three Important Tectonic Processes of Qinghai-Tibet Plateau

The main composition of Qinghai-Tibet Plateau is not the split terrane of Gondwana Continent, but the MABT formed by the two-way subduction of Tethys Ocean in different periods of Phanerozoic, which was combined and converged by multi-stage collision orogeny of multiple island arcs. The original geological bodies of Qinghai-Tibet Plateau were constantly changed and transformed, and new ones were constantly formed, which are manifested in three important tectonic processes: First, the shrinking and destruction of the lithosphere of Tethys Ocean and the arc-arc or arc-land collision orogeny; second, the plateau is growing with the basin-mountain transformation and the continuous formation of orogenic belts; third, the substantial shortening and thickening of the earth crust with intracontinental convergence and plateau uplift. Due to the frequent transformation and evolution of the paleostructure and paleogeographic environment of Qinghai-Tibet Plateau, the multiple transformation and repositioning of material and energy flows are conducive to the formation of various mineral resources.

2.2 Space–Time Structure of Sanjiang Tethys Multi-Arc-Basin-Terrane (MABT)

2.2.1 Space–time Structure and Evolution of Yidun Arc-Basin System

The Yidun Arc-Basin System is mainly composed of Yidun Island Arc Zone on the west side of Ganzi-Litang Ophiolitic Melange Zone. As the product of the westward subduction of Ganzi-Litang Oceanic Crust, Yidun Island Arc Zone is mainly formed on a long-term spread and thinned continental crust. Volcanic-magmatic arc chains are distributed in parallel on the west side of Ganzi-Litang Junction Zone, running through the north and south, and stretching intermittently for 500 km. Yidun Island Arc Zone is not only a typical volcanic-magmatic island arc zone, but also an important polymetallic metallogenic zone of tungsten, tin, molybdenum, copper, gold, lead, zinc and silver.

2.2.1.1 Space–time Structure of Yidun Arc-Basin System

The Yidun Island Arc Zone (Fig. 2.4), which lies between Zhongza-Shangri-La Block in the west and Ganzi-Litang Junction Zone in the east, started in Late Carnian of Late Triassic, and was initially formed on a partially spread oceanic crust or transitional crust. Its main structure was built on the thinned continental crust with the characteristics of graben-horst system, and it experienced a complicated development history of compression-spreading alternation.

Fig. 2.4

Tectonic framework and deposit distribution of Yidun Island arc zone

2.2.1.1.1 Formation of Ganzi-Litang Ocean Basin and Development of Yidun Island Arc

The limestone in Ganzi-Litang Melange Zone contains Late Permian fossils. Radiolarians from Early Triassic to early Late Triassic have been found in siliceous rocks associated with oceanic ridge basalt. Early Triassic radiolarians (Yanagia Chinensis Feng, Paurinella Sinensis Feng); Middle Triassic-early Late Triassic radiolarians (Triassocampe cf. nova Yao, Pseudostylo-sphaera nazarovi Kozur, Squinabolella? sp., Hinedorclls sp., Muelleritortis cochleata tumidospina Kozur, Pseudostylosphaera nazarovi (Kozur et Mostler), Triassocampe coronata Bragin, Astrocentrus Pulcher Kozur et Mostler et al. (1:200,000 Gongling Map Sheet, 1984; 1:200,000 Litang Map Sheet, 1984). The ⁴⁰Ar/³⁹Ar plateau age of Panyong ocean ridge pillow basalt in Ganzi-Litang Melange Zone is (231.3 ± 6.7) Ma, which is equivalent to the transition period from Middle Triassic to Late Triassic. It shows that Ganzi-Litang Ocean Basin was formed in Permian-early Late Triassic (the peak of the spreading of Ganzi-Litang Ocean Basin), with a width of about 480 km. Ganzi-Litang Ocean Basin was further spread on the basis of the deep-water rift basin deposited in Carboniferous.

Ganzi-Litang Oceanic Crust began to subduct westward in the middle Late Triassic under Zhongza-Shangri-La Block, forming a typical supporting pattern of Yidun Island Arc-back-arc-basin system on the west side, and its closure and filling destructed at the end of Late Triassic (Liu et al. 1993; Mo et al. 1993). Yidun Island Arc is mainly occupied by volcanic-sedimentary rock series formed in Late Triassic and granite basement formed in Late Indo-China—Yanshanian and is characterized by volcanic-intrusive complex combination developed. The subduction arc-forming occurred in 238–210 Ma, which is equivalent to the formation time of arc volcanic rocks in the Late Triassic. The collision arc-forming (collision orogeny) occurred in 208–138 Ma, which is closely related or distributed with volcanic arc granite in Late Triassic. The back-collision spreading occurred in 138–75 Ma, which is equivalent to the formation time of type A granite in Late Yanshan.

2.2.1.1.2 Spatial Distribution of Yidun Arc-Basin System

Yidun Arc-Basin System is developed with Ganzi-Litang Ophiolite Melange Zone, Que’er Mountain-Daocheng Outer Volcanic-Magmatic Arc Zone, Changtai-Xiangcheng Intra-arc Rift Basin Volcanic Zone, Dege-Changdagou Inner Volcanic-Magmatic Arc Zone, Baiyu Kongma Temple-Mianlong Back-arc Spreading Basin Volcanic Zone and Gaogong-Cuomolong Back-arc Inter-plate Volcanic-Magmatic Zone from east to west (Fig. 2.4).

• (1) Ganzi-Litang Ophiolite Melange Zone

It is an indisputable fact that the westward subduction of Ganzi-Litang Oceanic Crust is the fundamental factor leading to the formation and development of Yidun Island Arc (Liu et al. 1983; Luo et al. 1998; Xu et al. 1992). Ganzi-Litang ophiolite intermittently appears along Ganzi-Litang Junction Zone in the areas of Yushu Zhishimen, Yulong, Ganzi, Litang and West Muli. Most of them have been dismembered into ophiolite melange due to multiple structural deformations. In the range of 5–20 km in width, mafic rocks, ultramafic rocks, basic basaltic volcanic rocks, gabbro-diabase sheeted dike swarms, abyssal radiolarian siliceous rocks and turbidites are mixed with tectonic slices and melange formed by tectonism and slump. The mafic–ultramafic rocks in the zone are mostly mixed in Triassic basalt series, which are not too much, generally 100–500 m long and 10–500 m wide. The gabbro-diabase body is small in quantity but large in area, and its bedding intrusion is located in the basalt series of Quga Temple Formation. The basic basalt series is the most widely distributed volcanic rock in the Late Triassic, and pillow basalt is developed. In the area north of Ganzi, pillow basalt is dominated by Late Triassic Qugasi Formation, which is widely distributed along the fracture zone. In the Litang Area, the basic volcanic rocks were mainly formed in the Middle and Early Triassic, and in Tuguan Village in the southern member, the basic volcanic rocks were formed in the Late Permian (Mo et al. 1993). The thickness of the whole basalt system along the fracture zone is relatively stable, generally 600–1500 m. The sequence of ophiolite complex in Litang, Muli and other places is clear and complete.

Ganzi-Litang Ocean Basin was formed in Late Permian-early Late Triassic, and subducted westward in the middle Late Triassic under Zhongza-Shangri-La Block, forming a typical supporting pattern of Yidun Island Arc-back-arc-basin system on the west side, and its closure and filling destroyed at the end of Late Triassic. Ganzi-Litang Ophiolite Melange Zone is an important gold metallogenic zone, for example, Cuo’a Gold Deposit, Gala Gold Deposit, Niduocun Gold Deposit, Madake Gold Deposit, Xionglongxi Gold Deposit and other gold deposits.

• (2) Que’er Mountain-Daochengwai Volcanic-Magmatic Arc Zone

The outer-arc volcanic-intrusive complex zone is located on the west side of Ganzi-Litang Ophiolitic Melange Zone, which is composed of the first sub-cycle neutral volcanic rocks and its adjacent intermediate-acid plutonic rocks-subvolcanic rocks. The outer-arc volcanic rocks, mainly composed of andesite and andesitic pyroclastic rocks, are caused by the first arcing activity in Carnian of Late Triassic, with an average Rb–Sr isotope age of 220 Ma.

Intermediate-acid plutonic rocks and subvolcanic rocks appear in pairs with andesite line, and they are distributed in dependence, forming the Niyajiangcuo-Daocheng Granite Zone. This zone starts from Niyajiangcuo in the north, passes through Cuojiaoma, Yongjie, Changtaishan to Gongbala and Jiangcuo in Daocheng, and stretches for hundreds of kilometers. There are dozens of rock masses, which are distributed in a strip in SN direction. These rock masses are complex massif formed in multiple stages and caused for many reasons, and the rocks are composed of diorite, quartz diorite, granodiorite, biotite granite and biotite monzogranite. The volcanic-magmatic arc was mainly formed in the early and late Indo-China, with an average age of 226 Ma and 204 Ma, respectively. The representative rock masses in early Indo-China include Niyajiangcuo rock mass (K–Ar age of 221.1 Ma) (the regional survey report of Sichuan Geology and Mineral Bureau Regional Geological Survey Team), Jiaduocuo rock mass (K–Ar age of 224–225 Ma) (the regional survey report of Sichuan Geology and Mineral Bureau Regional Geological Survey Team), part of Cuojiaoma rock mass (K–Ar age of 227 Ma) and part of lithofacies of Dongcuo rock mass. The representative rock masses in late Indo-China include part of Dongcuo rock mass (K–Ar age of 206 Ma; Rb–Sr age of 208 Ma; U–Pb age of 200 Ma), Ajisenduo rock mass (K–Ar age of 201 Ma) (the regional survey report of Sichuan Geology and Mineral Bureau Regional Geological Survey Team) and Ranxigong rock mass (K–Ar age of 207 Ma) (the regional survey report of Sichuan Geology and Mineral Bureau Regional Geological Survey Team). The above data show that the diorites in early Indo-China, which are absolutely dominant in the granite zone, are located in Middle Carnian of Late Triassic, and their genetic type is mainly type I. The location period is basically the same as the formation time of the volcanic rocks on the west side, which indicates that subduction and arcing volcanism-magmatism started in the early and Middle Carnian of Late Triassic. The granitoids in late Indo-China are mainly light-colored granitoids, and the petrogenetic type is type S, which indicates that the arc-land collision and arcing volcanism-magmatism occurred in Rhaetian of Late Triassic (Fig. 2.5). Minerals in the outer-arc volcanic-intrusive complex zone are concentrated in Shangri-La Arc Area at the southern end of the island arc, mainly porphyry and skarn copper, molybdenum and gold polymetallic deposits related to intermediate-acid subvolcanic rocks, for example, porphyry copper, molybdenum and gold deposits in Pulang, Chundu, Xuejiping and Lannitang, and skarn copper polymetallic deposits in Hongshan and Langdu.

Fig. 2.5

Volcanic-intrusive complexes in different secondary tectonic units of Yidun Island Arc. BA—back-arc area; IA—inner arc; IAF—island arc rift; OA—outer arc. 1—Arc granite; 2—Collision granite; 3—Diabase sheeted dike swarm; 4—Diorite; 5—Arc andesite; 6—Basalt-rhyolite bimodal assemblage (coexisting); 7—Basalt-rhyolite bimodal assemblage (interbedded); 8—Arc andesite; 9—Shoshonite-rhyolite bimodal assemblage

• (3) Volcanic zone of Changtai-Xiangcheng Intra-arc Rift Basin

The volcanic zone of the island rift basin is located on the west side of the outer volcanic-magmatic arc zone, and the volcanic-intrusive complex is composed of “bimodal” volcanic rocks and diabase sheeted dike swarms associated with them, which are intermittently distributed in the axial zone of the island rift. The bimodal rock assemblage was caused by the second sub-cycle volcanic activity in Gacun, which occurred in the middle part of Gacun Formation. The Rb–Sr isochron age of basalt is 217 Ma, and that of ore-bearing rhyolite series in Gacun Deposit is 232–203 Ma. The strongly altered ore-bearing dacite volcanic rocks in Gayiqiong Deposit have a K–Ar age of 221–210 Ma, and the ore lead isotope model age of the rock ore is 229–211.8 Ma. Therefore, it can be inferred that the age of acid volcanic rocks in bimodal rock assemblage is 229–212 Ma, which indicates that the “bimodal” volcanic activity occurred in the middle and late Carnian of Late Triassic, which followed the outer arc volcano-intrusive complex. The diabase sheeted dike swarms mainly developed in the northern member of island arc rift zone, that is, between Changtai and Zengke Volcanic Rock Development Area, and penetrated into volcanic-sedimentary rock series of Genlong Formation and the lower member of Gacun Formation in a bedded manner. The intra-arc rift basin has been an important ore-bearing basin for Sedex (black ore) silver-lead–zinc polymetallic deposits, including Gacun Silver Polymetallic Deposit, Gayiqiong Silver Polymetallic Deposit, Shengmolong Sliver Polymetallic Deposit, Quekailongba Silver Polymetallic Deposit, Dongshanji Silver Polymetallic Deposit, etc.

• (4) Dege-Chandagou Inner Volcanic-Magmatic Arc Zone

The inner volcanic-magmatic arc zone is located on the west side of the volcanic zone in the intra-arc rift basin. It is composed of third sub-cycle intermediate-acid volcanic rocks (Norian in Late Triassic), diorite-diorite porphyry and biotite granite-granite porphyry associated, and is the main mass of the inner arc, with dacite-andesite dacite and pyroclastic rocks as the main mass.

The intermediate-acid volcanic rocks are mainly composed of dacite-andesite dacite and pyroclastic rocks. Compared with the outer arc “andesite line”, the dacite line is much smaller in scale, and its acidity is obviously higher, with a large number of lava domes and subvolcanic rocks accompanying it. Diorite-diorite porphyrite is distributed in groups and zones, concentrated in Zengke Area, mainly in small rock strains or lumps and mainly distributed on the west side of the volcanic rock series, and closely coexisted with them. Some of them intruded into the volcanic rock series, and some of them intruded into and penetrated the gabbro-diabase in the island arc rift. Most of the rock masses are covered by the strata of Miange Formation, which revealed that its formation period was the same as that of the third sub-cycle volcanic activity of Gacun Formation, but it might be a little later, indicating the island arc environment of original arc-land collision.

• (5) Volcanic zone of Baiyu Zhama Temple-Mianlonggou Back-arc Spreading Basin

The volcanic zone of the back-arc spreading basin is generally located on the west side of the inner volcanic-magmatic arc zone, with local overlap. The back-arc volcanic rocks in Yidun Island Arc belong to Miange Formation in the Upper Triassic, and the volcanic-stratigraphic horizon is equivalent to that of Lana Mountain Formation (T₃) previously divided. The volcanic rock series ranges from Denglong Town, Baiyu County in the north to Dulonggou in the south, with a width of 5–10 km, spreading for about 120 km along the strike in a strip manner in NNW direction, parallel to Changtai Volcanic Arc. The back-arc volcanic rock series is composed of upper and lower members. The lower member is composed of basaltic lava and basaltic tuff, which are small in volume and not spread far away, and are concentrated in Miange, Labagou and other places. The upper member is composed of an acidic volcanic rock series, which consists of dacite lava and dacite clastic rocks in the lower part and rhyolitic welded tuff in the upper part, mainly rhyolite rock series. The scale of acid volcanic rock series is much larger than that of basic rock series, with a thickness of several hundred meters, and it is covered by the overlying black slate series. Generally speaking, the back-arc area is characterized by “bimodal” volcanic activities.

The bimodal volcanic rock series in the back-arc-basin of Yidun Island Arc developed in a significant extensional tectonic environment on the continental crust basement, which has the tectonic conditions for the formation of volcanic rock hygroscopic low-temperature gold and silver polymetallic deposits. Along the volcanic zone (Miange Formation), besides the middle-sized gold-silver polymetallic deposit in Nongduke and the large mercury deposit in Kongma Temple, the regional geochemical exploration and microwave remote sensing data also show a number of comprehensive mineralization anomalies in Tage, Darike and Dulonggou, which shows a good metallogenic prospect in this volcanic zone.

• (6) Gaogong-Cuomolong Back-Arc Inter-Plate Volcanic-Magmatic Zone

The back-arc inter-plate volcanic-magmatic zone is located on the west side (continental side) of the volcanic zone in the back-arc spreading basin, which is distributed in a strip shape and roughly parallel to the volcanic rocks in the back-arc-basin, and is separated by the strata of Lana Mountain, which is dominated by psammite formed later. The inter-plate volcanic strata belong to the Tumugou Formation (T₃t) formed in the Late Triassic, which is similar in horizon to the Miange Formation in the back-arc-basin. The volcanic rocks are rhyolite. The inter-plate magmatic rocks developed in the narrow area between the Keludong-Xiangcheng Fracture Zone and the Aila-Riyu Fracture Zone, starting from Gaogong, Shiqu in the north and reaching Batuoren, Batang in the south, and constitute the second large granite zone, namely Gaogong-Cuomolong Granite Zone. The rock mass intrudes in Tumugou Formation of Upper Triassic, showing obvious intrusive contact; most of the rock masses are complex rock masses, with porphyritic moyite in early stage and porphyritic biotite monzogranite and moyite in late stage. The genetic type of rock is type A, which reflects the stress system of continental crust uplift and spreading after arc-land collision, and indicates the formation environment after collision orogeny; the duration of magma varies from 8 to 29 Ma. For example, the early diagenesis of Gaogong Rock Mass is 115.8 Ma, and the late diagenesis is 86.9 Ma; the early diagenesis of the Cuomolong Rock Mass is 84.8 Ma, and the late diagenesis is 76.8 Ma. The peak of the magmatic event is around 87 Ma.

The rock mass in the back-arc inter-plate volcanic-magmatic zone is often accompanied by tin and silver polymetallic mineralization, and many tin and silver polymetallic deposits have been found, such as medium-sized Lianlong (Xizhigou) Tin, Silver and Bismuth Polymetallic Deposit and large-sized Xiasai Silver Polymetallic Deposit, forming a large Tin and Silver Polymetallic Mineralized Granite Zone. In addition, the large pyrite-type silver, gold, copper and lead polymetallic geophysical and geochemical anomalies related to inter-plate volcanic rocks show an extremely broad mineral exploration prospect.

2.2.1.2 Formation and Evolution of Yidun Arc-Basin System

2.2.1.2.1 Characteristics and Properties of Yidun Island Arc “Basement”

The ancient metamorphic basement in Yidun is composed of pre-Sinian Qias Group, exposed in “Qias Fault Uplift”, which is equivalent to the Hekou Group on Yangtze Platform. The overlying strata are deposited by platform-type Sinian Guanyinya Formation and Dengying Formation, which indicates that Qias Fault Uplift was once a part of the western margin of Yangtze Platform. The sedimentary facies characteristics and biological features of Ordovician-Lower Permian stratigraphic units are similar to those of Yangtze Platform, indicating that the Zhongza Block and Yidun Area are the components of the western margin of Yangtze Platform. Carbonate and clastic sediments of Upper Permian-Middle Triassic reveal that they were formed in the continental margin basin or slope environment. The Qugasi Formation formed in the Upper Triassic changed from conglomerate and coarse sandstone to sand shale from bottom to top, reflecting the evolution of its formation environment from shoreland to continental shelf, and its basic volcanic rocks also showed the affinity of inter-plate basalt. Therefore, Yidun was once on the western margin of Yangtze Platform. Estimated that the thickness of the basement continental crust of Yidun Island Arc was 20–25 km according to the restriction of magma density on magma eruption and intrusion, which indicated that Yidun Island Arc had the characteristics of thin continental crust basement.

The island arc basement is a continental crust basement that has been stretched for a long time. It is further stretched and thinned in Xiangcheng Area, and a new oceanic crust is produced by local stretching and cracking. The spreading center is located in the Panyong-Baisong in the western part of Xiangcheng, and the age of the oceanic crust is about 231 Ma. Ocean crust fragments marked by mafic–ultramafic rock masses, gabbro-diabase sheeted dike swarms, pillow-massive basalt, abyssal radiolarian siliceous rocks, etc., are widely found in Chizhong-Muyu in the east and Fanyong-Baisong in the west. With the opening of Jinsha River and the westward subduction of the oceanic plate, the passive continental margin of Yangtze River in Yidun developed a graben-horst system under the tension, which controlled the stratigraphic distribution and sedimentary facies characteristics. The graben of the graben-horst system in Yidun Area is located on the east side of Zhongza Landmass and Ganzi-Litang Zone, respectively, and the horst is located on the west side of Ganzi-Litang Zone and Zhongza Block, respectively; the horst has carbonate platform deposits, while the interior of the graben is composed of fine terrigenous clastic deposits and a small amount of siliceous rocks. In Early Carnian of Late Triassic, the graben-horst tectonic pattern in Yidun remained basically unchanged, but a secondary graben-horst system developed in the graben basin (Fig. 2.6).

Fig. 2.6

Graben-horst tectonic pattern and sediment distribution in fore-island arc period in Changtai area

2.2.1.2.2 Formation and Evolution of Yidun Arc-Basin System

On the basis of the ocean basin formed by the spreading of Ganzi-Litang Zone in Late Permian-early Late Triassic, the strata subducted westward under Zhongza-Shangri-La Micro-landmass in the middle Late Triassic. With the westward subduction of Ganzi-Litang Oceanic Plate, Yidun Area entered a new development period on the basis of the graben-horst tectonic pattern in the fore-island arc period, and started the generation, development and evolution of Yidun Arc-Basin System. Generally, it experienced the following processes (Fig. 2.7).

Fig. 2.7

Tectonic evolution of Yidun arc-basin system

2.2.1.2.2.1 Period of Subduction Orogeny (238–210 Ma)

The products of subduction orogeny include Ganzi-Litang ophiolite melange zone, volcanic arc granite zone, main volcanic arc andesite zone, volcanic zone of intra-arc rift basin, back-arc bimodal rock assemblage zone and back-arc intraplate volcanic-magmatic rock zone, etc. They are distributed from east to west across Yidun Island Arc Zone, and can be divided into the following stages:

• (1) Formation period of outer volcanic-magmatic arc (early and middle Carnian in Late Triassic). With the westward subduction of Ganzi-Litang Oceanic Crust, the metasomatism of dehydrated fluid from subduction plate to mantle source induced mantle rock melting, forming a low-density and low-viscosity incipient melting zone below the frontal volcanic zone, and the diapiric fold rising and magma segregation formed calc-alkalic magma, followed by crystallization differentiation to form calc-alkalic volcanic rock series, which constituted the so-called east “andesite line”.

In Xiangcheng in the southern member of the area, the crust was relatively thin and magma differentiation was poor, forming a volcanic arc dominated by andesite-basaltic andesite; in Changtai in the northern member of the area, the crust was relatively thick and magma differentiation is sufficient, forming the volcanic arc dominated by andesite-dacite. On both sides of the volcanic arc running through the whole area, sedimentary rock series characterized by turbidite fans were developed. On the outer side (the east side) of the frontal volcanic arc, the mantle rocks and even the rocks in the crust-mantle transition zone, which are metasomatism by dehydrated fluid and mixed with SZC, partially melt, forming the volcanic arc intermediate-acid magma, which was located on the outer side of the volcanic arc, forming Niyajiangcuo-Daocheng Intermediate-acid Intrusive Rock Zone, and forming the magmatic arc coexisting with the volcanic arc. The age of magmatic granite is 237–208 Ma, and volcanic arc andesite occurred in Tumugou Formation in Upper Triassic, which have similar REE distribution patterns and geochemical characteristics of trace elements. The latter was mainly caused by the partial melting of mantle wedge contaminated by SZC metasomatism, while the former was caused by large quantity of crust-derived materials. The development of granite-andesite volcanic-magmatic arc marks the formation of Yidun Island Arc Orogenic Belt, suggesting that Ganzi-Litang Ocean Basin was reduced and closed in early Late Triassic.

• (2) Development stage of island arc (inter-arc) rift basin (middle and late Carnian of Late Triassic) Basic characteristics of island rift: ① Typical “bimodal” volcanic activities occurred. The “bimodal” volcanic activity occurred after the early arc volcanic activity and was strictly limited to the narrow zone sandwiched by inner and outer volcanic-magmatic arcs. ② With “bimodal” volcanic activities, the significant emplacement of basic magma from shallow to ultrashallow occurred, forming the diabase sheeted dike swarms closely associated with bimodal rock assemblage. ③ There are two units of bimodal rock assemblages, the basic rock of the lower unit is composed of tholeiite with high Fe content and low Mg content, and that of the upper unit is composed of tholeiite with high Mg content and low Ti content. ④ In the island arc rift, the faulted basins are mostly graben or semi-graben, and the basin margin fractures are mostly extensional fractures in the fore-island arc period, spreading in NNW direction. Currently, Zengke Basin, Gacun Basin and Changtai Basin have been identified, which constitute a fracture subsidence zone with different fracture subsidence time, scale and depth. ⑤ Deposition in the island arc rift zone shows clastic rock facies on the slope margin of volcanos, sand shale facies in rift basin and mudstone facies in silled basin. ⑥ The silled basin or depression in the island arc rift zone is an important ore-bearing place for the exhalative-sedimentary volcanic-associated massive sulfide deposit. Formation mechanism of island arc rift: The development of rift depends on the thermal structure of crust and mantle below it. In the continental rift, the temperature near Moho is about 500–700 ℃; in the island arc zone, the crust and mantle are in a high thermal state, and the ground temperature at Moho is as high as 900 ℃. According to the paleotemperature of its adjacent area—West Panzhihua, the paleotemperature of Yidun Island Arc Zone is close to that of the paleo-ocean. The power source of the arc rift may be related to the subduction angle of the plate and the mode of action between the upper and lower plates. The oceanic plate subducted steeply at a large angle, and decoupling occurred between the obducted plate and the subducted plate. The tensile stress field appeared in the back-arc zone. The steep and deep plate subduction often naturally leads to subsidence, breaking and seaward movement of subducted plates. The oceanic trench may cause the island arc plate to be in a tensile state, forming an arc rift or even a rift basin, and accepting the mudstone facies of silled basin or sand shale facies of trough basin.

• (3) Formation of inner volcanic-magmatic arc (Late Carnian-Early Norian in Late Triassic). Due to the regional compression, Ganzi-Litang Oceanic Crust Plate, which once stopped subducting or moving seaward, subducted westward again at the end of Carnian in Late Triassic. As a result, the magma source rock of the island arc rift stopped melting due to the pressure increase, and the island arc rift was destroyed, while its dehydrated fluid and SZC were metasomatism and mixed mantle rocks again. Due to the intervention of water fluid, mantle rocks melted, forming calc-alkalic magma, which was separated and crystallized, possibly mixed with magma or mixed with the crust, forming an inner arc volcanic rock series. Perhaps due to the thickening of the crust, some calc-alkalic magma was partially emplaced from shallow to ultrashallow, forming a large number of subvolcanic rocks and diorite-diorite porphyrite, which constituted the main mass of volcanic-magmatic arc. Although the assemblage of inner arc and outer arc volcanic rocks formed by the early and late arcing volcanism is similar, their geochemical and geodynamic backgrounds are not completely the same. The early outer arc volcanic-magmatic rocks are subducted, and the late inner arc volcanic-magmatic rocks may have been formed in a short period after Ganzi-Litang Ocean closed and started small-scale arcing (Yidun Island Arc-land (Yangtze Plate) collision), which represents the island arc environment during the initial arc-land collision.

• (4) Development stage of back-arc spreading basin (middle and late Norian of Late Triassic) Geophysical research shows that after the initial collision between the continent and the island arc, the oceanic plate will undergo subsequent subduction. In Yidun Island Arc, after the closure of Ganzi-Litang Ocean and even the initial arc-land collision, the subduction of the subducted oceanic plate may also take place. On the one hand, the SZC component mixed with mantle rock and the water fluid from the subduction zone metasomatism mantle rock, on the other hand, the tensile stress field appeared in the back-arc area, and the back-arc spreading basin was formed, which was mainly manifested by the development of the shoshonite-rhyolite bimodal rock assemblage and the sedimentation of the black sand-slate series in the faulted basin. On the west side of Changtai Island Arc in the north member, Miange Back-arc Spreading Basin was developed, the spreading center was located on the line of Kongma Temple-Miange-Nongduke, and developed “bimodal” volcanic activities, forming a high-potassium rhyolite-shoshonite assemblage, which formed the same Rb–Sr isochron, with an age of 213.7 Ma, indicating that Miange Back-arc-Basin was formed in the middle and late Norian of Late Triassic. Wanrong-Qingdarou Back-arc Spreading Basin is developed on the west side of Xiangcheng Island Arc. The basin was mainly filled with clastic rock series of Tumugou (Gacun) Formation and Lamaya Formation, with little or no volcanic activities. According to the stratigraphic correlation of biological fossils and strata, the basin filling sequence was formed in Norian in Late Triassic. On the west side (continental side) of the back-arc spreading basin, the back-arc inner-plate volcanic zone is developed, which is mainly a sequence of acidic volcanic rock series, which is distributed in a strip shape and roughly parallel to the volcanic rocks in the back-arc spreading basin. The volcanic rock stratum belongs to the Tumugou Formation (T₃t) formed in the Late Triassic, and its horizon is equivalent to that of Miange Formation in the back-arc spreading basin. The volcanic rocks have typical characteristics of inter-plate rift magmatism. The back-arc spreading basin is another important ore-bearing place of volcanic shallow-low temperature gold-silver polymetallic deposits. Along the volcanic zone (Miange Formation), besides the middle-sized gold-silver polymetallic deposit in Nongduke and the large mercury deposit in Kongma Temple, the regional geochemical exploration and microwave remote sensing data also show a number of comprehensive mineralization anomalies in Tage, Darike and Dulonggou, which shows a good metallogenic prospect in this volcanic zone.

2.2.1.2.2.2 Collision Orogeny Period (208–138 Ma)

The collision orogeny period is marked by the successive development of collisional granite, orogenic uplift and late granite, accompanied by complex deformation tectonics formed by island arc and crust compression and contraction and shear strain. The magmatic activity during collision orogeny period is interdependent or closely associated with the granite zone of 237–208 Ma. The rock masses of interdependent distribution are distributed at the north and south ends of the Cuojiaoma-Daocheng Granite Zone, and the closely associated rock masses are small in scale and often intrude into the interior and edge of the granite base of 237–208 Ma.

At the end of Late Triassic, Ganzi-Litang Ocean Basin contracted into a residual basin along the subduction zone, and the main mass of Yidun Island Arc rose, partially accepting delta distributary channel facies and coastal swamp facies deposits, forming the molasse-miscellaneous debris tectonics. The high-density oceanic crust dragged the low-density continental crust to subduct along the subduction zone, resulting in the shortening of the continental crust and the arc-continent convergence collision. With the downward subduction and the change of P–t conditions, significant dehydration occurred, which on the one hand resulted in the double thickening of the collision zone crust, and on the other hand formed the collision granite with crust source, which penetrates into or was located in the volcanic arc granite zone and its vicinity. With significant shortening of the continental crust and the double thickening of the crust, the collision zone rose greatly, and the island arc magma in the crust, which may have been “frozen” in the island arc orogenic belt, was “activated” again, resulting in hypabyssal rock—ultrahypabyssal rock emplacement.

The syn-collision granites were relatively large in quantity, interdependent or closely associated with the 237–208 Ma granite zone, and distributed at north and south ends of Cuojiaoma-Daocheng Granite Zone. The main rocks were two-mica granite and moyite, followed by monzonitic granite and monzodiorite, which were the continental crust remelting granite. Late orogenic granites mainly occurred in the western granite zone of Shangri-La, and the dating data of granites vary greatly, ranging from 138 to 200 Ma, indicating that the granites were formed and located after the syn-collision granites. The magma evolution sequence is quartz monzonite diorite porphyrite → quartz dioritic porphyrite → quartz monzonitic porphyry → monzonitic granite porphyry, which belongs to crust-mantle granite. Minerals in collision orogenic period are concentrated in concentrated in Shangri-La Arc Area at the southern end of the island arc, mainly porphyry and skarn copper, molybdenum and gold polymetallic deposits related to intermediate-acid subvolcanic rocks, for example, porphyry copper, molybdenum and gold deposits in Chundu, Xuejiping and Lannitang, and skarn copper polymetallic deposits in Hongshan and Langdu.

With significant shortening of the continental crust, a series of axial vertical syn-cleavage folds in NWW-SN to south-north direction were formed, and the shear strain from west to east occurred, forming an arc-shaped ductile shear zone and phyllonite zone with gentle dip to the west in nearly SN direction, accompanied by the stretching lineation of recumbent folds and vertical shear zones.

2.2.1.2.2.3 Post-Orogenic Extension Period (138–75 Ma)

The typical magmatic product of post-orogenic extension is type A granite in Late Yanshanian, which developed in the narrow area between Keludong-Xiangcheng Fault Zone and Aila-Riyu Fault Zone, and was spatially distributed in the back-arc zone, forming another important granite zone in this area, namely Gaogong-Cuomolong Granite Zone. Type A granites formed in Late Yanshanian are often accompanied by tin and silver polymetallic mineralization, and many tin and silver polymetallic deposits have been found, such as medium-sized Lianlong Tin, Silver and Bismuth Polymetallic Deposit and large-sized Xiasai Silver Polymetallic Deposit, forming a large Tin and Silver Polymetallic Mineralized Granite Zone. A significant tin-bearing granite zone has been formed in the back-arc area of Yidun Island Arc, showing a broad mineral exploration prospect.

Since the concept of type A granite was put forward, its connotation and extension have changed greatly, and its genetic types and tectonic environments are also varied. Eby divided type A granites into two types, namely type A1 and type A2, based on the geochemical characteristics and tectonic environment of type A granites all over the world. The former type is geochemically similar to the magmatic hearth mantle of ocean island basalts (OIB), and formed in the continental rift and mantle plume (hot point). The latter type is geochemically similar to continental and island arc basalts, and the magma is derived from the crust or island arc and formed in the extensional tectonic environment after collision or orogeny. Gaogong-Cuomolong Granite Zone reflects that it is equivalent to inter-plate extensional granite, that is, type A granite defined by Pearce generally shows the geochemical characteristics of type A2 granite.

Due to double thickening of the crust, rock metamorphism in the lower crust led to high-density mineral assemblages (eclogite facies), and shortening of the continental crust led to thickening of the crust and lithosphere, resulting in the temperature of the mountain root being relatively lower than that of the asthenosphere. The hot material structure produced potential gravity instability, which led to rooting or lower crust subsidence. As a result, the hot asthenosphere at the lower part upwelled greatly, replacing the cold lithosphere, and led to partial melting of the crust, resulting in significant magmatism, that is, the significant development of type A granite. The main mass of post-orogenic spreading in this area occurred in 138–75 Ma, and the peak of spreading may be 80 Ma.

2.2.1.2.2.4 Himalayan Intracontinental Orogeny (65–15 Ma)

The main Himalayan intracontinental orogeny occurred in Qinghai-Tibet Plateau, manifested as the collision uplift of Tibetan Plateau. Its long-range effect in Yidun Island Arc Collision Orogenic Belt showed thrust nappe structure and significant strike-slip translation, as well as the formation of pull-apart basin and Himalayan granite emplacement.

Yidun Island Arc Collision Orogenic Belt has basically been shaped after the subduction orogeny in Late Triassic, collision orogeny in Jurassic-Cretaceous and subsequent post-orogenic spreading. In Cenozoic, due to the northward wedging collision of the Indian Plate and its intracontinental convergence orogeny, the main mass of Qinghai-Tibet Plateau uplifted and was orogenic, while the post-orogenic spreading of Yidun Island Arc Collision Orogenic Belt stopped and then vertically uplifted rapidly, and significant thrust nappe occurred. The significant translational strike-slip accompanied formed a series of beaded strike-slip pull-apart basins, accepting the Paleogene and Neogene red bed deposits. Himalayan granitic magmatic activity occurred along the nearly SN direction translational fracture, forming a nearly north–south granite belt. Although this sequence of granite is also type A granite, which shows the characteristics of intraplate extensional granite, the geochemical difference between this sequence of granite and type A granite during post-orogenic spreading indicates that they may have different magma sources and dynamic backgrounds. Himalayan granitic magma shows more characteristics of crust-mantle mixed source, which is mainly controlled by Dege-Xiangcheng Deep Fault (which changed to strike-slip translation fracture in Himalayan); the granite magma in Late Yanshanian mainly originated from continental crust remelting and was restricted by the spreading of regional orogenic belts.

Himalayan granite emplacement was the latest magmatic event of the Yidun Island Arc Collision Orogenic Zone. The exposed rock masses in this area include Batang Genie rock mass, Xiangcheng Cilincuo-Riyong rock mass, Changtong rock mass, Shangri-La Chuliang rock mass, etc. Among them, Genie rock mass is distributed in Gaogong-Cuomolong Granite Zone, and other rock masses constitute the main mass of Cilincuo-Chuliang Granite Zone. Genie rock mass intruded in Triassic sand-slate series and Hagela rock mass (39–81 Ma), and its main mass is porphyritic medium-fine moyite, with a ⁴⁰Ar/³⁹Ar plateau age of 15 Ma. Cilincuo-Riyong rock mass intruded in the sand-slate system of Lamaya Formation in Upper Triassic, and is the compound batholith. The main rocks are porphyritic fine-grained monzonitic granite and hornblende biotite monzogranite. The age of Cilincuo rock mass is 65 Ma, while that of Riyong sheeted dike is 60 Ma. Generally speaking, the Himalayan magmatism lasted for a long time (50 Ma), but the period of magmatism in each rock mass was short.

All ore deposits formed in Yidun Island Arc Zone in different ages were shaped in the process of intracontinental orogeny, and were superimposed and reformed by tectonism and magmatic activity to varying degrees, showing the characteristics of multiple minerals and complex and diversified deposit types in an ore deposit.

2.2.2 Temporal and Spatial Structure and Its Evolution of Jinsha River Arc-Basin System

On a regional basis, the Jinsha River tectonic zone and Ailaoshan tectonic zone belong to the ocean basin subduction and arc-land collision junction zone. Both of them are comparable in terms of the formation, subduction and destruction of ocean basin and the evolution history of the continental margins on both sides. Thus, they belong to the same arc-land collision junction zone and are also an important polymetallic metallogenic zone of copper, gold, lead and zinc.

2.2.2.1 Temporal and Spatial Structure of Jinsha River Arc-Basin System

Jinsha River Tectonic Zone (Fig. 2.8) between Qamdo-Lanping Landmass in the west and Zhongza-Shangri-La Landmass in the east experienced the splitting of Devonian -Carboniferous strata, spreading of Late Carboniferous-Early Permian Ocean Basin, subduction and destruction of Permian stratum, and collisional orogeny of Triassic stratum. The strata in the zone suffered from significant tectonic deformation and metamorphism, which destroyed the original sequence and relationship of strata. Except for the volcanic arc zone and stable landmass on both sides that are relatively complete, the extrusion fracture, schistosity and mylonitization were extremely developed in Jinsha River Zone. Tectonic melanges such as mafic–ultramafic rocks, carbonate rocks, siliceous rocks and basic volcanic rocks were found everywhere in the period from Devonian to Permian; the matrix was flysch sand-slate and intermediate-basic pyroclastic rock in Permian stratum-Triassic stratum.

Fig. 2.8

Distribution of tectonic–magmatic rocks of Jinsha river zone. 1—Granite (γ4-5); 2—red sandstone distribution area (J-K); 3—clastic rock sedimentation area (T3); 4—intermediate-acid volcanic zone in continental margin arc (T1-2); 5—volcanic zone of Jiangda-Deqin-Weixi superimposed rift basin (T2²–T3¹); 6—intermediate-basic volcanic zone of Ardenge-Nanzuo continental margin arc (P1²–P2); 7—intermediate basic volcanic zone of Zhubalong-Benzilan intra-oceanic arc (P1²–P2); 8—Deqin-Shimianchang tectonic melange zone; 9—Jinsha river tectonic melange zone; 10—Qingnidong-Haitong overthrust zone; 11—Qamdo-Lanping block; 12—Zhongza-Shangri-La block

2.2.2.1.1 Formation Age of Jinsha River Back-Arc Ocean Basin

In recent years, radiolarians formed in Late Devonian-Early Permian have been found in siliceous rocks associated with oceanic ridge basalt in Jinsha River Melange Zone. Radiolarians in Early Carboniferous, such as Albaillella paradoxa defladree, Astroentactinia multispinisa Won; radiolarians in Early Permian, such as Albaillella sp., Pseudoalbailla sp., etc.; radiolarians in Late Devonian-Early Carboniferous, such as Entactinia sp., Entactinosphera sp., Entactinia parva Won, E. tortispina Ormiston et Lane, Entactinosphera foremanae Ormiston et Lane, En. cometes Foreman, En.deqinensis Feng, Belowea varibilis (Ormiston et Lane), Astroentactinia multispiosa (Won) etc. The U–Pb age of zircon in oceanic ridge-quasi-oceanic ridge basalt in Jinsha River melange zone is (361.6 ± 8.5) Ma indicating that zircon was nearly formed in Early Carboniferous. The Rb–Sr isochron age of cumulate in Jiyidu is (264 ± 18) Ma (Mo et al. 1993), indicating that cumulate was nearly formed in Early Maokou of Early Permian. It shows Jinsha River back-arc ocean basin was formed in Early Carboniferous, and Jinsha River back-arc rift basin in Late Devonian was developed into the late development stage with the embryonic form of back-arc ocean basin. Jinsha River back-arc ocean basin was developed into the peak stage in the early stage of Early Permian, with a width of about 1800 km (Mo et al. 1993).

Jinsha River Ocean Basin began to subduct westward in the late stage of Early Permian and then destruct below Qamdo-Lanping Landmass, forming an intra-oceanic arc in Zhubalong-Yangla-Dongzhulin and a back-arc-basin of Xiquhe-Xueyayangkou-Jiyidu-Gongnong in the west of the intra-oceanic arc (basement of oceanic crust, P1²–P₂) from east to west (basement of oceanic crust, P1²-P₂); Jiangda-Deqin-Weixi continental margin volcanic arc and Qamdo-Lanping back-arc-basin (base bed of oceanic crust, P1²–P₂) in the west of the continental margin arc. The Early and Middle Triassic formed Jiangda-Deqin-Weixi volcanic-magmatic arcs and the Qamdo-Lanping arc-back foreland basin (T_1–2) on the west side of the volcanic arc through oblique subduction collision; the superimposed rift basin of Chesuo Township-Xu Zhong-Luchun-Hongpo-Cuiyi was formed in the late stage of Middle Triassic to the early stage of Triassic (T₂²–T₃¹).

The so-called angular unconformity between Upper and Lower Permian stratum in Gerongna, Batang was once considered as the product of oceanic crust destruction and ocean basin close to Jinsha River Paleo-Tethyan Ocean in Early Permian and “Late Variscan Orogeny”. After investigation and tracing for three lines, it was determined as a slip nappe composed of sandshale-limestone formed in Late Permian mixed with volcanic rocks from east to west, which overlaid on the melange with garnet mica quartz schist as the matrix.

2.2.2.1.2 Spatial Distribution of Jinsha River Arc-Basin System

Jinsha River Arc-Basin System can be divided into the following zones in space from east to west: passive margin fold-thrust zone, ophiolite tectonic melange zone, volcanic zone of superimposed rift basin, Shimianchang Tectonic Melange Zone and continental margin arc volcanic zone (Fig. 2.8).

2.2.2.1.2.1 Passive Margin Fold-Thrust Zone

The passive margin fold-thrust zone is located on the western margin of Zhongza-Shangri-La Block, with developed fault tectonics, significant thrust nappe-extensional detachment tectonics, and multi-stage tectonic transformation and transformation, which is characterized by tectonic slices of thrust in different stratigraphic ages overlaying westward in an imbricate way, or are limited to the tight folds in shear fracture zones. With its unique limit Smith stratigraphic configuration, this zone overthrusts westward on the non-Smith stratigraphic system of Jinsha River Tectonic Melange Zone, while it is dominated by the nappe-detachment fault in the east, which is different from the Smith stratigraphic sequence in Zhongza-Shangri-La Block. Silurian stratum-Devonian stratum are composed of carbonate rocks mixed with clastic rocks and are developed into stromatoporoid reef in Middle and Late Devonian; Carboniferous stratum-Permian stratum are a sequence of collapse accumulation of carbonate rocks, turbidite mixed with basalt, basaltic andesite, pyroclastic rocks and carbonate rock, which are characterized by stratigraphic sequence and rock assemblage in the passive margin rift basin in the western part of Zhongzan-Shangri-La Block.

The western passive margin fold-thrust zone in Zhongza-Shangri-La Block starts from Dongpu in Dege Country in the north, and extends to Nixi, Tuoding and Shigu in Shangri-La in the south through Batang, Zhongza and Derong. The main body of this zone extends from north to south along the east bank of Jinsha River, and is characterized by developed imbricate thrust nappe, extensional detachment fault and detached block tectonics, accompanied by significant tectonic mylonitization, rheid fold, flow cleavage and dynamic metamorphism. It is not only a huge nappe-detachment tectonic zone with a length of 600 km, but also a metallogenic zone of copper, lead, zinc and other nonferrous metals. Tuoding Copper Ore Deposit (medium-sized carbonate rock in Devonian stratum), Gelan Copper Ore Deposit in Shangjiang (Pt₂ small-sized low-grade metamorphic rock), Sanjia Village Lead–Zinc Ore Deposit (medium-sized carbonate rock in Cambrian stratum) and Najiao Lead–Zinc Ore Deposit (medium-sized carbonate rock in Cambrian stratum).

2.2.2.1.2.2 Ophiolite Tectonic Melange Zone

The ophiolite melange zone is on the west side of the passive margin fold-thrust zone, and is the main body of Jinsha River Junction Zone. The reason why the ophiolite melange zone is described here is that in addition to ocean basin sediments, many sedimentary blocks in the passive continental margin in the west of Zhongza-Shangri-La Block are also mixed in the ophiolite melange zone. For example, the study on Yangla Copper Ore Deposit showed that a sequence of continental slope clastic rock formed in Devonian stratum, mixed with carbonate rock sediments was developed between oceanic ridge basalt in Late Carboniferous and intermediate-basic volcanic rocks in Permian, which was an overturned fold and was in contact with surrounding rocks in a way of fault. The similar condition is common on both sides of Jinsha River Valley, of which the distribution is consistent with that of the ophiolite melange zone, namely, a north–south distribution. The ophiolitic melange zone can be roughly divided into two subzones as seen from the surface. The eastern subzone extends from Bengzha to Derong in Batang, generally including E’aqin Group, some of Zhongxinrong Group and Gajinxueshan Group divided by the regional geological mapping with an original scale of 1:200,000. From the perspective of lithologic assemblage, it is a sequence of metamorphic basic volcanic rocks, ultrabasic rocks and oceanic ridge basalt; the main body is a sequence of mafic and ultramafic rock assemblage of the lower oceanic crust. The western subzone extends through Zhubalong-Yangla-Gongka and is composed of the upper oceanic crust substances with the characteristics of intra-oceanic arc, such as basaltic andesite, amphibolite basaltic andesite, siliceous slate, fine clastic rock, siliceous rock, etc. Taking Xiquhe Bridge and Yangla Copper Deposit as an example, it is composed of Gajinxueshan Group and the members a + b of the mining area, which are mainly composed of sedimentary rocks, mafic rocks and andesite, and its volcanic rocks have the characteristics of island arc volcanic rocks. Generally speaking, the oceanic crust is mainly composed of the lower oceanic crust with a large thickness, while the members of the upper oceanic crust are composed of sedimentary rocks, oceanic island volcanic rocks and island arc volcanic rocks. At the rapidly spreading oceanic ridge, the thickness of the lower oceanic crust is relatively large and has certain sequences; at the slowly spreading oceanic ridge, the thickness of the lower oceanic crust is very small and has no sequence. Compared with the distribution characteristics of the ophiolite melange zone in Jinsha River, it can be seen that Jinsha River Ocean Basin is a slow-spreading tecnotics. Besides the later tectonic reworking, the three groups mentioned above shall be regarded the same in composition, and the difference is that the eastern zone is dominated by lower oceanic crust substances while the island arc (intra-oceanic arc) volcanic rocks are accumulated in the western zone.

According to the study of the surrounding rock of Yangla Copper Deposit, the age of oceanic tholeiite is 361.6 Ma (zircon U–Pb method), indicating that Jinsha River Ocean Basin has spread into ocean in Early Carboniferous, and began to subduct in the late stage of Early Permian, so intra-oceanic arc volcanic rocks (257 Ma, amphibole K–Ar method) were found in the west side of the ocean basin; and radiolarians in Maokou were formed in the associated siliceous rock in Zhubalong intra-oceanic arc volcanic rocks (Peng Xingjie); continental margin arc volcanic rocks in the late stage of Early Permian were formed in Deqin-Weixi area on the western margin of ocean basin, rocks in Maokou were formed in limestone associated with arc volcanic rocks, and 220 Ma (biotite K–Ar method, Yunnan Bureau of Geology and Mineral Resources) to 280 Ma (zircon U–Pb method) island-type granodiorites were formed in Baimangxueshan area, all of which are the evidence that the Jinsha River Ocean Basin began to subduct and destruct westward. It can be seen that Jinsha River Ocean Basin was mainly spread in the early stage of Carboniferous-Early Permian, and subducted and destructed from the late stage of Early Permian to Early and Middle Triassic. Jinsha River-Ailaoshan Ophiolitic Melange Zone is an important metallogenic zone of precious and nonferrous metals such as gold, chromium and copper, including Ailaoshan Super-large Gold Ore Deposit (ophiolitic melange), Yangla Large Copper Ore Deposit (intra-oceanic arc volcanic-sedimentary rocks) and a sequence of copper and gold ore occurrences.

2.2.2.1.2.3 Volcanic Zone of the Superimposed Rift Basin

The volcanic zone of the superimposed rift basin is on the west side of the ophiolite melange zone, distributed in Xuzhong of Yanjing County, Luchun-Jijiading area of Deqin County, Reshuitang-Sizhuangzi Bridge area, Pantiange-Qiaohou area of Weixi, and developed in the continental margin volcanic arc of Jiangda-Weixi and its marginal zones. It is a superimposed rift basin formed by extension in the post-collisional island arc orogenic belt, with the formation age from the late stage of Middle Triassic to the early stage of Late Triassic. There are volcanic-sedimentary rocks with well-developed geological sections in Luchun-Jijiading area of Deqin County, Reshuitang-Sizhuangzi Bridge area of Deqin County and Pantiange-Cuiyibi area of Weixi County. In the basin, a sequence of tholeiite and alkaline rhyolite assemblage with the characteristics of “bimodal” volcanic rocks and a large number of gabbro-diabase dyke groups under extensional background are developed, accompanied by bathyal siliceous rocks, radiolarian siliceous rocks, tuffaceous turbidite, tuffaceous-siliceous turbidite, arenopelitic flysch, fine clastic rocks and limestone lens. The initial value for 87Sr/86Sr of basalt in Luchun Deposit varies from 0.7065 to 0.7194, and the Rb–Sr isochron age is 236 Ma. The initial value for ⁸⁷Sr/⁸⁶Sr of rhyolite varies from 0.7099 to 0.7213, and the Rb–Sr isochron age varies from 224 to 238 Ma (on a scale of 1: 200,000 in Deqin country, 1985); the initial value for ⁸⁷Sr/⁸⁶Sr of rhyolite in Pantiange area of Weixi is 0.7074, and the Rb–Sr isochron age is 235 Ma. It can be seen that the “bimodal” volcanic rocks (Cuiyibi Formation) in the Pantiange area of Weixi and the “bimodal” volcanic rocks (Renzhixueshan Formation) in Luchun Deposit in Deqin have the same geotectonic background and volcanic rock development sequence. Basalt and rhyolite are from the same magma source, and the volcanic rocks were formed in the early stage of Late Triassic.

The superimposed rift basin has become an important ore-bearing basin for sedimentary exhalative massive sulfide copper–gold-silver-lead–zinc polymetallic ore deposits, such as Zuna Lead–Zinc (Silver) Ore Deposit in Shengda-Chesuo Basin, Zhaokalong Large Siderite Type Silver-Rich Polymetallic Ore Deposit and Dingqinnong Copper–Gold (Lead–Zinc-Silver) Ore Deposit; Luchun Zinc-Copper-Lead (Silver) Polymetallic Ore Deposit in Luchun-Hongpo Basin, Hongponiuchang Copper–Gold (Lead–Zinc) Polymetallic Ore Deposit and large gypsum ore deposit in Lirenka-Bamei area; Laojunshan Medium-sized Lead–Zinc (Silver) Ore Deposit and Chugezha Large Siderite Type Ore Deposit in Reshuitang-Cuiyibi Basin.

2.2.2.1.2.4 Shimianchang Tectonic Melange Zone

Deqin-Shimianchang Tectonic Melange Zone is on the west side of the volcanic zone in the superimposed rift basin, and distributed on the west side of Xuzhong and passes through Deqin County, Shimianchang-Yanmen area. It is the product of the destruction of the rift valley basin on the margin of Qamdo Landmass and its post-arc-land collision in Carboniferous-the early stage of Early Permian. This zone in Shimianchang area is a melange mainly composed of serpentinite ultrabasic rocks, schistose basalt, gabbro-diabase, siliceous rocks and limestone as blocks and mica schist, mica quartz schist and chlorite quartz schist as matrix. Slumping blocks composed of continental shelf carbonate rock, clastic rock and slope carbonate rock, as well as basinal turbidite, contourite, radiolarian siliceous rock (P₁), ultrabasic rock and gabbro-diabase were developed in Early Permian from west to east on the geological section of Yonghongrongqiu, Deqin, showing the sequence characteristics of continental margin rift (valley) basin. Shimianchang asbestos ore deposit related to serpentinite ultrabasic rocks has been found in Shimianchang Tectonic Melange Zone.

2.2.2.1.2.5 Continental Margin Arc Volcanic Zone

As an important part of Jinsha River Arc-Basin System, the continental margin arc volcanic zone is located on the west side of Shimianchang Tectonic Melange Zone and on the east margin of Qamdo-Lanping Landmass. In spatial terms, its main body is distributed on the east side of Zongla Mountain Pass in Markam County, Qamdo and area between Nanren-Bucun-Nanzuo in Deqin County and Badi-Yezhi area in Weixi County, which is called Jiangda-Deqin-Weixi Continental Margin Arc in Permian, including the upper part of Jidonglong Formation and Shamu Formation overlaid on it. The earliest arc volcanic activity was found in Nanren-Feilaisi area of Deqin County in the late stage of Early Permian, and Neomisellina aff.douvillei (Gubber), N.aff.sichuanesis Yang, Kahlerina sp., Reichelina sp. were found in the limestone containing bioclast in the upper thin bed of Jidonglong Formation, which was an arc volcanic activity happened in the late stage of Maokou of Early Permian and lasted until Late Permian. Tholeiite → calc-alkaline → potassium basalt rocks were developed in the volcanic rocks from morning to night. The rock types include quartz tholeiite, intermediate potassium andesite, dacite, rhyolite and its pyroclastic rocks. The properties of volcanic rock mark the complete process of generation, development and maturity of island arc (Mo et al. 1993).

Columnar joints of basaltic andesite formed in Permian were developed on the east side of Zongla Mountain Pass in Markam County, Qamdo, which belongs to continental eruption. The volcanic rocks in Adengge, Deqin are mixed with siltstone and carbonate rocks, forming a semideep-neritic environment. Very developed columnar joints of basaltic andesite were found on the west side of Feilaisi, Deqin, which belongs to continental eruption. The volcanic rock-carbonate rock assemblage with pillow structure developed in the volcanic rock in the Nanzuo-Bucun area belongs to a semideep-neritic environment. Volcanic rocks in the Shamu area are associated with sandshale containing plant fossils and brachiopod fragments, reflecting the marine-continental environment. Volcanic turbidite and volcanic source turbidite of submarine fan facies are developed in Yanmen Township. In Badi-Yezhi area, Weixi Country, a sequence of flysch sand-slate, metamorphic volcanic rocks and conglomerate, intermediate-acid volcanic breccia, slump breccia, marlstone and sedimentary sand-slate with incomplete Bouma sequence were found in Kangpu, Weixi and Xidagou, Jicha, indicating that arc volcanic rocks have entered the marginal slope-basinal deep-water environment. It shows that the distribution environment of arc volcanic activities is quite different in space. Arc volcanic rocks have varied lithofacies and diverse sedimentary types in space, and the topography of island arc fluctuates greatly. There is a land with terrestrial plants and columnar joints outcropped from the water surface, and there are also carbonate rock plateau and abyssal valleys grown underwater. There are various sedimentary facies and types of sediments, varying from continental facies-marine-continental transitional facies-neritic facies-plateau slope facies-basinal deep-water facies, so as to form a tectonic paleogeographic pattern in an island-chained distribution way.

Jiaduoling-Dongka Medium-sized Iron-Copper Ore Deposit, Renda Copper Ore Deposit, Azhong Gold Ore Deposit, Nanzuo Medium-sized Lead–Zinc (Copper, Silver) Ore Deposit, Lirenka Large-scale Lead–Zinc (Copper, Silver) Ore Deposit, and a sequence of ore occurrences and gold anomaly areas have been found in Jiangda-Deqin-Weixi Continental Margin Arc Volcanic Zone in Permian, with various ore deposit types.

2.2.2.2 Formation and Evolution of Jinsha River Arc-Basin System

2.2.2.2.1 Geological and Paleogeographic Features of “Metamorphic Soft Basement” in Pre-Devonian

Regionally, Yangpo Formation of the former Shigu Group in Shigu area of Zhongza-Shangri-La Block on the east side of Jinsha River Zone is a sequence of metamorphic rocks of high-grade greenschist facies-amphibolite facies, among which the Nd model age of plagioclase amphibolite is between 1343.9 and 1369.9 Ma, and its metamorphic Rb–Sr isochron age is (996.1 ± 33.7) Ma which is quite consistent with the curing time limit (800–900 Ma, Jingningian) of the metamorphic basement in Yangtze Landmass. Therefore, it is the basement of Yangtze Landmass, and the quite stable plateau-type sediments formed in Paleozoic remain on it. In Qamdo-Lanping Block on the west side of Jinsha River, the age of single mineral zircon in the gneiss of Ningduo Group in Changqingke area, Yushu, Qinghai Province is 1870 Ma, which can be compared with that of Qias Group in Western Sichuan and Ailaoshan Group in Western Yunnan. The age values of the two types of granites intruded are 1780 Ma and 1680 Ma, respectively. There are tillites in the upper part of the Caoqu Group and basalts in its lower part, both of which are two important characteristics of plateau-type sediments in Sinian and Yangtze Landmass in the south extending into the Sanjiang. The main metamorphic age of Ningduo Group and Caoqu Group is 640 Ma, which is equivalent to the time limit of Chengjiang Orogeny during the final consolidation of the basement of Yangtze Landmass. Therefore, Qamdo-Lanping Block shall also be the basement of Yangtze Plateau, that is, Qamdo-Lanping Block and Zhongza-Shangri-La Block on both sides of Jinsha River were a unified block during Sinian- “Pan-Yangtze Landmass”.

In Early Paleozoic, the areas of Western Sichuan, Western Yunnan and Eastern Tibet on the unified landmass had the “sedimentary cover formed in Sinian and later age” (Liu et al. 1993). Aulacogen was formed along Jinsha River and its two sides in Caledonian of Early Paleozoic, and its representative sediments are mica phyllite, mica-quartz phyllite mixed with marble and amphibolite (schist) in Longba Formation of original Shigu Group, and the representative source rocks are composed of semi-pelagic siliceous and arenopelitic flysch and semi-pelagic flysch clastic rocks in Qingnidong Group of Middle and Lower Ordovician stratum. Qamdo-Lanping Landmass and Zhongza-Shangri-La Block on both sides of aulacogen of Jinsha River maintain a relatively stable plateau-type sedimentary environment; in Early and Middle Ordovician, neritic carbonate rocks and clastic rocks were deposited in Yanjing and Markam areas of Eastern Tibet, while the Upper Ordovician stratum is lacking, and the Silurian stratum is composed of neritic sandy arenopelitic sediments. Apart from clastic rock and carbonate rock sediments of trilobites in Cambrian, there are also continuous biocarbonate rock sediments of Ordovician stratum-Silurian stratum in Zhongza-Shangri-La area. The fossils are characterized by neritic coral and reef-building coral of bryozoa and stromatopora; there are also other abundant benthos. The neritic plateau type sediments of Lower Paleozoic stratum “can be compared with that of Yangtze Plateau in both biological and sedimentary characteristics” (on a scale of 1: 200,000 in Shangri-La 1985).

The aulacogen of Jinsha River was closed and rose to form land along with the stable landmass on both sides at the end of Caledonian, which is characterized by Devonian stratum being unconformably overlaid on or being disconformably overlaid on underlying strata (Middle Ordovician stratum and Upper Silurian stratum) regionally. At the same time, Caledonian orogeny caused extensive and intense deformation and metamorphism of Pre-Devonian stratum. In Early Devonian, it began to enter the formation, development and evolution stage of Paleo-Tethys of Jinsha River on “metamorphic soft basement” in Pre-Devonian.

2.2.2.2.2 Formation and Evolution of Jinsha River Arc-Basin System

By taking Qiangtang-Jitang-Chongshan-Lancang Residual Arc on the west side of Qamdo-Lanping Block as the frontal arc, Jinsha River Arc-Basin System entered a new development period at the beginning of Devonian on the basis of the metamorphic “soft basement” of Early Paleozoic, namely the generation, development and evolution of Paleo-Tethyan Jinsha River Arc-Basin System (Fig. 2.9). It generally includes several stages as below.

Fig. 2.9

Geologic evolution history of Jinsha river arc-basin system. 1—acid intrusive rock; 2—marginal clastic rock; 3—plateau carbonate rock; 4—basinal volcanic rock; 5—basalt; 6—continental crust basement; 7—intermediate-basic volcanic rock of island arc; 8—intermediate-acid volcanic rock of island arc; 9—abyssal flysch; 10—ore body; 11—tectonic melange; 12—rhyolite; 13—“bimodal” volcanic rock; 14—plateau clastic rock; 15—oceanic crust; 16—subduction direction

2.2.2.2.2.1 Rift (Valley) Basin Stage (D)

In Early Devonian, Western Sichuan, Western Yunnan and Eastern Tibet had been connected with each other and formed the plateau type sediments on the “Pan-Yangtze Landmass”, and seawater flows into the Sanjiang area from the south and north of Jinsha River Zone; except for the Jitang-Chongshan-Lancang Remnant Arc Zone, where Lincang archicontinent and Riwoqê archicontinent (island arc mountain system) were formed at the north and south ends respectively, the areas on both sides of Jinsha River Zone generally descend to be sedimented, forming terrigenous alluvial, alluvial-proluvial and diluvial clastic sediments first and then developing into littoral-neritic clastic rock and carbonate rock sediments. In Jinsha River Zone, Early Devonian stratum may be continuously deposited on Silurian underlying strata, which was formed by neritic continental-shelf carbonate-clastic rocks.

In Middle Devonian, the transgressive area was spread, and local extension and rift occurred on the background of the extension of neritic continental shelf in Early Devonian. In the Yangla-Benzhilan-Xiaruo-Tacheng area, the neritic-semi-pelagic carbonate rocks and siliceous-arenopelitic flysch formation were formed and intermediate-basic volcanic rocks erupted in the rift basin. In Late Devonian, the basin was extended and rifted significantly. In the rift basin with Yangla-Dongzhulin-Shigu area as the central axis, the bathyal sediments represented by radiolarian siliceous rocks-thick-heavy limestone-sandstone-mudstone assemblage are developed, accompanied by the eruption of extensional continental tholeiite and intermediate-basic volcanic rocks, showing that the extension and rifting have deepened the seawater and developed volcanic activities in the rift basin, namely rift basin stage formed by the thinning and rift of the continental crust. The stable landmasses on both sides of Jinsha River (Qamdo Landmass on the west and Zhongza-Shangri-La Landmass on the east) are composed of plateau carbonate rock-clastic rock formations in epicontinental neritic basins.

2.2.2.2.2.2 Ocean Basin Formation Stage (C₁–P₁)¹

The period from Carboniferous to the early stage of Early Permian is an important period for the spreading of Jinsha River Back-arc Ocean Basin. On the basis of the rift basin in Late Devonian, it further spread to form an ocean basin in Carboniferous-early stage of Early Permian, that is, the evolution and development of Jinsha River Ocean Basin began. With the formation of Jinsha River Ocean Basin from Carboniferous to the early stage of Early Permian, the Qamdo Landmass split off from the “Pan-Yangtze Landmass” to form an independent micro-block. In Early-Middle Carboniferous and Late Devonian, rift basins continued to spread and the continental crust split off, forming Jinsha River Initial Ocean Basin in Early-Middle Carboniferous, in which volcanic, endogenous and terrigenous low-density turbidites represented by radiolarian siliceous rock-thick bedded limestone-black mudstone-tuff assemblage were deposited and oceanic ridge-oceanic island basalt erupted, belonging to semi-pelagic-abyssal carbonate rocks, volcanic rocks and arenopelitic-siliceous flysch formation.

In Late Carboniferous-early stage of Early Permian, with the increase of crustal splitting intensity, Jinsha River Ocean Basin spread rapidly in an asymmetric tectonic pattern, forming a mature ocean basin. The lithofacies in the ocean basin are composed of mid-ocean ridge mafic–ultramafic rocks, oceanic ridge-oceanic island tholeiite assemblages, and form the ocean basin ophiolite complex in Gajinxueshan, Gongka, Jiyidu and other places together with radiolarian siliceous rocks, along with which the abyssal sediments represented by the arenaceous-siliceous flysch formation composed of radiolarian siliceous rocks, siltstone, silty mudstone, black arbonaceous shale assemblages (rich in iron and manganese) in Gajinxueshan, Yangla, Gongka, Jiyidu-Xiaruo and Xinzhu area in Late Carboniferous-Early Permian were deposited in basins, so it is a sequence of volcanic, endogenous and terrigenous low-density turbidite.

The main body of Qamdo Block on the west side of Jinsha River Ocean Basin is a metastable plateau type sediment formed by carbonate rocks and clastic rocks mixed with intermediate-basic volcanic rocks in epicontinental neritic basins. Closing to the ocean basin, the eastern margin of Qamdo Block formed marginal rift (valley) basin in the Jiangda-Deqin-Shimianchang area with the significant spreading of Jinsha River Ocean Basin from Middle and Late Carboniferous to the early stage of Early Permian, in which slope carbonate rock slump accumulation at the margin of the continental shelf, submarine turbidite fan sedimentation, semi-pelagic basinal turbidite, radiolarian siliceous rock, contourite, as well as extensional basic and intermediate-basic volcanic rocks were developed. By the early stage of Early Permian, volcanic rocks had the characteristics of “bimodal” assemblage (Mo et al. 1993), showing the sequence characteristics of the rift (valley) basin at the margin of the continental shelf. The main body of Zhongza-Shangri-La Landmass on the east side of Jinsha River Ocean Basin is stable plateau type sediment formed by carbonate rocks-clastic rocks in epicontinental neritic basins, and the continental shelf marginal facies near the basin is composed of metastable carbonate rock-clastic rock mixed with intermediate-basic volcanic rocks.

2.2.2.2.2.3 Oceanic Crust Subduction and Destruction Stage (P₁²–P₂)

The tectonic geology background of Jinsha River Zone changed greatly from the late stage of Early Permian to Late Permian. On the basis of the ocean basin formed by spreading in Late Paleozoic, Early Carboniferous-the early stage of Early Permian, Jinsha River Basin subducted and destructed westward significantly in the late stage of Early Permian, marking the development of volcanic rocks in the intra-oceanic arc and the formation of back-arc-basin.

Due to the subduction and destruction between oceanic crusts (its formation mechanism is related to fracture and detachment and subduction inside the oceanic crust plate), Xiquhe Bridge-Xueyayangkou-Dongzhulin-Jiyidu-Gongnong Back-arc-Basin (oceanic crust basement) on the west side of the volcanic arc and its intra-oceanic arc in Zhubalong-Yangla-Dongzhulin area in the late stage of Early Permian-Late Permian were formed respectively in the central axis area of Jinsha River Ocean Basin. Neritic carbonate rocks and clastic rocks and semi-pelagic arenaceous-siliceous flysch formation were deposited in Zhubalong-Yangla-Dongzhulin intra-oceanic arc environment. Island-type volcanic rocks composed of quartz tholeiite-basaltic andesite-andesite-dacite (a small amount) were developed in the volcanic arc from morning to night. The middle part (oceanic crust basement) of Xiquhe-Xueyayangkou-Dongzhulin-Jiyidu-Gongnong Back-arc-Basin is on the west side of the intra-oceanic volcanic arc. The semi-pelagic-abyssal siliceous-arenopelitic flysch formation is formed in the back-arc spreading basin, accompanied by the diabase-sheeted dyke swarm in the back-arc spreading environment and the assemblage of tholeiite and basaltic tuff on it.

The oceanic crust subducted and destructed westward under Qamdo-Lanping Block on the west side of Jinsha River Ocean Basin, and then Jiangda-Deqin-Weixi continental margin volcanic arc of Permian and Qamdo Back-arc-Basin (continental crust basement) on the west side of the continental margin arc were formed respectively in the late stage of Early Permian to Late Permian. Volcanic-sedimentary rocks in the continental margin volcanic arc have varied lithofacies and diverse sedimentary types in space, and the topography of island arc fluctuates greatly. There is a land with terrestrial plants and columnar joints outcropped from the water surface, and there are also carbonate rock plateau and abyssal valleys grown underwater. There are various sedimentary facies and types of sediments, varying from continental facies-land-sea transitional facies-neritic facies-plateau slope facies-abyssal basin, so as to form a tectonic paleogeographic pattern in an island-chained distribution way. Tholeiite → calc-alkaline → potassium basalt volcanic rocks were developed in the arc volcanic rocks from morning to night. The properties of volcanic rock marks the complete process of generation, development and maturity of island arc (Mo et al. 1993). Qamdo Back-arc-Basin (continental crust basement) is on the west side of Jiangda-Deqin-Weixi continental margin volcanic arc, in which the metastable marine-continental biterrigenous coal-bearing clastic rock and volcanic rock formation, littoral biterrigenous clastic rock and volcanic rock formation, and neritic carbonate rock, clastic rock and volcanic rock formation are deposited.

The main body of Zhongza-Shangri-La Block on the east side of Jinsha River Ocean Basin maintains neritic carbonate rock sediments of plateau facies, the western margin of Zhongza-Shangri-La Block corresponds to the island arc-basin system on the eastern active margin of Qamdo Landmass; Permian is the evolution and development period of passive continental margin rift basin; the slope carbonate rock slump accumulation at the margin of the continental shelf, submarine turbidite fan sedimentation, semi-pelagic arenopelitic-siliceous shale flysch, and extensional basic and intermediate-basic volcanic rocks were developed in the passive continental margin rift basin in Zhiyong-Fulong Bridge-Nixi-Tuoding, and the lithogeochemistry characteristics of volcanic rocks and sandstones show that they are passive continental margin environment (Mo et al. 1993). The formation of the rift basin on the western margin of Zhongza-Shangri-La Block corresponds to the development of the island arc-back-arc-basin system on the active margin of Jiangda-Deqin-Weixi Block in Permian on the eastern margin of Qamdo Landmass.

Due to the westward subduction and destruction of the oceanic crust of Jinsha River, the spatial configuration structure of Jinsha River Arc-Basin System was formed during the period from the late stage of Early Permian to Late Permian, namely the intra-oceanic volcanic arc-back-arc-basin and the continental margin volcanic arc-back-arc-basin. This process is not only the conversion of the oceanic-continental lithosphere, but also the process of significant adjustment, recombination and conversion of material constituents.

2.2.2.2.2.4 Arc-Land Collision Stage (T₁–T₂¹)

In the Early and Middle Triassic, the tectonic and sedimentary environment of Jinsha River Arc-Basin System, Qamdo Block and Zhongza-Shangri-La Block on the east and west sides changed dramatically. Jinsha River Ocean Basin was destroyed and closed, and then the oceanic crust disappeared at the end of Late Permian. Jinsha River Zone turned into an arc-land collision development stage in Early and Middle Triassic, which is marked by the development of Jiangda-Deqin collisional continental margin volcanic arc and the formation of Qamdo Back-arc Foreland Basin and the residual marine basin (marginal sea) of the Jinsha River.

Jinsha River Ocean Basin was subducted and destroyed in Permian and then was closed at the end of Late Permian, with the arc-land collision and land-land docking. On the basis of the original Jinsha River Ocean Basin of Permian, Jinsha River Ocean Basin entered into the development stage of the residual marine basin (marginal sea) in the Early and Middle Triassic, in which the semi-pelagic fine clastic turbidite composed of endogenous and volcanic sources and mixed with spillite-keratophyre, radiolarian siliceous rock and marlstone assemblages was formed, belonging to carbonate rock, siliceous-arenopelitic flysch and volcanic rock formation. Collisional island arc intermediate-acid volcanic rocks, subvolcanic rocks and intrusion appeared in Shusong-Tongyou, which resulted from the subsequent development of the intra-oceanic arc from Permian to Middle Triassic. The main body of Zhongza-Shangri-La Landmass on the east side of Jinsha River Residual Marine Basin is dominated by the uplift and denudation area, but is lack of Middle Triassic stratum and its subsequent strata. Clastic rock sediments were formed in the epicontinental neritic basin in the Early Triassic, the basal fluvial conglomerate disconformably overlaid on or unconformably overlaid on Permian stratum, and the middle and upper part is composed of littoral-neritic carbonate rock and clastic rock formation.

The Jiangda-Gebo-Xuzhong area on the eastern margin of Qamdo Landmass is on the west side of Jinsha River Residual Marine Basin. Due to the arc-land collision and land-land docking, collisional continental margin volcanic arc was formed in Early-Middle Triassic and superimposed on the subduction type continental margin volcanic arc in Permian, and the volcanic rock assemblage of basaltic andesite-andesite-dacite-rhyolite series with island arc nature was developed. The arc volcanic rock was formed following the piedmont purplish red conglomerate in the Early Triassic, and consists of facies assemblage of alluvial-proluvial facies → littoral-neritic facies → marginal slope facies → basin facies from bottom to top and from west to east. It was turned into the spatial pattern of the fore-arc, inter-arc and back-arc-basins in the Middle Triassic, and bathyal volcanic rocks, terrigenous and volcanic turbidite were developed in the basins.

Qamdo Landmass on the west side of Jiangda-Gebo-Xuzhong collisional continental margin volcanic arc in Early and Middle Triassic changed from the back-arc-basin in Permian into the back-arc foreland basin. Most areas were uplifted due to the arc-land collision, and lack of Early and Middle Triassic strata. In the early stage of Early Triassic, fluvial and littoral clastic rock and intermediate-acid volcanic rock formations were formed in the marginal zone of the back-arc foreland basin only in Markam area near the island arc in the east of the block, and disconformably overlaid on underlying strata.

2.2.2.2.2.5 Superimposed Rift Basin Stage (T₂²–T3¹)

Jinsha River Residual Marine Basin (marginal sea) disappeared during the period from the late stage of Middle Triassic to the early stage of Late Triassic, and the geological pattern of Jiangda-Deqin-Weixi Island Arc Orogenic Belt changed from extrusion into extension, and the possible conversion mechanism of its mechanical properties - lithosphere delamination, which caused continental crust to be stretched due to the extensional collapse caused by thinning; the superimposed rift basin split mainly formed in the early stage of Late Triassic in the original volcanic arc and its marginal zone due to extension and splitting; it was formed after the subduction and destruction, arc-land collision and land-land docking and collision of Jinsha River Ocean Basin and before the large-scale and large-area accumulation of molasse formation in Jinsha River in time; the upper main body is superimposed on Jiangda-Deqin-Weixi Island Arc Orogenic Belt in space, belonging to the post-collision extensional tectonic background.

From the late stage of Middle Triassic to the early stage of Late Triassic, the superimposed rift basin was characterized by the development of semi-pelagic volcanic turbidite, tuffaceous turbidite, tuffaceous-siliceous turbidite and argillaceous flysch, as well as “bimodal” volcanic rocks and gabbro-diabase dykes and dyke swarm composed of basalt and rhyolite assemblage. The geochemistry characteristics of volcanic rocks show that they are rift basin environments under extensional background. In the early development stage, the rift basin was composed of the assemblage of neritic to semi-pelagic basalt, basaltic tuff, sandstone, sandy mudstone, tuffaceous siliceous rock and marlstone, in which a large number of gabbro-diabase dykes and dyke swarm were developed. In the middle development stage, the rift basin was composed of the assemblage of semi-pelagic basalt, basaltic tuff, rhyolite, rhyolite tuff, rhyolite volcanic breccia, sandy mudstone, tuffaceous siliceous rock and marlstone, in which a large number of gabbro-diabase dykes and dyke swarm were developed. In the late development stage, the rift basin was composed of the assemblage of semi-pelagic and neritic (continental rock had appeared in some areas) rhyolite, rhyolite tuff, rhyolite volcanic breccia, sandstone, sandy mudstone and marlstone, in which gabbro-diabase dykes and dyke swarm were distributed. At the end of its development, the geological pattern of the rift basin changed from extension and rift into extrusion, the basin gradually shrank and destructed, forming the littoral-neritic clastic rocks (with the nature of molasse) mixed with intermediate-intermediate-acid volcanic rocks and pyroclastic rocks, in which a large number of gypsum-salt sediments composed of gypsum, barite and siderite deposits were developed.

From the late stage of Middle Triassic to the early stage of Late Triassic, the tectonic paleogeographic environment of the superimposed rift basin had changed greatly both in time and space. In terms of time, in the early and middle development stage, the rift basin had significant extension and rift, volcanic activity erupted in the deep water, and the gabbro-diabase dykes and dyke swarm in the extensional tectonic background as well as subvolcanic gabbro porphyrite were developed. In the late development stage, the rift basin had slight extension and rift, and was closed with acidic volcanic activity, volcanic rocks were formed in the shallow water, and even continental eruption and columnar joints occurred. At the end of its development, it turned into an extrusion environment, and ended by the appearance of intermediate-intermediate-acid volcanic rocks. Moreover, a large number of gypsum-salt sediments were developed. The rift basin is composed of continental-adlittoral volcanic islands and faulted extensional basins spatially, with abyssal sediments developed, forming a tectonic paleogeographic pattern dominated by “graben and horst”. Shengda-Chesuo Township-Xialaxiu volcanic-sedimentary basin, Xuzhong-Luchun-Hongpo volcanic-sedimentary basin and Reshuitang-Cuiyibi-Shanglan volcanic-sedimentary basin can be roughly found from north to south.

By taking geochronologic scale as the age standard and based on the correlation diagram (Fig. 2.10) drawn depending on data about K₂O and rifting and extension speed in basalt of Ethiopia provided by Mohr and Berberi et al. and data about K₂O and rifting and extension speed in basalt of Kenya-Tanzania provided by Mohr et al. and Villims, the extension and rifting distance of three volcanic-sedimentary basins were measured. As for Shengda-Chesuo Township-Xialaxiu volcanic-sedimentary basin in the north member, the average value (1.43%) of w (K₂O) from six basalts is adopted, which is set at point A in the figure, with the rifting velocity (V_P) of 0.27 cm/a and the rifting distance (D) of 63 km; as for Xuzhong-Luchun-Hongpo Volcanic-Sedimentary Basin in the middle member, the average value (0.48%) of w (K₂O) from 10 basalts is adopted, which is set at point B in the figure, with the rifting speed (Vp) of 0.43 cm/a and the rifting distance (d) of 140 km. As for Reshuitang-Cuiyibi-Shanglan volcanic-sedimentary basin in the south member, the average value (0.81%) of w (K₂O) from four basalts is adopted, which is set at point C in the figure, with the rifting speed (Vp) of 0.36 cm/a and the rifting distance (d) of 116 km. Mo et al. (1993) estimated that the spreading width of three volcanic-sedimentary basins is 49.5 km in Chesuo Basin, 113 km in Jiaojiading Basin and 81 km in Cuiyibi Basin, with similar results.

Fig. 2.10

Relationship between Basalt w (K₂O) and rifting speed in rift basin. A—rifting speed of Shengda-Chesuo township-Xialaxiu volcanic-sedimentary basin; B—rifting speed of Xuzhong-Luchun-Hongpo volcanic-sedimentary basin; C—rifting speed of Reshuitang-Cuiyibi-Shanglan volcanic-sedimentary basin

According to the critical spreading rate (0.5–0.9 cm/a) proposed by Sleep and Kuzmir, at which the magma under the ridge is formed, the rifting speed of Chesuo Basin in the north member (0.27 cm/a) is lower than that of Jijiading Basin in the middle member and that (0.43 cm/a and 0.36 cm/a) of Cuiyibi Basin in the south member. Therefore, the rifting intensity of Jijiading Basin in the middle member and Cuiyibi Basin in the south member is relatively high, with “bimodal” volcanic rock assemblage appearing in the basin, while the rifting intensity of Chesuo Basin in the north member is relatively low, with only tholeiite appearing in the basin but no “bimodal” volcanic rock. In addition, the rifting speed (0.27 cm/a, 0.43 cm/a, 0.36 cm/a) of the three basins in the north, middle and south members are all lower than the critical spreading rate (0.5–0.9 cm/a) of the magma under the ridge, so the spreading ridge ophiolite assemblage was not formed.

2.2.2.2.2.6 Foreland Basin Stage (T₃²–K)

In the middle and late stage of Late Triassic, Jinsha River Zone entered the comprehensive intracontinental collision orogeny stage. Clastic rocks, molasses and coal-bearing formations were formed in marginal foreland basin in Jinsha River Orogenic Belt and its rear margin due to accumulation, and unconformably superimposed on Jinsha River Tectonic Melange. In Qamdo Landmass on the west side of Jinsha River Junction Zone, the fluvial-littoral clastic molasse formation was formed in the back-arc foreland basin in the early stage of Late Triassic, and unconformably covered and underlaid on the strata of different ages; its back-arc foreland basin continued to develop and evolve to form the formation varying neritic carbonate rocks from marine-continental coal-bearing clastic rocks in the middle and late stage of Late Triassic. By the Late Cretaceous, the foreland basin gradually shrank and destructed.

2.2.2.2.2.7 Intracontinental Convergence Stage (E–Q)

Cainozoic is the formation and uplift period of the Sanjiang area and even Qinghai-Tibet Plateau. The last orogeny formed a large-scale overthrust nappe, a large-scale strike-slip and the layered detachment of the surface and lithosphere formed by overthrust nappe and stretching. On the one hand, some fracture, depression, strike-slip, stretching and pull-apart basins were formed; on the other hand, the mountains formed in the early stage were superimposed and reworked, and the crust was thickened significantly, accompanied by significant magmatism, metamorphism, tectonism and mineralization of nonferrous metals and precious metals. All ore deposits formed in Jinsha River Zone in different ages were shaped in the process of intracontinental orogeny, and were superimposed and reformed by tectonism and magmatic activity to varying degrees, showing the characteristics of multiple minerals and complex and diversified deposit types in an ore deposit.

2.2.3 Temporal and Spatial Structure and Its Evolution of Zhongza-Shangri-La Block

Zhongza-Shangri-La Block lies between Jinsha River Junction Zone on the west side and Yidun-Xiaqiaotou Back-arc-Basin Orogenic Belt on the east side. In the Paleozoic, it was a part of the western passive margin of Yangtze Continent and a horst on the west side of Muli-Haidong Graben. In the middle and late stage of Late Paleozoic, it split from Yangtze Landmass with movement of Dege-Shangri-La Micro-landmass due to the opening of Ganzi-Litang Ocean, forming a stable block in the micro-landmass. Before splitting, this block moved up and down with the movement of Yangtze Landmass, so that its movement is related to but has difference with the sedimentary characteristics of Yangtze Landmass. As a stable block, Zhongza-Shangri-La Block was formed in the Middle and Late Cambrian. This block experienced three major development stages. That is, basement formation stage, stable block formation stage and reverse polarity orogenic stage of block fold and uplift (Li et al. 2002).

2.2.3.1 Basement Formation Stage

This block has a typical dual structure of basement and caprock. The basement is a metamorphic crystalline basement, with the first rock formation of the primary Shigu Group in the south member and Chamashan Group in the north member. The caprock is the Paleozoic stratum-Triassic stratum.

The basement (Shigu Group) in the south member of the block is mainly distributed in Shigu, Lijiang-Tacheng outside Weixi in the west of Yinchanggou Anticline in Shangri-La Block and in the east of Ludien Granite Zone in Jinsha River Tectonic Zone. It is divided into three groups from bottom to top: Yangpo Group, Longba Group and Tacheng Group. The study on Shigu Group is unknown and its stratigraphic division and formation age are still controversial. By our observation, Shigu Group can be roughly divided into three rock formations according to the degree and characteristics of deformation. The first rock formation is Shigu Group outcropped in Liming Township, Lijiang, which is the lower member or lower part of Yangpo Formation. With high-grade metamorphism, it belongs to high-grade greenschist-low amphibolite facies, and granite veins formed by partial melting are seen locally. The deformation level is also relatively deep, S₀ has been completely replaced by S₁ and many folds are seen locally. The second rock formation is Shigu Group outcropped in the east of Taiping Bridge, Judian-Lamaluo, Shigu and in the west of Baita Village, Judian-Wuhou Village-Hetaoping, Shigu; with low-grade metamorphism and belonging to the middle-grade greenschist facies, so it is equivalent to the upper part of Yangpo Formation and Longba Formation. The deformation level is relatively low, S₀ has not been completely replaced by S₁ and it is characterized by flysch sedimentation. A series of sharp edges or congruous folds that reverse eastward is formed. It is separated from the first rock formation by the mylonite zone with ductile shear. The third rock formation is distributed in the east of Baita Village-Hetaoping, which is equivalent to marginal Tacheng Formation. It is mainly composed of mica schist and mica quartz schist, so its metamorphism is lower than that of the second rock formation and the layers are still visible, forming a series of folds that reverse westward and contacting with the second rock formation by fault, which is just opposite to the second rock formation. In addition, a large sequence of metamorphic basic volcanic rocks is found in the rock formation, and siliceous rocks are found locally. This is very similar to the Carboniferous stratum and Permian stratum in the passive margin zone of Jinsha River in Tacheng-Tuoding in the north. Therefore, we consider that the first rock formation belongs to the real metamorphic crystalline basement of Shangri-La Block; the second rock formation belongs to the miogeosynclinal sedimentation at the passive margin, may also belong to the contemporaneous heterotopical sedimentation on the block, which is formed by the stable block-type shallow water in Cambrian-Silurian changing into the abyssal environment on the western margin, and may also contain some Precambrian strata. The third rock formation is regionally connected with Late Paleozoic stratum in Tacheng-Tuoding in the north and distributed in zones from north to south, forming the passive margin sedimentary zone in the eastern part of Paleo-Tethyan Ocean in Jinsha River. The lithogeochemistry characteristics of its basic volcanic rocks show that it is continental margin rift type (Li et al. 1999a; b), so the third rock formation may be Late Paleozoic stratum.

The basement of the northern member of the block is Chamashan Group. It is a sequence of metamorphic carbonate rocks and intermediate-basic volcanic rocks, located under the Cambrian stratum and in contact with Cambrian stratum by faults. It should be pointed out that some suspect that the Chamashan Group was formed in Late Paleozoic due to upward thrust of Cambrian stratum and distributed on the passive margin of Jinsha River. As the strata on the Zhongza-Shangri-La Block overthrust westward on Jinsha River Junction Zone, the thrust superposition relationship mentioned above is reasonable. In the case of the above statement, Chamashan Group is not the real basement of the block.

To sum up, up to now, the direct contact relationship between the caprock and the basement of the block has not been found. However, in terms of metamorphism and deformation, as there was a tectonic change between them, it is reasonable to take the first rock formation-Shigu Group as its crystalline basement. It shows that the block is kept stable after a tectonic event.

2.2.3.2 Stable Block Formation Stage

The stable block was formed in Middle and Late Cambrian and ended at the end of Permian. The whole Paleozoic stratum is an assemblage of clastic rocks and carbonate rocks, but the Lower Paleozoic stratum is composed of basic and intermediate-acid volcanic rock, which is relatively active, while the Upper Paleozoic stratum is relatively stable due to no volcanic rocks mixed. Therefore, its evolution can be divided into two stages, namely Early Paleozoic and Late Paleozoic.

2.2.3.2.1 Evolution Stage in Early Paleozoic

From Middle and Late Cambrian to Silurian, the block is mainly composed of the sedimentary sequence of carbonate rocks-clastic rocks-carbonatite, which generally shows the stable coastal-continental shelf plateau-type sedimentary environment.

The lower bed of Middle Cambrian stratum is composed of dark-gray thin laminated dolomitic silty slate, containing trilobite locally, which shows an offshore environment; the middle bed is composed of feldspathic quartz sandstone mixed with dolomitic debris, sandstone mixed with dolomitic debris, and feldspathic quartz sandstone, in which swash bedding can be found, so it shows a coastal environment; the upper bed is composed of dolomite mixed with sandstone, which shows a littoral-neritic environment. The Upper Cambrian stratum is composed of secondary dolomite and contains biodetritus, belonging to a neritic carbonate rock gentle slope environment. The analysis on sedimentary facies shows that Middle and Late Cambrian strata in Zhongza-Shangri-La Block were located in the shore and offshore tectonics, with shallow water but high energy. Paleocurrent data (wave ridge strike: 160°, 150°) indicates that the coastline at that time generally extended from north to south. Considering that the west of this area is Proto-Tethyan Jinsha River, it is speculated that the west at that time should be a front slope zone of plateau and continental slope zone inclined to the west, while this area was kept in the marginal shoal environment of plateau. The main lithology of Ordovician stratum is quartz sandstone or feldspathic quartz sandstone mixed with sandy slate, with dolomitic limestone lens in the lower bed. Abraded wave ripple, cross bedding and parallel bedding were developed, showing a littoral-neritic environment. Early sediments of Early Silurian are mainly composed of gray, medium-layer medium-grained feldspathic quartz sandstone, showing a foreshore environment. In the middle and late stages, the stratum is composed of siltstone and slate. It is an inshore-offshore environment. The Middle and Late Silurian stratum is composed of crystal powder dolomite, showing a tidal flat environment. In a word, the Early Paleozoic stratum has high energy and shallow sea water and is generally located in a stable environment on the plateau margin.

2.2.3.2.2 Late Paleozoic Evolution Stage

The Early Paleozoic stratum in this area is a part of the western margin of Yangtze Landmass, and Caledonian orogeny still has an influence on it. It is mainly characterized by the conformity of Devonian stratum and underlying stratum. Lugu Lake-Erhai Lake on the east side is a depression zone with continuous sediments and deepening water body, and the block still has the tectonic pattern of early horst-shaped uplift.

In the early stage of Early Devonian, the stratum is composed of feldspathic quartz sandstone sediments, with plate-like cross bedding and lenticular basal conglomerate, belonging to a fluvial environment. In the middle and late stage, the stratum is a thin layer of sediments composed of quartz sandstone and silty shale, with well-developed horizontal bedding, belonging to a littoral-neritic environment. The Middle and Late Devonian stratum is composed of limestone and bioclastic reef rock with fore-reef collapse. It is a bordered carbonate rock plateau environment. The sea level dropped at the end of Late Silurian, causing the extensive exposure of the western land margin of Yangtze Landmass and the uplift of carbonate rocks on the plateau margin. Aulacogen was developed in Lugu Lake-Erhai Lake on its west side, in which the carbonate turbidity current and abyssal radiolarian siliceous rocks can be found. Therefore, the paleogeographic pattern of Zhongza-Shangri-La Block at that time was dominated by a high-energy zone on the margin of a continental shelf, with the fore-reef and back-reef colluvium developed on the west side. There is a continental shelf lake in the east of the high-energy zone; with synsedimentary fracture, the sea water is extremely deep and changes suddenly. The Lower Carboniferous stratum is composed of light gray and dark gray, thin-medium-thick micritic limestone and silty limestone, some of which have become crystalline limestone, belonging to an open plateau environment. The Middle and Upper Carboniferous stratum is composed of light gray bioclastic limestone and oolitic limestone. In the northern Zhongza area, the Lower Carboniferous stratum is also composed of bioclastic limestone and oolitic limestone. Therefore, it shows that Zhongza-Shangri-La Block in the whole Carboniferous is a marginal shoal environment of carbonate plateau, and the local area is an open plateau environment. The Carboniferous stratum on the east side of the block is dominated by limestone, but also contains argillaceous limestone, flint limestone and siliceous rocks; the sea water is relatively deep, especially in Lugu Lake-Erhai Lake area, where an aulacogen was formed and manganese ore and radiolarian rock can also be found. Like the Carboniferous stratum, the Permian stratum is also a neritic carbonate rock environment. There is also an aulacogen in the east, in which siliceous rocks and corrosive fluid sediments can be found. In Late Permian, basalt erupted on the east side of the block, which marked the complete splitting of the block from the Yangtze Landmass and formed an independent block.

It can be seen from Paleozoic sedimentary evolution that Zhongza-Shangri-La Block is a part of the western margin of Yangtze Landmass, which constitutes a complete plateau and passive continental margin. This passive continental margin is not a simple epicontinental sea inclined to the ocean, but always maintains a graben-horst pattern of two highs and one low, two shallows and one deep, namely marginal reef (shoal) of continental shelf, marginal moraine (shoal) of land and marginal slope of plateau. In several areas with sea level drop, shoal areas are nearly exposed and subject to erosion and even incised valley sedimentation. This pattern lasted until the late stage of Paleozoic, and ended upon the formation of Ganzi-Litang Back-arc Ocean Basin.

2.2.3.3 Reverse Polarity Orogenic Stage

The uplift of Zhongza-Shangri-La Block in Late Permian (local uplift to form land) shows that Jinsha River Oceanic Crust had been destroyed, marking that it has entered the land-arc and land-land collision stage and the foreland uplift has appeared. Coarse clastic molasses are accumulated in some members of the Late Triassic block, such as the northern Lalashan and the southern Tuoding area, which are characterized by sedimentation of the post-orogenic foreland basin. However, due to the tectonic inversion of Jinsha River Zone, that is, the back-thrust spreading of the junction zone and the passive margin zone (see Sect. 2 below), the strata on Zhongza-Shangri-La Block thrust westward on the passive margin zone and even the junction zone of Jinsha River Tectonic Zone, resulting in the tectonic inversion of the foreland basin (Liu et al. 1993), forming a back-thrust spreading foreland basin of Late Triassic in Qamdo-Pu’er Block, so the development of foreland basin in Zhongza-Shangri-La Block was quickly ended and only a trace of the initial embryonic form of foreland basin was left.

The collision between Zhongza-Shangri-La Block and Qamdo Landmass also led to the fold deformation of Paleozoic stratum on the block, forming a series of nearly north–south short-axis geanticlinal tectonics, and making Late Triassic stratum unconformably cover on it. Its tectonic deformation pattern is from the center to the west of the block and from the wide and gentle isopachous fold without cleavage to the overturned fold with the close cleavage and same inclination, showing a change from weak to strong and a kind of reversed orogeny. This reverse polarity orogeny overthrusts Zhongza-Shangri-La Block westward, forming an important regional thrust sheet on the east side of Sanjiang area. Due to the large-scale westward thrust nappe of the block, a thrust fault also appeared at its rear margin, which causes the strata on the block (such as Devonian stratum) to thrust eastward on Mesozoic stratum on the east margin of the block, showing a horst on a cross section.

Reverse polarity orogeny of Late Triassic in Zhongza-Shangri-La Block may be further intensified during Yanshanian-Himalayan. This is because the overthrust nappe on the east side of the whole Qamdo-Pu’er Basin reached a peak in Yanshanian-Himalayan, which ended the development of the back-thrust spreading foreland basin but started the orogeny. Moreover, this may also be proved by the fact that Zhongza-Shangri-La Block had not sedimented since the Late Triassic.

We believe that Zhongza-Shangri-La Landmass was always in a horst state in the graben-horst tectonic pattern on the western margin of Yangtze River from Ordovician–Silurian to Early Carboniferous and was separated from Yangtze Landmass with the opening of Ganzi-Litang Ocean until Carboniferous to Permian. The Late Permian uplift is related to the closure of Jinsha River Ocean, which is characterized by foreland uplift. It was a sequence of plateau-type adlittoral sediments from Middle and Late Cambrian to Permian, showing the inherent characteristics of stable blocks, while its tectonic deformation was relatively significant and the reverse polarity orogeny occurred, which was different from the normal blocks. This seems to be the common characteristic of different blocks in the Sanjiang area in terms of deformation characteristics, which may be related to the significant extrusion applied on the Sanjiang” area.

2.2.4 Temporal and Spatial Structure and Evolution of Qamdo Basin

Located between Jinsha River Junction Zone and the Northern Lancang River Fracture Zone, Qamdo Basin is a composite back-arc foreland basin system developed and formed on Proterozoic-Lower Paleozoic basement in Late Paleozoic and Mesozoic as the same name, and is Paleo-Tethyan Basin in the northern member of Sanjiang area in southwest China.

2.2.4.1 Composition and Characteristics of the Upper Crust Tectonic Bed

The upper crust of Qamdo Block has the tectonic characteristics of “double basements” and “three caprocks”, the double basements refer to Proterozoic crystalline basement and Lower Paleozoic fold basement and the three caprocks are composed of Upper Paleozoic, Mesozoic and Cainozoic caprocks. According to the tectonic-rock assemblage and tectonic orogeny cycle, Qamdo Block can be divided into five tectonic beds from bottom to top (Table 2.1).

Table 2.1 Tectonic bed characteristics of Qamdo block

2.2.4.1.1 Basement Tectonic Bed

It is composed of Proterozoic crystalline basement (I) and Lower Paleozoic fold basement (II). The Proterozoic crystalline basement is outcropped in Xiaosumang and Xiariduo in the northeast of Qamdo Basin, which is composed of the Paleo-Mesoproterozoic Ningduo Group and Neoproterozoic Caoqu Group and may also include a part of metamorphic rocks in the lower bed of Xiongsong Group. The rock assemblage in Ningduo Group is composed of (garnet) biotite plagioclase gneiss, plagioclase amphibolite, biotite/binary quartz schist, granulite, etc. The metamorphic grade is of low amphibolite facies and the rock has significant multi-stage tectonic deformation. Caoqu Group is a sequence of metamorphic rocks of high-grade greenschist facies, with the lithology of schist mixed with metamorphic volcanic rocks; and its lower bed contains metamorphic conglomerate. There is no direct contact relationship between Caoqu Group and Ningduo Group, but they are obviously different in lithologic assemblage and metamorphic grade, so they may be unconformable.

Proterozoic Xiongsong Group in Boluo-Xiongsong on the west side of Jinsha River is composed of gneiss and schist members in the lower bed and marble members in the upper bed. The gneiss in the lower bed has the same lithology as that of Ningduo Group. Abundant colonial coral, bivalves, lamellibranch and crinoidal biological fossils are found in the marble in the upper bed of continuous Xiongsong Group in Xiugeshan on the west side of Xiongsong. Among them, the coral is identified as Lithostrotion, which was formed in Early Carboniferous.

The source rock age of Ningduo Group is 1680–2200 Ma (zircon U–Pb method, based on data of a scale of 1: 200,000 in Dengke and Laduo respectively), and the age of plagioclase amphibolite (source rock is volcanic rock) in the lower bed of Xiongsong Group is 1594 Ma (SM–Nd whole rock isochron age, based on data of a scale of 1: 200,000 in Wanxiong), which was basically formed in (Paleo) Mesoproterozoic. The volcanic rocks in Caoqu Group are earlier than 876–999 Ma (rock U–Pb method, based on data of a scale of 1: 200,000 in Dengke), which was basically formed in Mesoproterozoic and Neoproterozoic. The gneiss in the lower bed of Proterozoic Xiongsong Group obtained two metamorphic ages of 857.4 Ma ± 143 Ma (Rb–Sr method) and 611–669 Ma (Rb–Sr method, on a scale of 1: 200,000 in Xiongsong), respectively, which were equivalent to the period from Jingningian to Chengjiang, indicating that Jingningian orogeny and Chengjiang orogeny occurred during the formation period of the crystalline basement of Qamdo Block. The fold basement on the crystalline basement, namely the second tectonic bed (II), is composed of Middle-Lower Ordovician stratum of Lower Paleozoic and a small amount of Silurian stratum. Middle and Lower Ordovician strata are mainly outcropped in the Qingnidong-Haitong-Duojiban, and mainly composed of low-grade metamorphic sand-slate mixed with carbonate rocks, with significant deformation and development trendline as the same as that of tight-inclined folds. The Lower Devonian stratum obviously unconformably covers it in Jiangda and Jueyong. The fold basement was formed in Caledonian, which not only caused folds in Lower Paleozoic stratum, but also caused the plagioclase granite intrusion with the age of 462 Ma in Jiefang Township, Jiangda.

2.2.4.1.2 Upper Paleozoic Tectonic Bed

The Upper Paleozoic tectonic bed (III) (including Devonian stratum, Carboniferous stratum and Permian stratum) is well developed and is the first stable caprock sediment in Qamdo Block. In the interior of the block, it is a sequence of stable neritic plateau-transitional carbonate rock and clastic rock sediments, with the thickness between 3000 and 5000 m, and the east and west margins of the block are dominated by metastable-active sediments with large sediment thickness. There is no regional tectonic unconformity interface in the Upper Paleozoic tectonic bed except the local parallel unconformity interface between Devonian and Carboniferous strata and Upper and Lower Permian strata, which represents a complete regional transgressive–regressive sedimentary cycle.

Hercynian orogeny ended the development of this tectonic bed. Wide and gentle folds and brittle deformation are mainly developed in the Upper Paleozoic stratum, and the metamorphic grade is generally low. However, there is significant deformation and metamorphism locally in the orogenic belts on the east and west sides of the block.

2.2.4.1.3 Mesozoic Tectonic Bed

Mesozoic tectonic bed (IV) is the second caprock developed on Qamdo Block, which was formed in the reversed foreland-intracontinental basin after Hercynian-Early and Middle Indosinian orogeny. Except for Lower and Middle Triassic strata, the main body of Mesozoic stratum was composed of continuous Upper Triassic stratum to Cretaceous stratum, which constitute the sedimentary sequence of foreland basin, and are fully overlaid on Proterozoic, Paleozoic and Lower and Middle Triassic strata, with almost more than 80% of Qamdo Block. The relationship between Lower Triassic stratum and Upper Paleozoic stratum, and between Upper Triassic stratum and Lower (Middle) Triassic stratum in this tectonic bed is tectonic unconformity.

Mesozoic tectonic bed was formed on the erosion surface after Hercynian-Early and Indosinian orogenies, and was composed of transgressive–regressive cycles developed in the eastern part of Early and Middle Triassic strata and the regional transgressive–regressive cycles in Late Triassic-Cretaceous strata, which were the evolution history of Qamdo Basin developed into foreland basin. Influenced by the tectonic activities of orogenic belts on both sides of the block, the eastern part of the Mesozoic stratum is dominated by the Triassic stratum and is characterized by the development of a large number of volcanic rocks. On the basis of the Upper Paleozoic stratum, the central and western parts are foreland basin filling sequences starting from Late Triassic molasse formation, which are dominated by stable sedimentary assemblages. After the end of the late stage of Late Triassic, the foreland basin moved westward, and Jurassic-Cretaceous strata were extremely thick molasse sediments accumulated in a significant intracontinental depression.

Rocks of the Mesozoic stratum were not metamorphic and dominated by simple, wide and gentle folds. Significant superficial brittle deformation occurred during Himalayan intracontinental extrusion.

2.2.4.1.4 Cainozoic Tectonic Bed

Cainozoic tectonic bed (V) is a strike-slip pull-apart basin sediment formed in Cainozoic intracontinental convergence stage and mainly distributed in Paleogene/Neogene basins such as Gongjue, Nangqian, Jiqu and Lawu. Paleogene stratum in Gongjue and Nangqian basins is a sequence of red molasses with a thickness of 4000 m, and also contains gypsum-salt, accompanied by the intrusion of intermediate-acid volcanic rocks and alkaline porphyry/vein rocks. The Paleogene/Neogene stratum in Jiqu Basin is mainly a sequence of fluviatile-lacustrine red clastic rocks; the lower bed of Neogene stratum in Lawu Basin is a sequence of latitic-trachytic volcanic rocks, and the upper bed is a sequence of fluviatile-swampy coal-bearing clastic rock sediments. Cainozoic tectonic beds are limited in distribution, formed before the large-scale uplift of the plateau, and mainly composed of Paleogene stratum. There is an unconformity relationship between the Paleogene stratum and the underlying strata of different ages as well as between the Paleogene stratum and the Neogene stratum. Influenced by the intracontinental convergence, the Paleogene/Neogene stratum is mainly characterized by tectonic deformation such as stratigraphic tilting, local folds and strike-slip faults.

2.2.4.2 Temporal and Spatial Structure of Qamdo Basin System

The Qamdo Composite Basin System is represented by the above two tectonic beds of the Upper Paleozoic and Mesozoic, both of which belong to different types of basins controlled by different tectonic stages and mechanisms, and have experienced different development and evolution processes, respectively, with different temporal and spatial structures.

2.2.4.2.1 Late Paleozoic Lithofacies Paleogeography and Basin Structure

Late Paleozoic basin was formed on the erosion surface of fold basement formed by Caledonian orogeny in Qamdo Block, and Lower Devonian stratum unconformably overlaid the underlying Lower Ordovician stratum, on which the stable Devonian, Carboniferous and Permian sedimentary assemblages were continuously developed, but volcanic active formations representing rift basins were developed in different degrees on the east and west sides.

2.2.4.2.2 Mesozoic Lithofacies Paleogeography and (Back-Arc) Foreland Basin Structure

The Mesozoic Qamdo Basin was formed on the erosion surface of the above-mentioned Late Paleozoic basin after the Hercynian-Middle and Early Indosinian orogeny. Mesozoic basin was located on the subduction plate on the west side of Jinsha River Subduction Orogenic Belt, in which the foreland basin filling sequence starting from Piedmont molasse formation was developed. Therefore, the basin has the nature of a back-arc foreland basin. The Mesozoic basin was formed in the Early and Middle Triassic, reached the peak of development in the Late Triassic, and then shrank in the Jurassic, and finally closed in the Cretaceous. It is characterized by long evolution time, obvious dynamic change, diverse basin types and complicated temporal and spatial structure.

2.2.4.2.2.1 Early and Middle Triassic Lithofacies Paleogeography and Basin Properties

During the Early and Middle Triassic, Qamdo Block was generally in an uplifting state, and only a few small depressed basins existed in the eastern orogenic belt of the block. Jiangda-Walasi area is the only basin with continuous Early and Middle Triassic sediments. Pushuiqiao Formation in the lower bed of Lower Triassic stratum unconformably overlaid on Hercynian granite body by eluvial purplish red granitic basal conglomerate, in which a sequence of continental, littoral and continental shelf coarse sandstone containing gravels, sandstone mixed with oolitic limestone, andesitic volcanic rocks and dacite volcanic rocks was deposited, and then was developed into a sequence of shoal-tidal flat assemblage (Quxianong Formation) with the upper transition, which is composed of oolitic limestone, sand or calcarenite and micrite and mixed with few volcanic rocks; in addition, littoral-neritic → slope brecciated limestone, volcanic turbidite, oolitic limestone, calcarenite, siliceous rocks mixed with intermediate-acid volcanic rocks were developed in the upper bed (Serongsi Formation). In the Middle Triassic Walasi Formation, a sequence of volcanic, endogenous and terrigenous turbidites with a thickness of 2000–2500 m containing intermediate-acid volcanic rock interbeds developed, which vertically constituted repeated regressive and progressive submarine fan superposition. It shows that the basin had sustained subsidence from the Early Triassic to the Middle Triassic. There are a series of small basins of Early Triassic distributed in the northwest narrow strip along Xiariduo-Malasongduo-Jiaolongqiao at the western rear edge of the orogenic belt. The lower member of Lower Triassic (Malasongduo Formation) is littoral-neritic grayish black, variegated sandstone, siltstone mixed with marl and limestone, which is unconformity with Carboniferous-Permian, and the upper member is a sequence transitional-continental of grayish green and locally purplish red acidic dacite-rhyolitic volcanic rock mixed with quartz sandstone and black shale erupted. The basins may have been squeezed by the eastern orogenic belt and soon destroyed at the end of Early Triassic.

The above basins were formed on the erosion surface of Hercynian movement, all of which were isolated small basins, which were not connected with each, and had different filling sequences and evolution histories. They belong to tectonic basins trapped in orogenic belts and thrust depressions on the western edge. Judging from the difference in the time of basin destruction and the general existence of collision-type intermediate-acid volcanic rocks, the collision orogeny in the eastern part of Qamdo Block in the Early and Middle Triassic was significant, and the difference between the uplift and depression of the crust was quite obvious. The small basins and intermediate-acid volcanic activities in this period were just the response of collision orogeny.

2.2.4.2.2.2 Late Triassic Lithofacies Paleogeography and Basin Properties

After the general uplift in Early and Middle Triassic, the Qamdo Block began to sink integrally in Late Triassic, resulting in the complete overlap of molasse formation in the lower part of the Upper Triassic, and the sedimentary sequence of littoral-neritic clastic rocks, shelf carbonate rocks and land-sea transitional clastic rocks was successively deposited upward, forming a complete transgression-regression cycle. In Carnian-Norian of Late Triassic, except for two NW-NNW parallel ancient lands in Boluo-Xiongsong on the west of Jinsha River and Xiariduo-Qingnidong-Haitong on the west, the whole Qamdo Block is a wide sea basin. After the Norian, the eastern margin of the block rose, and Boluo-Xiongsong Ancient Land spread to the north and south and to the west. At the end of the Late Triassic, the eastern part of the whole basin rose further from east to west, and the sedimentary area of the Late Triassic became land.

The Late Triassic Carnian was bounded by Xiariduo-Qingnidong-Haitong Ancient Land in the northwest direction. The east and west sides were affected by the different tectonic activities, so the paleogeographic environment, basin types and sedimentary assemblages are quite different. Post-collision tectonic spread occurred in the eastern orogenic belt, forming a superimposed volcanic-sedimentary basin, and on its west side, a rift basin was formed by stretching along the back-margin fracture of the main arc of the early continental margin. The central and western part of the block belongs to an intracontinental neritic basin.

In Late Triassic Carnian, Jiangda-Gebo superimposed volcanic-sedimentary basins were distributed on the early continental margin arc, that is, Gelashan-Jiangda-Gebo, and spread southward to Xuzhong-Deqin, Yunnan. A sequence of active volcanic-sedimentary assemblages was developed in Carnian, which consisted of Dongdu Formation, Gongyenong Formation and Dongka Formation from bottom to top, which was equivalent to Jiapila Formation in the same period in the central and western parts of Qamdo Block, so it was also called Large Jiapila Formation. The Dongdu Formation in the lower part was a sequence of fluvial and littoral amaranth-variegated clastic rock molasse, with a small amount of intermediate-acid volcanic rocks locally, which was pseudo-integrated with the Middle Triassic, and featured angular unconformity with Proterozoic and Paleozoic. It contained multiple layers of conglomerate mainly composed of quartzose mylonite and locally composed of metamorphic rocks, obviously its provenance was mainly from the orogenic belt at that time. The Gongyenong Formation in the middle part was composed of neritic-shelf medium-thin to thick limestone; the Dongka Formation in the upper part was mainly composed of neritic-transitional sandstone and siltstone and locally composed of volcanic turbidite and intermediate-acid volcanic rocks.

The research on the fine structure of the basin in the early Late Triassic shows that the basin was formed on the continental margin arc with island chain structure in the early stage, with an undulating basement and complicated paleogeographic environment. In the northwest, the basin was generally a shallow sea basin except the Xiaosumang-Caoqugulong zone composed of Proterozoic stratum, and in the southeast, it was Kagong-Songxi-Gebo bay basin connected with the shallow sea in the northwest between Boluo-Xiongsong Ancient Land and Xiariduo-Qingnidong-Haitong Ancient Land in the west. Massive volcanic activities in the basin occurred in Carnian, and the magmatic activities quickly ended at the end of Carnian, and then the volcanic-sedimentary basin was transformed into a stable neritic basin. In Norian-Rhaetian (Bolila-Adula-Duogaila) in Late Triassic, stable neritic carbonate rocks to transitional clastic rocks developed in this zone, and the sedimentary facies of the whole block tended to be consistent during this period.

Shengda-Chesuo Rift Basin was located in the west of the volcanic-sedimentary basin mentioned above. The Shengda-Chesuo-Songxi zone to the east of Xiariduo-Qingnidong-Haitong Uplift spread in NW direction for about 300 km, with a width of only 5–10 km. It is a series of limited rift basins in the Late Triassic, stretching and intermittently developing along Chesuo-Deqin Fracture on the west side of Jiangda-Mangling continental margin arc orogenic belt in Late Paleozoic, and is similar in nature to Deqinluchun Rift Basin in the south. As the Upper Triassic in the southwest of the basin overlaps the Devonian and Carboniferous-Permian, the basement of the basin may be mainly the Upper Paleozoic. In the northern part of the basin, along Xialaxiu-Moditan-Shengda, the Upper Triassic is called Shengda Group (T_3Sd), and the lower part is clastic rocks with a small amount of fluvial-lacustrine to neritic basic basalt (equivalent to Quezhika Formation and Chayekou Formation), and neritic limestone/marl (equivalent to Niangken Formation, Nilenong Formation and Mianda Formation-Luose Formation). The middle and upper parts are a sequence of grey-dark grey low-grade metamorphic sand-slate, sandwiched with multiple layers of alkaline olivine basalt (equivalent to Caijunka Formation, Bama Formation, Zhaga Formation and Zaileda Formation). In Ridanguo, Zaileda and other places, alkaline olivine basalt can be found in multiple interlayers, with a thickness of about 1500 m, accompanied by the intrusion of basic and ultrabasic rocks such as olivine diabase. The Bouma series is widely developed in clastic rocks, belonging to flysch turbidite formation dominated by land debris, with a total thickness of 6000 m (top and bottom not seen). In the slope zone on the east side of the basin, according to the observation of Shengda and other places, a sequence of transitional deposits has been developed, which is composed of angle gravelly slump limestone, channel deposits and calcareous turbidite fans of slope-marginal basin facies, and there are load casts in the sand-slate.

The southward rift basins are small. In Chesuo and its northwest area, the Upper Triassic in the same period is a sequence of mottled/dark grey clastic rocks from shallow water to semi-deep water, with a sedimentary thickness of nearly 4000 m. A large number of pillow-shaped breccias, conglomerates and lava mainly composed of (intermediate) basic volcanic lava are developed in the upper part, with a volcanic rock thickness of nearly 2300 m, accompanied by diabase vein intrusion. The thickness of volcanic cycle from bottom to top is increasing, which indicates that the extension of the basin is increasing. In Songxi-Chetai area in the northeast of Gongjue County, a sequence of volcanic turbidite is developed in the lower part of the Upper Triassic (Jingu Formation-Waqu Formation), and a large number of grayish green-grayish black block-bedded, pillow-shaped, almond-shaped intermediate-basic volcanic lava, agglomerate and breccia are developed in the upper part, with andesite at the top, and the volcanic rock thickness is up to 2000 m. A large number of diabase and gabbro intrusion occurred at the same period.

In the west of Xiariduo-Qingnidong-Haitong ancient land, the Carnian of Late Triassic started to sink and transgress as a whole from the ancient land to a vast shallow sea basin, and a sequence of sub-stable molasses to stable littoral-neritic carbonate rocks and clastic rocks was developed, which is called Jiapila Formation. At the bottom of Jiapila Formation, there are gray-purple composite conglomerates and gravelly sandstones deposited by piedmont debris flow and fluvial alluvial fan, and they transit upward to littoral-neritic medium coarse to coarse-grained sandstones, which are unconformity with all old strata. The middle and upper parts are composed of fuchsia to gray-purple, grayish-yellow to gray-green debris of quartz sandstone, siltstone and mudstone of different thickness, interbedded with limestone or lens, and belong to littoral-neritic sedimentation.

The Upper Triassic Jiapila Formation, also known as Small Jiapila Formation, developed in the area west of Xiariduo-Qingnidong-Haitong Ancient Land, is at the same horizon as the so-called “Large Jiapila Formation” in the eastern part of the block, that is, Dongdu Formation, Gongyenong Formation and Dongka Formation. They are isochronous and heterogeneous. Compared with the active “Large Jiapila Formation” in the eastern area, the sedimentary assemblage is very different, and the Small Jiapila Formation in the western area is the sub-stable to stable sedimentation, with simple lithology and lithofacies and almost no volcanic rocks. The sedimentary characteristics of Jiapila Formation show that the Jiapila period is the primary stage of transgression, with abundant material sources, extremely fast deposition and poor sorting. The eastern part of the basin is dominated by clastic sediments, and the grain size of sediments becomes smaller from east to west and from bottom to top. The structure is dominated by continental facies to littoral-neritic facies, which indicates that the paleogeographic environment experienced changes from Piedmont river, alluvial plain to clastic shelf from east to west and from bottom to top. However, the grain size of sediments in the western part of the basin is relatively fine, with the littoral-neritic gray layer accounting for a significant proportion, and there are more carbonate rock deposits in the middle and upper parts, which may have experienced the evolution from alluvial fan and alluvial plain to clastic rock, carbonate rock shelf and plateau. The provenance filling from the eastern orogenic belt may mainly control the sedimentary basin. The analysis of the huge difference in the sedimentary thickness of Jiapila Formation reveals that the basin was formed on the undulating basement in this period, with Xiariduo-Qingnidong-Haitong Uplift in the east, Dangbala-Machala Uplift in the west and Tuoba Uplift in the middle. The uplift-depression pattern of the basin basement may have been strengthened and enlarged by tectonic activity in the Jiapila period, and the deposition thickness of Jiapila Formation in the depression (such as Dangga) between Tuoba Uplift and Machala Uplift has doubled, which proves this aspect. The sporadic volcanic rocks in the middle and lower horizons of Jiapi Formation coincide with the volcanic rocks developed in the contemporaneous active volcanic-sedimentary basin and rift basin in the east, which reflected the contemporaneous tectonic–magmatic activity in the western basin. From the volcanic rock assemblage, the eastern part of the basin is dominated by (alkaline) basalt, while the western part is dominated by intermediate-acid volcanic rocks. Further studies regarding whether magmatic activity is related to extensional tectonic mechanism are required.

In Norian (Bolila) of Late Triassic, the spread of the eastern orogenic belt of Qamdo Block stopped and the difference of EW tectonic activity disappeared, which was also the period of the largest transgression range and the most stable sediment in Qamdo Basin. On the vast clastic shelf formed by the accumulation and filling of terrigenous clastic materials in the Jiapila Period, a wide sea carbonate shelf plateau with gentle inclination from east to west was formed, and a sequence of stable neritic-platform carbonate deposits (called Bolila Formation) was developed, integrated on Dongka Formation (east) or Jiapila Formation (west), covering almost the whole Qamdo Block. The Bolila Formation has a single lithology, consisting of gray-dark gray limestone, arenaceous limestone mixed with micrite-silty limestone, nodular limestone, arenaceous limestone, bioclastic limestone and dolomite, mixed with a small amount of argillaceous siltstoue. The upper and lower parts are mainly thin and medium-thick layers, and the middle part is mainly thick blocks. The thickness of the western part is generally greater than that of the eastern part, and the thickness of the eastern edge drops sharply to only tens of meters, which is directly unconformity with the underlying strata. It shows that the sea water overlaps from west to east.

During Late Norian-Rhaetian of Late Triassic (Adula-Duogaila), the orogenic belts on the east and west sides of Qamdo Block rapidly uplifted, the erosion rate was accelerated, the material supply was sufficient, and a large amount of terrigenous clastic materials were injected into the basin at an accelerated rate and accumulated on the early carbonate shelf, resulting in the rapid filling and silting of the basin and the relative lowering of sea level. It mainly formed a regressive sedimentary sequence, which was composed of Adula Formation (lower part) and Duogaila Formation (upper part), also called Bagong Formation. Adula Formation was in conformity contact with Bolila Formation in the lower part and Duogai Formation in the upper part. The sedimentary assemblages of Adula-Duogaila Formation are similar, mainly belonging to the neritic-shelf to transitional terrigenous clastic deposits, with shed coal (layers) locally, which are widely distributed with little change, and the total thickness is generally 500–1000 m. According to the stratum distribution of Adula-Duogai Formation, the deposits at that time mainly occurred in the west of Xiariduo-Qingnidong Uplift, and there were only some connected to semi-connected local depressions or finger-shaped bays in the east. Except that the local depressions were deep and the deposition thickness reached 3000 m, the deposition in most other areas was not thick. During the same period, the difference between the internal basins was obvious, with significant local persistent depression and large accumulation. For example, the thickness of Adula-Duogai Formation in Niaonong Area between Tuoba Uplift and Qingnidong Uplift surged to over 3000 m.

Since Early Jurassic, the tectonic pattern of Qamdo Basin has changed greatly. On the one hand, the Late Triassic sedimentary area in the eastern part of the basin has been uplifted into land, and the parallel orogenic belt in the eastern part of the basin has retreated westward and moved to the west of Maozhuang-Tuoba-Juelong; on the other hand, the sea water withdrew gradually and entered the development stage of intracontinental depression basin. Because there was continuous sedimentary transition between Jurassic and Upper Triassic, and no new tectonic sequence has been found, the transition occurred by gradual “peaceful evolution”. During Early Jurassic-Middle Jurassic, the basin was still in a relatively open environment. With the change of sea level, seawater flooded in from time to time, and continental and transitional deposits alternately appeared. In Late Jurassic, all seawater finally withdrew and completely evolved into a continental basin, with the development of fluvial-lacustrine deposits representing the basin destruction.

Jurassic-Cretaceous stratum is a series of transitional to continental clastic rocks deposited continuously on Upper Triassic. The deposition thickness is generally between 1500 and 3000 m, the thickness in the east is small and gradually increases to the west. Jurassic stratum is composed of lower Chalangga Formation, middle Tutuo Formation, Dongdaqiao Formation and upper Xiaosuoka Formation.

E’aipu Fracture in the east of the basin (the western boundary fracture of Tuoba Thrust) played an important role in controlling the whole Jurassic sedimentation. The northeast side of the fracture is basically the tidal flat, mainly thinner red clastic rock deposits, while the southwest side of the fracture is mainly silty, muddy and marlaceous lacustrine deposits with a dark color. A delta is formed in Xiangdui Area in southwest of the southeast member of the fracture. The Jurassic deposits are over 4000 m thick, and the Cretaceous deposits integrated on it are over 5000 m thick. It shows that the northeast member of the fracture is in an active state of relatively rising and the southwest member is relatively falling, and the foreland thrust activity on the east side of the basin started from the end of Late Triassic to the west and lasted until Jurassic. In addition, in Randui-Angchong to the east of Lancang River on the western edge of the basin, a large number of composite conglomerates and sandy conglomerates deposited in the alluvial foothill debris flow-river appeared in the Middle Jurassic to Upper Jurassic. It shows that the eastward thrust uplift of the orogenic belt in the western margin of the basin was also very significant.

Cretaceous stratum is mainly distributed in Cuowa-East Markam in the southeast of the basin, which indicates that the basin has shrunk from northwest to southeast. The Cretaceous stratum is in conformity with the underlying Jurassic stratum, mainly developing a sequence of red continental molasse deposits upward coarsening, which generally shows the characteristics of coarse grain size of sediments on both sides of the basin and relatively small grain size of sediments in the center. It shows that at that time, with the accelerated speed of mountains on both sides of the basin, a large number of bioerosion was formed and transported to the depressed basin, resulting in the final siltation and closure of the basin. The Cretaceous stratum is well exposed in Markam area, and the lower series (namely Cuowa Formation, Laoran Formation or Xiangdui Formation) is mainly composed of purple sandstone, mixed with light gray, grayish yellow-grayish green calcareous sandstone containing argionite, calcareous sandstone, siltstone and fine conglomerate. From bottom to top, the reverse cycles of siltstone → sandstone → conglomerate form an upward coarsening sedimentary sequence. The upper series (namely Markam Formation and Hutoushan Formation) is composed of fuchsia fine and medium grained quartz sandstone, feldspar quartz sandstone, argillaceous siltstone, mudstone mixed with conglomerate and glutenite, and oblique bedding and cross-bedding are well developed. The above Cretaceous strata are continental basin fluvial-lacustrine deposits.

2.2.4.3 Evolution and Orogeny of Qamdo Basin

Based on the above analysis of the space–time structure and sedimentation of Qamdo Basin, the tectonic evolution and orogeny process of the reshaped basin are as follows.

2.2.4.3.1 Evolution of Epicontinental Sea Basin During the Cleavage of Paleo-Tethyan Ocean in Devonian-Early (Middle) Carboniferous

From Early Devonian to Early Carboniferous, a geodynamic mechanism of extension caused by mantle uplift opened the prelude to the tectonic evolution of the Paleo-Tethys. The development of Jinsha River Rift split Qamdo Block from Yangtze Plate, and Qamdo Block was brought into the Paleo-Tethyan ocean-land system. In the same period, there are indications that the west side of the block is roughly along Tienailie-Denglongnong and Leiwuqi Gangzi-Kagong areas in Nangqian County, Qinghai Province, and the North Lancang River rift or ocean basin also developed.

Compared with the active continental margin slope-deep water basin deposits developed in the rift basins on the east and west margins, the Qamdo Landmass located between the two oceans is a shallow sea basin, which is mainly composed of stable neritic-shelf deposits.

2.2.4.3.2 Evolution of Arc-Basin System Under the Subduction of Permian Paleo-Tethyan Ocean

According to the typical calc-alkaline series arc volcanic rocks in Permian, Jinsha River Ocean may have started to subduct under Qamdo Block in the late period of Early Permian. The eastern part of Qamdo Block forms an active continental margin arc-basin, and the western part is a contemporaneous back-arc-basin, which generally constitutes a complete arc-basin system. At that time, the subduction trench in the northern member may be very close to the continental margin and subducted directly under Qamdo Block. The Permian oceanic crust subduction in the south of Gobo occurred in the sea east of the continental margin, belonging to the intra-oceanic subduction. Therefore, the continental margin island arc in the eastern part of the block developed on the basis of different crust, and there are obvious differences in the properties and crustal structure between the southern and northern arc members.

2.2.4.3.3 Collision Orogeny in Early and Middle Triassic

With the westward subduction of Jinsha River Oceanic Basin closed at the end of Late Permian, the evolution of Late Paleozoic basin ended. Collision orogeny in Early and Middle Triassic started. The important basis is as follows: First, Qamdo Block generally uplifted at the end of Late Permian, which is reflected in the upward coarsening of Permian stratigraphic sequence as a regression sequence; second, in the eastern part of Qamdo Block, there are sporadic Lower (Middle) Triassic unconformity overlying the Upper Paleozoic or Hercynian intrusive rocks, followed by the overall overlapping of Late Triassic deposits, and the lower part of Lower Triassic and Upper Triassic are molasse deposits; third, in Jiangda in the eastern area and Xiariduo, Malasongduo, Zhiba, Reyong and Xuzhong in the western area, high-silica, high-potassium dacite-rhyolite and homologous intrusive S-type granites of Early (Middle) Triassic are developed.

As the east and west sides of Qamdo Block in Early Triassic and Middle Triassic are dominated by collision orogeny, Qamdo Block is uplifting as a whole under the tectonic compression.

2.2.4.3.4 Development and Evolution of Mesozoic Composite Foreland Basin in Qamdo Block

2.2.4.3.4.1 Extensional Tectonics After Collision in the Early Period of Late Triassic

After the collision orogeny and uplift in Early Triassic and Middle Triassic, Qamdo Block was once again subjected to transgression and sedimentation in the early period of Late Triassic. In the eastern part of the block, with the detachment of mountain roots and upwelling of mantle, the orogenic belt was stretched and collapsed. In Carnian of Late Triassic, a superimposed volcanic-sedimentary basin was formed in the orogenic belt, and a limited rift basin was developed in Shengda-Chesuo-Songxi on the west side of the basin.

2.2.4.3.4.2 Evolution of the Composite (Back-Arc) Foreland Basin in Qamdo Block in Late Triassic-Cretaceous

In the early period of Late Triassic, Qamdo Block was transformed into a composite (back-arc) foreland basin located between the Jinsha River Collision Orogenic Belt in the east and Leiwuqi-Dongda Mountain Orogenic Belt in the west on the basis of back-arc-basin formed in early stage, except for the active volcanic-sedimentary basin formed by spreading in the east. During the whole Late Triassic, the basin was mainly descended and uplifted, and from east to west, it experienced a tectonic cycle from slow descending to slow uplift. At the end of Late Triassic, the dynamic mechanism of basin evolution began to change, and the eastern margin arc orogenic belt of Qamdo Block was further squeezed and uplifted, and pushed westward, so that the Upper Triassic stratum began to be drawn in the orogenic belt. During the Jurassic-Cretaceous, the foreland basin also moved westward parallel to the orogenic belt, and the sedimentation center was roughly located on Markam, Qamdo. In the same period, westward intracontinental subduction occurred along the North Lancang River Fracture, and a significant relative depression appeared in the northeast side. The general tectonic framework of Jurassic-Cretaceous is that the mountain chains on both sides of the basin are constantly uplifting and pushing backward into the basin, while the central part of the basin is continuously depressed, with continuous thick deposits from Upper Triassic to Cretaceous. During the Cretaceous, the basin gradually retreated and destructed from northwest to southeast. Under the continuous compression and collision, the orogenic belts on both sides of the basin developed significant intrusion of S-type intermediate-acid magma, forming Dongda Mountain and Dizhong deep-seated granite bases, respectively.

2.2.4.3.5 Intracontinental Convergence and Its Tectonic Response in Himalayan

Since Late Yanshanian-Himalayan, with the subduction and closure of Neo-Tethyan Ocean and the continuous significant jacking of Indian Plate into Eurasia, the whole Qinghai-Tibet Plateau started the comprehensive intracontinental convergence and compression and significant uplift. Qamdo Basin is located in the eastern part of Qinghai-Tibet Plateau. During Himalayan, it was generally in a tectonic system dominated by compression. With the readjustment of massive crustal materials, the tectonic deformation of the block occurred from the early transition from strike-slip to extension to the late massive horizontal shortening and the sharp uplift and thickening of the crust.

2.2.4.3.5.1 Massive Right Strike-Slip Structure

During Late Yanshanian-Early Himalayan, given the N-NNE orogeny of Indian Plate and the relative S-SW movement of Yangtze Plate, Sanjiang composite orogenic belt between the two plates became a giant strike-slip shear system. Qamdo Block is located on the southwest side of the strike-slip system, with Jinsha River Fracture and deep NNW fractures such as Chesuo-Deqin Fracture as boundaries. It is mainly controlled by the near NS stress field from Indian Plate, which is mainly manifested in the right-handed orogeny between the main body of the block and its eastern orogenic belt along the above fractures, resulting in a series of NW-NNW folds and alternate compressional and torsional fracture structures in the middle-east of Qamdo-Markam Composite Basin in Upper Triassic. Its strike intersects with the NNW main strike-slip fracture in an acute angle in the shape of Chinese character “入” (Fig. 2.11), among which Xiariduo-Yulong-Haitong Oblique Fold Tectonic Zone developed on the west side of Chesuo fracture is the most representative, and it plays an important role in controlling the distribution of porphyry in Himalayan.

Fig. 2.11

Secondary fold reflecting dextrorotation in upper triassic on the east side of Tuoba fracture zone. 1—Limestone; 2—Anticline; 3—Syncline; 4—Twist direction

2.2.4.3.5.2 Formation of Strike-Slip Basin and Deep-Seated Magmatism in Paleogene

During the Paleogene Eocene, strike-slip extensional tectonic activities took place in Eastern Tibet, and a series of strike-slip extensional basins were formed on the surface, such as Gongjue, Nangqian, Jiqu and Lawu basins. Paleogene crustal extension was coupled with mantle uplift, and crustal decompression led to the melting of lower crust or upper mantle. Regional extensional fractures promoted magma to rapidly rise into the crust, and at the same time as the formation of strike-slip extensional basins on the surface, deep volcanic-magmatic activities, which were mainly alkaline, developed. Volcanic activities mainly occurred in basins with significant strike-slip tension. In the Gongjue, Nangqian and Lawu basins, there were Paleogene-Neogene volcanic rocks with rough-coarse texture in varying degrees. In the same period, the relative uplift of the surface, such as Xiariduo-Haitong Belt and Gaoji-Tuoba Belt, has become a favorable environment for the emplacement of intrusive rocks due to its tectonic dilatancy, forming small porphyry bodies distributed in groups and strips, constituting important tectonic intrusive rock belts, which were closely related to the formation of porphyry copper deposits.

2.2.4.3.5.3 Hedging System with the Giant Thrust Belt of North Lancang River as the Main Structure

The giant thrust belt of North Lancang River was mainly formed in Late Yanshan-Himalayan, which had an important influence on the tectonic deformation in Qamdo Block. During Himalayan, Leiwuqi-Dongda Mountain composite island arc orogenic belt on the west side took on the huge stress from Indian Plate in N-NE direction, which not only caused its own significant tectonic deformation, forming a series of imbricated sheets, but also took the North Lancang River Junction Zone as the main thrust belt, resulting in an overall thrust nappe to northeast (Fig. 2.12). Due to the significant thrust nappe, a part of the west side of Qamdo Basin has been swallowed up by overthrust fracture. Due to the transmission and application of tectonic stress to NE-NNE Qamdo-Markam Basin, Qamdo Basin has been greatly shortened and narrowed in NE-NNE direction, and the significant tectonic deformation zone has been developed at the front edge of the main thrust fracture, with a series of axial planes inclined to the W-SW and closed deflection-overturned fold and a series of secondary thrust fractures of Upper Triassic and Jurassic-Cretaceous. A large number of detached blocks pushed from the west were developed. The horizontal compression range of the basin increased from northwest to southeast, and tended to tip out near the Sanjiang waist in the south of Yanjing. It shows that the amplitude and intensity of thrust nappe compression in Qamdo-Markam Basin in Leiwuqi-Dongda Mountain Island Arc Zone increased from northwest to southeast, and the nappe direction also changed from NE-NEE direction to nearly EW direction, mainly nearly EW compression in Yanjing. It is precisely because the island arc zone completely squeezed the Qamdo-Markam Basin that the strike-slip extensional basin formed in Paleogene quickly destroyed, and some of them were further transformed into depressed basins. Corresponding to Lancang River on the west side, the eastern orogenic system has further thrust westward on the basis of the foreland thrust zone (Fig. 2.13). Its frontal zone was Tuoba-E’aipu Fracture Zone in the north and Laoran Yanjing Fracture in the south. The two-sided thrust system with the basin as the central axis has been formed (Fig. 2.14), which has become the most spectacular tectonic deformation in Himalayan intracontinental convergence of Qamdo Basin.

Fig. 2.12

Jiaoba Mountain-Zhuka lancang river Nappe zone overthrust eastward over Jurassic-Cretaceous stratum

Fig. 2.13

Haitong-Markam Nappe thrust westward over Jurassic-Cretaceous stratum

Fig. 2.14

Profile of hedging system on two sides of Qamdo basin

2.2.4.3.5.4 Nearly EW Tectonic

The EW tectonic in Qamdo Block is hidden but exists widely. It is composed of a series of small NWW to nearly EW left-lateral shear fracture bundles, which cut through all strata including Paleogene and Neogene, mainly belonging to brittle surface and formed in Himalayan. There are two important zones on the surface: First, Tongtian River located on the northern edge of Qamdo Block and its south area. The Tongtian River Fracture in the region is connected with Xianshui River Fracture in Western Sichuan, and it strikes in the NWW direction. Since Himalayan, there has been a significant left-hand strike-slip, which has significantly transformed Yushu-Luoxu Member of Jinsha River Junction Zone. It is also clearly reflected in Qamdo Block on the south side, and a series of secondary EW fractures have been developed. Second, the structure is located in Xiaya-Chaya-Latuo, spreading in the NWW direction with different characteristics in the east and west members. The west member shows NWW left-hand orogeny in Xiaya and its north and south sides, which results in the dislocation of Luwuqi-Dongda Mountain Island Arc Zone and the east or NEE arc protrusion of the main thrust fracture along Wokadi-Zhuka-Yanjing.

In Chaya-Latuo, the basement block is dislocated, and the traction tectonics formed on the surface layer makes the NW-NNW fracture and stratigraphic strike line change to NWW-SEE direction obviously, and then merges with NW-NNW tectonics in the south of Latuo, and the connected Gongjue-Mangcuo Paleogene Gongjue Basin may be disconnected due to dislocation compression. According to the observation of the Paleogene-Neogene EW left strike-slip fractures between Gongjue County and Latuo, the horizontal dislocation distance of a single fracture reached tens to nearly hundreds of meters, and the continuous dislocation displacement caused by group faults resulted in a large total displacement in the whole zone. Most of the EW left strike-slip fractures took the regional NW fracture tectonic as the boundary line, and occurred in the divided blocks and strips, and a few of them cut the NW fractures. The understanding of the nearly EW tectonic in Qamdo Block is consistent with the remote sensing of Sanjiang Region recently made by the Institute of Mineral Deposits of Chinese Academy of Geological Sciences (according to the “Ninth Five Year Plan”, Sanjiang and “Research on Yidun Island Arc Tectonic Evolution and Metallogenic Regularity”), and the two large EW linear tectonic zones interpreted for Eastern Tibet coincide with the above generally: First, it extends from Shangka in the west to the east through Latuo, Ziga and Ailashan passes in Western Sichuan. It has obvious aeromagnetic anomalies (treated by ΔT polarization) and gravity anomalies (treated by residual gravity anomalies), showing frequent Himalayan activities. Second, it is located in Chayu-Bitu-Daocheng in the south. In addition, a series of small and medium EW linear tectonics have been found in Eastern Tibet. The EW left strike-slip fracture is the result of the interaction of the inhomogeneity of materials in the block and the asymmetric compression stress in EW. It is the result of adjustment and transformation between crustal materials and stress field after the NS compression was blocked in Himalayan, and it has become an important manifestation of Himalayan tectonic deformation in Qamdo Block, which is also in line with the characteristics of tectonic stress field in this area.

2.2.5 Space–Time Structure and Evolution of Ailaoshan Arc-Basin System

For many years, it has been considered that Ailaoshan Arc-Basin System is roughly in the same back-arc ocean basin subduction—arc-land collision orogenic belt as Jinsha River Arc-Basin System, which is mainly based on the fact that both systems are located on the east side of Qamdo-Lanping Block. However, the east side of Ailaoshan is directly adjacent to the southwest continental margin of Yangtze Continent, and the east side of Jinsha River is adjacent to Zhongza-Shangri-La Block which is divided from Yangtze Continent.

2.2.5.1 Space–Time Structure of Ailaoshan Arc-Basin System

Ailaoshan Orogenic Belt is mainly composed of (from northeast to southwest) Red River Fracture, Ailaoshan Fracture, Tengtiao River Fracture, Jiujia-Anding Fracture and Amo Jiang-Lixian River Fracture, and their basement high grade metamorphic zone, arc-land collision ophiolite melange zone (commonly called low-grade metamorphic zone), Mojiang-Lvchun Arc Volcanic Zone and foreland thrust belt. Generally speaking, the belt converges to the northwest and spreads to the southeast (Fig. 2.15).

Fig. 2.15

Sketch of geological tectonics of Ailaoshan. 1—Jurassic-Paleogene continental sedimentation; 2—Yiwanshui formation in upper Triassic; 3—Triassic Ganbatang group; 4—Paleozoic Madeng group complex; 5—Devonian-lower carboniferous (low-grade metamorphism); 6—Devonian; 7—Silurian (not metamorphic); 8—Proterozoic Ailaoshan group complex; 9—Ultrabasic rock; 10—Albitite basalt; 11—Pyroxene basalt; 12—Metamorphic basalt; 13—“Bimodal” volcanic rocks; 14—Basalt-andesite; 15—Andesite; 16—Latite; 17—Rhyolite porphyry; 18—Rhyolite; 19—Gabbro-diorite; 20—Granite; 21—Geological boundary; 22—Main fracture and its number; 23—Red river fracture; 24—Ailaoshan fracture; 25—Tengtiao river fracture; 26—Jiujia-Anding fracture; 27—Amo Jiang-Lixian river fracture; 28—Babian river fracture; 29—Other fractures; 30—Unconformity boundary; 31—Gold deposit (point)

2.2.5.2 Formation Period of Ailaoshan Back-Arc Ocean Basin and Lvchun Continental Margin Arc

The main Ailaoshan low-grade metamorphic zone is the ophiolite melange zone. Early Carboniferous radiolarian Albaillella paradoxa (?), Deffaadree, Astroentactinia multispinisa Won. can be found in siliceous rocks. The silicon isotope (δ³⁰Si) content of radiolarian siliceous rocks in Laowangzhai is 0.2‰, which is close to the 0.6 ‰ to 0.18 ‰ (average 0.16‰) of that in abyssal environment (δ³⁰Si) classified by Ding. The ΣREE of siliceous rare earth elements in Devonian and Carboniferous is (47.56–137.93) × 10^–6, the fractionation of light and heavy rare earth elements is obvious, the LREE is 7.07–9.46, and the (La/Yb)_N is 0.61–10.63. The rare earth partition is light rare earth enrichment right, and the negative δEu is abnormal. δEu ranges 0.58–0.79, indicating the abyssal to abyssal environment, which can represent the characteristics of abyssal siliceous rocks in the arc ocean basin. Fuchsia siliceous rocks (C₁) can be found in Pingzhang, Xinping County. It is generally believed that fuchsia radiolarian siliceous rocks represent the vast ocean deposit. In this zone, ocean ridge volcanic rocks can be also identified, and gabbro and plagioclase granite are formed in Shuanggou Ophiolite Formation. The U–Pb age of this zone is 328–362 Ma, which is basically consistent with the U–Pb age of Shusong plagioclase granite, Jinsha River Zone, which is 340–294 Ma. It reveals the initial formation period of ocean floor expansion at the turn of Devonian/Carboniferous, which represents the development period of Ailaoshan Back-arc Ocean Basin from Early Carboniferous to Early Permian.

The volcanic rocks of Panjiazhai in Late Permian, which occurred in the flysch sedimentary strata composed of metamorphic sandstone, phyllite and quartz schist on the northeast side of Ailaoshan low-grade metamorphic zone, have been metamorphosed into green schist and “blue schist”. Its lithochemistry and geochemistry characteristics indicate that it is composed of continental rift rough basalt and basalt. It is characterized by low Si content and high Ti and alkali content. The total amount of rare earths is high, and the fractionation of light and heavy rare earths is obvious; The REE distribution is of right enrichment, similar to Emei Mountain basalt. Therefore, Panjiazhai metamorphic volcanic rocks belong to continental rift volcanic rocks. However, they have been involved in the block of melange, and the glaucophane in the blue schist is arfvedsonite and blue tremolite. In addition, according to the Middle Triassic Ganbatang flysch sedimentation with metamorphic deformation in the ophiolite melange zone, it shows that the back-arc ocean basin finally destructed and the melange was formed in Middle Triassic, and it was covered by Late Triassic molasse tectonic unconformity.

The Ailaoshan Back-arc Ocean Basin began to subduct southwest in the early period of Late Permian, and the eastern edge of Lanping-Pu’er Block in the southwest formed the Mojiang-Lvchun Continental Margin Arc. Mojiang-Lvchun Continental Margin Arc is mainly characterized by volcanic-sedimentary rocks formed in Late Permian-Late Triassic and granite intrusion formed in the late period of Late Triassic. The subduction arc was formed before 260–250 Ma, which is equivalent to the time of arc volcanic rocks formed in Late Permian; the collision arc was formed before 240–210 Ma, which is consistent with the development age of arc volcanic rocks formed in the Late Triassic.

2.2.5.3 Spatial Distribution of Ailaoshan Arc-Basin System

The spatial structure and tectonic pattern of Ailaoshan Arc-Basin System have been significantly reformed by the significant collision between India Plate and Eurasia since the end of Cretaceous and Paleogene. The rotation and slip of the landmass and the overlapping displacement of different tectonic stratigraphic units lead to the blurring of boundaries of some tectonic stratigraphic units, but the spatial configuration of volcanic arc and ophiolite melange zone can basically be identified.

2.2.5.3.1 Active Basement Thrust Zone in the Southwest Margin of Yangtze Continent

Since the Red River Fracture and Ailaoshan Strike-slip-Thrust Ductile Shear Zone are mainly composed of the high grade metamorphic zone of Ailaoshan and Jinping overlying fracture block units, according to the research of Wang during the “Eighth Five-Year Plan”, Ailaoshan Group Complex is divided into Xiaoyangjie Rock Formation, Qingshui River Rock Formation and Along Rock Formation. Xiaoyangjie Rock Formation is an aluminum-rich rock series assemblage, it is mainly composed of biotite plagioclase gneiss, garnet fibrolite biotite plagioclase gneiss, disthene biotite plagioclase gneiss, etc., sandwiched with granulite, and locally sandwiched with olivine dolomite marble lens and boudin. Amphibolites are rare. Folding layers, bedding recumbent folds, synclinal inverted fold and plunging fold are well developed. Flow fold and rootless fold formed by plastic rheology are more common, and they are mostly in the form of boudin migmatite, shadow migmatite and homogeneous migmatite. It shows that the rock has experienced local melting under deep conditions, and even formed in-situ/quasi-in-situ low-intrusion remelting granite rock mass. In Yuanyang and Langdi, the intrusion of magnesian ultramafic rock mass is also found in the rock formations, which indicates that there was the intrusion of anatectic magma in the upper mantle. It shows that this tectonic-rock stratum was once a part of the deep crust or at least the middle crust.

Qingshui River Rock Formation and Xiaoyangjie Rock Formation are separated by a regional tectonic detachment surface, mainly composed of plagioclase amphibolite gneiss, amphibolite plagioclase gneiss and granulite, with biotite plagioclase gneiss, granite gneiss and heronite, and amphibolite and marble in the lower part. The strata are characterized by the widespread development of folding layers, bedding recumbent folds, and bedding ductile shear zones. The plastic rheological characteristics of deep local melting in this tectonic-rock stratum are rare. Generally, the grade of metamorphism can reach high greenschist facies, with amphibolite facies locally, and its source rock is volcanic-sedimentary rock assemblage, which shows obvious difference from Xiaoyangjie Rock Formation. Along Rock Formation is a calcium-rich rock assemblage, mainly composed of marble, with a small amount of amphibolite, amphibolite plagioclase gneiss and calcium silicate, etc. The intrusion of mafic dikes and granite veins has resulted in metamorphism and deformation. Marble is characterized by folding layers and bedding shear zones, with obvious solid plastic rheology, which is actually recrystallized carbonate mylonite. The metamorphism and deformation of this rock formation is similar to that of Qingshui River Rock Formation.

Up to now, no valuable isotopic age data has been obtained in Ailaoshan Group Complex. Only the Institute of Geology and Geophysics, Chinese Academy of Sciences has measured the whole-rock apparent isochron age of Rb–Sr (839 ± 0.739) Ma in this group complex, and the K–Ar age of gneiss and granulite in Along Rock Formation includes three ages in Late Paleozoic, Mesozoic (171–85 Ma) and Cenozoic (47.7–11.5 Ma). It may reflect the superposition of tectonic thermal events such as the formation of Ailaoshan Arc-Basin System, arc-land collision and post-orogenic spreading and exposure.

The Jinping Unit above Ailaoshan Metamorphic Group Complex is characterized by the development of Paleozoic-Triassic sedimentation, and it should be an integral part of the overlying strata at the western margin of Yangtze Plate. The oldest exposed stratum is the Ordovician stratum, which is mainly sand-mud formation with flysch-like characteristics, with slight metamorphism. It is covered by parallel unconformity of Middle Silurian, and the Middle and Upper Silurian stratum is basically a sequence of magnesium carbonate rocks. The Devonian stratum lacks the lower series, and the middle and upper series are carbonate rocks mixed with dolomite, argillaceous siliceous rocks and shale. The Carboniferous stratum is composed of limestone and biological limestone. The Lower Permian stratum is composed of biological limestone and cryptocrystalline limestone. The lower part of the Upper Permian is Emei Mountain basalt and pyroclastic rock equivalent to the western margin of Yangtze Plate, which is eruption unconformity with the lower series, with a thickness of 4536 m; the upper part is the transitional coal-bearing stratum. The Lower Triassic is in lacuna, and only the Middle and Upper Triassic strata are exposed. The Middle Triassic is dominated by limestone and dolomite, with tuffaceous sandstone, shale or basalt occasionally, directly overlying the Upper Permian basalt; the Upper Triassic is dominated by sand shale, locally mixed with limestone and dolomite, containing coal and rich biological fossils. The strata exposure, contact relationship and lithologic and lithofacies characteristics of Jinping Overlaying Unit are similar to those of Haidong, Dali. It seems that it was originally a part of the same tectonic unit and was dismembered into two parts by the left-handed strike-slip tectonic of Ailaoshan-Red River Fracture in the later period, with a strike-slip distance of about 350 km. Therefore, together with Ailaoshan Group, it formed the early passive margin subduction unit of orogenic belt and has been transformed into an overlapping block unit since the end of Late Triassic.

2.2.5.3.2 Ailaoshan Ophiolite Melange Zone

Ailaoshan Ophiolite Melange Zone is located in the low-grade metamorphic zone between Jiujia-Anding Fracture Zone and Ailaoshan Fracture (Fig. 2.15). There are many studies on this zone. In recent years, more oceanic ridge basalt, pyroxene-diorite cumulate, lherzolite and radiolarian siliceous rock (C₁) have been found in studies. Based on previous studies, a relatively complete ophiolite sequence can basically be established, with metamorphic peridotite (including lherzolite and harzburgite), cumulate complex (including pyroxenite, gabbro, gabbro diorite and plagioclase granite), diabase, basic lava (including albitite basalt and pyroxene basalt) and radiolarian siliceous rock from bottom to top. Compared with the typical ophiolite in the world, the layered gabbro and diabase sheeted dike swarms in the cumulate are undeveloped.

The mafic–ultramafic rocks are distributed in groups and strips in a melange zone with a length of 400 km and a width of several kilometers along the NW strike (Fig. 2.15). Basic volcanic rocks, mafic–ultramafic rocks, and radiolarian siliceous rocks are mixed with each other in the muddy flysch matrix in various sizes. There is obvious tectonic contact between rocks with different genesis and composition and their matrix, showing the appearance of ophiolite melange. Generally, the matrix is mainly sandy shale with turbidite sedimentary, but the significantly deformed limestone blocks can be seen directly in the sandy shale layer, and their attitudes are very discordant, the rocks formed in different environments are mixed, and some limestone blocks may be slumping rocks on the slope edge. There are biological fossils in limestone blocks, including conodonts of Early Carboniferous and Ordovician–Silurian. In Pingzhang, Kudumu and other places, Carboniferous-Permian biogenic limestone of shallow plateau origin with block length greater than 500 m can be seen in lower Carboniferous argillaceous limestone, fuchsia radiolarian siliceous rocks and black sandy argillaceous rocks mixed with siliceous rocks deposited in deep water, showing obvious mixing tectonics. The exposed strata along this zone, which are relatively complete and have the basis of biological fossils, such as the lower Carboniferous black thin-bedded argillaceous limestone or nodular limestone, are exposed in sporadic fragments, and the rocks are significantly squeezed and deformed, forming complex folds. However, the source rock features are well preserved, and there are abundant conodont fossils. In the aleuropelitic turbidity sediment along the east side of Jiujia-Anding Fracture Zone, a sequence of conglomerates with the same composition as the sediments are intermittently exposed in the range of about 50 km from Yakou Street, Heping to Xiangyang Mountain, Shuanggou, with poor sorting and grinding, mixed size, different gravel size, different shapes and single composition. Such sediment is the gravity sediment of submarine fan channel in slopes.

According to Wang Yizhao’s observation, the macroscopic tectonic deformation of this zone is mainly characterized by lateral tectonic displacement, and the formation of tectonic community related to bedding shear, which is characterized by bedding recumbent folds, bedding shear zones and folding layers, and by the extensive development of S1 lateral displacement of S0 in the region. Generally, the replacement is thorough, forming a regional permeable foliation composed of S1. Due to the significant compression during the orogenic period, S1 and S0 are widely developed in the area, forming a regional tectonic style composed of synclinal inverted folds and imbricate thrust fractures associated with thrust fractures. The axial plane of the inverted fold generally falls to the SW and tends to the NE, reflecting that the thrust nappe orogeny is in NE-SW direction. The tectonic characteristics of metamorphic rock formations and mineral paragenesis assemblage show that the two foliation replacements are basically in the tectonic deformation environment of low greenschist facies. Near Ailaoshan Fracture Zone in the northern member, the low-grade metamorphic rock belt formed phyllonite and tectonic schist due to significant tectonic deformation, and the significant strain zone developed. The low-grade metamorphic rocks generally reach the low greenschist facies, and it shows the single-phase dynamic metamorphism with no progressive metamorphic zones, except that it can reach the high greenschist facies locally. It is worth noting that the source rock of the albite chlorite schist containing glaucophane, which is exposed from the tectonic block at 244 km between Mosha and Malutang, is almond-shaped basalt, which was formed by high-pressure and low-temperature metamorphism. The green schist obviously belongs to mylonite formed by ductile shear deformation of basic volcanic rocks. The rock has obvious S-C fabric. The ductile shear zone represented by this green schist has been severely damaged and reformed by the later fractures, which means that it developed into an important part of the thrust tectonic melange zone during the arc-land collision on the basis of the oceanic subduction.

2.2.5.3.3 Mojiang-Lvchun Continental Margin Arc Zone

This zone is located in Mojiang-Lvchun, which is in the west of Jiujia-Anding Fracture Zone on the southwest boundary of Ailaoshan Ophiolite Melange Zone, and in the east of Amo Jiang-Lixian River Oblique Thrust Fracture Zone. In the orogenic stage of the arc-land collision, this zone was basically involved in the overlapping landmass unit of the orogenic belt. It consists of Paleozoic and Mesozoic strata and Late Triassic granite. Silurian-Lower Carboniferous strata developed graptolite shale, clastic turbidite, radiolarian siliceous rocks, thin argillaceous limestone and reticulate limestone. In Early Carboniferous, a sequence of “bimodal” volcanic rocks, sandstone, siliceous shale and siliceous rocks, which are composed of basic volcanic rocks and acidic volcanic rocks, developed in Bulong-Wusu of Mojiang, and were characterized by the passive margin rift on the east side of Pu’er Block. The Middle and Upper Carboniferous to Lower Permian strata are a sequence of unstable plateau carbonate rocks and coal-bearing clastic rocks, showing the uplift of fracture blocks. In the Late Permian, a sequence of arc volcanic rocks developed along the Taizhong-Lixian River. In the Late Triassic, a sequence of collision acidic volcanic rocks and lag arc volcanic rocks developed in Gaoshanzhai and Keping of Lvchun, respectively.

The “bimodal” volcanic rock assemblage in Mojiang-Wusu shows two eruptive cycles (Fig. 2.16). Both eruptive rhythms of the lower cycle are all composed of lava, with basalt first, rhyolite later, and basalt pillow tectonic developed; both eruptive rhythms of the upper cycle are rhyolitic tuff, indicating the action of volcanic eruption. There is a sequence of rhythmic sediments of siliceous shale-sandstone-siliceous rock between two volcanic activities. The top and bottom of the member are deep-water siliceous rocks and flysch-like sand shale, and the petrochemical and geochemical characteristics of the volcanic rocks are double rift type “bimodal” volcanic rocks (Fig. 2.17). The basalt has high TiO₂ content, which mainly falls in the intraplate basalt area in ATK diagram, indicating that the “bimodal” volcanic rocks were formed in the continental margin on the east side of Pu’er Block.

Fig. 2.16

Section of Mojiang-Wusu at 314 km. 1—Massive basalt; 2—Pillow basalt; 3—Pyritized basalt; 4—Almond-shaped basalt; 5—Jasper filled pillow basalt; 6—Rhyolite; 7—Pyritized rhyolite; 8—Rhyolitic tuff; 9—Sandstone; 10—Feldspar coarse sandstone; 11—Silty siliceous rocks; 12—Siliceous shale; 13—Fracture

Fig. 2.17

TAS diagram (Li et al. 1999a, b). S₁—Coarse basalt; S₂—Basaltic andesite; S₃—Coarse andesite; T—Trachyte; Pc—Picrite basalt; B—Basalt; O₁—Basaltic andesite; O₂—Andesite; O₃—Dacite; R—Rhyolite; I—Irvine boundary (the upper part is alkaline and the lower part is subalkaline); 1—Ailaoshan oceanic ridge basalt; 2—Ailaoshan quasi-oceanic ridge basalt; 3—Wusu-Bulong volcanic rocks; 4—Main arc volcanic rocks; 5—Collision volcanic rocks; 6—Lagged arc volcanic rocks

The spatial overlap of Late Permian arc volcanic zone with Mojiang-Wusu “bimodal” rift zone can be divided into three types of volcanic rocks: main arc volcanic rocks, collision volcanic rocks and lagged arc volcanic rocks.

Volcanic rocks are widely exposed in the main arc period, represented by Taizhong volcanic rocks, Nanwenqiao volcanic rocks and Lixian River volcanic rocks (Fig. 2.18) formed in the Late Permian. There are mainly almond-shaped rough basalt, plagioclase basalt, basaltic andesite, biotite andesite and pyroclastic rocks. Basalt interbedded with andesite. The tholeiite series coexisted with the calcium alkali series. The content of Ti of clinopyroxene in basalt is obviously lower than that of pyroxene in continental tholeiite. The chemical composition and petrochemical composition are shown in the ATK diagram (Fig. 2.19) and lgτ-lgσ (Fig. 2.20), almost all of which fall in the island arc orogenic belt, and the rare earth pattern and trace element pattern are similar to those of island arc volcanic rocks (Figs. 2.21 and 2.22). According to the spatial distribution, from east to west, the general trend is from tholeiite series to calc-alkali series, and the content of Al₂O₃ and (K₂O + Na₂O) gradually increased, reflecting the westward subduction of the ocean crust.

Fig. 2.18

Profile of Lixian River-Xin’an Volcanic Arc. 1—Basaltic andesite; 2—Basaltic volcanic breccia; 3—Andesite tuff; 4—Limestone-bearing breccia basalt; 5—Slump sedimentary rock; 6—Limestone; 7—Sandstone; 8—Shale; 9—Fracture

Fig. 2.19

ATK Diagram. I—Oceanic basalt area; II—Continental basalt and andesite area; III—Island arc, basalt and andesite area in orogenic belt. 1—Ailaoshan oceanic ridge basalt; 2—Ailaoshan quasi-oceanic ridge basalt; 3—Main arc volcanic rocks in Taizhong-Lixian River; 4—Wusu volcanic rocks; 5—Pinghe latite; 6—Panjiazhai metamorphic basalt

Fig. 2.20

lgτ-lgσ Illustration. A—Volcanic rocks in intraplate stable area; B—Volcanic rocks in subduction zone; C—Alkaline volcanic rocks evolved in areas A and B 1—Bulong-Wusu volcanic rocks; 2—Taizhong-Lixian River main arc volcanic rocks; 3—Lagged volcanic rocks; 4—Collision volcanic rocks; 5—Panjiazhai volcanic rocks

Fig. 2.21

REE distribution pattern in volcanic rocks in Taizhong-Lixian River. 1—Rhyolite (Wusu); 2—Tholeiite (Wusu); 3—Dolerite (Nanwen Bridge); 4—Basaltic andesite (Lixian River); 5—Biotite andesite (Taizhong); 6—Plagioclase basalt (Baliu); 7—Rhyolite porphyry (Gaoshanzhai); 8—Latite (Pinghe); 9 and 10—Metamorphic basalt (Panjiazhai)

Fig. 2.22

Geochemical distribution pattern of trace elements in volcanic rocks in Taizhong-Lixian River. 1—Tholeiite (Wusu); 2—Dolerite (Nanwen Bridge); 3—Basaltic andesite (Lixian River); 4—Biotite andesite (Taizhong); 5—Plagioclase basalt (Baliu); 6—Latite (Pinghe); 7 and 8—Metamorphic basalt (Panjiazhai)

The collision volcanic rocks, represented by Lvchun Gaoshanzhai volcanic rocks, are a sequence of acidic (rhyolite porphyry) assemblages, dating from the Late Triassic. Their petrochemistry is characterized by high SiO₂ (73.99%) content and K₂O (5.20%) content (Fig. 2.17), which is completely the same as that of the collision volcanic rocks in Sanjiang Region (Mo et al. 1993), and also similar to the collision rhyolite in the eastern zone of western United States.

Post-collision extensional volcanic rocks, represented by Late Triassic volcanic rocks in Lvchun-Pinghe, are mainly a sequence of neutral to intermediate-acid pyroclastic rocks mixed with some lava (alkaline andesite, Fig. 2.17), with a volcanic explosivity index Ep of 0.7. They are a series of high-potassium volcanic rocks with petrochemical characteristics, which can be compared with T32 latite in the South Lancang River. The REE distribution pattern (Fig. 2.21) is similar to that of continental volcanic rocks, and the trace element distribution pattern is similar to that of island arc volcanic rocks (Fig. 2.22), which indicates that it was far away from plate boundaries and close to continental volcanic island arcs.

2.2.5.4 Formation and Evolution of Ailaoshan Arc-Basin System and Arc-Land Collision Orogenic Belt

The Ailaoshan zone experienced the transformation of different tectonic systems and tectonic deformation of different tectonic levels from Paleozoic to Mesozoic and Cenozoic. According to the space–time structure, rock composition and deformation characteristics of the above-mentioned tectonic units, they generally experienced the following stages.

2.2.5.4.1 Ocean-Land Transition Stage

The expansion and contraction of ocean lithosphere is marked by the appearance of arc-basin system. Ailaoshan Arc-Basin System and Jinsha River Arc-Basin System developed in the same way, with Jitang-Chongshan-Lancang residual arc on the west side of Qamdo-Lanping-Pu’er Block as the front of the northeastern Tethyan Ocean, and entered a new development period at the beginning of Devonian on the basis of Early Paleozoic subduction complex or metamorphic “soft basement” deposited on the western margin of Yangtze Continent. In Middle-Late Devonian neritic shelf carbonate plateau background, local tension occurred. On the west side of Jiujia-Anding Fracture Zone, the Middle-Upper Devonian semi-pelagic mud-sand mixed with siliceous sediment were deposited in the rift basin. Ocean crust represented by oceanic ridge basalt appeared in Shuanggou-Pingzhang-Laowangzhai in the Early Carboniferous. On the west side of the back-arc ocean basin, Bulong, Mojiang-Wusu experienced a “double-peak” volcanic eruption in the rift zone that split off the edge of the landmass. Carboniferous-Early Permian is the period of further expansion and finalization of Ailaoshan back-arc ocean basin. Carbonate rocks in Carboniferous-Early Permian plateau near the land margin were distributed in isolated island chains, with siliceous flysch mixed with basalt. Lanping-Pu’er Block split from the Yangtze Landmass to form an independent micro-landmass.

In Late Permian, the back-arc ocean basin stopped spreading and turned into significant subduction and reduction to the west Lanping-Pu’er Block, forming the Mojiang-Lvchun continental margin volcanic arc.

2.2.5.4.2 Left Strike-Slip Arc-Land Collision Orogenic Stage

At the end of Late Paleozoic-Late Triassic, Yangtze Landmass wedged westward, and Ailaoshan Back-arc Ocean Basin was the earliest orogenic belt formed by the arc-land collision after the subduction and destruction of Hoh Xil-Jinsha River-Ailaoshan Zone. As a result of the arc-land collision, Taizhong-Lixian River collision continental margin volcanic arc was superimposed on or on the east side of Late Permian subduction continental margin volcanic arc, and a sequence of basaltic andesite-dacite-rhyolite and other calc-alkaline volcanic rock assemblages with island arc properties developed.

During the Middle-Late Triassic, Lanping-Pu’er Basin began to deposit on the side of Jinsha River-Ailaoshan, receiving the material source supplied by the low-grade metamorphic belt. At the bottom of Yiwanshui Formation in the Upper Triassic, a large amount of gravel from the low-grade metamorphic zone can be found, including basic-ultrabasic rock gravel. Yiwanshui Formation also contained red sand and mudstone gravel deposited in its early stage. As mentioned above, Yangtze Plate subducted westward, and the foreland basins should have developed on the edge of Yangtze Block, but they all developed on the eastern edge of Lanping-Pu’er Block, and the tectonic position of the foreland basin was reversed. In the low-grade metamorphic zone and the whole ophiolite melange zone, the westward obduction nappe collision orogeny can be found anywhere. At this time, Ailaoshan Thrust Nappe Fracture has been formed, and a group of imbricate thrust nappe tectonics developed in the Ailaoshan Group, then the Paleozoic strata in Jinping in the lower plate were significantly reversed westward. In Fenggang, Yuanyang, Ailaoshan Group foliation, pegmatite and arizonite developed along the previous foliation also formed westward inverted folds, and the fragments in mylonite belt also indicated the upward thrust of the upper plate. The foreland folds and Permian heavy folds in Permian–Triassic strata in Changdong, Zhendong Village, Zhenyuan may have been formed in this period. The thrust stacked mountains moved into the foreland basin, which also resulted in the denudation and redeposition of the lower or early sediments of Yiwanshui Formation. However, the Ailaoshan Group of high grade metamorphic zone was not denuded out of the surface at that time, because no gravel from the high grade metamorphic zone was found in Triassic-Jurassic strata.

2.2.5.4.3 Stage of Lithosphere Extension and Basement Stripping

At this stage, the mountain may rise sharply due to the significant orogeny of the overthrust in the previous stage, and the reverse slip or spreading may be formed at the rear edge of the overthrust zone due to the imbalance of gravity, so that the metamorphic basement of Ailaoshan Group and Cangshan Group was gradually exposed by the tectonics, resulting in a series of tectonic deformations found in the metamorphic core complex. In Xinjian-Jiayin in the southern member of Ailaoshan, along the contact interface between Xiaoyangjie Rock Formation and Qingshui River Rock Formation, as well as within Qingshuihe Rock Formation and Along Rock Formation, regional or secondary detachment fault zones occurred, resulting in bedding (foliation) slip, forming pudding/brecciation zones, bedding ductile shear mylonite zones, folds, sheath folds and domino tectonics, all these reflect the spreading and stripping in northeast direction. Its spreading may occur at the same time as that of Cangshan, that is, after Late Triassic and Jurassic. Through extensional tectonic stripping and erosion of overlying rocks, Cangshan Group and Ailaoshan Group are exposed on the surface and subjected to erosion, resulting in gravel in high grade metamorphic zones in subsequent strata. The exposure mechanism of Ailaoshan Group and Cangshan Group is similar to that of the overthrust of Kangma metamorphic core complex and Ximeng metamorphic core complex at the rear edge of Himalayas and Dashan, Gengma.

2.2.5.4.4 Overthrust and Left Strike-Slip Stage

At the end of Late Cretaceous to Paleogene, due to the closure of Yarlung Zangbo River Ocean Basin and the northward compression of Indian Plate, the whole Qinghai-Tibet Plateau and even the Sanjiang Region entered a stage of comprehensive intracontinental convergence, and the northeast horn of Indian Landmass and Yangtze Landmass collided with each other in Sanjiang Region, forming a transverse wasp waist-shaped tectonic knot. As the northeast horn of India Landmass was slightly offset to the north relative to the southwest corner of Yangtze Landmass, both twisted during the collision, which made the overall strike of the tectonic belt in Sanjiang Region reverse in an “S” shape. What’s more, the above-mentioned Xilaping-Diancang Mountain Overthrust of Yangtze Landmass appeared in Sanjiang wasp waist zone. There is a significant left strike-slip in Ailaoshan Zone in the south wing of the arc overthrust zone, and vertical foliation zone, nearly horizontal tensile lineation and inclined vertical folds were developed in the high and low-grade metamorphic zones. However, in Diancang Mountain, which is located at the top of the nappe arc, there is no sign of left strike-slip, but mylonite foliation and westward inverted folds formed in Cangshan marble. The Triassic nappe of Lanping Lead–Zinc Deposit is overlaid on the Paleocene Yunlong Formation and covered by its sediments, while the Cangshan nappe overthrust on the Paleogene and Neogene strata of Lanping-Pu’er Basin, indicating that its overthrust nappe occurred in the Paleogene and later.

The whole-rock K–Ar age of felsic mylonite from the northeast of Jinshan Bealock in the northern member of Ailaoshan is 18.7 ± 1.9 Ma. Schearer et al. measured the U–Pb isotopic age of yttrium ore and monazite with two samples of light-colored granite in Ailaoshan Metamorphic Zone and found that the result was the same (23.0 Ma ± 0.2 Ma). These two ages can represent the upper limit time of the end of left strike-slip ductile shear. This left strike-slip process transformed the early tectonics of Ailaoshan Zone more thoroughly in the northwest member than in the southeast member. Some members in the south of Yuanyang in southeast member still have traces of early thrust and extension, and it is difficult to find early deformation traces in Jinshan Bealock in middle member. The mylonite foliation is steep or inclined to the northeast, and the tensile lineation is nearly horizontal, with a direction angle of 327°, which is consistent with the direction of the regional tectonic line. The interbreeding of striped mylonite and large and small eyeball mylonite reflects the interbreeding of relatively strong and weak strain zones. The porphyroclast pressure shadow shows the left strike-slip characteristics of the northeast plate slipping to the northwest, and then to northwest, according to the fact that the ophiolite melange zone and low-grade metamorphic zone gradually destructed and the high-grade metamorphic zone narrowed, it seems that the closer it is to the nappe front, namely Diancang Mountain, the more obvious overthrust than strike-slip, which was probably a westward oblique thrust.

2.2.5.4.5 Right Strike-Slip Fracture Stage of Red River

The overthrust tectonics made Diancang Mountain and Ailaoshan rise to the present height of more than 3000 m. After Miocene, due to gravity, anti-slip normal fractures were formed along Red River Fracture, which was accompanied by right strike-slip orogeny. The formation of the right strike-slip fracture may be caused by the India Landmass pushing into the Qinghai-Tibet Plateau, resulting in the uplift of the plateau, the eastward movement of the materials in the plateau, forcing the Yangtze Landmass to rotate counterclockwise and sliding eastward along Red River Fracture. In Middle Pliocene sediments developed in Red River Valley, there are a large number of gravels of high-grade metamorphic rock series in Ailaoshan Group, which indicates that it is in the stage of intense rising and denudation, which is similar to the time of rapid cooling of 20–19 Ma, and it also indicates that it is closely related to normal faulting along Red River Fracture.

2.2.6 Temporal and Spatial Structure and Evolution of Lanping Basin

The Lanping Basin is located in the south of Chaya-Jiangcheng Depression Zone in Mesozoic, sandwiched between Jiangda-Weixi Lvchun volcanic arc zone (east) and Yunxian-Jinghong volcanic arc zone (west). The basin has experienced two important evolution stages since its formation in Mesozoic: the back-arc foreland basin stage (Mesozoic) and the strike-slip pull-apart basin stage (Cenozoic).

Before the back-arc foreland basin was formed, the Qamdo-Pu’er Block was Jinsha River-Ailaoshan Ocean in the east and Lancang River Ocean in the west. From Late Carboniferous to Permian, the two ocean crusts began to subduct under the landmass between them, forming volcanic arcs on both sides of the landmass and a common back-arc-basin in the middle. During Triassic collision orogeny, the tectonic reversal occurred, which caused the rocks of Dongda Mountain-Lincang Landmass and Yangtze Landmass on the other side of the above two ocean basins to thrust on Qamdo-Pu’er Block, and started the foreland basin stage.

In Cenozoic, as the India Plate continued to push northward and northeasterly after the Indus River-Yarlung Zangbo River Ocean destructed, and the Yangtze Landmass pushed westward possibly due to the subduction of the Pacific Ocean, the two facies collided and squeezed, and the wasp waist zone contracted. All Mesozoic basins in Sanjiang Region contracted in a beaded shape, and the blocks crawled northward and southward or were squeezed, forming a massive strike-slip, which made the basin enter an evolution stage dominated by the development of strike-slip pull-apart basins.

2.2.6.1 Back-Arc Foreland Basin Stage (Mesozoic)

Lanping Basin lacks the Lower Triassic stratum, the Middle Triassic stratum is mainly developed in the Xiaomojiang Area between Jinggu and Pu’er in the basin except in the volcanic arc zones on both sides, and in Xiapotou Village of Yunxian in Pu’er and Laogongzhai of Batang Village in Zhendong Town, respectively, which is angular unconformity with C₃ and P₂, reflecting that the basement of the basin experienced significant tectonic deformation and denudation after Permian. Xiapotou Formation (T₂x) and Dashuijing Mountain Formation (T₂d) are a sequence of littoral-neritic clastic rocks and carbonate rocks, with composite conglomerate at the bottom. In Xiapotou Cundi conglomerate, the gravel mainly comes from the siliceous rock, sandstone, mudstone and volcanic rock of the underlying Longdong River Formation (C₃l), which is supported by matrix, without mud and paleosoil at the bottom. It is a kind of transgression overlying sediment that has been cleaned by water. The Choushui Formation (T₂c) is composed of mudstone and carbonate rocks, and some carbonate rocks may be large sliding blocks with turbidite limestone and sand-slate on them, reflecting the deepening of the water body in the sedimentary basin, the appearance of a slope zone, and the overlapping to the north and east. The basin uplifted and suffered erosion after this period. During the Late Triassic, a sequence of clastic rocks (feldspar lithic sandstone, siltstone and mudstone, calcareous mudstone) and marl were deposited on the parallel unconformity, with basal conglomerate, fine conglomerate containing volcanic debris and boulder conglomerate at the bottom. In the area of Yiwanshui on the eastern edge of the basin, there was a huge thick molasse deposit at the bottom of Yiwanshui Formation in the Upper Triassic, and fluvial sediment appeared on the composite molasse deposit. On the unconformity surface of the other section, a sequence of thick conglomerate accumulated by fuchsia sand shale gravel can be seen, which indicates that the Mesozoic red beds developed before the deposition of Yiwanshui Formation. This fully shows the significant orogeny of Ailaoshan Zone and the movement of orogenic belt to basin. After that, the basin sediment changed from continental facies to marine facies, and a sequence of neritic clastic rocks and carbonate rocks were deposited. The Upper Triassic stratum is obviously divided into three parts, the lower part is continental-marine coarse clastic-fine clastic rock, the middle part is neritic limestone, the upper part is delta front sand shale and delta wedge sandstone, and finally coal-bearing clastic rocks. The Jurassic stratum is still a sequence of sand shale mixed with limestone in neritic tidal flat environment, occasionally deposited in continental sedimentation, and the sea area in Middle Jurassic once expanded, with transgressive overlapping to the east and west sides; The Cretaceous stratum is composed of a sequence of typical fluvial-lacustrine sand shale and conglomerate, and the basin obviously shrinks from the east and west sides to the center. Besides the east–west change, the sedimentary thickness also changes from the north to the south, reflecting that the sedimentary basin shrinks into a beaded shape. Due to the continuous migration of orogeny on both sides to the center of the basin, strike-slip pull-apart basins were superimposed in the center and edge of the basin in Paleogene and Neogene, and Paleogene and Neogene red clastic rocks, gypsum-salt and coal measures were deposited.

The unconformity between Upper Eocene Baoxiangsi Formation (E₂b) and Lower-Middle Eocene Guolang Formation occurred in the basin. This is the first regional unconformity in the Mesozoic basin since the Late Triassic, which is considered as the beginning of the first act of the Himalayan Orogeny. However, except for the sea area on both sides of the basin expanded in the Middle Jurassic and the sea water invaded the orogenic belts on both sides, the mountain zones on both sides were kept in an orogeny status and moved toward the basin, which mainly occurred in the Late Triassic and Cretaceous. Late Triassic is characterized by the early basin sediments being involved in orogenic belts and then being uplifted, denuded and re-deposited; Cretaceous is mainly characterized by basin contraction. Fluvial sandstone and coarse conglomerate can be found near the orogenic belt (Ailaoshan Zone). Due to denudation caused by mountain uplift and the overthrust toward the basin, no unconformity between Cretaceous internal strata and the underlying strata was found. But the unconformity between different horizons of Paleogene and Neogene strata and Cretaceous and Jurassic strata can be found. Pliocene Sanying Formation (N₂s) and Cretaceous Jingxing Formation (K₁j) (Liantie Township, Eryuan Country), Pliocene (N₂) strata unconformably underlaid on Middle Jurassic Huakaizuo Formation, and the unconformity between Paleocene Yunlong Formation (E₁y) and Cretaceous Hutousi Formation (K₂h) can be even seen in Longtang, Lanping Country. However, Yunlong Formation conformably underlaid on Cretaceous strata, and it is difficult to divide them completely in Yunlong and other basins. This reflects the gradual progradation of orogeny from the margin to the center of the basin, but does not indicate that no Yanshanian orogeny occurred in the basin. Regionally, Yanshanian magmatic activity indicates that Yanshanian orogeny occurred, while the sedimentary records show that there is a gradual progradation process between Yanshanian and Himalayan orogeny.

Generally speaking, the evolution process of the foreland basin includes the early abyssal-bathyal flysch sedimentation stage, the middle marine molasse sedimentation stage and the late continental molasse sedimentation stage.

According to the study on foreland basins in the Sanjiang area of Southwest China, foreland basins can be divided into two types, namely marginal foreland basins and back-arc composite foreland basins. The former can be divided into early marginal foreland basin (marginal marine basin after the ocean basin disappears) and late marginal foreland basin (orogenic foreland basin) depending on its two significant development stages. The early marginal foreland basin/residual marine basin were developed between the passive continental margin of the western part of Yangtze Landmass and Changtai-Xiangcheng Volcanic Arc Zone, that is, the marginal marine basin after Ganzi-Litang Ocean disappeared in the late stage of Late Triassic. The marginal sea changed from the early deep-water flysch to the late shallow-water sedimentation, and was finally silted up and folded for orogeny, so that the post-orogenic Sichuan foreland basin developed on the eastern part of Yangtze Block (Liu et al. 1993). It should be noted that the formation and evolution of Sichuan foreland basin, especially in its early stage, are mainly controlled by the northern and northwestern Qinling-Qilian-Kunlun Orogenic Belts, and may also be influenced and controlled by the passive continental marginal orogenic belt in the western margin of Yangtze Landmass after the closure of Ganzi-Litang Ocean at the end of Late Triassic and in Jurassic. Due to lack of sediments formed in the Jurassic and later ages in the passive continental marginal zone of the western margin of Yangtze Landmass, a large number of continental molasses sediments had developed in Longmen Mountain Foreland Basin since Jurassic. Marginal foreland basins with the same evolution model can also be found in Baoshan Block in the west of Changning-Menglian Junction Zone in Yunnan in the Sanjiang area. After the Changning-Menglian Ocean disappeared in Late Permian, the residual sea was formed on the passive continental margin in the east of Baoshan Block, which was located in the foreland uplift part and was lacking sediments formed in Late Permian. Its sedimentary evolution process generally develops from the early deep-water flysch, carbonate rock and radiolarian siliceous rock (T₁) to shallow-water sediments, and post-orogenic Shuizhai-Muchang Mesozoic foreland basin/depression zone (1999a, b; Liu et al. 1993) were developed in the east of Baoshan Block since Late Triassic.

However, Lanping Basin is a special back-arc composite foreland basin, which does not completely conform to the evolution model of this normal foreland basin. Lanping Basin was developed on the obduction plate with opposite subduction between ocean basins and oceanic crusts on both sides, and its main body is located on the back-arc-basin zone, but it is not completely similar to Dickson back-arc foreland basin (1974). Especially in the early stage of Middle Triassic, the sedimentary basins were mainly in the forearc zones on both sides, which are consistent with the early marginal foreland basins in terms of tectonics, that is, there should be an early abyssal-bathyal flysch sedimentation stage. No typical deep-water flysch sedimentation was found in Shanglan Formation or Manghuai Formation in the west of the basin, or Xiapotou Formation and Dashuijingshan Formation in the central Pu’er, that is, there was no foredeep development, and the carbonate rocks deposited by turbidity current were found only in Choushui Formation in the upper part of Middle Triassic strata in Yunxian, Pu’er-Zhendong, showing the change of water body from shallow to deep, rather than the overall change from deep to shallow (the sedimentary changes from shallow to deep also occur in the area where the depression center moves to the foreland). This may be the reversed orogenic polarity tectonics in the collision orogenic belt due to subduction on both sides, that is, the orogenic action of the passive margin mountain zone of the subduction plates on both sides has no progradation to the foreland in both east and west sides, but has progradation to the obduction plate-Lanping-Pu’er Block located between them in the reversed polarity, forming the orogenic pattern of the opposite overthrust nappe, resulting in Late Paleozoic volcanic arc zone and back-arc-basin zone on both sides in the early stage being developed into Mesozoic foreland basin as the basement for the development of foreland basin. Moreover, the central axis of the basin is characterized by a foreland uplift, with foreland depression and foreland thrust zone on both sides, forming a typical foreland basin tectonic pattern. Due to reversed orogenic polarity, the reverse orogenic overthrust nappe covered the early abyssal-bathyal flysch sediments at the front margin, and caused the sediments to be overlaid on the volcanic arc zone in the opposite direction, resulting in unconformity with the volcanic arc zone. The provenance is mainly from the volcanic arc zone, for example, the gravel of the basal conglomerate of Longdi Village, Eryuan is mainly supplied by the volcanic arc zone. At the same time, the depression center of the basin moves to the middle of the basin over time and allodapic limestone of Choushui Formation is deposited in Yunxian, Pu’er-Zhendong. The appearance of allodapic limestone also indicates that there is a marginal carbonate rock plateau environment on the margin of the foreland zone.

Although the evolution of Lanping Foreland Basin has certain particularity, its evolution history can still be discussed from the perspective of sedimentology.

2.2.6.1.1 Abyssal-Bathyal Flysch Sedimentation Stage

In this stage, when the thrust wedge emerged from the water surface or had not reached its maximum height, it lagged behind sedimentation due to sedimentation; the sedimentation rate of the foreland basin is higher than the accumulation rate of sediments, so the basin is in a starved state, mainly forming a deep-water sedimentation type and also forming a gradually shallowing sedimentary system toward the foreland uplift at the same time. In the early development stage of foreland basin, the thrust wedge was thrust on the margin of Craton, where the crust thickness decreased rapidly and the stiffness decreased significantly. Under the action of gravity and horizontal extrusion, the crust flexed downward greatly. Therefore, the formed foredeep depression is deep and narrow and is often covered by the late overthrust nappe in the evolution process.

The sediments that occurred in this stage shall belong to Shanglan Formation and Manghuai Formation of the Middle Triassic in terms of the spatial development tectonics of the basin and are generally located between the volcanic arc zone and the passive margin zone, namely the tectonic position of the residual sea. However, just as mentioned above, the sedimentary characteristics show that the sedimentary characteristics of this stage can be found only in Shanglan Formation in the northeast of the basin, and the early abyssal flysch sediments in the lower part of the rest of the area may be covered (may be covered by (T₁) due to the reversed orogenic polarity. No sedimentary evolution sequence from deep to shallow is found, while the evolution from shallow to deep appears, which may just reflect the conversion period of reversal in orogenic polarity, so that Lanping Basin was depressed on both sides and had a shared foreland uplift in the middle during the Middle Triassic, bringing the evolution of foreland basin into the second evolution stage with the reversal in orogenic polarity.

2.2.6.1.2 Marine Molasse Sedimentation Stage

In this stage, when the thrust orogenic belt crossed the hinge line of the continental slope to the thick rigid Cratonic crust, the flexural settlement formed by the tectonics gradually decreased compared with the previous stage, so a shallow and wide foredeep depression was formed and the thrust orogenic belt was finally developed to its maximum height and remained in a relatively stable status for a long term. At this time, a retrograde orogeny occurred in the provenance area of the foreland basin, developing from the sediments with high maturity mainly from Craton in the early stage into the sediments with low maturity mainly from the thrust zone in this stage. When the flexural settlement is fixed or gradually reduced, and the basin gradually rises to the sea level and keeps in the stable state for a long term, stable environmental sediments with high maturity will appear. The development period of Lanping-Pu’er Composite Foreland Basin in this stage varies roughly from the sedimentation period of Choushui Formation/Pantiange Formation to the sedimentation period of Huakaizuo Formation. Before the thrust orogenic belt is formed, this stage is mainly continental alluvial sedimentation, such as alluvial fan and fan delta sedimentation of Waigu Village Formation. Littoral-neritic coarse clastic rock sediments, such as littoral-neritic quartz sand or delta sediments of Maichuqing Formation and Huakaizuo Formation; carbonate rock and evaporite sediments in the euxinic environment, such as limestone of Sanhedong Formation.

In the development process of the foreland basin, there may be three ways for the thrust of the fold orogenic belt to Cratonic Block: ① slow climbing; ② fast parallel overthrust; ③ skipping overthrust. The thrust block of Lanping-Pu’er Composite Foreland Basin may belong to skipping overthrust, resulting in several cycles from sea to land: Shanglan Formation (T₂s)—Waigu Village Formation (T₃w), Sanhedong Formation (T₃s)—Yangjiang Formation (J₁y); Huakaizuo Formation (J₂h)—Bazhulu Formation (J₃b), Jingxing Formation (K₁j)—Nanxin Formation (K₁n) and above. Due to the skipping and suddenness of thrust and the lag of sedimentary facies behind settlement, many fast deepening starved sedimentary events have been caused, such as turbidite of Choushui Formation, black shale of Waigu Village Formation, turbidite and carbonaceous shale of Walu Formation, and even the dark gray bed at the top of Huakaizuo Formation. This phenomenon also reflects the result of the continuous skipping migration of the foredeep into Craton, and many unconformities have been formed at the margin of the basin and many “piggyback” basin covers have also been formed due to this thrust form.

2.2.6.1.3 Continental Molasse Stage

With the further migration of the thrust orogenic belt toward Craton, most areas of the basin of Early Cretaceous (Nanxin Formation K₁n) were occupied by an alluvial environment. Rivers from the thrust orogenic belt flowed into the catchment area of the foreland basin, forming a lake environment at the catchment center of the basin and a delta environment at the margin of the basin. It should be noted that the foreland depressions on both sides of the basin are not completely evolved synchronously. However, due to the accuracy of stratigraphic correlation and the incompleteness of stratigraphic preservation, the corresponding relationship between the two sides in terms of evolution shall be further studied.

2.2.6.2 Strike-Slip Pull-Apart Basin Development Stage (Cenozoic)

Since Paleogene, Lanping Basin had entered the strike-slip pull-apart basin development stage. This type of basin is generally an axial basin distributed along a large strike-slip fracture zone, which is mostly rhombic or rectangular, and is a deep basin. Therefore, the sedimentary rock series is extremely thick, and the basin has multiple filled facies and wide varieties, such as clastic rock piles, landslides, alluvial fans, braided rivers, meandering rivers, fan deltas, seashores, shallow lakes and deep lakes, turbidity currents, chemical sediments, and algal limestones. Because of the continuous lateral movement of faults at the margin of the basin, the provenance also changes over time, so the filled facies are complex and diverse. At the same time, synsedimentary deformation and unconformity are also very common. The basin is distributed along the margin of synsedimentary translational motion in a significant asymmetric way, with alluvial fans dominated by small clastic flows, which contain coarse breccia and conglomerate sediments. While large-scale alluvial and fluvial sediments are developed along the inactive or slightly active margins, including fine conglomerate, but few breccia.

Yunlong Formation (E₁y) is the earliest sedimentary stratum in the strike-slip pull-apart basin and forms a sedimentary cycle from coarse to fine with Guolang Formation (E₂g¹⁻²). Due to the reworking caused by the late strike-slip fault, the original appearance of the whole basin cannot be restored, and the alluvial fan facies, fluvial facies, shallow lake facies and semi-deep lake facies can be found in terms of the sedimentary facies. Based on a scale of 1: 200,000 in Lanping, Yongping and Weixi or other regional survey data, the thickness of Yunlong Formation varies greatly, reaching 2025 m in Shijing, Lanping, but decreasing to 266 m in Laomujing. The sedimentary basement is also different. It contacts the underlying Hutousi Formation (K₁h) in a parallel unconformity or unconformity manner in Yunlong, Baofeng and Longtang Village, Lanping. It overlies on Nanxin Formation (K₁n) and Shanglan Formation (T₂s) respectively in Lanping and Qiaoshi, and even overlies on Cangshan Group in the east. At that time, the basin was actually composed of a sequence of deep aulacogens from north to south. For example, in Shunchuanjing-Yanqu Village-Mishajing-Qiaohoujing, Jianchuan County, a large number of broken rocks (mainly limestone) on both sides of the fault fell down, forming angular gravels of different sizes. The basement of this formation is mainly composed of conglomerate, of which the composition is mostly mudstone, siltstone and shale, and few marlstone. Conglomerate is round or sub-angular, and its particle size changes greatly. For example, the particle diameter on Laomujing section in Lanping County is 0.5–1 cm, is 3–5 cm in Lajing, Lanping County, and the largest one can reach tens of centimeters, all of which are supported by matrix. It is worth noting that even mudstone and siltstone with poor abrasion resistance and conglomerate are angular, indicating that conglomerate is not formed through long distance motion but may be deposited by collapse or alluvial fan. The basal conglomerate of Yunlong Formation in Longtang Village, Lanping County is quite special, and the conglomerate is well ground and oval in shape, with different sizes (varying from several centimeters to tens of centimeters in diameter) and it is mainly supported by particles. The interstitial materials are coarse sand with thin mudstone sandwiched in the middle, and the conglomerate is composed of sandstone, limestone, flint, etc., and also contains glutenite, sandstone and mudstone (extremely thin) in the upward beds, forming a sequence of upward-thinning cycles, namely, braided fluvial sediments. There is lenticular feldspathic quartz sandstone on it, in which the cross bedding can be found and the sedimentary characteristics of braided river can also be found.

The middle part of Yunlong Formation is mainly composed of purplish-red fine sandstone and siltstone mixed with mudstone. In the purplish-red sandstone near Guquan Bridge in Lajing, Lanping County, abraded wave ripple and fluvial unidirectional current ripples and launder can be found, and even planar cross bedding composed of fine conglomerate and coarse sandstone can be found, which should be fluvial-deltaic or littoral-neritic sediments. The upper bed of Yunlong Formation is mainly composed of purplish-red siltstone, mudstone and marlstone, mixed with a small amount of medium and thin layers of fine sandstone. Yangcen in Jianchuan County is more representative. The observation reveals that this sequence of stratum is the product vertically aggraded in the center of the basin. Some intervals are composed of purplish-red silty mudstone mixed with yellowish-green shale, occasionally mixed with 1–2 cm thick fine sandstone. Some horizons contain many layers of 10–20 cm thick graded bedding varying from argillaceous fine sandstone and argillaceous siltstone to mudstone, belonging to low density turbidite sediments in the lake basin. Some yellowish-green shales are sandwiched with 1–2 cm thick flat marlstone lens or thin marlstone. A 0.2 mm thick horizontally laminated bed can be found on the limestone bed section, and the horizontally laminated bed can also be found in shale. The lower part of the laminated bed is rich in debris (silt-sized) and the upper part is rich in mud (clay or marl), which is varves sediments. As the yellowish-green shale was formed in the period with low debris injection, which is conducive to calcium precipitation, it coexists with marlstone; however, purplish-red siltstone mixed with fine sandstone was formed in the period with high debris injection, which is not conducive to calcium precipitation, so it does not coexist with marlstone. It can be seen that the yellowish-green shale and purplish-red siltstone appears alternately on the section. Each cycle appears as purplish-red siltstone and ends with yellowish-green shale and off-white marlstone. To sum up, it should be sediments of deep lake-semi-deep lake facies.

The overlying Guolang Formation is in conformable contact with Yunlong Formation, the lower bed is interbedded with sand and mud, and the upper bed is mainly composed of sandstone, with cross bedding and few gravels, belonging to the product generated due to lake water gradual shallowing. The top stratum of this formation is incomplete and covered by the overlying Baoxiangsi Formation in an unconformity manner.

New definition of Baoxiangsi Formation: ① similar to Lijiang Formation; ② the upper bed is the original Baoxiangsi Formation and the lower bed is the original Meile Formation (Yunnan Bureau of Geology and Mineral Resources, 1996). Therefore, Baoxiangsi Formation actually includes two cycles from coarse to fine. Most of the lower strata of Baoxiangsi Formation are in unconformable contact with Shigu Group. The bottom bed is a thick layer of grayish-purple and purplish-red massive gravel bed. The gravel is mainly composed of limestone and quartz schist, followed by vein quartz, purplish-red siltstone and basic rock, with extremely poor separation and moderate rounding. The lower bed is breccia, which gets smaller from bottom to top, the middle-upper bed is brick-red and off-white sandstone containing feldspar, with well-developed large plate-like cross bedding, and a thickness of strata series of 1–5 m and a cross bedding dip angle of up to 30°, and no fossils were found (Regional Geological Survey Report on a scale of 1: 200,000 in Weixi). In Liming Township, Lijiang County, the bottom bed is composed of pebbly conglomerate, containing a lot of quartzite gravel, with well rounding but poor separation. In Labazhi Copper Ore Deposit, the basal conglomerate is composed of a thick layer of massive conglomerate bed, varying from 1–2 cm to 10–20 cm in diameter. With complex composition, it is mainly composed of limestone and few quartzite and schist, etc., mixed with coarse sandstone lenses or wedges. The former is a fluvial sediment on the inactive or relatively inactive margin in the strike-slip pull-apart basin, while the latter is an alluvial fan sediment on the active margin in the strike-slip pull-apart basin. The middle and upper strata are generally considered as large fluvial cross bedding. It is found through observation of the section of Liming Township that the sandstone is characterized by good degree of separation and extremely high roundness and is free of mud and mica sheets. In addition, the thickness of cross bedding is large, and the dip angle of foreset bed is steep (30°) and the direction of dip is stable, all of which are the typical characteristic of eolian dune. Therefore, it may be the desert sediments. It may be aeolian dunes on the marginal banks of rivers and lakes, as seen on both sides of Yarlung Zangbo River Valley in Tibet at present.

In whole Neogene, the fault activity had been greatly reduced, the scale and number of strike-slip pull-apart basins had also been greatly reduced, and the sedimentation range had been reduced, while small basins dominated by rivers and lakes had appeared and swamps had developed in large numbers, forming important coal-bearing strata. The Miocene stratum is the Shuanghe Formation, Pliocene stratum is Jianchuan Formation and Sanying Formation, all of which are fluvial and lacustrine sediments. For example, Jinding Township, Lanping is a small north–south nearly rhombic basin, in which Sanyingxian fluviatile-lacustrine sediments were deposited and it unconformably overlaid on the underlying Paleogene Yunlong Formation and its previous strata, with the top being outcropped or covered by Quaternary strata.

2.2.7 Temporal and Spatial Structure and Its Evolution of Tenasserim Arc-Basin System

Bangong Lake-Shuanghu-Nujiang River-Changning-Menglian Mage-suture Zone is a relic of the destruction of Tethys Ocean. The eastern part of this zone roughly includes Jitang Residual Arc, Northern Lancang River Junction Zone, Riwoqê-Dongda Mountain Magmatic Arc, Lincang Magmatic Arc, Southern Lancang River Junction Zone, Yunxian County-Jinghong Volcanic Arc, etc. all of which constitute Tenasserim MABT, and Qamdo-Lanping-Pu’er Block is located in its east side.

2.2.7.1 Temporal and Spatial Structure of Tenasserim Arc-Basin System

The understanding of the temporal and spatial structure and formation process of Bangong Lake-Shuanghu-Nujiang River-Changning-Menglian mage-suture zone plays a very important role in the study of the formation and evolution of geological structures in Eastern Tethys. Due to differences in perspectives, collected data and understanding, the nature and tectonic attributes of ocean basin restored in this zone have long been a subject of debate in the study on Sanjiang Orogenic Belt.

2.2.7.1.1 Ocean Basin Formation Age and Tenasserim Islands Development Age

Tectonic stratigraphic units of Devonian to Middle Triassic radiolarian siliceous rocks have been found in Menglian, Changning, including radiolarian rock represented by the earliest Monograptus uniformis zone of Devonian, Archocyrtium menglianesis Wu. Ar. delicatum et al. (C₁). There is a sequence of siliceous rocks deposited by deep-water flysch on the western slope of Meri Snow Mountain, such as Early Carboniferous Palaeory phosty.lus uar spina. Late Carboniferous-Permian Albaillea SP., Pseudea I bailla sp. were found in the siliceous rocks in Zhayu-Bitu. Interdisciplinary comprehensive research on sedimentary geochemistry of siliceous rocks, REE, stable isotopes and radiolarian paleoecology (Liu et al. 1993) showed the sedimentary environment of the abyssal ocean basin. Radiolarian siliceous rocks of Devonian, Carboniferous and Early Permian in Manxin, Menglian, Gengmanongba and other places have three characteristics of associated rock assemblage, namely flysch sandwiched with siliceous rock, limestone and mudstone, sandwich bed in ocean floor basalt or exposed caprock above pillow basalt, and sandwich bed in basalt above oceanic island volcanic rocks, which form siliceous rock-basalt-limestone assemblage with limestone without terrigenous clast on oceanic island. The amphibole K–Ar isochron age of gabbro in the Tongchangjie ophiolite is 385 Ma (Cong et al. 1993), which represents the geology of Middle Devonian. Therefore, most scholars believe that the Paleo-Tethyan Ocean in Changning-Menglian was formed since the spreading in Devonian stratum. However, the following important geological records allow us to reconsider the age of Tethys ancient ocean basin represented by Dingqing-Bitu-Changning-Menglian zone (Pan et al. 1997, 2004).

• (1) The landmasses on both sides of Changning-Menglian have different basements. The lowest outcropped stratum in Baoshan Block is the Cambrian Gongyanghe Group, with fluvial submarine fan-slope environment in its lower bed and basinal siltstone, shale sandwiched with siliceous rock in its upper bed. Significant tectonic activities at the end of Cambrian and significant intermediate-acid magma intrusion occurred in this area, which represented by Pinghe granite body, with isotope age between 495 and 525 Ma and uneven metamorphism and deformation. Baoshan Block is actually the northern extension member of the Shan Landmass. Mogok gneiss, Shan Landmass and Tengchong Landmass of Gaoligong Mountain Group have similar basement to India Landmass, which can be regarded as the accretion part of the northern margin of Gondwana affected by Pan-African Events. Lincang Landmass, Jitang Block and Qamdo-Lanping Block in the east of Changlian-Menglian Zone all have the same basement characteristics with that of island arc accretion in Neoproterozoic Rodinia supercontinent convergence stage, and have the characteristics of volcanic magma accretion in Early Paleozoic as they are all split fragments in the western margin of Yangtze Continent. However, granite intrusion of 750–900 Ma, metamorphic event of 800–900 Ma, sedimentation of Yangtze plateau in New Nanhua-Sinian, glacial sedimentation, etc. on Yangtze Continent unconformably contact with the Mesoproterozoic underlying tectonic strata, as well as the phosphorus-bearing sediments of Early Cambrian are obviously different from Shan Landmass which is close to Gondwana. It shows that Shan (including Baoshan) Landmass has been separated from Yangtze Continent by ocean at least in Neoproterozoic and later. The analysis of paleomagnetic data by Zhuang reveals that the latitude difference between Baoshan Block and Yangtze Landmass of Cambrian-Ordovician was great in Early Paleozoic. The former was around 18° south latitude and the latter was near the equator, which was consistent with the environment reflected by geological records. Shan-Baoshan Block is not the spreading block of Changning-Menglian Ocean Basin due to splitting from Lincang or Qamdo-Pu’er Block on the west side of Yangtze Landmass in Devonian, but the tectonic relationship formed by shrinkage, final subduction and destruction of Tethys due to northward drift of Shan-Baoshan Block.

• (2) Subduction event of Tenasserim Frontal Arc in Early Paleozoic. Yong research on Tenasserim Mountain on the west side of the North Lancang River showed that a sequence of metamorphic rock with greenschist facies in Youxi Group of Lower Paleozoic stratum was decomposed from Jitang Group of Precambrian, with the source rock of basalt and dacite mixed with arenopelitic debris, among which dacite Rb–Sr age was (371 ± 50) Ma, being the evidence of metamorphic age and active continental margin island arc environment obtained after geochemistry research on rocks. ⁴⁰Ar/³⁹Ar plateau age of granodiorite mineral of Biluo Snow Mountain is between 418 and 424 Ma, and ⁴⁰Ar/³⁹Ar plateau age of the glaucophane of the metamorphic basic volcanic rocks in Lancang Glaucophane High-pressure Metamorphic Zone is 410 Ma (Zhang et al. 1990). 433 Ma coarse-grained biotite granite and 422 Ma fine-grained biotite granite are found in Lincang Granite Basement. Most of the geological records of these tectonic thermal events in Jitang Group Complex, Chongshan Group and Lancang Group in Precambrian show the remnants of accretionary wedge under the island arc, such as volcanic-magmatic arc formed by the subduction of Tethys Ocean in Early Paleozoic toward the northeast (present position). This continental crust zone with soft basement may have been originally a series of mountains along the southwest margin of Yangtze Landmass of Pan-Huaxia Continental Group. At the beginning of Devonian, this continental margin arc chain entered the formation and evolution stage of the MABT of Qiangtang-Sanjiang in the form of Japan-Ryukyu Islands splitting, and Jitang Residual Arc-Chongshan Residual Arc-Lancang Residual Arc are referred to as Tenasserim Frontal Arc, of which the southwest side is the residual Proto-Tethys.

• (3) Sedimentary records of Baoshan Block. Cambrian to Permian strata in Baoshan Block had kept the passive margin strata of continental shelf-plateau, and the strata formed in each era are mostly in conformable contact with each other, except for any hiatus between Lower Carboniferous stratum and Devonian stratum. It lacks the Middle Carboniferous stratum. The upper Carboniferous stratum is composed of glacial debris flow at the continental margin, siltstone and shale in the lower bed, and intraplate rift basalt mixed with limestone lens in the upper bed (see the section on Structure and Evolution of Baoshan Block for details). These sedimentary records also reflect the continuity of Paleo-Tethyan Ocean from one aspect, which restricts the overall continuity of Paleozoic passive margin strata in the Shan-Baoshan Block in the southwest of the ocean.

Therefore, Shan-Baoshan Block may have become an independent block in Paleozoic (when Rodinia supercontinent decomposed) and was separated from Yangtze Landmass by the Tethyan Ocean. The Paleo-Tethyan Ocean in Changning-Menglian was not reopened after the Proto-Tethyan Ocean was destroyed, but developed and evolved based on the Proto-Tethyan Ocean.

2.2.7.1.2 Spatial Distribution of Tenasserim Arc-Basin System

According to the present tectonic pattern of Hengduan Mountain, it is divided into two members (namely, southern member and northern member), which are briefly described from west to east in turn.

2.2.7.1.2.1 Dingqing-Zhayu-Bitu Junction Zone

This zone is located in the curved turning part (extending from NWW-SEE to NS) of the eastern part of Bangong Lake-Nujiang River Junction Zone. According to the available data, Carboniferous-Permian ophiolite, Triassic ophiolite, Early Jurassic ophiolite and radiolarian siliceous rock have been found in Suruka, Dingqing Country, Nuxilaka, Luolong Country and Zhayu, Zuogong Country, which are mixed with various greenschists or flysch sand-slate of Carboniferous-Permian and Late Triassic-Early Jurassic, respectively. The main remnants of Carboniferous and Permian ophiolite are distributed in Nuxilaka on the east side of Jiayu Bridge Residual Arc and Suruka-Tongka in the middle zone. At present, only metamorphic peridotite (serpentinite), gabbro-cumulate, metamorphic lava (with pillow rock locally) and overlying abyssal turbidite mixed with recrystallized metamorphic siliceous slate are found. Ophiolite has been significantly decomposed, with lenses of different sizes and in contact with the matrix in form of faults. The ophiolitic melange zone in Suruka is of low greenschist facies, with metamorphism; jadeite + quartz mineral assemblage are found in quartzite, and Permian spore fossil are found in fine-grained limestone in the tectonic stratum. Late Triassic-Early Jurassic ophiolite in Dingqing area has been widely studied. Although the trinity ophiolite, such as metamorphic peridotite, cumulate, mafic complex, dike swarm and pillow lava, has been descomposed, its remnant is complete. In addition, many radiolarian siliceous rocks in Late Triassic-Early Jurassic are mixed in the sedimentary deformation bed of Quehala Turbidite Fan representing the outer arc ridge slope sediment. A considerable part of Quehala Assemblage should be classified as the fore-arc accretionary wedge in Jiayu Bridge Residual Arc. The so-called Meng’axiong Carbonate Rock Plateau is only a limited fore-arc plateau between the inner side of accretionary wedge and Jiayu Bridge Island Arc. Restoration of Tethyan Ocean Basin by siliceous rocks, Carboniferous and Permian ophiolite and Late Triassic-Early Jurassic ophiolite in accretionary wedge is a continuous evolution process. Dingqing Ophiolite Melange Zone bends toward the southeast arc and then overlaps with Suruka-Nuxilaka-Tongka Ophiolite Melange Zone in Xialinka through Jizhong and Bangdaxi, and finally connects with Zhayu-Bitu Ophiolite Melange Zone. The cumulate gabbro and diabase in Late Triassic-Early Jurassic ophiolite belong to glassy andesite, and were formed in the intra-oceanic arc environment and oceanic intraplate environment of the ocean ridge. The existence of the intra-oceanic arc also indicates that the ocean represented by Dingqing ophiolite had existed much earlier than the Triassic. Middle Jurassic molasse unconformably overlies on the whole ophiolitic melange unit, which indicates that Dingqing-Nujiang River Zone started subduction and destruction of oceanic crust in Late Triassic and then collided and closed at the end of Early Jurassic.

Dingqing ophiolite melange zone bends toward the southeast arc and then overlaps with Suruka-Nuxilaka-Tongka ophiolite melange zone in Xialinka through Jizhong and Bangdaxi, and finally connects with Zhayu-Bitu Ophiolite Melange Zone.

Located between Chayu Landmass and Qamdo Landmass, Bitu Junction Zone is a tectonic melange zone composed of some tectonic blocks, with a width of about tens of kilometers. According to its tectonic characteristics, it can be roughly divided into three parts. Roughly distributed along both banks of Nujiang River, the western part is dominated by a thrust zone composed of Late Devonian-Early Carboniferous sand-slate, marble and siliceous rocks, in which the rock metamorphism generally reach low greenschist facies, and flysch rhythms, flute cast and abyssal cellular arenicolite are developed, showing the characteristics of turbidite sedimentary fans on the continental slope and continental basement, which may be the accretionary wedge of flysch on the eastern margin of Chayu Landmass. The middle part, such as Lawu Village, Zhayu Town, is dominated by a large rock mass composed of neritic continental shelf carbonate rocks of Middle and Upper Devonian and Carboniferous, on which Carboniferous limestone is mixed with a small amount of basalt and limestone is rich in corals, brachiopods and fusulinid. The east and south parts are dominated by a sequence of melange composed of Carboniferous and Permian flysch sand-slate, phyllite, siliceous rock, tholeiite and ultramafic rock. Early Carboniferous Palaeoryphosty., Lus uar spina are found in a sequence of siliceous rocks deposited by abyssal flysch on the west slope of Meri Snow Mountain; Late Carboniferous-Permian Albaillella sp, Psiudeal bailla sp. and Carboniferous conodonts are found in the siliceous rocks in Zhayu-Bitu; basalt is a sequence of sub-alkaline tholeiite, with w (SiO₂) between 47.57 and 50.73%, w (TiO₂) between 1.18 and 2.96%, w (K₂O) between 0.36 and 0.98%, w(P₂O₅) between 0.14 and 0.36%, w (Rb) between 9 and 27 × 10⁻⁶, w (Sr) between 150 and 226 × 10⁻⁶, w(Ba) between 75 and 229 × 10⁻⁶, w (Th) between 1.85 and 9.15 × 10⁻⁶; REE is distributed flatly with weak enrichment in ΣLREE, with ΣREE of (55.32–79.85) × 10⁻⁶, w (Sm)/w (Nd) between 0.23 and 0.36, w (Ce)/w(Yb) between N1.01 and 2.05, Eu between 1.04 and 1.35; it is included in ocean ridge and ocean island basalts on the diagram of TiO₂–P₂O₅ and Zr-TiO₂.

The above basalts is generally of low-grade metamorphism and the metamorphism in the south of Bitu reaches the grade of greenschist facies. Pumpellyite is found in altered basalts and metamorphic basalts in Zhayu and Bitu, accompanied by ultramafic rocks formed by tectonic intrusion, so it may be a transitional type of medium–high pressure facies series. The research made by Zhang on the phengite in Jiayu Bridge Metamorphic Zone shows that bo = 0.9032 nm (based on 20 chlorite samples) and RM = 0.09, both of which are all projected in the high-pressure zone II in MgO-RM diagram. In addition, pumpellyite-actinolite assemblage is also found in Rutog-Gêrzê Metamorphic Zone in the west of Jiayu Bridge, forming the medium–high pressure tectonic metamorphic zone in Bangong Lake-Dingqing-Bitu accompanied by ophiolite melange zone.

The eastern part of Bitu Junction Zone is dominated by a large-scale ductile shear zone and a large nappe thrust eastward. Among them, Bitu and Wapu-Chawalong Melange Nappe is the largest in scale.

2.2.7.1.2.2 Jitang Residual Arc and Caprock

As the important part of Tenasserim Frontal arc, Jitang Residual Arc is composed of Jitang Group, Chongshan Group and Early Paleozoic Youxi Group with high greenschist facies-amphibolite facies. The arc is bounded by Northern Lancang River Fault in the east and Bangong Lake-Nujiang River Junction Zone in the west (east member), respectively, and is adjacent to Southern Qiangtang Basin in the north being divided by Sanduo-Duocai NNW Fault, in which there are few stratigraphic units and metamorphic rocks of Tenasserim Mountain and northern Lancang River are outcropped in its east, while a large area of Upper Triassic stratum and few Jurassic stratum are outcropped in its west.

Metamorphic rocks in North Lancang River Zone can be divided into Precambrian Jitang Group Complex and Lower Paleozoic Youxi Group. The Jitang Group Complex is composed of biotite plagioclase gneiss of amphibolite facies, biotite granulite, amphibolite plagioclase gneiss, banded (striated) migmatite and local mixed tonalite mixed with quartz schist, sillimanite garnet plagioclase schist, biotite feldspar schist, plagioclase amphibolite and marble, with an apparent thickness of more than 4000 m. Being reworked by multiple metamorphism and deformation, some sections are characterized by significant plastic flow deformation. The age of gneissic granitic intrusive rocks is 338 Ma, and the latest migmatization age is 212.5 Ma (K–Ar method). The source rock is composed of a sequence of arenopelitic rocks mixed with island-type intermediate-acid and basic volcanic rocks. Youxi Group is composed of various schists and multi-layer composite conglomerates. The source rock of the schist is arenopelitic rock and intermediate-acid arc volcanic rock mixed with basic volcanic rock formed by dynamothermal metamorphism of greenschist facies at the active continental margin. However, it is of low-grade metamorphism in the west of Dongda Mountain and mainly composed of thin interlayer of quartz sandstone, slate, schist and marble, with an apparent thickness of more than 3000 m, a whole rock Rb–Sr metamorphic age of 371.1 Ma and angular unconformity with Jitang Group, so it may be an accretionary wedge formed by Paleo-Tethyan subduction and destruction.

A number of Permian granitoids have been found in places such as Lajiang, Riwoqê County, which are composed of “tonalite, granodiorite-monzogranite”, and are intruded into Jitang Group. The U–Pb age of monzogranite is (269 ± 18) Ma, which may be related to the early island arc magma series restricted by the northeastward subduction of the Tethyan Oceanic Crust of North Lancang River and the intrusion of alkali granite in the later stage (113–90 Ma).

A sequence of volcanic-sedimentary metamorphic complex is outcropped in strips along Biluo Snow Mountain-Chongshan, and is located between Lancang River Fault and Biluo Snow Mountain Fault. Among them, the metamorphic rocks are composed of schist, granulite, gneiss mixed with marble and plagioclase amphibolite (schist), which is called Chongshan Group Complex (Pt2). Its age-dating by Sm–Nd and K–Ar varies from 1100 to 1000 Ma, 956 Ma and 738–72 Ma, and multiple sets of data. Tectonic strata show products of Rudinia supercontinent convergence stage and decomposition stage, and also show collisional monzogranite intrusion in Late Triassic.

Only the upper stratum is found in the Triassic stratum of Zuogong Basin in the western part of the residual arc, and it can be divided into Dongdacun Formation, Jiapila Formation, Bolila Formation and Bagong Formation from bottom to top. Dongcun Formation is distributed eastward along Zuogong County and ended by Wuqida in the north, with the middle and lower beds of 120–150 m thick composing of sandshale and limestone in three rhythms; that is, the upper bed is composed of sandshale mixed with a thin layer of limestone, and the bottom bed is composed of 0.6–3 m thick composite conglomerate and is rich in Carnian corals and bivalve fossils, with a thickness of 750–1000 m and overlaying on the metamorphic rocks of Youxi Group in a way of angular unconformity. The Jiapila Formation is composed of a littoral-neritic purplish-red sandshale mixed with few gray-green rocks, with two sequences of thick limestone in the middle bed and multi-layer 1000–1400 m thick conglomerate assemblages in the middle and lower beds, which is in a conformable and transitional relationship with the underlying Dongdacun Formation. Bolila Formation on it is a thick layer of limestone mixed with dolomitic limestone and silty mudstone, partly mixed with andesite and rich in Carnian-Norian fossils, with a thickness of 200–500 m. The top bed (e.g., Bagong Formation) is dominated by rhythm interbeddings with greyish-black and black sandshale and grey and green fine sandstone and siltstone; the semideep sedimentary tectonics can be found in the lower bed; sandstone and lithic sandstone are relatively common and phytoclasts are found in the upper bed, with a thickness of more than 3000 m. It is considered as the foreland depression product formed by the westward oblique subduction of destructed residual arc landmass of Dingqing-Bitu Ocean Basin.

The Jurassic stratum is only distributed on a small scale in the south and north of Xiaya and the east of Tiantuo in Zuogong County. Jurassic strata in Xiaya-Jingtang-Beishan belong to the middle strata, which is composed of littoral purplish-red sandstone, mudstone mixed with variegated silty mudstone and unstable limestone and unconformably overlays on Triassic strata, and the thickness of about 1000 m. The Jurassic strata in southeast Xiaya are all purplish-red sandy mudstone sandwiched in the fracture zone, which should be the south extension member of the Middle Jurassic stratum in Beishan, Guojing. Preserved on the fault block between faults, Jurassic strata in the northeast of Tiantuo is also purplish-red clastic rock without top and bottom beds.

Late Triassic-Middle Jurassic sedimentary strata in Zuogong may be the foreland basin product formed by destruction of Dingqing-Bitu Ocean Basin and the southwestward oblique subduction of destructed residual arc landmass, and the fold deformation is characterized by the tectonic pattern of foreland fold-thrust zone.

2.2.7.1.2.3 Northern Lancang River Junction Zone

This zone runs from the northwest of Ulaan-Uul Lake in the northwest end to Zhayu-Bitu Junction Zone in Zhayu, Zuogong County in the south through the east side of Jitang Residual Arc in the southeast, and the Southern Lancang River Zone runs along the east side of Meri Snow Mountain-Biluo Snow Mountain Residual Arc in Deqin County, Baijixun, Weixi County and Yingpan, Lanping County, and runs along Lancang River Fault in its south and then extends out of the frontier southward through Banpo, Jinggu Country, Yakou, Lancang County and Jinghong Country, with the whole length of about 1400 km. Lancang River Zone bends westward in the north member and roughly extends to the southwest of Rola Kangri, which may be connected to Jinsha River Junction Zone. According to the outcropping and spatial distribution characteristics of basic and ultrabasic rocks and tectonic strata as well as the assemblage sequence of geological bodies, this zone can be divided into two members, namely Northern Lancang River Zone and Southern Lancang River Zone.

The Northern Lancang River Zone is distributed from NNW to NW. Jadeite and glaucophane are found in Shitou Mountain and Heixiong Mountain, etc. in the northwest of Ulaan-Uul Lake in the northern part of this zone, and ultramafic rocks (on a scale of 1: 200,000 in Riwoqê 1992) and mid-ocean ridge basalt invaded in Carboniferous stratum are found in Riwoqê. Most members are covered by Permian-Middle Triassic volcanic arc in Zadoi-Riwoqê-Dongda Mountain on the east side due to their westward thrust, and no ophiolitic melange/melange has been outcropped yet.

In the middle-south member, such as Riwoqê and Jitang, turbidite of abyssal sedimentary basins is found in the upper bed of Riazenong Formation and Majunong Formation in Carboniferous Kagong Group defined by predecessors, which is a sequence of siliceous-calcareous-argillaceous sediments and is coexisted with tholeiite-rhyolite assemblage, with different tectonics from stable coal-bearing formations that have no volcanic rocks of original Kagong Group in Qamdo Stratigraphic Zone on the northeastern side or east side. Riazenong Formation is composed of a “bimodal” volcanic rock series, which is the product of the rift valley (back-arc) where the crust is stretched and thinned. The incompatible elements of basalt conform to the distribution pattern curve of single uplift, and the rare earth elements are of nearly flat distribution pattern and slightly enriched in light rare earths. Vi/V value (between 20 and 40) and the results of multiple element discrimination show that it has the characteristics of E-MORB. Therefore, it should be the product of oceanic crust formed by back-arc spreading. The lithochemistry data of basalt in Junmanong Formation indicate that it belongs to an oceanic island with an intra-oceanic hot spot environment.

The so-called Kagong Group in Northern Lancang River Junction Zone is significantly deformed and is a stratified disordered group complex, in which S0 has been replaced by tectonic foliation and axial flow cleavage has developed. Therefore, it is the product of subduction, collision and extrusion mechanism. This deformation is closely related to the arc-arc collision between Jitang Residual Arc and Dongda Mountain-Zhuka Continental Margin Arc in the western margin of Qamdo Landmass. Late Triassic arc volcanic rocks unconformably cover the stratum in Northern Lancang River Junction Zone (such as Junda-Jueyong). The east side of the junction zone is composed of clastic rock-dacite-rhyolite arc volcanic rock assemblage from Jiaoba Mountain to Zhukabing Station and Late Triassic Type-S granite, and similar arc volcanic rock assemblage is still found in Riwoqê and Jitang on the west side of the junction zone, indicating that subduction and destruction of the oceanic crust of Northern Lancang River is characterized by two-way subduction. Similar to the two-way subduction of oceanic crust that occurred in Maluku Sea in Southeast Asia today, this two-way subduction is the only example of arc-arc collision occurring in the global tectonics.

A significant superposition of left-lateral ductile shear occurred in the Northern Lancang River Zone in Early Cenozoic.

2.2.7.1.2.4 Volcanic-Magmatic Arc in Riwoqê-Dongda Mountain

Controlled by two-way subduction of Northern Lancang River Ocean Basin in the east and west directions, volcanic-magmatic arc in Riwoqê-Dongda Mountain was developed on the western margin of Qamdo Landmass, and the molasses in the lower bed of Middle Triassic stratum unconformably overlaid on the island arc volcanic rocks dominated by Upper Permian basalt and andesite. And then, a sequence of Middle Triassic rhyolite, dacite and few pyroclastic rocks was developed. With w (SiO₂) of 67.26–76.91%, w (TiO₂) of 0.13–0.6%, w (Al₂O₃) of 11.96–14.52%, w (CaO) of 0.61–1.83%, w (K₂O) of 3.19–6.17%, w(K₂O) > w (Na₂O), it is highly rich in potassium, and rocks is of aluminum supersaturated type and calc alkali series. With W (Rb) of (143–190) × 10⁻⁶, w(Sr) of (35–295) × 10⁻⁶, w(Ba) of (227–1039) × 10⁻⁶, w(Th) of (15–37) × 10⁻⁶, ∑REE of (154.81–680.46) × 10⁻⁶, it is rich in LREE; with w(Sm)/w(Nd) of 0.15–0.22, it is similar to that of Type-S granites. Watering Formation in Upper Triassic stratum is composed of a sequence of andesite, alkaline basalt, potassium basalt and few trachyte and single-spotted chalcocite. With w (SiO₂) of 48.36–72.07%, w (K₂O) + w (Na₂O) of 6.81–12.1%, w (K₂O) > w (Na₂O), w(Al₂O₃) of 11.00–16.10%, w(TiO₂) 0.55–0.90%, it is enriched in Rb, Sr, Ba, with ∑ REE of 439.7 × 10^–6, w (La)/w (Yb) of 21.33, w (Sm)/w (Nd) of 0.16, belonging to LREE-significantly enriched type and the product of the late continental margin collision orogeny.

Dongda Mountain granite batholith is characterized by type-I subducted granite, and quartz monzogranite, quartz diorite and tonalite and other small rock masses in Late Permian to Early-Middle Triassic stratum are distributed in the main granite batholith of Late Triassic and its margins in scattered shape. Late Triassic granite batholith is composed of 3 units, namely plagioclase granite, granodiorite and monzogranite. The Rb–Sr age of granodiorite is between 215.5 and 219.6 Ma and the K–Ar age of monzogranite is 194 Ma. With w(K₂O)/w(Na₂O) > 1.5, A/CNK of 1.1–1.2 and standard mineral C of 0.7–4.97%, the rock is of the aluminum supersaturated series. They all are included in the Type-S granite zone in the A-C-F diagram. w (Sm)/w (Nd) is between 0.14 and 0.17 and the initial value of ⁸⁷Sr/⁸⁶Sr is between 0.7125 and 0.7233. All characteristics above indicate that the main granite of Dongda Mountain is Type-S granite after syn-collision. A series of tectonic patterns inclined westward and thrust eastward, such as tectonic foliation, pinacoidal surface of overturned fold with same inclination, and ductile shear zone, can be found in Dongda Mountain-Zhuka.

2.2.7.1.2.5 Changning-Menglian Junction Zone

It can be divided into three different tectonic-lithofacies zones, different melange degrees and different deformed and metamorphic zones from west to east (Fig. 2.23).

Fig. 2.23

Geological sketch of changning-Menglian-Southern Lancang River zone. 1—Ocean ridge and ocean island basalt; 2—Ultramafic rock; 3—Potassium/sodium arc volcanic rocks; 4—Magmatic arc granite; 5—Bathyal-abyssal facies of landmass and passive margin; 6—Back-arc-basin facies (turbidite); 7—Deep-water facies of ocean basin; 8—Bathyal-abyssal facies; 9—Ocean Island and carbonate rock plateau; 10—Accretionary wedge; 11—Significant metamorphism and deformation zone; 12—T₃-Q stratum

• (1) Gengma-Cangyuan Tectonic Deformation Zone.

This zone is mainly composed of Devonian-Triassic strata. The Devonian Wenquan Rock Formation is mainly composed of a sequence of turbidite sediments of continental uplift slope submarine fan, such as graptolite shale and arenopelitic-siliceous rocks. The lower bed of Early Carboniferous Pingzhang Formation is dominated by andesite basalt, and the upper bed is composed of three sedimentary environments: turbidite channel sediments, carbonate rock plateau sediments without terrigenous clast on ocean islands, and carbonate rock platform of landward seamount. The Late Carboniferous-Early Permian Yutangzhai Formation is dominated by oceanic island carbonate rocks regionally, but locally overlaps on Pingzhang Formation or Early Devonian Wenquan Formation. The Upper Permian Nanpihe Formation is composed of a sequence of arenopelitic clastic rocks, siliceous rocks, argillaceous-siliceous rocks, carbonaceous shale, thin limestone and breccia limestone. However, as Permian radiolarians are developed in siliceous rocks, the age of Nanpihe Formation is still controversial. Radiolarians from Late Devonian to Early Permian are found in clastic siliceous rocks in Kenong in the southeast of Mengsheng Town. Sporopollen from Late Devonian to Early Carboniferous is found through analysis of the prophyte fossils. Recently, we have found Early Carboniferous radiolarian Albaillella sp., Entactinosphaera foremanae and Schrfenbergia turgida in the siliceous rocks in the eastern member of the section. The area around Sipaishan, Gengma is complex in tectonics and developed in shear thrust tectonics, so it is not a continuous section. Nanpihe Formation may originally include part of Late Devonian and Early Carboniferous strata. Papai Formation containing Late Permian and Early Triassic radiolarians, Early Triassic bivalves and ammonites is similar to Nanpihe Formation in lithologic assemblage and sedimentary characteristics. Devonian, Carboniferous and Early Permian radiolarians are also included in the siliceous debris of clastic siliceous rocks. Its horizon may be partly equivalent to that of Nanpihe Formation. The clastic flysch, limestone and thin argillaceous limestone and siliceous rocks outcropped in Ximeng, Cangyuan, Gengmadashan-Mengtong are a sequence of passive continental margin sediments from the end of Precambrian to Early Paleozoic, and no unconformity relationship with the overlying Late Paleozoic strata has been found so far. It seems that the east side of Baoshan Block has a long history of passive margin development from Early Paleozoic to the end of Late Paleozoic. Parallel unconformity of Upper Permian and the underlying strata and the inclusion of D-P₁ radiolarian siliceous debris with different ages in the clastic siliceous rocks from Late Permian to Early Triassic indicate that Late Permian Changning-Menglian Ocean Basin has been subducted and destructed into the development stage of the foredeep depression, and the eastern basinal zone has been partially uplifted and denuded. Nanpihe Formation and Papai Formation may be formed in this foreland depression zone, because the only source of siliceous rock debris containing D-P₁ radiolarian can be the eastern basin facies zone, and no D-P₁ radiolarian siliceous rock sediments are found in Baoshan Block on the west side.

• (2) Ophiolite Melange Zone in Niujingshan, Shuangjiang-Manxin, Menglian.

The zone is narrow in the north but wide in the south, and has been outcropped in Changning-Menglian, and then extended out of the frontier southward. Most strata are in a disorderly and mixed state. In Manxin, Menglian, the stratigraphic sequence in some areas is relatively well preserved. It was basically a sequence of arenopelitic-siliceous rock formation from Devonian to Middle Triassic, and radiolarian fossils D₁-P₁, Archocyrtium menglianesis Wu, Ar. delicatum Cheng (C₁) were found in a sequence of not-too-thick strata, which indicates that most of the basins were in a starved state or some of them were faulted. At the same time, a sequence of ocean ridge/quasi-ocean ridge basalt (C₁), ocean island basalt and plateau-type carbonate rocks (C₁-P₁) formed on oceanic islands are outcropped in the zone. They can form the basin lithofacies zone together with the above arenopelitic-siliceous rocks. Changning-Menglian Ophiolite Zone is mainly distributed in Tongchangjie-Manxin and Yingpan-Yutangzhai-Gengma on its west side. It is further confirmed through relatively systematic research that there are not only ocean ridge/quasi-ocean ridge volcanic rocks, but also oceanic island type and continental margin rift-type basic volcanic rocks in this zone (Fig. 2.24), and the temporal and spatial distribution relationship between them is preliminarily clarified. Ocean ridge volcanic rocks are mainly found in Tongchangjie and Manxin, continental margin rift basalt is mainly found in Yingpan, Yutangzhai and Gengma, and ocean island basalt is found in Tongchangjie and Yiliu. Ocean ridge volcanic rocks are dominated by tholeiite series, including quartz tholeiite, olivine tholeiite and alkaline basalt containing hypersthene, which are associated with metamorphic peridotite (serpentinite), metamorphic cumulate (metamorphic pyroxenite, pyroxenite), metamorphic diabase and radiolarian siliceous rock. Lithochemistry and geochemistry characteristics indicate that these basalts belong to ocean ridge basalts (Figs. 2.25 and 2.26). REE distribution patterns of olivine tholeiite and quartz tholeiite in Manxin, metamorphic olivine tholeiite and quartz tholeiite in Tongchangjie are nearly flat, with w(La)/w(Yb)_N of 0.84–1.87 and no Eu anomaly, so it basically belongs to T-MORB type; sub-alkaline picrite basalt, olivine tholeiite (the horizon may be slightly higher than before) and alkaline basalt containing Hy in Manxin are of moderately-enriched LREE type, with w(La)/w(Yb)_N of 5.18–9.32 and no Eu anomaly, so it basically belongs to E-MORB type. Oceanic island volcanic rock is composed of alkaline basalt series, including alkaline olivine basalt, picrite basalt, basanite, potassic trachybasalt and sodium trachyte basalt, with lithochemistry characteristics of ocean island basalt (Figs. 2.25 and 2.26). Since the lithochemistry and geochemistry characteristics of ocean island basalt are similar to those of continental overflow basalt, only oceanic island carbonate rocks (no terrigenous clast) developed on the basalt in Yiliu and the basalt associated with ocean ridge/quasi-ocean ridge basalt in Manxin are identified as ocean island basalt. Although alkaline basalts distributed in Yingpan, Yutangzhai, Mengsheng-Xiaoheijiang River area in the west have similar lithochemistry and geochemistry characteristics to ocean island basalts and some of them are covered by carbonate rocks, it is also necessary to make a further study to confirm whether they are ocean island basalts. They are mainly distributed in the passive margin zone of the eastern margin of Baoshan Block, associated with clastic flysch, siliceous rocks and carbonate rocks in the passive margin zone, so they are temporarily classified as continental margin rift type. As for the alkaline basalts on Wuma Highway in Laochang and Tongchangjie, which have been determined by radiolarians to be of Late Permian, from the geological development history of the whole area, the tectonic background of their formation is different from that of oceanic island and passive continental margin rift, which may be formed in the period of subduction and forthcoming destruction of oceanic crust and in the period of foreland fault block uplift on the original passive margin. The formation age of volcanic rocks in this zone was controversial before. It can be confirmed by observing the section and studying the biological fossils that ocean ridge/quasi-ocean ridge basalt was formed in Early Carboniferous. Early Carboniferous radiolarian fossils are found in the associated siliceous rocks in Manxin, and basalt is locally found under Middle and Late Carboniferous strata. The metamorphic volcanic rocks in Tongchangjie area were originally identified to be formed in Early Carboniferous, and the K–Ar age of amphibole in gabbro is 383 Ma (Zhang et al. 1990), which indicates that it existed until Late Devonian. Metamorphic basalt in the borehole in Laochang area may be similar to that in Tongchangjie area, and Middle Carboniferous conodont fossils Niognathodus symmetricus (Lane), Hindeodilla sp. are found in the limestone on it. It can also be determined that the ocean island basalt is covered with carbonate rocks of Middle Carboniferous in Yiliu, so it was undoubtedly formed in Early-Middle Carboniferous. The volcanic rocks in the rift valley of the continent on the west side were mainly formed in Early Carboniferous, with basalt being underlaid the limestone in Lower-Middle Carboniferous strata at the place 14–15 km away from Cangyuan (Mengsheng) Highway. Alkaline basalts in Laochang and Wuma Highway were formed in Late Permian just as mentioned above. In addition, the gravity flow slump breccia limestone is agglutinated or wrapped by alkaline basalts locally at the end of Early Permian in Manxin area. This basalt may also be formed in the Late Permian. All types of basic rocks, serpentinite, metamorphic cumulate, peridotite, basic lava, pillow basalt, amphibolite schist, glaucophane, two-mica quartz schist, phyllite, various limestones, siliceous rocks of different ages and other rock assemblages with different sizes, different genetic types and different metamorphic deformation characteristics are in contact with each other by taking tectonic shear as the interface, and disorderly and fragmented melange can be found in the field. It is particularly worth pointing out that after the predecessors discovered the high-pressure blueschist in the late 1980s, Yunnan Institute of Geological Survey carried out a survey on a scale of 1:250,000 in Lincang County and Gunlong in recent years, and found and determined three high-pressure glaucophane schist zones in Changning-Menglian Subduction Complex Zone for the first time, and confirmed the existence of Ophiolite in Niujingshan.

Fig. 2.24

Sequence comparison of late Paleozoic volcanic rocks in changning-Menglian zone (Shen 2002). 1—Basalt; 2—Trachytebasalt; 3—Olivine basalt; 4—Amygdaloidal basalt; 5—Pillow basalt; 6—Picrite; 7—Cumulate; 8—Metamorphic peridotite; 9—Basaltic tuff; 10—Basaltic tuffaceous breccia; 11—Basaltic agglomerate; 12—Tuffaceous siltstone; 13—Tuffaceous mudstone; 14—Mudstone and shale; 15—Siliceous rock; 16—Limestone; 17—Paleo-karst surface; 18—Mine; 19—Radiolarians and other fossils; 20—Sampling position. OIB—ocean island basalt; MORB—mid-ocean ridge basalt; F—fault

Fig. 2.25

Distribution pattern of rare earth elements in volcanic rocks and spider diagram of trace element in changning-Menglian. 1–2 refer to ocean ridge basalts; 3–6 refer to ocean island basalt

Fig. 2.26

Identification map of volcanic rock tectonic environment in changning-Menglian (Shen 2002). IAB- island arc basalt; MORB—mid-ocean ridge basalt; OIB—ocean island basalt 1–2 refer to ocean ridge basalt; 3–6 refer to ocean island basalt

• (3) Fore-Arc Accretionary Wedge Thrust Zone.

This zone is mainly composed of proximal turbidite in the southern formation of Carboniferous and fore-arc slope turbidites of Permian Laba Formation. The turbidite and siliceous rock sedimentary bed of Laba Formation generally contains intermediate-basic and intermediate-acid pyroclastic rocks and tuff interbedded with basaltic andesite cuttings; moreover, calcirudyte formed by gravity flow is found in many places. The limestone breccia contains C₂-P₁ fossils, and the siliceous rocks contain Late Permian radiolarian Follicucullus assemblage and Neoalballella optima assemblage. There are also Late Permian ammonites and bivalves buried in situ, and Middle Triassic radiolarians are found in the upper siliceous rocks. In the southern formation, it unconformably overlaps on Lancang Group. The data above show that Lincang-Jinghong Remnant Arc Zone in the east was in a continental uplift state in Carboniferous, with C₂-P₁ slope formed, slope turbidite developed and carbonate gravity flow accumulated in the west margin of the island arc, and intermediate-acid volcano erupted in Permian. This tectonic lithofacies zone may have extended to Shuangjiang area to the north, and then outcropped in Xiaoheijiang River Bridge as a sequence of sandstone, slate, and carbonate rock and breccia field stone (no fossils are found) accumulated by gravity flow in gully distributed from north to south; the sedimentary characteristics are similar to those of the southern member and Laba Group, indicating that the eastern member of Changning-Menglian area was adjacent to the margin of continental margin arc in Carboniferous, and reworked into fore-arc-basin in Permian, and then reworked into fore-arc accretionary wedge in the Early Middle Triassic.

2.2.7.1.2.6 Lincang Magmatic Arc

The Stratum between the Lancang River Tectonic Zone and Changning-Menglian Tectonic Zone is characterized by wide outcropping of intermediate-acid intrusive rocks, forming Lincang Composite Batholith Zone with the largest scale in Yunnan. The batholith zone generally extends from north to south, ranging from 10 to 48 km from east to west, with an average width of 22.5 km. The continuous outcropping length from north to south in Yunnan Province is 350 km, forming a very striking tectonic magma zone. Lincang granite is a wedge inclined to the west and slightly concave in the middle. Generally, the extension depth is relatively shallow in the east, about 0.5–2 km; while the extension depth is relatively deep in the west, about 8–12 km. There is a huge ductile shear zone with thrust nappe between the east side of the batholith and Upper Paleozoic and Triassic strata. The west side of the batholith is mostly in intrusive contact with the Mesoproterozoic Lancang Group. In addition, xenoliths containing metamorphic rocks of Lancang Group in granite can be found in many places on Douge-Mengku Highway. The batholith is often unconformably covered by Huakaizuo Formation of Middle Jurassic stratum. For many years, a lot of isotopic dating data have been collected: The Rb–Sr isochron age of the whole rock is between 236 and 348 Ma (Yunnan Bureau of Geology and Mineral Resources 1990), mainly varying from 279 to 297 Ma (Early Permian), and the U–Pb age of zircon is between 212 and 254 Ma (Early Middle Triassic), and the K–Ar age of single minerals is between 223 and 288. The batholith is mainly composed of medium-grained-coarse-grained biotite monzogranite, followed by biotite granodiorite. The chemical composition of rocks is relatively stable, and most of them are of the aluminum supersaturated series, namely faintly acid calc-alkaline rocks (some of them are of the normal series). Their acidity and alkalinity are lower than the average value of similar rocks, while the contents of compositions magnesium, iron and calcium are higher than the average value, indicating that the rocks are intermediate. Lincang composite granite batholith is dominated by Type-S granite, and also mixed with Type-I granite (Regional Geology of Yunnan 1990), and the middle member (Lincang member) is characterized by the coexistence of Type-S granite and Type-I granite. The characteristic values of lithochemistry vary widely, indicating that the provenance of batholith is characterized by polyphyly and heterogeneity. In addition to substances in the upper and lower crust, there may also contain substances from the upper mantle, reflecting at least that the depth of the provenance area where the partially remelted granite were formed during the long-term diagenesis is different. Intrusions formed by two mechanisms: one is the anatectic highly-intrusive rock formed under the control of dike spreading, that is, hypomagma rises through the fracture to the upper magma reservoir and then cools down and crystallizes (there may be “thermal ballooning” magma intrusions locally); the other is that with the repeated opening activities of faults, magma migrates from the deep crust to the shallow crust for many times, finally forming a whole rock masses with the rock mass extending along a certain direction, namely a large pluton with irregular plane and uniform internal tectonics.

The early stage of batholith formation was the subduction arc-building period (P₁), during which the lower crust was partially melted to form intermediate granitic magma possibly mixed with substances from the upper mantle, and then a rock mass dominated by Type-I granite (this rock mass contains two-pyroxene granulite, hypersthene granulite or granulite enclaves) was formed with spreading and rising to the middle and upper crust of the fault. With subsequent intensification of the arc-land collision and extrusion, melting occurred along different horizons of the crust (mainly in the middle and upper bed of the crust) and along the ductile shear zone, forming a large-scale arc-land collision-type intermediate-acid magma, which rose and invaded to form the main body of Type-S granite. According to the occurrence of Type-S granite in batholith, there should be two types: high intrusion and para autochthone, both of which shall have the same mechanism and homology.

It shall be pointed out that “satellite-shaped” rock mass was outcropped in Mengsong and Bulangshan at the southern end of Lincang-Menghai Granitic Batholith, of which the lithology is quite special and dominated by muscovite granite and two-mica granite; and the lithology of each rock mass is quite different. The lithology is dominated by muscovite granite, two-mica granite, and mixed with monzonitic granite locally. Biotite is represented by siderophyllite. Being rich in Si and Al but poor in Fe and Mg, muscovite belongs to phengite. Besides zircon and apatite, the accessory minerals include scheelite and cassiterite. This type of rock mass is closely related to tungsten and tin mineralization, and the tin ore deposit in Mengsong is related to this kind of granite. This kind of rock mass belongs to alkaline rock, accounting for 35.7%, which is the magma product of post-orogenic spreading.

2.2.7.1.2.7 Southern Lancang River Melange Zone

This zone is distributed along the Lancang River Valley. According to the observation of the outcropped tectonic-stratum (about 4 km) in the west of Reshuitang, Simao-Lancang Highway, by taking a sequence of dark gray thin-layer shale and gray medium-grained-coarse-grained turbidity greywacke as matrix, the zone is mixed with limestone and siliceous rock lenses, and also contains basic volcanic rocks and ultrabasic rocks locally, among which limestone rock contain Maokou fossils. There are also gray thick-layer medium-grained-coarse-grained lithic sandstones, gray thin-layer fine-grained lithic greywacke, gray thin-layer fine-grained quartz sandstone, and dark gray, greyish-green thin-layer shale, all of which have been sheared significantly, so it is difficult to restore their original sequence. However, there are still tectonic stratigraphic sequences with weak strain, so some scholars think that this sequence of strata is mainly composed of abyssal turbidite sandstone, and also mixed with clastic sandstone related to contour current and argillaceous rocks in distal-basins. Turbidite sandstone is closely related to volcanic island arc, and the rock debris content is more than 50%.

The abyssal sediments between Lincang Magmatic Arc and volcanic arc on its east side are incomplete due to tectonic damage, and partly covered due to the overlapping of Triassic volcanic arcs. As a result, it is difficult to restore the basin prototype of this abyssal basin distributed along the present Southern Lancang River Fracture Zone. Some scholars believe that it is a fore-arc-basin sediment (Nan Jinhua, doctoral dissertation 1993), while we think it may be a back-arc-basin based on the regional analysis. The reasons are as follows:

• (1) In the western margin of Pu’er Block, Daxinshan Formation composed of Late Paleozoic strata is a tectonic stratum, in which the LRE distribution pattern of Late Permian basalt is flat type, with the depletion of incompatible elements such as Rb, Ba, U and Th, etc. and the enrichment of elements such as Ti, Co and Ni, etc., showing the characteristics of oceanic crust tholeiite. This fact indicates that the eastward subduction of Tethyan Ocean in Changning-Menglian has formed a tectonic process of back-arc ocean basin spreading on the east side of Lincang Magmatic Arc.

• (2) Basic and ultrabasic rock mass intruded in groups and zones along Southern Lancang River Zone from Chahe, Waili, Banpo in the west of Jinggu County, as well as Nanlian Mountain, Manshuai, Manhuai, Manshan, etc. in the south of Jinghong County. In the field of Waili, significantly foliated dunite (serpentinite) is sandwiched with bedded gabbro (cumulate), and basic and ultrabasic rock tectonics intrude into Late Paleozoic strata, mainly Daxinshan Formation. Because the ophiolite in the inter-arc deep basin was significantly decomposed during the orogenic process, it was destroyed to be incomplete and some tectonics intruded into the volcanic zone in Yun Country-Jinghong Arc.

• (3) Nanguang Village which is located in the southeast suburb of Jinghong and near Lancang River is the coarse clastic rocks of Nanguang Formation of Middle-Upper Devonian strata, which was once considered as the symbol of Caledonian orogeny, but it was actually a sequence of Late Devonian abyssal submarine fan glutenite after field research by, which marked the initial spreading of Southern Lancang River back-arc-basin. Pillow lava and Early Permian radiolarian siliceous rocks (Follicucullus sp., Pseudoalbaillella sp.,) were found in Jinghongxiaojie (Fig. 2.27). The geochemistry study of rocks shows that an ocean ridge volcanic zone of tholeiite series existed along the Daxing Mountain-Xiaojie line, of which the pattern diagram of rare earth elements and trace elements (Fig. 2.28) is consistent with that of N-type slowly spreading ocean ridge basalts (Shen 2002). According to the analysis of the distribution of Daxinshan Formation starting from the west piedmont of Wuliang Mountain and running through Anle, Minle, Yongping, Mengyang and then extending into Padang where it extends into Myanmar, Daxinshan Formation in Reshuitang (P₁) should be an interarc and abyssal sedimentary basin between Lincang Magmatic Arc and volcanic arc on the east side. Geochemistry study on trace elements shows that the primitive sedimentary characteristics of basalt in Reshuitang and volcanic rocks in intra-oceanic arc (Shen 2002) are distal abyssal turbidite mixed with basic lava, which represents the significant post-arc spreading occurred in the early stage of Early Permian. Volcanic rocks and continental arc volcanic rocks in Daxing Mountain-Reshuitang Intra-oceanic Arc show different geochemistry characteristics (Table 2.2).

  Fig. 2.27

  

  Profile of the place 500 m away from Xiaojie, Henan and Dongzhai in Northwest direction (Shen 2002)

  Fig. 2.28

  

  Pattern diagram of rare earth elements and trace elements in rift-ocean ridge volcanic rocks of Southern Lancang River zone (Shen 2002)

  Table 2.2 Comparison of geochemistry characteristics of three types of arc volcanic rocks in Southern Lancang River zone

The characteristics of geophysical field and the evidence of field geological observation show that a large-scale thrust nappe ductile shear zone is found along the east side of Lincang-Menghai Granite Batholith Zone, which shows the thrust of granite from west to east. The geological interpretation of gravity data inversion shows that granite is a rootless nappe. The same understanding has been obtained based on the recent magnetotelluric sounding data. A ductile shear zone is found between granite and Daxinshan Formation at the fault on the east side of Lincang Granitic Mass along the river in Qianliu and Mangpa, Lancang County. On the one hand, due to the significant reworking and compression of the later tectonic orogeny, Lancang Fracture Zone shows significant thrust nappe characteristics from west to east, that is, a large mylonite zone was formed along the fracture zone, indicating that it has the nature of a ductile shear zone formed by deep tectonic orogeny and makes Huafeng rock mass in Lincang be a rootless nappe. On the other hand, a strike-slip fault with a curved vertical section is found along Lancang River Valley, which was considered by Wang to be the product of intense intracontinental reworking and deformation in Himalayan.

2.2.7.1.2.8 Yunxian-Jinghong Volcanic Arc

The volcanic arc tectonic rock zone is located between the Lancang River Junction Zone and Jiufang Fracture, and the oldest exposed stratum in the zone is the Middle-Upper Devonian, which is distributed in Jinghong-Nanguang. According to plant fossils and pyroclastic rocks, the Nanguang Formation in Upper Devonian was considered as continental sediment. The on-the-spot investigation reveals that there are mainly a sequence of fluvial sand body sandstone, submarine fan conglomerate, fine clastic turbidite, siliceous turbidite and siliceous rocks, with well-developed Bouma Sequence. It is equivalent to the sediment of low-stand system tract and the condensation section in the maximum flooding period, reflecting that Nanguang Formation is a sedimentary environment from slope to basin margin. There are carbonized plant imprints in the sandstone and volcanic breccias in the breccias, reflecting an ancient land uplift with volcanic activity to the east, and Lamping-Pu’er Basin may have an Early Paleozoic folded basement beneath it as in Qamdo Basin. Carboniferous-Permian is an island arc volcanic-sedimentary formation composed of sand-slate, marl, limestone, pyroclastic rock, basalt, andesite and rhyolite. In Bangsha, Jinghong, a sequence of volcanic-sedimentary rock series of continental margin arc was originally in the Middle Triassic, and the Late Permian radiolarian Neoalbaillella ornithoformis assemblage was found in its siliceous rocks. There are Carboniferous-Permian island-like isolated carbonate plateau and deep-water trough of intra-arc rift in the area around Ganlanba, which has typical island arc geomorphological characteristics.

The Middle and Upper Triassic stratum is a sequence of volcanic-sedimentary rock series dominated by volcanic rocks, with the lower volcanic rocks being acidic and the upper volcanic rocks being lagged arc volcanic rocks. This volcanic-sedimentary rock zone spreads from Xiaodiding and Minle to Nanguang in the south. The Middle Triassic Manghuai Formation and Upper Triassic stratum are the Xiaodixing Formation and Manghuihe Formation. According to the research of Zhang the volcanic eruption in this sequence of strata can be divided into three cycles: The first cycle (Manghuai Formation) formed a sequence of high-potassium rhyolitic volcanic rocks; the second cycle (Manghuihe Formation) is caused by “bimodal” volcanic eruption, from rhyolitic volcanic rock → subalkaline basalt → medium-long basalt → potassium coarse basalt → rhyolite; the geochemical element gradients and trace element diagrams of the first and second cycles show the tectonic environment of continental margin volcanic arc; the “bimodal” volcanic rocks in the third cycle show the back-arc rift environment after hard collision. In this volcanic-magmatic arc, besides the volcanic rocks mentioned above, there are some mafic–ultramafic rocks and diorite developed along the volcanic arc. Its rock assemblage and occurrence characteristics belong to Alaska mafic–ultramafic rocks and peridotite-diorite mafic–ultramafic rocks. The former is mainly distributed in Banpo, Jinggu, while the latter is mainly distributed in Jinghong-Damenglong. The lithochemistry characteristics are similar to those of the same type of California peridotite-diorite. Its general characteristics are similar to those of calc-alkalic basaltic-andesite in the orogenic belt and mafic rocks in some Alaska rocks. The formation of mafic–ultramafic and peridotite-diorite rock bodies in this area is directly or indirectly related to the source areas of magmatic activities formed by volcanic-magmatic arcs in this area. Combined with their close symbiosis in space, it shows that they should be the products of magmatic intrusion in island arcs/orogenic belts.

The tectonic deformation of this volcanic-magmatic arc is relatively significant, generally characterized by eastward thrust fractures, and the folds extending linearly in the same direction can be found. Near Lancang River Fracture, the Jurassic-Cretaceous strata have undergone local metamorphism and deformation to form slate, which formed primary bedding which is significantly replaced by axial cleavage and were mainly related to the late action of the Lancang River Fracture. The Late Paleozoic-Triassic strata have ununiform metamorphism and deformation, and the strata close to the Lancang River Fracture have deepened metamorphism and increased deformation, some of which can reach slate and phyllite, and the highest grade of metamorphism can reach schist with low greenschist facies. It can be seen from the highways in East Jinghong, the part with significant metamorphism and deformation is often closely related to thrust nappe, and a series of imbricate thrust nappe tectonics and synclinal inverted folds with westward reversal and eastward axial tilt are formed, which indicates the kinematic characteristics of nappe from east to west, which may be related to the superimposed transformation of Jinghong left strike-slip fracture in NWW direction.

2.2.7.2 Evolution of Tenasserim Arc-Basin System

It is difficult to understand the tectonic evolution of Tenasserim Arc-Basin System only by a simple open-close evolution. If the Bangong Lake-Nujiang River-Changning-Menglian Ocean Basin is only considered to be formed from the early period of Mesozoic to Paleozoic, it is difficult to properly explain the geological processes in each period in this zone. In our model, the Bangong Lake-Shuanghu Lake-Nujiang River-Changning-Menglian Mage-suture Zone is defined as the northern boundary of Gondwana. As the relic of the continuous evolution and destruction of Proto-Tethys to Paleo-Tethys, Qamdo-Lanping-Pu’er Block was the southwestern margin of Yangtze Landmass in the Early Paleozoic. Jitang Group (including Xixi Group), Chongshan Group, Lancang Group, etc. are all formed by the subduction of the Proto-Tethyan Ocean in the western margin of Yangtze Landmass to the accretionary wedge. In essence, these metamorphic bodies include the products of the convergence and collision of Neoproterozoic Rodinia supercontinent, which can disintegrate island arc volcanic rock-pyroclastic rock assemblage, Early Paleozoic Suyi-Nanlang blueschist assemblage, and residual arc volcanic rock assemblage such as Youxi Group.

Baoshan Block on the west side of Changning-Menglian Junction Zone has been transformed into plateau evolution since Late Cambrian, while the sediment of Gengma-Cangyuan passive continental margin in Early Paleozoic has been developed on the east edge of Baoshan Block. From Late Cambrian (Manggao Rock Formation) to Silurian (Mengdingjie) group complex, flysch sediment occurred. There are typical turbidites in Mengdingjie Group Complex, which reflects the ocean in the east of Baoshan Block. This ocean is the Proto-Tethyan Ocean in the Early Paleozoic. Therefore, when analyzing and studying the geological records of the Proto-Tethyan Ocean in the northern member, the researchers should not only seek from the composition of Bangong Lake-Dingqing-Bitu Ophiolite Melange Zone, but also identify and understand the ocean from the basement, overlying strata, characteristics of MABT, similarities and differences of crustal lithosphere structure and the evolution of ocean-land transition. The consistency of Paleozoic plateau sediment in Gangdise and Himalayan periods reflects that there is a wide passive continental margin sedimentation in the northern part of India Continent, which is quite different from the active continental margin in the northern part of Bangong Lake-Nujiang during Paleozoic. Therefore, Gangdise Block, Tengchong-Baoshan Block (Danbang Microcontinent) and India are part of Gondwana Continental Group, while Qamdo-Lanping-Pu’er Block (Indo-China Microcontinent) and Yangtze Continent are part of Pan-Huaxia Continental Group.

The Tethyan Ocean is located between Gondwana Continental Group and Pan-Huaxia Continental Group since Phanerozoic and is an inherited ocean. The development conforms to the life cycle law of ocean evolution history of more than 600 Ma, rather than the closure of Proto-Tethys and the reopening of Paleo-Tethys.

The subduction of Tethys Ocean to the northeast in the early Late Paleozoic formed Tenasserim Frontal Arc. A series of stable blocks and island arcs behind the frontal arc and arcs to the ocean basin formed the MABT. The restored Paleo-Tethyan Ocean Basin of Dingqing-Zhayu-Bitu-Changning-Menglian includes ocean ridges, abyssal basins, ocean islands, intra-sea arcs and intra-oceanic arcs. The sea mountain carbonate plateau is a sign of the shrinking of the ocean basin in Tethyan Ocean during ocean-land transition. The “bimodal” back-arc rift in Early Carboniferous in North Lancang River to the back-arc ocean basin, and the back-arc ocean basin in South Lancang River have only a few tens of millions of years of life. From Late Permian to Triassic, the back-arc-basin shrank and subducted in both directions, and magmatic arcs appeared on both sides, which led to the formation of continental margin volcanic arcs on the east side. The arc-arc collision in Late Triassic and basin-mountain transition in Jurassic-Cretaceous have been transferred to the intraplate evolution.

2.2.8 Space–Time Structure and Evolution of Baoshan Block (Northern End of Danbang Micro-Landmass)

Baoshan Block is located between Changning-Menglian Junction Zone and Nujiang River Fracture Zone (south member). According to the exposure of strata and sequence composition, it is generally a stable plateau.

At the end of Precambrian and the beginning of Paleozoic, there were significant tectonic thermal events in Baoshan Block: granite in Laojiezi, Ximeng, with an Rb–Sr isochron age of 687 Ma, two-mica granite in Zhibenshan, Luxi, with an Rb–Sr isochron age of 645 Ma (Zhang et al. 1990), and granite in Pinghe, with an Rb–Sr isochron age of 529.9 Ma, which may represent the product of Pan-Africa events. The strata of this landmass are well developed. The oldest stratum is outcropped in the western part of the landmass, namely Gongyang River Group (Z-∈₂), which is composed of bathyal and neritic sandstone, mudstone (shale) with a small amount of silty siliceous rock and thin limestone, which are slightly metamorphic. It was caused by rift development. It locally conformably contacts the Upper Cambrian stratum, and most part is overlapped by Lower Ordovician and Middle Permian strata. The Lower Paleozoic strata in the landmass is well developed, and it is the continuous sediment. It includes Hetaoping Formation (∈₁), Shahechang Formation (∈₂), Baoshan Formation (∈₃), Laojianshan Formation (O₁), Shidian Formation (O_1-2), Pupiao Formation (O_2-3), Renhe Bridge Formation (O₃-S₁) and Lichaiba Formation (S_2-3) from bottom to top, and is composed of neritic-tidal sandstone, shale, limestone with a total thickness of 5000 m. Graptolite shale can be found in Renhe Bridge Formation. It has abundant paleobios and the mixed biological characteristics of North China and South China.

The Upper Paleozoic stratum is dominated by stable plateau carbonate rocks, which are deposited continuously with the underlying Silurian stratum, with a total thickness of about 3000 m, including Xiangyangsi Formation (D₁), Heyuanzhai Formation (D₂), Dazhaimen Formation (D₃), Xiangshan Formation and Pumenqian Formation (C₁), Dingjiazhai Formation and Woniusi Formation (P₁), Bingma Formation (P₂) and Shazipo Formation (P₃). During the late period of Early Permian (Woniusi Formation), there were continental margin marine basic volcanic rocks, the thickest of which was up to 700 m. There is a parallel unconformity between the Lower Permian stratum and the Lower Carboniferous stratum, and the Upper Carboniferous stratum is in lacuna. The lower part of Lower Permian-Middle Permian (Dingjiazhai Formation-Bingma Formation) has Gondwana mudstone sediment and cold-water animal molecules represented by Stepanoviella and Eurydesma, indicating its significant Gondwana affiliated characteristics.

The Triassic stratum is composed of Hewanjie Formation (T_1–2) and Nanshuba Formation (T₃) basically inherits the carbonate environment of the Late Paleozoic plateau, but the basin has become closed and dominated by dolomite. In recent years, a large number of Early Triassic conodonts have been found in the lower part of Hewanjie Formation, which is deposited continuously with Permian stratum. At the end of the Late Triassic, some intermediate-basic and intermediate-acid volcanic rocks were active. The Mengjia Formation in the lower part of Middle Jurassic stratum is composed of red conglomerate, sandstone and mudstone, containing gypsum-salt, locally mixed with basalt, and unconformity with Triassic stratum and its lower strata. The Liuwan Formation and Longhai Formation on Mengjia Formation are neritic limestone, sandstone and shale (mudstone). The marine environment ended after the Late Jurassic and was followed by significant intracontinental orogenic activity, which accumulated Paleogene red molasse coarse clastic rocks. In the Neogene, small fault basins developed along various fractures, forming limnetic-alluvial-lacustrine clastic rocks with lignite beds.

Baoshan Block can be further divided into three tertiary tectonic units (Li et al. 2002).

• (1) Shuizhai-Muchang Foreland Depression Zone (P₂-T₃). The Late Permian–Triassic strata are mainly developed in this zone, and its lithofacies change from east to west, from continental to marine, and the Upper Triassic stratum overlaps eastward and unconformities on the underlying strata of different ages. This zone is a foreland depression formed on the front margin of Baoshan Block on the west side of Gengma-Cangyuan passive margin thrust fold zone after Danbang Micro-landmass subducted eastward and Tethyan Ocean closed.

• (2) Baoshan-Shidian Uplift Zone This zone is characterized by the lacuna of Late Permian stratum in Baoshan-Zhenkang Zone, and is an uplift zone formed in Mesozoic.

• (3) Liuku-Mengjia Mesozoic Foreland Depression Zone. This zone was developed on the basis of another depression on the west side of Triassic Shuizhai-Muchang Foreland Depression Zone. In the Early Jurassic, Dangbang (Baoshan) Micro-landmass was obliquely subducted to the west, the Nujiang Ocean was closed, and Gaoligong Mountain was pushed to the east to form the frontal depression zone. This zone is composed of Middle Jurassic continental molasses, marine clastic rocks and carbonate rocks, and some submarine volcanoes erupted.

This landmass (including the whole Dangbang Micro-landmass) belonged to Gondwana Continental Group in the Paleozoic, but it is still inconclusive whether it is attached to Australia or India in terms of tectonic attributes. On the whole, this zone seemed to be connected with an adjacent continent but not closed in Paleozoic. According to the Cambrian-Neogene outcrop and the research results of paleogeography characteristics of the sedimentary basins by Luo, it can be roughly divided into 3 stages: ① Cambrian-Silurian: This zone was mainly composed of stable neritic clastic rocks mixed with carbonate rocks; ② Devonian-Early Permian: This zone was mainly composed of stable neritic clastic rocks and carbonate rocks, and only basic volcanic rocks appeared in Carboniferous-Permian, which generally showed deceptive conformity with the previous strata; ③ Mesozoic: The sedimentary layer was superimposed and unconformity on the underlying strata of different ages, and this zone was composed of a sequence of clastic rocks and carbonate rocks mixed with intermediate-basic and intermediate-acid volcanic rocks, with magnesium carbonate rocks at the bottom and red molasse gravel at the top.

2.2.8.1 Formation Stage of Stable Plateau (C-S)

At the end of Precambrian and the early and middle Cambrian, a sequence of flysch submarine fan turbidite with a thickness of several kilometers was formed in the continental marginal sea. There were no volcanic rocks, and the stratum was slightly metamorphic, with no large fossils. Siliceous rocks and a small amount of limestone as well as spongy spicules and micro-paleobotany can be found. Sediments became thinner from southwest to northeast, forming the base of carbonate plateau and siliceous clastic rock pad, and changed into neritic-shelf sediments in Late Cambrian-Early Ordovician, mainly siltstone, slate, marl and limestone, with more purple shale in the upper part, rich in trilobites, brachiopods, graptolites and other biological fossils. In Middle Ordovician, the neritic-shelf yellow mudstone and marl alternately deposited, and lacustrine argillaceous dolomite sediments occurred in Lux. In Late Ordovician, there was mainly a regressive sequence, characterized by tidal flat mottled aleuropelitic sediments, which later transformed into estuarine graptolite shale sediments. The actinoceras cephalopoda in Renhe Bridge, Baoshan was Pararmocers cf., which was common in Nyalam, South Tibet.

The Ordovician stratum in Gengma-Cangyuan-Ximeng on the east side of Baoshan Block is a plateau margin to continental slope, which is composed of sericite slate, lithic sandstone and siliceous rocks with thin limestone.

The three series of Silurian are complete, the black shale developed in Lower Silurian, and a sequence of continental margin deep-water reticulate limestone with graptolite shale developed in Middle-Late Silurian, which reflects the sedimentary environment from the outer shelf to plateau margin slope.

2.2.8.2 Formation Stage of Basin Series (D-P1)

The sediments in Early Devonian were mainly composed of sand, argillaceous carbonate rocks and bioclastic limestone, which belonged to neritic plateau basin facies, and contain fossils of thin shelled tabasheer and graptolites and brachiopod with thin shell and fine stria. They were anoxic, hydrostatic low energy organisms in a weak reducing environment. Middle Devonian sediments were mainly littoral gravel and sandy argillaceous carbonate rocks. The upper part formed a stable neritic carbonate sediment, while the water body on the east side became deeper, and the sediment became muddy or marly carbonate rocks. The salinity and temperature of the sea water were normal, and the benthos were flourishing, and some reefs were formed. Late Devonian sediments were mainly composed of microcrystalline limestone mixed with siliceous rocks, argillaceous limestone and black shale, which belonged to neritic basin facies, with fossils of conodonts and brachiopod with fine stria.

Early Carboniferous carbonate plateau facies is distributed in NS direction and can be divided into two sub-facies zones according to sedimentary characteristics: The first one is an open plateau sub-facies zone, which is distributed in Baoshan, Shidian and other places. Its sediments are composed of osseous pelsparite, flint-banded strip pelsparite, bioclastic limestone and oolitic limestone, etc., which are rich in marine benthic fossils, forming biological beaches or reefs locally; the second one is plateau slope or basin sub-facies zone, which is distributed in Qingshuigou and Guanpo, Baoshan. Its sediments are composed of dark gray medium-thin pelsparite, argillaceous pelsparite and slump breccia limestone. The slump breccia is of different sizes, irregular shapes and disorderly arrangement, supported by miscellaneous matrixes. The breccia is composed of biosparite and bioclastic limestone, and the matrix is gray to gray-black micritic, with slumping deformation bedding and horizontal bedding developed. There are many pyrite crystals and siderite nodules in the rocks, and there are few biological fossils, mainly benthic and planktonic biota, which represent the sediments in the stagnant environment of deep water. Middle Carboniferous regional uplift sedimentary can be found locally. The Late Carboniferous stratum was influenced by Gondwana Continental Glaciation, and the continental margin glaciomarine debris flow sediments were developed in this zone, which were composed of fine conglomerate, gravel-bearing coarse sandstone and gravel-bearing mud shale. The gravel components were granite, limestone, gneiss, sandstone, etc., and the biological assemblage from cold water to warm water developed. The main biota assemblages in the early period were Eurydesma sp., Stepanoviella sp., Wikingia sp., Schizodus sp., Trigonotreta sp., Marginifera sp. and Lytvolasma. Eurydesma sp. and Stepanoviella sp. are the representatives of typical cold-water biota assemblage, which not only can be found in this zone, but also were widely distributed in Gondwana, the Gangdise, Himalayas, Pakista Salt Ridge and other places, and coexisted with moraine or ice water sediments. The sediments associated with this assemblage in this zone were also ice-water debris flow conglomerate, so this cold-water biological assemblage was closely related to the glacial climate and glaciation in Gondwana. The main biota assemblages at the end of Late Carboniferous were Triticites, Qua-sifusulina, Fenstella, Squamularia, Neospirifer and Marginifera, which have warm habits.

In the Early Carboniferous, Baoshan Biogeographic Region was a mixed biota with environmental characteristics at that time, and a typical cold-water biota assemblage was developed in Late Carboniferous, so Baoshan biota was a part of Gondwana.

The Early Permian stratum was characterized by the developed littoral sediments on the uplifted denuded basement at the end of Carboniferous. It was composed of iron-bearing aluminum sand shale, with basal conglomerate and animal and plant fossils. The Late Permian and Middle-Late Permian were characterized by the developed plateau subfacies, which were composed of limestone, dolomitic limestone and bioclastic limestone, with oolitic limestone and breccia limestone locally and benthic fauna.

2.2.8.3 Formation Stage of Marginal Foreland Basin (P2-T3)

The Tethyan Ocean Basin in Changning-Menglian subducted at the end of Early Permian, and then destructed due to the arc-land collision in Middle Triassic, which can be found on the passive margin of Gengma-Cangyuan in the west of the zone, forming the Shuichang-Muzhai Margin Foreland Basin and Baoshan-Shidian Foreland Uplift and their sedimentary records. In the early period of Late Permian, Gengma-Cangyuan Marginal Foreland Basin was characterized by coal-bearing sand shale sediments, and composed of littoral-neritic molasse. In the late period, it was carbonate sediment. The Baoshan-Shidian Foreland Uplift in the west of the basin was largely devoid of Late Permian and Triassic sediments.

The Triassic stratum was deposited in Shuichang-Muzhai Marginal Foreland Basin. The Hewanjie Formation in the Early and Middle Triassic was mainly a sequence of shallow clastic rocks, dolomitic limestone and limestone interbedded, rich in low salinity bivalves and gastropods fossils, 300–900 m thick, and composed of the supratidal and lacustrine sediment, which was in deceptive conformity contact with the underlying lower Permian stratum. According to recent research, the lower part of this formation contains Early Triassic conodont fossils.

The Late Triassic Nanshuba Formation, consisting of yellow-green silty shale, mudstone mixed with marl, has bivalves Halobia pluriradiata and H.yunnanensis assemblage and ammonite Tropites assemblage, with a thickness of 800–1400 m, and was composed of neritic clastic rock molasse.

2.2.9 Space–Time Structure and Evolution of Boxoila Ling-Gaoligong Arc-Basin System

The Baxoila Ling-Gaoligong Arc-Basin System, that is, the part where Gangdise MABT spreads eastward to the south and bends (bulges from the north to the east), is mainly composed of Baxoila Ling-Gaoligong Frontal Arc on the west side of the Nujiang Ophiolite Melange Zone.

2.2.9.1 Formation Period of Nujiang Ocean Basin and Development Period of Gaoligong Mountain Magmatic Arc

Because of the significant eastward thrust of Gaoligong Mountain Magmatic Arc Orogenic Belt in Late Mesozoic and Early Cenozoic, the Nujiang Ocean Basin overlapped the ophiolite melange zone in the middle part of Nujiang River, and covered the volcanic-magmatic arc of Biluo Snow Mountain and Baoshan Block in the south member, forming a strike-slip oblique thrust ductile shear zone composed of mylonite. Many geological records are denuded and destroyed, so it is difficult to determine the period of formation. The northern member is Dingqing-Zhayu-Bitu Junction Zone, while in the south member, only the remnants of ophiolite melange zone are preserved in Santaishan, Luxi on the side of the northeast Longling-Ruili Right-handed Strike-Slip Fracture Zone. It can be seen that ultramafic rocks intruded in Triassic-Early Jurassic deep-water turbidite and had the characteristics of ophiolite melange. There are significant differences in stratigraphic records, sedimentation, magmatic activity and metamorphic deformation between Tengchong Arc-basin System and Baoshan Block in Late Paleozoic and Triassic, which reflects that the both split from the ocean. Therefore, it is speculated that the ocean basin was finally destroyed in the Middle Jurassic.

Boxoila Ling-Gaoligong Frontal Arc was a relic of continental margin arc in the northern margin of India in Carboniferous. It was mainly composed of Late Triassic-Cretaceous granitic batholith, namely Type I granite, with features of island arc, and ages of mainly 227–210 Ma and 129 Ma. According to the biotite granodiorite in Changma, Yongsong, the Rb–Sr isochron age is (195.3 ± 7.0) Ma. Therefore, the Late Triassic stratum was formed by subduction arc, and Jiali-Parlung Zangbo Inter-arc-Basin and Yarlung Zangbo Back-arc Ocean Basin developed in the south and west of the island arc.

2.2.9.2 Spatial Pattern of Boxoila Ling-Gaoligong Arc-Basin System

From east to west, Gaoligong Overthrust Zone (Santaishan ophiolite melange zone), Boxoila Ling-Gaoligong Magmatic Arc and Bomi Parlung Zangbo Arc-arc Collision Zone are developed in the arc system.

2.2.9.2.1 Gaoligong Overthrust Zone (Santaishan Ophiolite Melange Zone)

Gaoligong Overthrust Zone spreads from Gongshan to Longling for more than 350 km in NS direction, and its southern member is restricted by Longling-Ruili NE strike-slip fracture zone, which is generally characterized by dipping westward of Gaoligong Group tectonic foliation and dipping eastward of a series of imbricate thrust nappe tectonics. The original system is the sedimentary cover on Baoshan Block, including Gongyang River Group and the overlapping. The whole-rock Sm–Nd isochron age of the amphibolite in nappe zone is (194.2 ± 20) Ma, the K–Ar age of mylonite biotite in strike-slip fracture is 203 Ma, and the molasse in Middle Jurassic foreland basin contains ultramafic rocks, all indicate that the thrust started at the end of Early Jurassic.

Near the 80 km monument of Luxi-Ruili Highway, along Santaishan and Nongbing in Luxi, ultramafic rocks, siliceous rocks and limestone can be seen in rock blocks, mixed with rocks with the matrix of Late Triassic-Early Jurassic turbidite. The folded axial plane and permeable foliation turned eastward indicate that the melange has also been significantly deformed, forming the imbricated thrust layer.

According to the research by Wang, the Lower Carboniferous marble limestone in the west of Gaoligong Mountain and near Tuantian in the west of Longchuan River, is similar in lithology and biological characteristics to the Lower Carboniferous stratum in Baoshan Block, but obviously different from the Lower Carboniferous stratum in Tengchong Landmass, which may be the tectonic window exposed by Gaoligong giant nappe tectonics. It can be inferred that most of the foreland basins on the west side of Baoshan Block are pressed under the metamorphic rock series of Gaoligong Mountain, while the Nujiang River Junction Zone should be in the west of Tuantian, which was overthrust eastward by at least 20 km in Paleogene.

2.2.9.2.2 Boxoila Ling-Gaoligong Magmatic Arc

The magmatic arc is based on the continental crust. Except for the Precambrian Gaoligong Group and Guqin Group, the Lower Ordovician limestone under the overlying strata is only exposed in Guqin in the northern member, and Late Paleozoic strata constitute the surrounding rock invaded by magmatic rocks.

Yingjiang Area in the southern member of Devonian stratum was a fluvial-littoral sandy sediment in Early Devonian, with slow ascending and descending, a small sediment range and a thickness of 62 m. Besides marine organisms, there were terrestrial organisms such as heterostraci, charophytes and ostracods. In Middle Devonian and Late Devonian, the subsidence and transgression expanded, and aleuropelitic clastic rocks and carbonate rocks were deposited. The northern area of Basu-Laigu was the coastal detrital sediment, basically forming a stable carbonate plateau. The semi-pelagic slope zone was developed on the west side of Nujiang River in the north member. Sediments were mainly composed of terrigenous sand, argillaceous strip and layered limestone and argillaceous limestone, and a large number of slumping tectonics and flexible folding layers were developed, reflecting the deepening of seawater from south to north. Nujiang River was the oceanward side.

On the northeast side of Bomi-Tengchong Ancient Land, significant transgression occurred since Early Carboniferous, and on the basis of carbonate plateau, the stratum developed into semi-pelagic to abyssal sediments. From southwest to northeast, the lithofacies zone can be divided into littoral-neritic zone in the outer margin of Chayu-Tengchong Ancient Land (the sediments are composed of sandstone, siltstone, argillaceous ribbon limestone and bioclastic limestone); semi-pelagic zone in Songzong, Laigu, Ranwu and Ridong (slump breccia limestone, terrigenous and volcanic fine turbidite and intermediate-basic volcanic rock sedimentary assemblage are developed). This zone can be divided into two sub-facies zones: the upper slope sub-facies zone and the lower slope sub-facies zone. The former zone is developed in Ranwu and other places, and the sediments are composed of limestone with eye ball-shaped siliceous structure, nodular limestone, fine lamellar limestone, calcareous siltstone, shale, and a large number of slump breccia limestone, with common horizontal bedding, convolute bedding, large cutting bedding, slumping and buckling deformation bedding, etc. The latter zone is developed on the east side of upper slope collapse breccia, and can be found in Zhongba, Ranwu and other places. The sediment is a sequence of fine terrigenous and volcanic turbidite sedimentary assemblage, and composed of fine sandstone, siltstone and siliceous mudstone, with Bouma Sequence, and generally layers a, c and e can be seen. There are a large number of volcanic materials, and the thickness of the low-density turbidite series is more than 1000 m, including basalt, andesite basalt and volcanic breccia. According to the petrochemistry analysis, the volcanic rocks of Early Carboniferous Nuocuo Formation in Bomi-Parlung Zangbo are formed in the continental margin rift, which may represent an arc-basin.

Significant climatic and environmental changes occurred in this zone during Middle-Late Carboniferous to Early Permian, and littoral-neritic gravel sandstone and sand-slate sediments developed in this zone. As in Baoshan, glacial marine sediments can be found in this zone.

The fluvial-littoral zone in Bomi, Ranwu and Tengchongin on the south side: The sediments are composed of grayish green gravel sandstone, feldspathic quartz sandstone, gravel slate, siltstone and shale. Sedimentary facies sequence of barrier island sand bank-tidal flat-estuary sand bank developed from bottom to top. From southwest to northeast, the neritic zone is distributed in Basu-Ridong, and the sediments are composed of gravel slate, siltstone, shale and marl. Gravel components of gravel sandstone and gravel slate in this zone include limestone, sandstone, vein quartz, granite, gneiss, etc., and all of them are near-shore glacier debris flows.

The Paleo-Tethyan Ocean Basin represented by Zhayu-Bitu Nujiang River Zone is located in the further northeast direction. Boxoila Ling-Gaoligong Magmatic Arc is dominated by Bomi-Tengchong Granite Zone, and the northern member of which is longer than 100 km is mainly composed of Zhaxi, Bomi, Zhongba and Dedanla Batholith. This arc is mainly composed of granodiorite, quartz diorite and monzonitic granite, invaded in Carboniferous-Permian strata, and distributed in the north of Parlung Zangbo-Guqin Fracture Zone (the arc-arc collision junction zone can be found in Parlung Zangbo) and the south of Bangong Lake-Nujiang River Zone.

Zhaxize batholith was formed in 108–127 Ma, and Dedanla batholith was formed in 120–128 Ma. The granites in later period are S-shaped and formed in 78–85 Ma. According to the petrochemical composition and digital diagram of the main rock mass, the rocks are mainly the subduction Type I granite and mainly formed in the Early Cretaceous. The southern member is located in eastern Tengchong and intrudes into Gaoligong Group along the granite batholith of Gaoligong Mountain. The U–Pb age of zircon in Naiwang rock mass is 249–258 Ma, and the whole-rock Rb–Sr isochron age of Menglian rock mass is (354.2 ± 154.2) Ma, which was caused by Carboniferous-Permian magma emplacement. The rock assemblage of Late Paleozoic granite is mainly monzonitic granite, followed by granodiorite. The assemblage belongs to the calc-alkaline rock according to the lithochemistry. During the field investigation in Tengchong-Lianghe in 1986, the researchers found rhyolite, crystal tuff and volcanic breccia in Menghong Group of Early Carboniferous, which constituted the host rock of the tin deposit on Siguangping stratum, and a large area of intermediate-basic and intermediate-acidic volcanic lava flow and eruption in Early Carboniferous in Ranwu in the north, which reflected that the Early Carboniferous-Permian Tethyan Ocean had subducted to the southwest to form continental magmatic arc. The Mesozoic rock masses in the west of the watershed of Gaoligong Mountain are widely distributed in the form of rock stocks and small batholith, mainly formed in Middle Jurassic-Late Jurassic (140–190 Ma) and Late Cretaceous (70–85 Ma). The west part was formed later than the east part.

The Middle-Late Jurassic granite was caused by collision orogeny, and the Late Cretaceous granite was caused by the post-orogenic spreading.

2.2.9.2.3 Parlung Zangbo Arc-Arc Collision Zone

This zone was found during the 1:250,000 regional survey in Bomi-Medog in recent years. It extends westward and has formed Yongzhu-Namu Lake-Jiali Ophiolite Melange Zone, which is an important tectonic zone between the Gangdise-Motuo-Chayu Magmatic Arc in the south and Boxoila Ling-Gaoligong Magmatic Arc. To the southeast, this zone is covered by the Dulong River-Nmai Hka-Longchuan River thrust nappe fracture zone dipping westward, and is not researched fully. No remnants of ophiolite melange have been found so far.

The ophiolite melange zone is exposed in Chongba Highland, Yabagou and Zhongkang in Bomi, such as olivine pyroxenite, amphibole pyroxenite, gabbro, gabbro-diabase, meta-basalt, quartzite, marble, siliceous rocks and its matrix greenschist mica quartz schist and albite actinolite schist, which are blocks with different sizes and shapes, and are emplaced and wrapped by granites after Cretaceous. The Rb–Sr age of the gabbro-diabase is (215 ± 63) Ma (on a scale of 1∶200,000 in Bomi, 1995).

The granites on the north side of this zone are all Type I granites with island arc characteristics, and their Rb–Sr ages are 227–210 Ma and (195.3 ± 7.0) Ma (Changba biotite granodiorite in Yongsong). This zone was formed in Triassic to Early Jurassic. The granitic pluton and the contact zone with surrounding rock are significantly deformed, and the ductile shear zone is developed. The south of this zone is dominated by S-type granite, which was mainly formed in the Cenozoic. The Upper Permian, Triassic and Lower Jurassic strata have not been found in Bomi-Ranwu, and there is angular unconformity of Mariposa Formation of Middle Jurassic on the underlying strata on both sides of the ophiolite melange zone. Therefore, it can be speculated that the residual ophiolite melange in Parlung Zangbo developed on the basis of the inter-arc-basin. The oceanic crust has appeared at least in the Late Triassic, and the arc-arc collision and destruction occurred at the end of Late Triassic and Early Jurassic.

2.3 Geological Evolution of Sanjiang Region and the Adjacent Tethys Multi-Arc-Basin-Terrane (MABT)

2.3.1 Evolution and Tectonic Framework of Sanjiang Multi-Arc-Basin-Terrane (MABT)

To study the tectonic evolution and mineralization of Yidun Island Arc, it is necessary to understand the evolution of the Sanjiang MABT. The evolution process of Sanjiang Tethys tectonic domain includes the formation and evolution of Sanjiang MABT, and this tectonic domain was formed due to the break-up → splicing → splitting of the Pangea. Its evolution includes 6 stages (Li et al. 1999a, b).

2.3.1.1 Evolution of Sanjiang Tethys Tectonic Domain

2.3.1.1.1 Destruction Stage of Pangea and Formation Stage of Proto-Tethys

In the Late Neoproterozoic, the global Pangea (Rodinia Supercontinent) was destroyed, forming the Gondwana Continental Group in the south, the Laurasia Continental Group in the north and the Pan-Huaxia Continental Group in the middle. The Proto-Tethyan Ocean lies between both continental groups, and the landmasses of the Pan-Huaxia Continental Group are scattered in the Proto-Tethyan Ocean, as shown in Fig. 2.2.

2.3.1.1.2 Formation Stage of Pan-Huaxia Continental Group

At the end of Early Paleozoic, the North China Landmass, Tarim Landmass, Yangtze Landmass, Cathaysia Landmass and the small ocean basins in their intervening areas closed, which made the landmasses of Pan-Huaxia Continental Group once separated combine to form the Pan-Huaxia Continent.

2.3.1.1.3 Formation and Evolution of Paleo-Tethys

In Late Paleozoic, Qiangbei, Zhongza, Qamdo-Pu’er and other blocks split off from the western edge of Pan-Huaxia Continent, forming the Jinsha River-Ailaoshan Ocean, Lancang River and Ganzi-Litang Ocean of the Paleo-Tethys, and then developed into the four ocean basins of Sanjiang Paleo-Tethys together with the continued Changning-Menglian Ocean, and forming the tectonic framework of MABT with the blocks, island arcs and ancient volcanic island chains. In Zhenyuan (Laowangzhai) Gold Deposit and its south area, Devonian carbonaceous, argillaceous and radiolarian siliceous rocks are developed, which indicates that the Jinsha River-Ailaoshan Ocean began to rift in the early period of Devonian.

2.3.1.1.4 Ocean-Land Transition of Paleo-Tethys

From the end of Late Paleozoic to the beginning of Mesozoic, the oceanic crust of ocean basins of Pan-Huaxia Continent dived either in two directions or in single direction, which made Changning-Menglian Ocean, Lancang Ocean, Jinsha River-Ailaoshan Ocean and Ganzi-Litang Ocean destructed synchronously at the end of Late Triassic, thus the Paleo-Tethyan Ocean has gradually closed from west to east, and the intracontinental convergence and orogeny of Sanjiang Region started. Subsequently, the inherited evolution of Bangong Lake-Nujiang Ocean in Late Triassic occurred, and the Neo-Tethyan Ocean in Yarlung Zangbo River was formed and ran through the whole Gondwana and Eurasia.

2.3.1.1.5 Closure of Neo-Tethyan Ocean and Intracontinental Convergence

With the complete disintegration of Gondwana and the formation and expansion of the Indian Ocean, the Gangdise Block and India Landmass moved northward, and Bangong Lake-Nujiang Ocean and Yarlung Zangbo River-Neo-Tethyan Ocean were completely closed at the end of Early Cretaceous and Late Cretaceous-Eocene respectively. Thus, the intracontinental convergence of Sanjiang Region and Qinghai-Tibet Plateau started.

2.3.1.1.6 Transformation and Orogeny of Cenozoic Sanjiang Intracontinental Tectonics

Due to the oblique collision between Indian Plate and Eurasian Plate in Sanjiang Region and the tectonic balance and stress release, large strike-slip, shear and nappe occurred in Sanjiang Region, resulting in a series of tectonic–magmatic-metamorphic zones, and the intracontinental tectonic transformation. The orogeny of the Sanjiang Region fully started since Cenozoic.

2.3.1.2 Structural Pattern of Sanjiang Region

According to the study of ophiolite melange zone, arc-basin system and its evolution of Sanjiang Region, the slicing inlaying of four major arc-land collision junction zones in Sanjiang Region, namely Ganzi-Litang, Jinsha River-Ailaoshan, Lancang River, and Nujiang River-Changning-Menglian, and their blocks, such as Zhongza-Shangri-La, Qamdo-Lanping, Lincang, Baoshan and Chayu-Gaoligong, can be determined. The geotectonics of Sanjiang Orogenic Belt and the basic pattern of metallogenic zone were formed, which were constructed at the waist, spread at both ends, and twisted in reverse “S” shape from east to west (Fig. 2.29).

Fig. 2.29

Geotectonic framework and mineral distribution of Sanjiang Region

Yidun Arc-Basin System, Zhongza-Shangri-La Block, Jinsha River Arc-Basin System, Ailaoshan Arc-Basin System, Qamdo-Lanping Block, Tenasserim Arc-Basin System, Baoshan Block and Boxoila Ling-Gaoligong Arc-Basin System form the Sanjiang Tethys MABT. Yidun Island Arc is located in the east of Sanjiang MABT and the west side of Ganzi-Litang ophiolite Melange Zone on the southwest margin of Yangtze Landmass.

2.3.2 Geological Evolution of Sanjiang Region and the Adjacent Tethys Multi-Arc-Basin-Terrane (MABT)

Scientists of stratigraphic paleontology, tectonics, petrology, geochemistry, sedimentology, geophysics and other disciplines have successively discovered more than 20 ophiolite melange zones in different periods and collision patterns on Qinghai-Tibet Plateau (including Sanjiang Region), and their subduction patterns are not always northward. The spreading ocean basin restored by the ophiolite melange zones does not have a specific development trending from north to south; a series of island arcs, continental margin arcs or residual arcs in different periods developed on one or both sides of the adjacent subduction collision zone; the sedimentary basins with different sizes and different tectonic environments can be found. Different tectonic regions have different crustal structures and lithospheric structures, as well as various stratigraphical interfaces at different depths. The new geological events have challenged the Tethys evolution of “conveyor belt” and “accordion” which had the greatest influence in the early period, and most of the basin prototypes represented by more than 20 ophiolite melange zones in Qinghai-Tibet Plateau only have the characteristics of “small ocean basin”, back-arc-basin and island arc marginal sea. According to the rock assemblages of different geological units in Qinghai-Tibet Plateau, the space–time structure of arc-basin system and the basic characteristics of the tectonic evolution of MABT in Southeast Asia, the evolution of Sanjiang Tethys MABT can be put into the geological evolution framework of the whole Qinghai-Tibet Plateau and its adjacent areas (Figs. 2.30 and 2.31).

Fig. 2.30

Distribution of archipelagic arc-basin tectonics in Qinghai-Tibet Plateau and its Adjacent areas. I—Pan-Cathaysian continent early Paleozoic arc-basin zone: I₁—Kunlun frontal arc of Pan-Cathaysian continent, I₂—Paleozoic subduction complex zone at the south margin of Kunlun Frontal arc, I₃—Late Paleozoic-Triassic volcanic-magmatic arc and fore-arc accretionary wedge of Burhan Budai-Ela mountain, I₄—Qaidam late Paleozoic residual back-arc-basin, I₅—Ordovician subduction zone at the north margin of Qaidam, I₆—Central Qilian-Datongshan Mesozoic residual Island arc, I₇—North Qilian back-arc ocean basin subduction zone, I₈—Corridor Island chain zone, I₉—Tarim late Paleozoic residual back-arc-basin; II—Pan-Cathaysian continent late Paleozoic-early Mesozoic arc-basin zone: II₁—Late Paleozoic Island chain frontal arc (Neoproterozoic-early Paleozoic accretionary wedge metamorphic basement of Pan-Cathaysian continent), II₁—Lumajiangdong Cuo deformed metamorphic unit (P, volcanic island chain), II₁—Central Qiangtang Island chain, II₁—Tanggula deformed metamorphic unit, II₁—Leiwuqi Island arc, II₁—Lincang Island arc (with accretionary wedge of Lancang group in the east side), II₂—Northern Qiangtan late triassic back-arc-basin, II₃—South Qiangtang foreland basin (J), II₄—Permian–Triassic composite back-arc-basin (with Kaixinling-Zadoi Island arc at the west side), II₄—Qamdo back-arc foreland basin (J-K), II₅—Lanping-Pu’er Late Paleozoic-triassic back-arc-basin, II₅—South Lancang river late Paleozoic-triassic volcanic-magmatic arc, II₆—Mojiang-Lvchun river permian–triassic volcanic-magmatic arc, II₆—Jiangda-Deqin Permian–Triassic volcanic-magmatic arc, II₇—Zhongza Paleozoic plateau (with Jinsha river junction zone at the west side), II₈—Yidun late Triassic Island arc, II₈—Samatuojiari late Triassic volcanic arc and Hoh Xil subduction complex in the north side, II₈—Ganzi-Litang junction zone, II₉—Yajiang late Triassic residual basin, II₉—Xianshuihe Melange zone, II₁₀—Early Paleozoic coastal mountain chain marginal basin at the west margin of Yangtze, II₁₀—Bayankala late Paleozoic-middle triassic back-arc-basin (T₂–T₃ indicate foreland basins); II₁₁—Xiqing mountain late Paleozoic Plateau; II₁₂—Gonghe triassic residual basin, II₁₂—Animaqin snow mountain Melange zone, II₁₃—Gemia early Paleozoic accretionary wedge, II₁₄—Ailaoshan junction zone, II₁₅—Indosinian microblock; III—Late Paleozoic–Mesozoic arc-basin zone at the north margin of Gondwana: III₁—Sibumasu microblock (with Changning-Menglian junction zone at the east side), III₂—Gaoligong mountain late Paleozoic frontal arc, III₃—Lhasa-Bomi-Chayu Mesozoic-Cenozoic volcanic-magmatic arc, III₄—Jiayu bridge late Paleozoic deformed metamorphic unit (with Jiayu Bridge-Zhayu-Bitu junction zone at the east side), III₅—Naqu Jurassic back-arc-basin, III₆—Nierong Paleozoic deformed metamorphic unit (with Anduo-Dingqing junction zone at the north side), III₇—Zenong-Wenbu early cretaceous volcanic-magmatic arc (with Guomangco-Namucuo Melange zone at the north side), III₈—Tsochen-Nyenchen Tanglha early Permian-Mesozoic Island chain, III₉—Gangdise Cretaceous-Paleogene volcanic-magmatic arc, III₁₀—Kongbogangri Cretaceous-Paleogene volcanic-magmatic arc, III₁₁—Anglonggangri Jurassic-Cretaceous magmatic arc (with Pangong Lake-Dongqiao junction zone at the north side, and Shiquan river Melange zone at the south side), III₁₂—Zhongba late Paleozoic Plateau (with Pulan Melange zone at the south side, and Gongzhucuo Melange zone at the north side), III₁₃—Laguigangri Metamorphic core complex zone, III₁₄—Continental margin splitting accretionary wedge (with Yarlung Zangbo river junction zone at the north side), III₁₅—High himalayan overthrust zone, III₁₆—Low himalayan overthrust zone, III₁₇—Burma central lowland volcanic arc, III₁₈—Najia-Arakan front accretionary wedge

Fig. 2.31

Schematic diagram of tectonics evolution of MABT in Qinghai-Tibet Plateau and its Adjacent areas. KL—Kunlun; CT—Qaidam; QL—Qilian mountain; QT—Qiangtang; ZQ—Central Qilian; NQ—North Qilian; NC—North China; SC—South China; GD—Gangdise; JS—Jinsha river; YZ—Yangtze; ZZ—Zhongza; GL—Ganzi-Litang; LM—Longmen mountain; LS—Lhasa; HM—Himalaya; IN—India; BG—Bangor-Jiali zone; BN—Bangong Lake-Nujiang river; YL—Yarlung Zangbo; Type Y—Indonesia and Southeast Asia split type; Type J—Kuril-Japan Islands split type; Type A—Andes split type

2.3.2.1 Frontal Arc of Pan-Huaxia Continental Group and Early Paleozoic Qinling-Qilian-Kunlun Multi-Arc-Basin-Terrane (MABT)

In recent years, Chinese geologists have made great progress in the research of complex tectonic domain composed of Kunlun Island Arc Orogenic Belt, Sanjiang residual island arcs, blocks and back-arc-basin oceanic crust subduction zone on Qinghai-Tibet Plateau, which confirmed the remains of Paleo-Tethyan Oceanic tectonic-rock assemblage. They also discovered the Early Paleozoic oceanic crust subduction and the corresponding active marginal rock assemblage remnants, which are called the Proto-Tethyan Ocean (Liu et al. 1993). The Kunlun Volcanic-Magmatic Island Arc Orogenic Belt on the north side of Proto-Tethyan Ocean is the Early Paleozoic frontal arc of Pan-Huaxia Continent.

In the Kudi ophiolite in West Kunlun, the pillow lava is composed of oceanic tholeiite, with the Nd age of 600–900 Ma. The age of acid magmatic rocks invaded in pillow lava and volcanic rocks is 458–517 Ma (including the age of U–Pb, Rb–Sr, and ⁴⁰Ar/³⁹Ar). The Precambrian metamorphic complex on the south side of the ophiolite melange zone is the basement and intermediate-acid intrusive rocks of different ages (mainly the granodiorite, diorite and granitoid rocks, with the age of 540–400 Ma and 260–200 Ma), which may be island arc thrust schists napped southward. The island arc magmatic zone extends eastward to East Kunlun. On the Qinghai-Tibet Highway to the south of Golmud, Late Devonian continental volcanic lava can be found. The plant fossils can be found in clastic rocks. The North Kunlun Island Arc Magmatic Zone extends over 1000 km from east to west and remains intact, while the Nachitai Group to the south of the Kunzhong Fracture is called the Early Paleozoic eugeosyncline sediment or classified as Ordovician–Silurian turbidite. Some scholars found stromatolites in the marble of the opening of Wanbaogou, and named it Wanbaogou Group separately from Nachitai Group, but in essence, it is still a Neoproterozoic melange zone containing ophiolite. Some scholars named this group “Wanbaogou Group” because the marble in this group is 5 km long and hundreds of meters wide, and most of them are lens blocks in a green schist series of different sizes. Just like Nachitai Group, Wanbaogou Group is a rock assemblage mainly composed of pillow lavas, volcanic rocks and ophiolites. According to the author’s observation, the sedimentary rocks containing Ordovician–Silurian fossils are actually blocks in the melange. There are Carboniferous-Permian fossils in Xiaonanchuan, and Xiaonanchuan Group (Bureau of Geological Exploration & Development of Qinghai Province, 1991) was also separated from Nachitai Group. A typical distal turbidite outcrop was found along the road where the east and west beaches meet to the north, indicating that it was composed of the mylonite with significant tectonic shear. From this point of view, the so-called Wanbaogou Group, Nachitai Group and Xiaonanchuan Group on the south side of Kunlun Magmatic Arc do not belong to Smith strata except for some specific slices (blocks) which maintain sequences, but are the non-Smith subduction complex zone which continuously subducts northward from Proto-Tethyan Ocean to Paleo-Tethyan Ocean (possibly since Ordovician). This subduction zone extends eastward with the frontal arc and bends significantly northward in Kuhai. The destruction of the Paleo-Tethyan Ocean Arc-Basin System can be found in the Triassic volcanic arc and fore-arc subduction zone in Burhan Budai-Ela Mountain.

On the north side of Paleozoic frontal arc in Kunlun, the geological history in Early Paleozoic in Tarim, Qaidam, Qimantag, Altyn Tagh and Qilian Mountains mainly includes back-arc sea floor spreading, back-arc-basin shrinkage, arc-arc collision and arc-land collision. Most of this zone has been transformed into land in Devonian, forming a part of the southwest margin of the North China Landmass of the Pan-Huaxia Continental Group. There are Carboniferous relic back-arc-basins only in Zongwulong Mountain, and Tarim and Qaidam are the largest ones. There are always different opinions on the formation environment and tectonic properties of marine volcanic rocks and ophiolite in Early Paleozoic in the Altyn Tagh-Qilian Mountain. Many scholars have clearly demonstrated the ocean ridge basalt in Qilian Mountain, but there are different opinions on subduction to the north or south. Thought that the North Qilian Mountain was a typical trench-arc-basin system in the Paleozoic active continental margin, and the subduction zone was composed of ophiolite and melange (including high-pressure and low-temperature blueschist and eclogite). Lai identified volcanic rock assemblages with different tectonic backgrounds, such as oceanic ridge, oceanic island (seamount) and island arc, in Qilian Mountain (including the northern margin of Qaidam) by geochemistry methods, and considered that there were mainly three oceanic ridge (oceanic island) volcanic zones and associated arc volcanic rocks. The assemblages were separated by Proterozoic crystalline basement, and constituted three independent Ordovician ocean basins in North Qilian, among which Yushigou-Dakecha Ocean Basin spread in Early Ordovician, while Sunan-Yongdeng Ocean Basin and Zhangye-Jingtai Ocean Basin spread mainly in Middle-Late Ordovician, and the widths of the three ocean basins were estimated to be 2400 km, 600 km and 640 km respectively.

The basic volcanic rocks of Xitie Mountain-Lvliang Mountain-Saishiteng Mountain in the northern margin of Qaidam, which were formed almost at the same time as the North Qilian, feature rock assemblages with high TiO₂ content and low K₂O content, and are similar to ocean ridges, while the intermediate-acid volcanic rocks mainly show the evolution trend of island arc calc-alkaline volcanic rocks, and the ocean basin in the northern margin of Qaidam is 1000 km.

Three typical ocean floor spreading basins in Qilian Mountain, including Hongliugou, Altyn Tagh-Lapeiquan Sea Floor Spreading Basin, Apa-Mangya Sea Floor Spreading Basin, the ocean basin at the northern margin of Qaidam and Qimantag Rift Basin, are all a series of back-arc-basins behind Kunlun Frontal Arc (Fig. 2.29). The tectonic paleogeographic pattern in Early Paleozoic (especially in the O-S₂ period) can be compared with Indonesia Island Arc and the archipelagic arc-basin pattern in the north of the arc. The formation of these basins was restricted by the two-way subduction of Proto-Tethyan Ocean and Paleo-Asian Ocean, similar to that of the MABT in Southeast Asia controlled by the two-way subduction of the Indian Ocean and Pacific Ocean. Tuolainanshan-Datongshan Micro-landmass in Central Qilian is similar to the Kalimantan Island Landmass in Southeast Asia. The volcanic rocks in different tectonic settings in Qilian Mountain are closely associated in space. The arc volcanic rocks with different maturity are produced in the same tectonic zone, and the melange zone composed of arc-basin subduction complex and ophiolite tectonic slice is the tectonic contact boundary, which are the main signs to identify the arc-arc collision in Qilian Mountain. In a word, in Qinling-Qilian-Kunlun Mountains, the ocean basins are mostly developed in Late Cambrian-Ordovician, and they are few small ocean basins and back-arc ocean basins. The volcanic-magmatic arcs were developed in Late Ordovician–Silurian, and the configuration and development of arc-basin systems are similar to those in Southeast Asia.

2.3.2.2 Late Paleozoic Qiangtang-Sanjiang Multi-Arc-Basin-Terrane (MABT) on the Southwest Margin of Pan-Huaxia Continental Group

The evolution of Tethyan Ocean and Pan-Huaxia Continent from Late Paleozoic to Mesozoic was concentrated in Qiangtang-Sanjiang Region. The available data show that the Kunlun Island Arc and Longmen Mountain-Kangdian are the Early Paleozoic coastal mountains in the southwest of Pan-Huaxia Continent. There are two types of basements in this mountain, the land side is composed of the pre-Sinian crystalline hard basement, and the outer side is mainly composed of the metamorphic soft basement of accretionary wedge in Neoproterozoic-Early Paleozoic Continental Margin. The soft basement is covered with Late Paleozoic overlying strata, which is in extensional or angular unconformity. In the western margin of the Yangtze Landmass, the Ordovician–Silurian stratum is a sequence of turbidite dominated by clastic rocks. Lower Ordovician pillow basalts can be found in Muli, and the Ordovician–Silurian stratum in Diancang Mountain-Ailaoshan to the south is composed of continental margin turbidites. Along the southeast margin of Yangtze Landmass, the Sinian-Ordovician stratum is also composed of siliceous stucco turbidite on the continental slope. Significantly, on both sides of Jinsha River, that is, in Haitong-Qingnidong of Qamdo Block on the west side, the Devonian stratum is in unconformity contact with the underlying Ordovician flysch turbidite. In Yidun-Baiyu of Zhongza Residual Island Arc on the east side, the plateau-type Late Paleozoic overlying strata and the Early Paleozoic strata with underlying metabasite basalt, volcanic breccia, trachyte and intermediate-acid volcanic rocks also show different deformation and metamorphism. A sequence of calc-alkaline island arc volcanic rocks and Type I tonalite are developed in the Lower Paleozoic (possibly including Precambrian) Boluo Group on the east side of Qamdo Block. In the ophiolite melange zone of Gemia, Vietnam (northeast of Indosinian Block), the Early Paleozoic stratum is composed of tuffaceous green schist, which was covered by Devonian stratigraphic unconformity. However, Kaixinling-Wuli in the northwest extension of Qamdo Block is an island arc system based on continental crust, although only the rock assemblage of seamount-ocean island-island arc of Early Permian is exposed.

In particular, in the west area of Pu’er-Lanping-Qamdo-Kaixinling, a sequence of metamorphic rocks of Lower Paleozoic Lancang Group, Chongshan Group, Precambrian Jitang Group and Lower Paleozoic Youxi Group are also exposed. The ⁴⁰Ar/³⁹Ar age of blueschist in Neoproterozoic-Lower Paleozoic metamorphic basic volcanic rocks of Lancang Group is 410 Ma, and the age of Lincang batholith on the east side of Lancang Group is 433–422 Ma (Zhang et al. 1990). Jitang Group is composed of intermediate-acid volcanic rocks mixed with fine debris and limestone, while Youxi Group is composed of metamorphic glutenite, sandstone and volcanic rocks with low greenschist facies, including chlorite albite schist, two-mica quartz schist and chlorite quartz schist (Rb–Sr isochron metamorphic age is 371 Ma ± 50 Ma) of basalt and dacite, showing the geochemical characteristics of island arc environment in Early Paleozoic. The deformed metamorphic basement in North Qiangtang is called Amugang Group, and the horizon of the lower gneiss is equivalent to that of Jitang Group. The middle and upper green schist, mica quartz schist, siliceous rock and volcanic rock members can all be compared with those of Youxi Group. The overlying strata are all stable cap rocks of Late Paleozoic, and the biological features are mainly warm-water organisms.

The above data show that ① The overlying strata of the stripped block group of North Qiangtang-Kaixinling-Qamdo-Lanping-Pu’er are the stable Devonian-Carboniferous stratum, and its underlying sedimentary strata are accretionary wedge-type greenschist metamorphic complex; ② Metamorphic complexes of Amugang Group in Qiangtang, Youxi Group in the west of Qamdo, Chongshan Group in the west of Lanping and Lancang Group in the west of Pu’er are all Precambrian-Early Paleozoic rock units, mainly the volcanic-magmatic arc formed by the subduction of Tethyan Ocean to the northeast (present position) and the remnants of accretionary wedge under the island arc. The Early Ordovician-Middle Ordovician passive marginal flysch sediments seen in Qingnidong-Haitong are the residual sediments after the expansion of Early Paleozoic back-arc-basin. ③ This continental crust strip may have started in Early Devonian and split off from the Early Paleozoic coastal mountains in the southwest of Pan-Huaxia Continent in the form of Japan-Ryukyu Islands back-arc expansion; ④ Following the idea of subduction and “splitting”, the history of archipelagic arc orogeny of back-arc expansion, arc-arc collision, and arc-land collision from the south of Kunlun and the west margin of Yangtze to the north of Tibet - Sanjiang Region in Late Paleozoic-Triassic can be found, rather than the so-called “opening-closing” evolution of the subduction and destruction of Proto-Tethyan Ocean and the opening of Proto-Tethyan Ocean. ⑤ Tenasserim Frontal Arc (including Norther Qiangtang Residual Arc, Jitang Group Residual Arc, Chongshan Residual Arc and Lancang Residual Arc) is the frontal arc of the archipelagic pattern on the southwestern margin of Pan-Huaxia Continent in Late Paleozoic, and the southwest side of the frontal arc is the Tethyan Ocean during the shrinking.

There are many arguments about Paleo-Tethys. Some scholars proposed that Late Paleozoic Tethys was developed on the basis of the expansion of the closed back-arc-basin of Proto-Tethyan Ocean; some scholars believed that the Paleo-Tethys was a new ocean basin formed by the drag and spreading of the subduction plate (i.e., the passive continental margin side), featuring the pattern of archipelagic ocean, which was composed of Yangtze affiliated block and Gondwana affiliated block group and the ocean basin between them; some believed that the Paleo-Tethyan Jinsha River-Ailaoshan Ocean might be the result of stretching and splitting on the basis of the closed residual sea or foreland depression of Proto-Tethys, while Paleo-Tethyan Lancang Ocean was formed by the spreading on the basis of Changning-Menglian Back-arc-Basin formed by the subduction of Proto-Tethyan Ocean. Based on years of research, the author believes that Late Paleozoic Paleo-Tethys is the inheritance and development of Proto-Tethyan Ocean, or it can be regarded as a residual ocean. The Late Paleozoic-Triassic island arc, back-arc-basin, margin magmatic arc and inter-arc-basin indicate that the lithosphere of Proto-Tethyan Ocean began to shrink since Devonian.

The ophiolite melange zones in Changning-Menglian, Western Yunnan and Bitu-Zhayu, Eastern Tibet, located between the Indosinian-Qamdo Block in Yangtze affiliated landmass and Sibumasu-Baoshan Block in Gondwana affiliated landmass, is a relic of the Paleo-Tethyan Ocean, which is composed of siliceous distal turbidite and radiolarian siliceous rocks in Devonian-Permian. The Late Paleozoic Paleo-Tethyan Ocean Crust subducted eastward, forming Lincang-Menghai Magmatic Arc, South Lancang River Arc-Basin, Late Paleozoic-Triassic volcanic-magmatic arc on the east side of Lancang River, Pu’er Late Paleozoic-Triassic Back-arc-Basin (converted into back-arc foreland basin from Late Triassic to Cretaceous), Mojiang-Lvchun Permian–Triassic Volcanic-Magmatic Arc and Ailaoshan Back-arc Spreading Ocean Basin in the southern member (Figs. 2.32 and 2.33). It should be noted that in addition to the spreading ridge tholeiite, a small amount of high alkalinity andesite basalt, Early Paleozoic subduction complex (ultramafic rock with a Rb–Sr age of 418 Ma) and residual blocks of remnant arcs can be found in the ophiolite melange zones caused by Late Paleozoic oceanic crust subduction in Ailaoshan. Late Paleozoic Tethyan Ocean subducted eastward to form Kaixinling-Zadoi Permian Volcanic Arc, Qamdo-Markam Permian–Triassic Back-arc-Basin (transformed into back-arc foreland basin in Jurassic-Cretaceous), Jiangda-Weixi Late Permian-Early and Middle Triassic Volcanic-Magmatic Arc, Jinsha River Back-arc Spreading Ocean Basin, Baiyu-Shangri-La Residual Island Arc Block and Late Triassic Volcanic Arc, Keludong-Xiangcheng Late Triassic Inter-arc-Basin, Que’er Mountain Late Triassic Magmatic Arc, Ganzi-Litang Late Permian-Early and Middle Triassic Subduction Margin Rift Ocean Basin, Yajiang Residual Basin and South Qinling Back-arc Ocean Basin.

Fig. 2.32

Schematic diagram of Sanjiang carboniferous-permian MABT paleotectonic plane

Fig. 2.33

Schematic diagram of Sanjiang carboniferous-permian MABT paleotectonic profile

The latest geological survey in Qiangtang, North Tibet shows that ophiolite melange with blueschist and greenschist as matrix has been found along Gangmacuo, Guoganjianian Mountain, Mayigangri, Jiaomuri, Qiagangcuo and Shuanghu Lake, with an extension of 350 km from east to west. The 40Ar/39Ar age of the blueschist is 222.5 Ma ± 3.7 Ma (Li et al. 1995), and the Middle Triassic radiolarian siliceous rocks covered by Late Triassic strata unconformity can be found in the melange zone. These records show that there is also an MABT based on a residual arc in the north side of the Paleo-Tethyan Ocean.

Arc-arc collision and arc-land convergence of East Tethyan Pan-Huaxia Continent mainly occurred in the Triassic. The oblique wedging of the Yangtze Landmass to the west was caused by the arc-arc collision and arc-land collision between Danbang Block (Hanbao Block) of Gondwana affiliated landmass and the split frontal arc of Yangtze Landmass as well as Indosinian, Qamdo and Zhongza blocks, including the shrinking and subduction of the back-arc-basin of the expanding ocean basin. The subduction polarity was mostly opposite to that of Paleo-Tethyan Ocean. Jinsha River experienced forward (westward) subduction collision (P₁-P₂) → left oblique subduction collision (T_1–2) and subsequent collision. This process is also manifested in the fact that in the western margin of Yangtze Landmass, Ailaoshan Back-arc Ocean Basin was destroyed at the end of the Middle Triassic, and the ocean basin developed northward in East Sichuan and East Tibet was destroyed in the Late Triassic. Correspondingly, in the northern margin of Yangtze Landmass, there is an oblique continuous collision in Middle Triassic-Late Triassic-the end of Late Triassic from West Qinling to East Kunlun and Hoh Xil. The eastern margin of Bayankala Late Paleozoic Back-arc-Basin showed the characteristics of a foreland basin in Latin (T₂)-Late Triassic (Xikang Group), which proved that the Yangtze Ancient Land was caused by westward, oblique and two-way subduction, which resulted in the closure of Paleo-Tethyan Bayankala Ocean Basin.

In a word, in Qiangtang-Sanjiang Region, the ocean basins developed mainly in Carboniferous-Early Permian, and are not large, with a width of only thousand kilometers. The volcanic arcs developed in Late Permian-Early and Middle Triassic, and Tenasserim frontal arc is similar to Kuril-Japan-Ryukyu Islands volcanic arc.

2.3.2.3 Gondwana Frontal Arc and Mesozoic Gangdise-Himalayan Multi-Arc-Basin-Terrane (MABT)

In recent ten years, many scholars have carried out much fruitful research on Jiayu Bridge “metamorphic complex” and its south extension in Nujiang River Junction Zone. According to the 1∶200,000 regional geological mapping, it is considered that the Bitu Junction Zone in the south is connected with Changning-Menglian Junction Zone through the western slope of Meri Snow Mountain, while the main part of Jiayu Bridge Metamorphic Complex is considered as the northeast continental margin arc of Gondwana. When Xu et al. started their international expedition to Qinghai-Tibet, they considered that the Gangdise-Boxoila Ling-Gaoligong Mountain should be Gondwana Late Paleozoic–Mesozoic frontal arc, and that Nierong Uplift and Jiayu Bridge Metamorphic Complex are the remnants of the frontal arc. The Late Paleozoic–Mesozoic Tibet Islands are located in the south of this forward arc. During the field investigation, many renowned scholars, such as Xu Jinghua, agreed to see the Bangong Lake-Nujiang River Suture Zone as the northern boundary of Gondwana. The recent research on the differences of geological and geophysical characteristics between the north and south sides of Bangong Lake-Nujiang River Suture Zone further confirms the rationality of the northern boundary of Gondwana in the Bangong Lake-Nujiang Suture Zone.

In Bomi, Songzong, Laigu and Ridong of Gangdise, gravity flow slump breccia limestone, volcanic fine turbidite and intermediate-basic andesite basalt of Nuocuo Formation in Early Carboniferous, and Laigu Formation, dacite, rhyolite and pyroclastic rocks in Middle Carboniferous can be found. Given a large number of Late Carboniferous intermediate-basic volcanic rocks found in Angjie Formation in Cuoqin, this volcanic-sedimentary rock series with volcanic arc characteristics is located intermittently in Gangdise-Nyenchen Tanglha Block (Pan et al. 2004), which may indicate that the North Gondwana transformed into an active continental margin since Early Carboniferous. In Leqingla on the northern margin of Linzhou Basalt in Gangdise, the Middle Permian basalt in Luobadui Formation shows the geochemical characteristics of island arc volcanic rocks (Fig. 2.34); although the main body of Gangdise-Nyanchen Thanglha Mountain Chain lacks Triassic sedimentary records, Middle Triassic arc volcanic rocks and Late Triassic island arc granite have been found in Quesang Temple and Menba in Lhasa. On the west side of Nujiang in Bitu, East Tibet, Jurassic continental magmatic arc in Sanmen Village, volcanic rocks of the Late Triassic Xieba Arc and Late Triassic flysch sediments in Quehala Formation in Gula Fore-arc-Basin can be found; in Dazi, Lhasa, a sequence of low-grade metamorphic calc-alkaline arc volcanic rocks and pyroclastic rocks (Yeba Volcanic Arc) developed in Early-Middle Jurassic in Yeba Formation. The volcanic rocks have the geochemical characteristics of arc volcanic rocks (Fig. 2.34), spreading about 300 km from east to west, with a residual width of only 30 km; in the further south area, the calc-alkaline arc volcanic rocks of Sangri Group (J₃-K₁) on the north side of Yarlung Zangbo Suture Zone (Sangri Volcanic Arc) spread more than 400 km from east to west, and the residual width is only 20–40 km. Due to Shiquan River Ophiolite Melange Zone between Anglonggangri Accretionary Arc (J₃-K₁) and South Gangdise found and explored in Gangdise, and the Guomacuo-Namu Lake-Jiali-Bomi Ophiolite Melange Zone between Guopucuo-Wenbu Volcanic-Magmatic Arc (J₃-K₁) and Bangor-Sangxiong-Boxoila Ling Magmatic Arc (J₃-K₁) discovered in Gangdise Zone in recent years, it can be found that Gangdise is not only a continental volcanic arc formed in Late Cretaceous-Paleogene, but also a volcanic arc formed in Middle-Late Carboniferous, Permian, Triassic and Jurassic. Therefore, the new island arc volcanic rocks in different periods in Gangdise are not only caused by the northward subduction of Yarlung Zangbo Oceanic Crust, but also related to the southward subduction of Bangong Lake-Nujiang River Oceanic Crust (Tethyan Oceanic Crust), which represents the northern boundary of Gondwana in earlier periods (Middle-Late Carboniferous, Permian, Triassic and Jurassic). The identification of these arc-arc collision zones and volcanic arcs in different periods in Gangdise indicates that Gangdise-Nyanchen Thanglha is not a simple terrane, but a Gangdise archipelagic arc and its back-arc (inter-arc) basin.

Fig. 2.34

Permian-Eocene arc volcanic rocks in Gangdise (Wood 1980)

The ophiolite melange zone between Gangdise Mountains and Himalayas is regarded as the Yarlung Zangbo River Suture Zone, and both are classified into the same one. In the investigation in 1994, it was found that this suture zone diverged in the western part of Saga, and Devonian-Middle Triassic Zhongba Neritic Plateau was mixed between two ophiolite melange zones. Gansser (1974) believed that Pulan Ophiolite Zone in the south was napped over the ophiolite from Gongzhucuo Ophiolite on the north side of the plateau to the south during the northward subduction of the Indian Plate. Some researchers also think that the Paleozoic stratum is the residual of giant nappe. These explanations are unsubstantiated because these zones cross a branch of suture. If we study this paleogeographic pattern from the archipelagic arc, we may find that the Zhongba Paleozoic Plateau is the detachment block that separates Pulan and Gongzhucuo back-arc-basins. What’s more, Pulan and Gongzhucuo ophiolite melange is caused by the back-arc-basin shrinkage, arc-arc or arc-land collision at the end of Mesozoic. The ophiolite of Yarlung Zangbo River in Mesozoic is the best-preserved and most complete “trinity” assemblage of ophiolite in Qinghai-Tibet Plateau and even in the Chinese mainland, but its thickness is smaller than that of the ophiolite in West Tethyan Orogenic Belt and some major ocean basins, and it is presented by geological and geochemical characteristics of small ocean basins called by Xiao.

In a word, in the south of Bangong Lake-Nujiang River Suture Zone, the ocean basins developed in Late Triassic-Early Cretaceous, and they are all small ocean basins and back-arc ocean basins. Volcanic arcs developed in the Middle and Late Carboniferous-Eocene. The Gangdise Zone should be Middle-Late Carboniferous Andes active continental margin on the south side of Tethyan Ocean represented by Bangong Lake-Nujiang River Suture Zone. The small ocean basin with ophiolite assemblage interbedded is a series of “entangled” back-arc or inter-arc-basins induced by the southward subduction of Tethyan Ocean.