Tectonic Framework of Sanjiang Tethyan Metallogenic Domain

Abstract

The Sanjiang Tethyan Metallogenic Domain refers to the Hengduan Mountain Range where the Nujiang River, Lancang River and Jinsha River run side by side, spanning western Yunnan, western Sichuan, eastern Tibet and southern Qinghai, including the eastern Qinghai-Tibet Plateau and the western Yunnan-Guizhou Plateau, with an area of about 50 × 10⁴ km². The range belongs to the eastern part of the global Tethys-Himalayan Tectonic Domain structurally, which is in the region where Gondwana and Eurasia collide significantly, and it is also the intersection of Tethyan tectonic domain and Pacific Rim tectonic domain. Influenced by the action of the Indian Ocean Plate, Pacific Plate and Eurasian Plate, it has experienced the complicated process of Tethys formation and evolution, India-Eurasia collision and plateau uplift. Therefore, the Sanjiang geological structure is extremely complex and diverse, with very favorable metallogenic conditions formed. This range is one of the most important metallogenic prospects of nonferrous metals, precious metals and oil and gas in the world and also one of the most critical areas to study the global structure.

1.1 Global Tectonic Background of the Formation of Sanjiang Tethyan Tectonic Domain

1.1.1 Enlightenment from the Formation and Evolution of the Atlantic Ocean, Indian Ocean and Pacific Ocean

It is known that the Atlantic Ocean, Indian Ocean and Pacific Ocean are in different stages of tectonic evolution, with different manifestations and evolutionary characteristics. The Atlantic Ocean is still in the period of spreading, and the passive continental margins on both sides are entering the stage of mature development; the Indian Ocean has begun to subduct unidirectionally in both contraction and spreading, the North Pacific Ocean is in the two-way subduction, and the South Pacific is in the two-way subduction and spreading, but in general has been contracted. Their evolution characteristics are shown as follows.

1.1.1.1 Evolutionary Trend of Closing (Closure), Merging and Transferring of the Ocean Basin

The Tethys tectonics in Mesozoic developed along the boundary between Eurasia and Gondwana is called the Meso-Tethys by Huang and Chen (1987) and the Neo-Tethys by Stocklin (1974) and. Its southern branch—Zagros-Yarlung Zangbo River—has not been completely closed to now, except that the junction zone of India Landmass and Eurasia Landmass was closed at the end of Mesozoic and the beginning of Cenozoic, and there was a subduction zone in the Arabian Sea in the west of India Landmass. A 260 km wide Mokelam subduction accretionary complex system (including fore-arc carbonate rocks, heterogeneous melange and flysch (E₂–N₁)) was formed from southeast Iran to southwest Pakistan, and subduction continues to the present. In the Gulf of Oman, there are still residual oceans between the Arabian Landmass and the Eurasia Landmass (Boulin 1991). Therefore, the Arabian Sea and the Gulf of Oman are both part of Meso-Tethys. Moreover, due to the subduction of the spreading ridge in the Arabian Sea, the ocean basin has stopped spreading, and the oceanic crust of the remaining Arabian Plate has obviously been merged by the Indian Ocean. The subduction of the Indian oceanic crust and the Australian Plate to the Eurasian Plate along the Andaman Islands-Sumatra-Java-Timor Island in the eastern India Landmass is actually a continuation of the subduction of the Meso-Tethys oceanic crust, and there may also be the merger of the Meso-Tethys Ocean to the Indian Ocean (Fig. 1.1). When the Indian Ocean and the Gulf of Oman are closed, different ocean basins will be closed in the same closed zone. Due to the subduction of the ocean basin, some areas are closed by the oceanic volcanic arc and turned into marginal seas, such as the Philippine Sea. According to Dr. Chen of Taiwan Province, an early intra-oceanic initial island arc was found in Luzon Island. He believes that the South China Sea was originally connected with the Philippine Sea and belonged to the marginal sea of the Pacific Ocean, but then Luzon Island moved northward to separate the South China Sea from the Philippine Sea. Chimei volcanic arc along the coast of Taiwan Province is the northern extension of Luzon Island, and an oblique arc-land collision occurred along the longitudinal valley of Taiwan Province. The Tethys region in Mesozoic, distributed in the area of Lesser Caucasus, is not only a continuation of the Paleo-Tethys, but also merged into the new Tethys Ocean formed in Mesozoic to the west and may be connected with Bangong Lake-Nujiang River to the east. The southern part of the ancient Atlantic Ocean, which was not closed at the end of Early Paleozoic, may have merged into the Paleo-Tethyan Ocean in Late Paleozoic. After the closure of Paleo-Tethyan Ocean, a nearly east–west Hercynian Fold Zone was formed in the southeastern USA, which can be used as an ancient example of ocean basin merging and transformation.

Fig. 1.1

Main suture lines in central Asia and South Asia 1—Caledonian suture zone; 2—Hercynian suture zone; 3—Kimerich suture zone; 4—Alps suture zone; 5—Regional fracture

The above statement indicates that the development of an ocean basin has a long and complicated evolution of closing, merging, transforming, connecting the past with the future, which makes the formation and evolution of continental orogenic belts more complicated and changeable, and it is increasingly difficult to reproduce its tectonic evolution history.

1.1.1.2 Destruction of the Oceanic Plate Caused by Three Types of Subductions

The subduction of the Pacific Plate is not the whole subduction, but the subduction of a series of broken small plates, the subduction of the mid-ocean ridge and finally the subduction of residual ocean basins. For example, the eastern margin of the Pacific Ocean has been subducted by a series of small plates such as the Gorda Plate, Cocos Plate and Nazca Plate, as well as by mid-ocean ridges that have subducted under the North American Continent, and the North Pacific Ocean is no longer spreading. The different subduction (or local obduction) speed, angle and direction (vertical and oblique movement) of small plates have caused the different ocean basin closure time on the same tectonic zone or plate junction zone in the orogenic belt and may cause the segmentation, heterochrony and difference of magmatic activities and the characteristics of the same volcanic-magmatic arc zone, for example, the Andes Volcanic Arc. The volcanic-magmatic arc of Jiangda-Weixi-Lvchun Zone and the volcanic arc of Kaixinling-Nanzuo-South Lancang River Zone in the Sanjiang region of southwest China, which match the Jinsha River Zone and Lancang River Zone, respectively, have the above three characteristics (Liu et al. 1993; Mo et al. 1993). Whether these three characteristics of magmatic activities of the volcanic-magmatic arc can be used in the orogenic belt to invert whether there may be subduction of small plates, or whether it is related to the destruction of multiple back-arc ocean basins, is a topic to be studied in depth with reference to the characteristics of magmatic activities of volcanic-magmatic arc in the eastern and western Pacific Ocean.

1.1.1.3 Coexistence of Subduction, Obduction, Strike-Slip, Accretion and Tectonic Erosion

The oceanic plate is characterized by subduction, obduction, strike-slip, subduction accretion and subduction tectonic erosion. In addition to plate subduction, there are many oblique subductions of micro-landmasses or strike-slip displacement collages of island arc terrane on both sides of the eastern and western Pacific Ocean, such as collage of many island arc terranes in the western USA, and the aforementioned northward slip of Luzon Island. The trench-arc-basin system of the western Pacific Ocean is developed with subduction accretion, and the western North America on the east coast of eastern Pacific Ocean has subduction accretion, such as the Francesco Melange Zone, but no melange zone is found in western South America, which shows the tectonic erosion or cutting of the South American Continent by subduction. Therefore, in an orogenic belt, there is often an intermittent discontinuity in the plate junction zone or ophiolitic melange zone, and the interrupted part is either a tectonic erosion zone or a large strike-slip ductile shear zone.

1.1.1.4 Three Types of Orogenic Belts After Ocean Basin Closure

When a long-developed ocean (such as the Pacific Ocean) finally closes, it may be manifested as arc-arc collision or arc-land collision orogeny caused by subduction of back-arc-basins in the MABT on both sides of the ocean. Professor Xu et al (1994) put forward the back-arc-basin collision orogeny and arc-arc collision orogeny after observing and studying many orogenic belts in the world. But this is only suitable for inter-continental collision orogenic belt after the ocean closure with two-way subduction. For the ocean basin with one-way subduction, the orogeny after closure is the back-arc orogeny on one side of the active margin or the passive margin orogeny, forming a composite orogenic belt with arc-land collision. Many majestic mountains are often formed on the passive continental margin, such as the Himalayas, Longmen Mountain in the west of Sichuan Basin and Gongga Mountain, which is more than 7000 m high. For the inter-continental trough or aulacogen without oceanic crust and subduction, it is purely the intracontinental orogeny by compression, such as Zongwulong Mountain in the northern margin of Qaidam. On the north side of Huaitoutala, the fold basement and unconformity of Early Paleozoic can be found and were merged into the folded mountain system formed from the end of the Late Paleozoic to the beginning of Early Mesozoic together with the sedimentary rocks of the inter-continental deep-sea trough which was further fractured in Late Paleozoic. Therefore, according to the destruction mode of ocean basins or basins, continental orogenic belts can be roughly divided into 3 types: ① Composite orogenic belts formed by two-way subduction and closure of oceanic crust in ocean basins, with arc-arc collision or arc-land collision orogeny on both sides of the active margin and different formation time. Of course, when the back-arc-basin near the continental margin is closed, there will also be arc-land collision orogenic belts, such as the closed orogeny of the Paleo-Asian Ocean in Inner Mongolia and the Bayankala Ocean. ② Composite orogenic belts formed by one-way subduction and closure of oceanic crust in ocean basins, with arc-land collision between active margin and passive margin, such as Himalayas and Gangdise orogenic belt, and the closure of Jinsha River-Ailaoshan and Lancang River. ③ Inter-continental compression orogenic belts formed after the closure of the inter-continental trough or aulacogen, with nonarc-land collision. The back-arc orogeny proposed by Xu et al. (1994) is only one type. As plate subduction is often characterized by oblique subduction and mass strike-slip in the late-collision orogenic stage, the orogenic belt finally formed is mainly a composite orogenic belt caused by subduction, collision and strike-slip, and most of the orogenic belts in Sanjiang region belong to this type.

The complexity of continental orogenic belts depends not only on the complexity of its internal material and structure and the complexity of lithosphere rheological stratification, but also on the complexity of ocean basin evolution. Since the orogeny after the closure of ocean basins is mostly manifested in arc-arc collision orogeny and arc-land collision orogeny, coupled with the subduction of small plates and strike-slip collage of blocks or island arc terranes, the cooling and solidification of the old oceanic crust and the increase in density are mostly abated by subduction and difficult to remain. Thus, in an orogenic belt, it is sometimes difficult to determine the main suture line for the final closure of a long-evolving ocean basin (its closure time may sometimes be earlier than that of some back-arc-basins). It may be a complex tectonic zone composed of not only one line, but two or more subduction accretion zones, such as the Paleo-Asian Ocean with two-way subduction, which has multiple suture zones of different periods with north–south symmetry, and the suture zone for its final closure at the end of Hercynian consists of not one but two zones, interspersed with Xilin Gol, Kiamusze, Mazong Mountain and other blocks of different sizes. The Bayankala Ocean with two-way subduction is finally closed in both north and south, with a “covered” folded thrust orogenic belt formed from the sediments of foreland depression. Several sections may not be closed; thus, there are residual ocean basins with subduction complexes as the base and filled with turbidite.

1.1.1.5 Nonophiolite in Ocean Basins

In the Atlantic Ocean, in addition to ophiolite formed by spreading ridges, in the transition zone between the ocean and land on its edge, the lithosphere can be stretched and thinned, and the underlying pyrolite can be stretched out, or after stretching and thinning, the underlying mantle peridotite can be exposed to the seabed through later thrusting, such as the serpentine mantle lherzolite in the transition zone of oceanic crust in Gahcia Bank section of the western Iberian continental margin (Lemoine and Trümpy 1987; Whitmarsh and Sawyer 1993), when the Atlantic Ocean is closed, the iherzolite may be easier to preserve in the orogenic belt than ophiolite with spreading ridges. The ultrabasic rocks like ophiolitic melange zone exposed in the south of Daofu in western Sichuan and the Muli-Kangwu area in the south of Ganzi-Litang Zone may belong to this type, because the associated pillow basalt is not oceanic ridge type but is the rift-type alkaline basalt on continental margins. Therefore, when studying orogenic belts, this type of ophiolite should be identified carefully, yet the existence of ocean basin shall not be denied due to the discovery of such nonophiolite.

1.1.1.6 Changes in Oceanic Crusts

Due to the continuous spreading of the mid-ocean ridges and the continuous generation of new oceanic crusts, especially when the oceanic crusts of ocean basins subduct, the old oceanic crust is continuously destructed. In the long-term development and evolution of the ocean basin, the oceanic crust has been changed. Therefore, the ophiolite formation age determined by radiolarians in siliceous rocks associated with oceanic crust basalt may not represent the age of initial ocean basin formation. Nowadays, there are numerous radiolarian siliceous rocks of Paleogene, Neogene and Quaternary in the Pacific Ocean, but it cannot be considered that the Pacific Ocean was formed in Paleogene, Neogene and Quaternary. Moreover, due to the decomposition and tectonic mixing of the ophiolite complex, the radiolarian siliceous rocks found may not be the same as those at the time of the initial ocean basin formation. Therefore, the age of ocean basin formation can be determined more reliably only when it is corroborated by historical data on the formation and evolution of passive continental margins or active margins on both sides.

Some of the evolution features of the three oceans can be used to deepen the understanding of the tectonic evolution of the Sanjiang Tethyan tectonic domain and its global tectonic setting.

1.1.2 Global Tectonic Setting

It can be seen from the geological maps of Europe and Asia that the giant Tethyan tectonic zone, which runs from east to west, is obviously inlaid with strips (orogenic belts) and blocks (landmasses), just like a giant “ductile shear zone” or “tectonic melange zone”. This unique tectonic zone, which distributes between the south and north continents, plays an important role in the global tectonic evolution and has always attracted much attention. The Sanjiang orogenic system in southwest China is in the eastern part of the Tethyan tectonic domain—Eastern Tethyan tectonic domain (east of Pamir), and its formation and evolution are closely related to the formation and evolution of three continental groups, especially Gondwana Continental Group and Pan-Cathaysian Continent Group and their continental margins. The history of global ocean-land evolution since the break-up of the Rodinia super-continent has been characterized by the coexistence of the three major landmass groups, namely Laurasia, Gondwana and Pan-Cathaysian landmasses, and three major oceans, namely Panthalassa Ocean (ancient Atlantic Ocean), Paleo-Asian Ocean and Tethys Ocean (Li et al. 1995; Lu 2004; Pan et al. 1997). The main body of the Eastern Tethyan tectonic domain, including the Sanjiang orogenic system, is in the basic framework—“one ocean and south and north continents”—of Gondwana Continental Group, Pan-Cathaysian Continent Group and the Tethys Ocean between them (Fig. 1.2).

Fig. 1.2

Schematic diagram of global ocean-land pattern of early Paleozoic (Ordovician)

1.1.2.1 Pan-Cathaysian Continent Group

1.1.2.2 Gondwana Continental Group

The Gondwana continent refers to the super-continent composed of several landmasses of East Gondwana (including India, Australia, South Asia, etc.) and West Gondwana (including South America, Africa, etc.) from the end of Neoproterozoic to the beginning of Paleozoic, with the destruction of Mozambique Ocean (Shackleton 1996) and the Pan-Africa orogeny (600–550 Ma) (Kröner et al. 1993; Kriegsman, 1995). The Gondwana continent is also called “the southern continent”, and its scope is much larger than that proposed by in The Face of the Earth. It includes South America, Africa, Australia, Antarctica, India Peninsula and Arabian Peninsula, as well as Iran, Turkey and Himalayas. In the Gondwana Continent, the Gondwana Rock is developed due to much glacial activities in Carboniferous-Permian, with biological characteristics of cold-water fauna spermatophyte in Carboniferous-Permian, and ferns-Gangamopteris-Glossopter is dominant in Permian and other Gondwana flora. In Late Triassic of Mesozoic, a narrow trench was formed in Madagascar in eastern Africa, and the Neo-Tethys in the northern margin of Gondwana continent spread. From the end of Jurassic to Cretaceous, the India Landmass and the Australian Landmass separated from the Antarctic Landmass, the Indian Ocean began to spread, the South American Landmass separated from the African Landmass, the South Atlantic Ocean began to spread, and Cenozoic gradually migrated to its present location.

1.1.2.3 Tethys Ocean

The Tethys, originally proposed by Suess (1893), refers to the vast ocean in Mesozoic between the ancient land of Angola in the north and Gondwana in the south. The destruction of the ancient ocean and the subsequent uplift formed the magnificent Alps-Himalayas. With the establishment of plate tectonics theory in 1960s, not only the Tethys area is larger than before, but also the formation time of Tethys Ocean dates to Paleozoic. From the time of Tethys evolution, the division of Proto-Tethys, Paleo-Tethys and Neo-Tethys can better reflect the evolution of Tethys spatial–temporal pattern: The Proto-Tethys (Sinian-Silurian) was mainly characterized by the dispersion of Pan-Cathaysian Continent Group and Laurasia Continental Group, the separation of Laurasia Continental Group from Gondwana continent and the spreading of Tethys Ocean (Li et al. 1995; Pan et al. 1997); the combination of “Pan-Cathaysian orogen” at the end of Early Paleozoic formed a unified Pan-Cathaysian continent (Lu et al., 2006) and the basis of the formation of Cathaysia flora (Xie et al., 1994; Pan et al., 1997); the Paleo-Tethys (Devonian-Middle Triassic) was characterized by the convergence of Pan-Cathaysian Continent Group and the Laurasia Continental Group, the connection between Laurasia Continental Group and Gondwana Continental Group, and the shrinking of the Tethys Ocean (Li et al., 1995; Pan et al., 1997), the East Asian continent and its marginal orogenic system were formed by the combination of Indosinian orogen from the end of Late Paleozoic to the Early-Middle Triassic and became an integral part of Pangea super-continent; the Neo-Tethys (Late Triassic-Eocene) was mainly characterized by the break-up of Pangea super-continent and Gondwana continent (Pan et al. 1997); the Tethys Ocean was destructed and transformed into continental lithosphere and entered the period of continental collision and orogeny.

The Sanjiang Tethyan tectonic zone was formed and evolved in global tectonic formation. It is located at the junction of Pan-Cathaysian Continent Group and Gondwana Continental Group and has gone through two times of Pangea break-up and three major development stages—the Proto-Tethys, Paleo-Tethys and Neo-Tethys. Its tectonic evolution is completed by continuous continent break-gathering evolution since Paleozoic, and its dynamic mechanism may be related to the southward migration of the earth’s mass center, the expansion of the southern hemisphere, the destruction of the Pangea and its northward drift. In the mantle convection of the asthenosphere that rotates clockwise from south to north and drifts the continent northward, apart from the vertical mantle convection, there may be horizontal vortices of different sizes, which can only promote the rotation of the landmass, the formation of the Pangea and its immediate destruction (Li et al. 1995; Pan et al. 1997).

1.2 Division of Main Tectonic Units

For the Qinghai-Tibet Plateau and the Sanjiang region, many scholars have divided these areas by tectonic units (Liu et al. 1993; Mo et al. 1993). In view of the improvement of geological survey, the deepening of basic research, the further determination of tectonic environment of some tectonic units, the discovery of some new tectonic units and the demand of metallogenic prediction and resource evaluation, this book is based on the above-mentioned division schemes of many scholars, combined with the latest research and the national division schemes of tectonic units. The main tectonic units in Sanjiang Tethyan Metallogenic Domain and its adjacent areas are divided into 4 first-class tectonic units (namely Yangtze Landmass, Sanjiang MABT, Bangong Lake-Shuanghu-Nujiang River-Changning-Menglian Mage-Suture Zone and Gangdise-Gaoligong Mountain-Tengchong Arc-Basin System) and further divided these units (Fig. 1.3). Also, the reference of “landmass” and “block” is further standardized. A landmass generally refers to a relatively stable area composed of consolidated land crust in the whole geological period, which is generally large in scope, and the paleogeographic features often undergo land-sea changes. A landmass may be an uplift denuded area or a sedimentary basin. A block refers to a small or very small continental crust block, whose geotectonics can be the product of a crack near the edge of a continental plate or an exotic terrane from other tectonic domains.

Fig. 1.3

Division of tectonic units in Sanjiang region of Southwest China. I—Yangtze landmass: I₁—Longmen mountain thrust zone, I₂—Bayankala foreland basin, I₃—Yajiang relict basin, I₄—Yanyuan-Lijiang continental margin depression zone, I₅—Chuxiong foreland basin; II—Sanjiang MABT: II₁—Ganzi-Litang junction zone, II₂—Dege-Xiangcheng Island arc (Yidun Island Arc): II_2–1—Que’er mountain-Daocheng outer arc zone, II_2–2—Jiegu-Yidun back-arc-basin zone, II₃—Zhongza-Shangri-La block, II₄—Jinsha river-Ailaoshan junction zone: II_4–1—Jinsha river Ophiolitic Melange zone, II_4–2—Ailaoshan Ophiolitic Melange zone, II₅—Qamdo-Pu’er block: II_5–1—Jiangda-Jijiading-Weixi continental margin volcanic arc, II_5–2—Qamdo-Markam bidirectional back-arc foreland basin, II_5–3—Zadoi-Dongda mountain continental margin volcanic arc, II_5–4—Mojiang-Lvchun continental margin volcanic arc, II_5–5—Lanping-Pu’er bidirectional back-arc foreland basin, II_5–6—Yunxian-Jinghong continental margin volcanic arc, II₆—Lancang river junction zone, II₇—Zuogong block, II₈—Lincang magmatic arc, III—Bangong lake-Shuanghu lake-Nujiang river-Changning-Menglian Mage-suture zone: III₁—Bangong lake—Nujiang river junction zone, III₂—Changning-Menglian junction zone, III₃—Jiayu bridge relic arc zone; IV—Gangdise-Gaoligong mountain-tengchong arc-basin system: IV₁—Baoshan block, IV₂—Shading-Luolong fore-arc-basin, IV₃—Bowo-Tengchong magmatic arc, IV₄—Xiachayu magmatic arc, IV₅—Yarlung Zangbo river junction zone

1.3 Basic Characteristics of Tectonic Units

1.3.1 Yangtze Landmass (I)

The Yangtze Landmass has pre-Sinian System crystalline basement and fold basement and Dahongshan Group, Yanbian Group, Huili Group, Kunyang Group and Ailaoshan Group, etc. in Mesoproterozoic. The basement tectonic layer is formed by arc-basin system development and arc-land collision. The Suxiong “bimodal” volcanic rift is developed in Nanhua. A wide range of carbonate rock plateaus was formed in Dengying, and sedimentary covers were formed in the Sinian System-Mesozoic and Cainozoic. Its basement is along Jianchuan-Dali and Ailaoshan fault in Diancang Mountain and Ailaoshan and overthrusts westward on the stratum in Mesozoic in Lanping-Pu’er Depression Zone and Ailaoshan Ophiolitic Zone, forming Diancang Mountain-Ailaoshan Basement Overthrust Zone.

1.3.1.1 Longmen Mountain Thrust Zone (I₁)

Longmen Mountain Thrust Zone is distributed in the northeast and spreads into Shaanxi Province along the northeast. It is cut by northwest-trending Sanhe Fracture to the southwest and enters Jinping Mountain Overthrust Zone in the southwest. This zone is mainly composed of formations in Sinian, Paleozoic, Mesozoic, Paleogene and Neogene, but the overall metamorphic degree of rocks is very low. The thrust nappe in this zone is significant, and thrust pieces of different sizes are developed, with many detached blocks formed. The Jiangyou-Dujiangyan Fracture Zone is the eastern boundary of Longmen Mountain Overthrust Zone. To the northwest, the Longmen Mountain Foreland Overthrust Nappe Zone, the Longmen Mountain Central Fold Nappe Zone and the Longmen Mountain Hinterland Arc Slip-nappe Zone can be further delineated. From northwest to southeast, the three zones show the characteristics of ductile to brittle.

1.3.1.2 Bayankala Foreland Basin (I₂)

The Bayankala Foreland Basin was transformed from the Yangtze passive margin in Paleozoic at the end of Middle Triassic, adjacent to the Sanjiang MABT tectonic zone in the west, and bounded by the Longmen Mountain-Jinping Mountain Fault in the east and the Yangtze Landmass. The main part of the basin is composed of flysch in the Triassic. In the Paleozoic, this area was a part of the Pan-Yangtze Landmass and from Ordovician to Devonian, with extremely thick clastic rocks and clastic flysch deposits developed. From Early Permian to Late Permian, the Ganzi-Litang Ocean gradually opened, and the carbonate gravity flow accumulation of slope facies was developed, accompanied by extensional basic basalt flow, and the passive continental margin began to form. The passive continental margin continued to develop in the Early Triassic and Middle Triassic, and the extremely thick clastic flysch and turbidite were deposited in the Late Triassic. After being transformed into a foreland basin, the distribution of lithofacies shows that the sediment is gradually deep from east to west.

1.3.1.3 Yajiang Relict Basin (I₃)

Yajiang Relict Basin (I₃) is a secondary basin in the south of Bayankala Basin and is separated by Xianshui River Strike-Slip Fault. Its basic characteristics are similar to those of Bayankala Basin. It entered the residual ocean stage from Late Permian to Middle Triassic and was closed at the end of Triassic. Its main part is the residual basin composed of abyssal sediments, turbidity sediments and neritic flysch sediments of Late Triassic. According to the pillow basalt in Late Permian in Jiulong-Muli area and the oceanic ridge basalt in Luhuo-Daofu area, it is speculated that it is the filling and destruction of the relict basin where the oceanic-continental transitional crust subducted westward. At the end of Late Triassic, with the closure, overall uplift or folding of Ganzi-Litang Ocean, few sedimentary records of Jurassic-Cretaceous can be found. In Paleogene, the right-handed strike-slip pull-apart basin, which only distributed along the narrow strip, accumulated continental molasse deposits. In the intracontinental convergence after the collision (since Jurassic), significant folds and thrusts were formed, and the Longmen Mountain-Jinping Mountain Nappe Zone was formed on the western edge of the Yangtze Block. Metamorphic core complex was formed in the rear edge of the nappe zone. The metamorphism in this area is not significant, mainly low greenschist facies. The magmatic activity is mainly caused by the collisional (Jurassic) terrestrial crust remelting granite except for the basic magmatic activity in the tensional period in Late Permian.

1.3.1.4 Yanyuan-Lijiang Depression Zone (I₄)

This depression zone is located in the southwest margin of Yangtze Landmass. From Sinian to Paleozoic, there are mainly littoral-neritic clastic rocks and carbonate rocks deposited stably. Only in the local depression during Ordovician–Silurian, there are deep-water graptolite shale and siliceous rocks deposited. The basalt in Permian began to erupt at the end of Early Permian. There were still littoral-neritic clastic rocks and carbonate rocks in the Triassic. At the end of Triassic, coal-bearing clastic rocks of marine-land transitional facies are deposited, which shows that the sedimentary is in shallow water, but the depression amplitude is large, and the sedimentary thickness can reach 6000 m. In the Middle Triassic and Late Triassic, in Xiangyun Area on the east edge of the depression zone, due to the fault of Chenghai, a local deep depression was formed, and clastic rocks and turbidites were deposited, accompanied by intermediate-basic pyroclastic rocks. The Jinping gliding nappe sandwiched between Ailaoshan Fault and Adebo Fault shows that the oldest stratum exposed is in Ordovician, and its sedimentary characteristics from Ordovician to Permian are similar to those of Yanyuan-Lijiang Depression Zone, which indicates that both of them are originally connected and belong to the Yangtze Block. The Yangtze Block was torn only because of the left slip along the Ailaoshan Fault, and then (possibly in Triassic) due to the southwest thrust of the Yangtze Block, the Ailaoshan Metamorphic Basement Thrust and exposed, which separated the block from the Yanyuan-Lijiang Depression Zone. The Ailaoshan Group is composed of a sequence of migmatitic gneiss, granulite, amphibolite, schist and marble, and the mylonite is transformed by the former rocks.

1.3.1.5 Chuxiong Foreland Basin (I₅)

Chuxiong Foreland Basin is located in the depression zone of the southwestern margin of Yangtze Block, and it was mainly a fault block depression sedimentary zone before Mesozoic. It was transformed into a foreland basin in the Late Triassic, and after the Jurassic-Cretaceous, especially the collision between India and Eurasia, there was a significant folding deformation. In the Yanyuan-Lijiang Depression Zone and Chuxiong Basin, there were many intermediate-acid rock masses and alkali-rich porphyries invaded in the Himalayan.

The continental overflow basalts and their corresponding intrusive rocks in Permian are mainly developed in the Yangtze Landmass, which are characterized by high TiO₂, Na₂O and K₂O and high enrichment of LREE. They are a part of Emei Mountain Igneous Province and have a genetic relationship with super-mantle plume. According to, the eruption period of Emei Mountain basalt is 262–258 Ma. In addition, in the area from east Erhai Lake to Red River, on both sides of Ailaoshan Fracture Zone, some intraplate deep volcanic rocks and small intrusive masses such as potassium-rich and high-magnesium lamprophyres, alkali-rich porphyry and potassium basalt in Himalayan are distributed intermittently, forming the post-collision alkali-rich porphyry and volcanic zone superimposed on the older tectonic–magmatic zone.

1.3.2 Sanjiang Archipelagic Arc-Basin System (II)

1.3.2.1 Ganzi-Litang Junction Zone (II₁)

The zone runs from western Xiewu Temple in the northwest to Ganzi in the southeast then to the south from Litang and Muli Yazui Ranch to the mouth of the Sanjiang at the junction of Sichuan and Yunnan and then turns to the west and spread to the south along Haba Snow Mountain and the west of Yulong Snow Mountain to Jianchuan, where it meets the Jinsha River Junction Zone extending to the south at the north of Qiaohou. Its northwest end may be connected with the Jinsha River Junction Zone in the west Deng Ke-Yushu. The southern end of the junction zone may be truncated or covered by the westward thrusting of the southwest Yangtze Landmass to the south of Jianchuan. The junction zone is over 500 km long and 5–30 km wide, and it is a tectonic melange zone composed of Late Permian (P₂)-Late Triassic (T₃) oceanic ridge tholeiite, picrite basalt, mafic and ultramafic cumulate, gabbro-diabase wall, serpentinite, radiolarian siliceous rock and flysch. The foreign sedimentary rock blocks are from Ordovician to Triassic, and the matrix is sand-slate and volcanic rocks of Late Permian and Late Triassic. Most ophiolites are decomposed to form ophiolitic melange blocks, but most of the basalts feature pillow structures, and their geochemical characteristics are similar to those of mid-ocean ridge basalts (MORB).

There is a well-preserved ophiolite sequence near Litang, and ophiolite in most places is decomposed. There are amphibole eclogite exposed in the south of Litang and glaucophane schists in Xinlong-Yiji Muli and Sanjiang estuary. Near Manigango in the north and Tuguan Village in the south, pillow basalts, massive basalts and abyssal sedimentary rocks (radiolarian siliceous rocks) are mainly exposed, as well as sporadic gabbro and pyroxenite. Basalt is usually pillow-shaped, characterized by low K₂O (average content of 0.19–0.37%), medium TiO₂ (average content of 1.38–1.63%) and average REE (rare earth elements) distribution pattern, which is similar to mid-ocean ridge basalt (MORB). The K₂O content in basalts in Tuguan Village area in the south is slightly higher, but the rare earth elements (REE) pattern is still average. The basalts in the south member (such as Tuguan Village) were formed in Middle Permian, those in the middle member (Litang) were formed in Early Triassic according to the radiolarian siliceous rock overlaid, and those in the north member (north of Ganzi) were formed in Late Triassic, indicating that the ocean basin opened between Middle Permian and Late Triassic. The ophiolite was localized before the Rhaetian of Late Triassic according to the age of arc volcanic rocks in Changtai-Xiangcheng (T₃¹⁻²) and the age of continental coal measure strata (T₃³). That is, the Ganzi-Litang Ocean Plate may have subducted since Late Triassic and the ocean basin closed at the end of Late Triassic.

1.3.2.2 Dege-Xiangcheng Island Arc (Yidun Island Arc Zone) (II₂)

This zone is located on the west of Ganzi-Litang Junction Zone, and the main exposed strata are Triassic and a few Paleogene and Neogene. The Middle Triassic and Lower Triassic stratum are clastic rocks mixed with carbonate rocks and siliceous rocks, with a thickness of nearly 5000 m; the lower part of Genlong Formation, Gacun Formation and Miange Formation of Upper Triassic is composed of extremely thick flysch and sand-slate with basic, intermediate-basic and acidic volcanic rocks and carbonate rocks, about 10,000 m thick; from the upper part of Mian Formation to Lamaya Formation, there are neritic clastic rocks and coal-bearing clastic rocks of land-sea transitional facies. To the east of Dingqu Fracture Zone in the south, slump breccia can be seen in the early stage. Most of the breccia are foreign blocks (flysch-like), as well as radiolarian siliceous rocks and back-arc basalts similar to oceanic crust, and ultramafic rocks are exposed. Paleogene and Neogene are molasses deposits in inter-mountain basins.

The tectonic deformation of this zone is significant, and the folding started from the end of Indosinian-Yanshanian, with syn-cleavage folding deformation and left translational ductile shear zone with nearly vertical attitude and the same strike as the tectonic line, and then high-angle positive ductile shear occurred. The Gacun Polymetallic Deposit was controlled by ductile strike-slip and ductile shear and was complicated. Since the Himalayan, there has been left strike-slip intense magmatic activities and well-developed magmatic rocks, which are composed of volcanic formation complex and intrusive rocks in four development stages.

There are crust structures similar to the Yangtze Landmass in the eastern Chas in the south, which are pre-Sinian basement, Sinian and sedimentary cover of later Paleozoic and Triassic, and they are island arc-continental crust basement. The Qias Group in pre-Sinian is composed of metamorphic intermediate-basic volcanic rocks mixed with carbonate rocks and clastic rocks, with a thickness of 2644–2818 m. Guanyinya Formation in Sinian is composed of clastic rocks, and Dengying Formation is composed of carbonate rocks, which are unconformably overlain by the Qias Group and are 300–1200 m thick. The Lower Paleozoic (with the lacuna of Middle and Upper Silurian) is mainly composed of clastic rocks, carbonate rocks mixed with intermediate-basic volcanic rocks and siliceous rocks that evolved from neritic to bathyal deposits and contact Sinian in a parallel unconformity manner. The Upper Paleozoic (with the lacuna of Upper Devonian) series are littoral-neritic clastic rocks and carbonate rocks with basic volcanic rocks and unconformably cover Sinian, Cambrian and Ordovician. The Triassic in Mesozoic is composed of marine clastic rocks, carbonate rocks and volcanic rocks, and the Upper Triassic stratum is composed of marine-continental coal-bearing clastic rocks.

The above stratigraphic characteristics show that Qias Group is equivalent to Hekou Group in Kangdian area of the Yangtze Block, and Sinian is similar to the Yangtze Block, which indicates that they were originally part of the Yangtze Landmass. At the end of Early Permian or from Late Permian to Early Triassic, with the opening of Ganzi-Litang Ocean Basin, this part separated from the Yangtze Landmass and formed the basement of Shaluli-Yidun Island Arc Zone. It also shows that the island arc zone developed on the graben of the continental crust basement of the Yangtze Landmass, and it contains blocks with old basement. This part can also be classified as a secondary tectonic unit-Chas fault uplift.

The Dege-Xiangcheng Magmatic Arc, which is nearly pod-shaped in the north–south direction, is mainly occupied by volcanic-sedimentary rock series in Late Triassic and granite basement of Indosinian-Yanshanian. Roughly bounded by the Yidun-Haizishan line, it can be divided into two arc volcanic-sedimentary basins in the south and north: Baiyu-Changtai Basin and Xiangcheng Basin. Volcanic activities occurred in three periods: the former island arc period, the main arc period and the later arc period. The main arc period can be divided into three stages: early arc-forming stage, mid intra-arc rift stage and late arc-forming stage. Before the formation of the volcanic arc in Early Carnian (T₁), a series of graben and horst is developed in the rock area, resulting in a rift-type alkaline-transition series basalt with high content of TiO₂ or a “bimodal” volcanic assemblage of basalt-rhyolite, which has similar petrogeochemical characteristics to Emei Mountain basalt in Yangtze Plate. The volcanic rocks in the main arc period (cycle in Gacun, T₃¹⁻²–T₃²⁻¹) are dominated by andesite in two arc-forming stages, with a small amount of calc-alkaline basalt and dacite-rhyolite, which have typical characteristics of arc volcanic rocks. Rhyolite-tholeiite “bimodal” volcanic assemblage is developed in the inter-arc rift stage. The development of intra-arc rift and corresponding “bimodal” volcanic rocks is the main feature of Yidun arc, which is different from other magmatic arcs and is also the basic tectonic-volcanic condition for ore-forming and ore-controlling of Gacun massive sulfide polymetallic deposit. This feature is most obvious in the middle and northern segment of magmatic arc. The volcanic rocks (cycle in Miange, T₃³) in the late arc-forming stage are only developed in the northern part of the arc, which is a “bimodal” assemblage of high-potassium basalt, shoshonite and rhyolite, with rhyolite as the main component, and were not exposed in the southern segment of the arc.

Que’er Mountain-Daocheng Outer Arc Zone (II_2–1), mainly composed of adamellite-granodiorite plutonic rock basement, is mainly distributed in the northern member of the magmatic arc. The diagenetic period is in Indosinian (237–195 Ma), but in some huge rock foundations (such as Cuojoma and Dongcuo), there are large xenoliths formed in Hercynian, which are granitoids in the same collision period. Therefore, the granitoids in the outer arc zone are mainly caused by collision, superimposed on the early island magmatic arc zone.

Jiegu-Yidun Back-arc-Basin Zone (II_2–2), located in Baiyu Gacun Basin in the north member, is symmetrically distributed with old parts on both sides and new parts in the middle; in the southern Xiangcheng Basin, the zone is mainly distributed in the west of the basin near the Zhongza Block. During the transition from the end of the former island arc period to the early stage of the main arc period, the sub-cyclic volcanic rocks in Chizhong, which are composed of high-MgO pillow tholeiite, massive island arc low-potassium tholeiite and high MgO, high-SiO₂ and extremely low-TiO₂ bonitite andesite, began to form obvious characteristics of arc volcanic rocks, but also retained some characteristics of late rift volcanic rocks. This sequence of volcanic rocks is only exposed in the middle area of Chizhong, Xiangcheng, located on the east side of cyclic volcanic rocks in Genlongya. Changdagou-Pulang Inner Arc Zone is mainly developed with calc-alkaline volcanic assemblage in the main island arc period and the associated intermediate-acid porphyry, which is distributed in Xuejiping-Pulang area of Shangri-La in the southern member of the magmatic arc, with dioritic porphyrite-monzonite porphyry. Its petrochemistry is characterized by low SiO₂, high CaO, high MgO, rich Na and poor K. It is a type I granite, a product of pressure magmatic arc, with porphyry copper deposits and polymetallic mineralization. There is intrusion of intraplate magma after collision.

In addition, there are post-collision granites in Yanshanian-Himalayan in the zone, which are superimposed in the distribution area of early arc magmatic rocks and syn-collision granites.

The Dege-Xiangcheng Island Arc (Yidun Island Arc Zone) has a close temporal and spatial relationship with the Ganzi-Litang Zone. In the northern member of the volcanic arc (Zengke-Changtai), the following spatial configurations can be seen from east to west: the disappeared Ganzi-Litang Ocean (Ganzi-Litang Ridge Volcanic Rock-Ophiolite Zone)—the fore-arc area (arc-ditch hiatus)—is the main arc area of the granite zone and the sedimentary rocks in Late Triassic in Que’er Mountain; the outer arc (east basaltic andesite zone), intra-arc rift (“bimodal” volcanic zone), inner arc (west basaltic andesite zone) and back-arc area (Miange Rhyolite—High-potassium Basalt Zone). The back-arc area is roughly divided from the main arc area by the Keludong-Dingqu River Fracture, along which there are many serpentinites of tectonic diapiric fold. Volcanic rocks are old in the east and new in the west, and volcanic activity centers migrate from east to west. In the middle-south member of the arc (Xiangcheng Area), the spatial configuration of volcanic rocks is roughly similar to that of the northern member. The difference is that there is no volcanic rock exposed in the late arc, and the back-arc area is occupied by volcanic rocks from the end of the fore-arc period to the beginning of the main arc period. The volcanic activity center tends to move from west to east, which may be due to the different dynamic boundary conditions and subduction mechanisms between the south and north members. It can be seen from the foregoing that the Ganzi-Litang Rock Zone and Yidun Volcanic Arc Zone are also closely connected in time. It can be seen that they are “double zones” organically linked, clearly marking the relative position of the active continental margin of the ancient Ganzi-Litang Ocean and the Zhongza Micro-landmass and pointing out the westward subduction direction of the ancient plate.

To sum up, the Dege-Xiangcheng Island Arc (Yidun Island Arc Zone) was developed since the continental rift grabon-horst system during the short Late Triassic due to the westward subduction of Ganzi-Litang Ocean Plate. Because of its history of alternating tension and compression and the existence of intra-arc rift, it is generally characterized by tension arc in the middle member and compression arc in the south and north members.

1.3.2.3 Zhongza-Shangri-La Block (II₃)

This block is bounded by Jinsha River Junction Zone in the west and Yidun Island Arc Zone in the east and is a long and narrow spindle-shaped fault block. The oldest exposed strata are Shigu Group (south) and Chamashan Group (north) in Sinian, with a thickness of 4800–11,700 m. Shigu Group is a sequence of fine clastic rocks of metamorphic flysch, which is covered by unconformity of Devonian. Chamashan Group is a sequence of metamorphic carbonate rocks and intermediate-basic volcanic rocks.

The exposed strata in the slope zone on the eastern edge of Zhongza-Shangri-La Block are Upper Permian to Triassic, with a total thickness of nearly 10,000 m. The Upper Permian is a sequence of clastic rocks mixed with basic volcanic rocks (mainly alkaline basalt), carbonate rocks, siliceous rocks and siliceous turbidite, and the number of upper basic volcanic rocks increases, with a large number of diabase invading and radiolarian siliceous rocks appearing. The Lower Triassic and Middle Triassic are mainly a sequence of flysch calcareous sand-slates, with multiple beds of muddy limestone and turbidite, and basic volcanic rocks in the lower part. The Upper Triassic is a sequence of flysch, flysch-like sand-slates with carbonate rocks, siliceous rocks and pyroclastic rocks, and slump breccia with more exotic rocks is developed in the middle-south member. It shows that the strata in this block belong to slope-to-uplift or abyssal plain facies from Late Permian to Early Triassic, and the basic volcanic activity is significant. The crust is extensional, which echoes the opening of Ganzi-Litang Ocean. In the Middle Triassic, there was the replacement of debris deposits from slopes to outer continental shelf and the tension and fault depression, as well as the formation of flysch and flysch-like rocks in the Late Triassic, which corresponded to the expansion of Yidun Back-arc-Basin, belonging to the western passive margin zone of the back-arc-basin.

The Lower Paleozoic in Zhongza Plateau is composed of clastic rocks and carbonate rocks mixed with basic and intermediate-acid volcanic rocks, with a thickness of nearly 10,000 m. From Cambrian to Silurian, clastic rocks gradually decreased, carbonate rocks gradually increased, and volcanic rocks changed from lower basic to upper intermediate-acid features. On the east side of Jinsha River Junction Zone, Silurian stratum has deteriorated into schist with significant structural deformation. It shows that the plateau was active and deposited in Early Paleozoic, with significant metamorphism and deformation. In the Late Paleozoic, the plateau was mainly littoral-neritic carbonate rock deposits, with weak metamorphism and deformation, with clastic rocks at the bottom. Devonian and Upper Permian inter-bedded basic volcanic rocks and were about 5000 m thick. Mesozoic is only distributed in the eastern margin of the block in a small amount, mainly clastic rocks and carbonate rocks of Triassic, with intermediate-acid volcanic rocks in the upper part of Triassic.

Shangri-La Plateau was stably developed in Paleozoic, mainly the sedimentary plateau of carbonate rocks. The basic volcanic activities in Devonian and Late Permian may be related to the extensional environment before the opening of Jinsha River Ocean in the west and Ganzi-Litang Ocean in the east, respectively. The strata deformation and metamorphism have been weak since Paleozoic, and the main part is compound anticline in Baiyinchang. The eastern margin of the plateau is thrust on Triassic, and a series of isoclinal folds and thrust faults that reverse westward are developed on the western side of the plateau from Tanglangding, Tuoding to Zhongcun, showing the sector profile of thrust on both sides of the block.

Volcanic activity in this block started from Cambrian. Volcanic rocks in Early Paleozoic are marine basalts with high TiO₂ content and low MgO content, sometimes accompanied by a small amount of dacite-rhyolite, forming a “bimodal” assemblage, formed in clastic rock or carbonate rock formation period and featuring the property of continental intraplate extensional volcanic rocks. Volcanic rocks in the Late Permian are the most important volcanic rocks in this block, mainly alkalinity continental intraplate extensional basalt with high TiO₂ content, which has many geochemical characteristics similar to Emei Mountain basalt in the Yangtze Landmass, reflecting that the mantle source areas of their magma may be related. According to the characteristics of symbiotic sedimentary facies, volcanic rocks mainly erupt in marginal trough (such as Boge west trough composed of basalt-deep-water carbonate rock-siliceous rock) and neritic plateau or bay (such as Derong-Guxue area, coexisting with neritic carbonate rocks). The above shows that the Zhongza Block was in continental intraplate extension in most time of Paleozoic (especially in the Permian), corresponding to the extension of the southwest Yangtze Plate, and the intraplate extensional alkaline to transitional basalt in the former island arc period of Shaluli-Yidun Arc Magmatic Rock Zone was also produced.

A few square kilometers of calc-alkaline volcanic rocks mainly composed of basaltic andesite and andesite breccia-agglomerate are exposed in the Lana Mountain, Yidun, which is in fault contact with the surrounding rock strata and may be a nappe from the Jiangda-Weixi volcanic arc in the west.

1.3.2.4 Jinsha River-Ailaoshan Junction Zone (II₄)

The northern member of Jinsha River-Ailaoshan Junction Zone may connected with Ganzi-Litang Junction Zone in the west Dengke-Yushu Area, and from then on, it extends to Yanghu and Guozhacuo areas in Northern Tibet to the West Xijir Ulan Lake. Then it goes south through Batang, Benzilan—the west side of Diancang Mountain, turns to Ailaoshan in the southeast longitude and extends out of the border, connecting with the Gemia Zone in Northern Vietnam.

The mafic–ultramafic rocks, carbonate rocks, basic volcanic rocks, slate, siliceous rocks and other melange in the junction zone range from Devonian to Permian, and the matrix is composed of flysch siliceous rock and siliceous rocks of Permian–Triassic. Ultramafic rocks are distributed in groups and strips, and sometimes chromite can be seen, but complete ophiolite profiles are rare. The volcanic rocks in ophiolite are oceanic ridge—quasi-oceanic ridge basalt, indicating the existence of oceanic crust. Ophiolite was formed from Early Carboniferous to Triassic. Significant deformation, fold, overthrust nappe and ductile translational shear are developed. Metamorphism is dominated by greenschist facies, and kyanite and sillimanite, which represent medium-pressure and high-temperature metamorphism, can be found locally. There are some high-pressure and low-temperature blue schists in the Ailaoshan Zone.

• (1) Jinsha River Ophiolitic Zone (II_4–1)

This zone is bounded by the Gaiyu-Zhongza Fault in the east, the narrow area to the east of Yushu Longbao Lake-Aila Mountain-Xiquhe Bridge-Yangla-Ludian Fault in the west, the Zhongza Block in the east, the Jiangda-Weixi Volcanic Arc in the west, Ludian in the south, Ganzi-Litang Zone in the north in Dengke Zone and then the Xijir Ulan Lake-Tongtian River Zone.

This zone is mainly composed of oceanic ridge basalt, quasi-oceanic ridge basalt, serpentinite (the source rock is harzburgite), conglomerate gabbro, diabase wall, radiolarian siliceous rock, etc., which constitutes decomposed ophiolite or ophiolitic melange. The oceanic ridge basalt is only seen near Jiyidu and is the tholeiite with low K₂O content, medium TiO₂ content and flat REE distribution. The value of w(Mg²⁺)/w(Mg²⁺  + Fe²⁺) is low (0.46). It is formed in Early Carboniferous (C₁). Quasi-oceanic ridge basalt is widely exposed in the rock zone, stretching for hundreds of kilometers from Gaiyuxi, Batang, Derong, Guxue, Benzilan, Luosha and Tuoding to the vicinity of Ludian. The geological environment of quasi-oceanic ridge volcanic rocks is similar to that of the oceanic ridge basalt, but it has more characteristics of continental overflow basalt or oceanic island basalt in lithochemistry and geochemistry. Compared with oceanic ridge basalt, its K₂O content is higher (greater than 0.50%), the TiO₂ content is high, the total REE and LREE enrichment are also high, which may come from the enriched mantle source area under the ridge. The quasi-oceanic ridge basalt in this zone is formed in the Early Carboniferous-Early Permian (C₁–P₁), and its lithology is quite stable, indicating that the ocean basin continues to expand at the same speed. The Zhubalong-Gongka Intra-oceanic Arc Volcanic Zone in Permian is located in Zhubalong, Markam, Xiquhe Bridge-Deqin Gongka and Dongzhulin Temple. The volcanic rocks in the main arc period are calc-alkaline andesite, basalt, basaltic andesite, sodium dacite, etc., which are characterized by high Al₂O₃ content and low TiO₂ content. In Gongka and Dongzhulin Temple zones, this zone forms melange with serpentinite, gabbro, diabase walls and radiolarian siliceous rocks. The back-arc volcanic rocks are only exposed between Zhubalong-Xiquhe Bridge and located on the west side of the main arc zone. They are composed of low-TiO₂ tholeiite, dense sill-like diabase wall, radiolarian siliceous rock, flysch, etc. The volcanic rocks in Zhubalong-Xiquhe Bridge were formed in Early Permian according to radiolarian fossils in Xiquhe Bridge Formation, and those in Dongzhulin Temple and Gongka were formed in a period spanning from Sakmarian, Early Permian to Middle Permian.

Given the above facts and the characteristics of Jiangda-Weixi Arc, it can be determined that Jinsha River Ocean had a prototype of ocean basin in Late Devonian, and it expanded significantly in Early Carboniferous to form an ocean basin. It began to subduct westward at the end of Early Permian and closed and collided from the end of Middle Permian to Early Triassic.

• (2) Ophiolitic melange zone of Ailaoshan (II_4–2)

This zone is bounded by Ailaoshan Fracture in the east and adjacent to Ailaoshan high-grade metamorphic zone in the basement of Yangtze Landmass and bounded by Amo Jiang Fracture in the west and adjacent to Lvchun Volcanic Arc in the eastern margin of Pu’er Micro-landmass, extends to Vietnam in the south and pinches out near Midu in the north, with a length of more than 240 km in China. Most people believe that the Ailaoshan Zone can be connected with the Jinsha River Zone.

The ophiolitic melange zone is composed of basic rock units such as metamorphic peridotite, cumulate, basic lava, radiolarian siliceous rocks, in which sheeted dyke swarms are not developed. Due to their tectonic decomposition, these rock units do not constitute a complete sequence. Metamorphic peridotite is composed of harzburgite representing depleted upper mantle and lherzolite representing primitive upper mantle. Two kinds of primary magma and two sequences of evolved rock series are generated from partial melting of lherzolite in different degrees: one is pyroxene basalt-gabbro-diabase evolved from primary tholeiite magma, and the other is picrite basalt-albitite basalt-basaltic andesite-gabbro diorite evolved from primary picritic basalt magma. That said, there is a close genetic relation among the mantle peridotite, cumulate and basic lava in the ophiolite complex. Most of the basic lava are characterized by oceanic ridge basalt or quasi-oceanic ridge basalt in terms of lithochemistry, trace and geochemistry for rare earth element and petrography, and their composition points are uniformly distributed in oceanic ridge basalt zone or ocean floor basalt zone on various discrimination diagrams of major, trace and rare earth elements. It is judged as the abyssal sediments based on the ecological environment of radiolarian assemblage in siliceous rocks and the characteristics of silicon isotope and rare earth elements in siliceous rocks. This point is further proved by the fact that the fuchsia radiolarian siliceous rocks were found in Lower Carboniferous stratum in Pingzhang, Xinping County. These characteristics indicate that Ailaoshan ophiolite has a property of oceanic crust.

About formation age of ophiolite. Radiolarian siliceous rocks and radiolarian fossils above Bailadu albitite basalt are identified to be formed in the Early Carboniferous. The data obtained by showed that the ⁴⁰Ar/³⁹Ar whole rock age of gabbro in ophiolite is 339 Ma (Early Carboniferous), and the U–Pb age of single grain zircon from plagioclase granite variant is 256 Ma (Early Permian). The age value obtained from the isotopic dating for gabbro, basalt and siliceous rock in ophiolite made by Yang and Mo (1993), and is between 345 and 320 Ma (Early Carboniferous). Therefore, it can be reasonably considered that the formation age of Ailaoshan ophiolite shall be earlier than the Early Carboniferous. On a regional basis, Yiwanshui Formation in the Upper Triassic stratum unconformably overlaid on ophiolite, and its basal conglomerate contains ophiolite and chromite debris. It is believed that Ailaoshan ophiolite was formed before the sedimentation in the Late Triassic (Yiwanshui Formation). It also matches with the formation time of the arc volcanic zone in Late Permian and the collisional acid volcanic rocks of Lvchun in Late Triassic.

1.3.2.5 Qamdo-Pu’er Block (II₅)

The development of Qamdo Block in the north of Qamdo-Pu’er Block and Pu’er Block in the south of Qamdo-Pu’er Block may be different in Early Paleozoic, but the evolution of these two blocks is basically similar since Late Paleozoic.

Qamdo Block is a double-bed structure of basement and caprock. Gneiss, schists and granulites of Precambrian Ningduo Group and Xiongsong Group are outcropped in Xiariduo, Xiaosumang and Jiangda in the east, and their source rocks are a sequence of clastic rocks and carbonate rocks mixed with basic volcanic rocks. In the Xiaosumang area, Caoqu Group is located above Ningduo Group, which is a sequence of low-grade metamorphic series. The Early Paleozoic stratum is mainly exposed in areas of Gebo, Qingnidong, Haitong of Markam and Duoji of Yanjing in Jiangda. The Gebo Group in the Gebo area may contain Late Precambrian and Early Paleozoic strata, which is a sequence of island-type volcanic-sedimentary rock series; and Middle-Lower Ordovician strata in Qingnidong and Haitong areas are a sequence of continental slope turbidite fan, clastic rocks of the lower continental shelf and upper carbonate rocks. The Lower Devonian stratum of Silurian is mainly found in Duojiban, Yanjing in the south, which is composed of a sequence of clastic rocks and carbonate rocks. It was developed into a stable carbonate plateau in the late period of Middle Ordovician to Devonian without Caledonian folding orogeny, and the Middle Devonian stratum in Qingnidong spread on the folded Ordovician with unconformity contact. Since Devonian, Qamdo Block has entered a stable stage of caprock development. Devonian stratum is composed of continental to neritic clastic rocks and plateau carbonate rocks and is developed into stromatoporoid reef in the south, but is developed into basic and intermediate-acid volcanic rocks in the Middle and Upper Devonian stratum of Jiangda area. Similar to that in Devonian, a sequence of continental clastic rocks and plateau carbonate rocks as well as sponge reefs was developed in Qingnidong area in Carboniferous. The Lower Carboniferous stratum is composed of coal-bearing clastic rock, and the Upper Carboniferous stratum is composed of carbonate rock mixed with volcanic rock on the west side of the Qamdo-Kaixinling area. A sequence of clastic rocks, clastic turbidite and carbonate turbidite belonging to continental shelf to slope fluvial sand bodies and shoreline sand bodies can be found in Deqin area on the east side. In the Permian, island-type volcanic-sedimentary rock series were developed on the east and west sides in Triassic, especially in Late Triassic, the strata on the south landmass correspond to that on north landmass.

The oldest stratum exposed in Pu’er Block is the Silurian stratum, which is found on the west side of Ailaoshan Zone. The Silurian stratum-Devonian stratum are composed of flysch and graptolite shale, Carboniferous stratum in Carboniferous is a sequence of flysch sand-slate, basic volcanic rocks and carbonate rocks, and Permian stratum is an island-type volcanic-sedimentary rock series; and a sequence of flysch sand-slate (turbidite), turbidite limestone, siliceous rocks and slump breccia accumulated due to gravity flow, basic and intermediate-acid volcanic rock is developed in Longdong River area in the west. The stratigraphic age is dominated by the Carboniferous-Permian. Radiolaria in Devonian is found in the Dapingzhang area of Pu’er. This sequence of stratum may contain Devonian stratum. The Devonian stratum is also a sequence of turbidite sand-slate, siliceous rocks, slump breccia and intermediate-acid volcanic rocks in Nanguang Formation of Jinghong in the south. In addition, it may contain Carboniferous stratum-Permian stratum. They may be connected in the north and the south to form deposition in the passive margin zone in the eastern part of the Lancang River in the early stage and then develop into back-arc-basin deposition after in the later stage.

Qamdo-Pu’er Block can be further divided into 6 tertiary tectonic units.

• (1) Jiangda-Jijiading-Weixi Continental Margin Volcanic Arc (II_5–1)

This arc is located in Jiangda County-Tongpu-Dongdu-Jiaduoling-Deqin-Jijiading-Pantiange. Arc volcanic rock is a marine-continental assemblage of alkaline basalt-andesite-rhyolite-dacite, and the age is from Early Triassic to Carnian in Late Triassic. It consists of three sequences of volcanic rocks, which are partially overlapped in space. The first sequence is collisional rhyolite-dacite with high contents of SiO₂ and Al₂O₃ in Early Triassic (Pubeiqiao Formation and Malasongduo Formation in Jiangda, which are located in Longqiao area of Markam) and Middle Triassic (Walasi Formation and Pantiange formation, which are mainly located in areas of Jiaolongqiao of Markam-Jiading-Pantiange-Shizhongshan), which is exposed in the south of Deqin along with collisional granite intrusion. Volcanic tuffaceous turbidite sand-slate, siliceous rocks and slump breccia, andesite and andesitic volcanic breccia lava can be found in Jiaolongqiao area. The second sequence is an assemblage of post-collisional (lagging) arc volcanic rock-type andesite-dacite porphyry-rhyolitic porphyry in Late Triassic (represented by Jiangda Formation), with marine-continental facies for the two sequences above. Located in Jijiading area in the middle north margin, the third sequence is an assemblage of basalt, basaltic andesite and radiolarian siliceous slate, while it is an assemblage of spilite, quartz keratophyre and radiolarian siliceous rock in Cuiyibi-Jigaiji areas in the south margin, which were formed in the Late Triassic (Cuiyibi Formation) and are characterized by a bathyal-abyssal environment. Volcanic rocks representing the nature of inter-arc rift basin are also formed after the collision. The Carnian stratum (Jiangda Formation) in the Late Triassic is the most developed. The volcanic rocks in Early Triassic were formed after the accumulation of basale molasses in Pushuiqiao Formation in Lower Triassic stratum, while the volcanic rocks of Carnian in Late Triassic were formed after the accumulation of basale molasses in Jiangda Formation in Upper Triassic stratum, so they are typical post-collisional (lagging) arc volcanic rocks. Rift basins such as Shengda-Chesuo Basin, Xuzhong Basin, Luchun-Hongponiuchang Basin and Reshuitang-Cuiyibi are developed in the superimposed back-arc rift basin from north to south; the superimposed back-arc rift valley basin is a “two-peak” assemblage of tholeiite spilite (low content of TiO₂) and rhyolite, with marine facies and pillow structure; moreover, siliceous rock, carbonaceous slate and a small amount of laminal limestone can be found in this basin. Jiangda-Weixi arc are characterized by the segmentation and inhomogeneity along the strike.

The temporal and spatial distribution of volcanic rocks shows that the Jiangda-Weixi arc was formed by the westward subduction of the oceanic plate of Jinsha River. The subduction started in Late Permian, and the collision started in Early Triassic and ended in Late Triassic. After the collision, the magmatic activity with the characteristics of arc magmatic rocks (lagging arc magmatic rocks) occurred.

• (2) Qamdo-Markam bidirectional back-arc foreland basin (II_5–2)

The back-arc foreland basin was formed on a stable block of Late Paleozoic and exposed on Ordovician stratum-Neogene stratum. Only the Lower Paleozoic stratum is exposed in the Lower Ordovician stratum, which is a sequence of flysch sand-slate mixed with carbonate rocks. Devonian stratum-Permian stratum is a sequence of continental to stable neritic carbonate rocks and clastic rocks mixed with a small amount of volcanic rocks, and the cold and warm water organisms coexisted in Carboniferous-Early Permian, but it is dominated by warm water organisms. The Upper Permian stratum in the Tuoba area is coal-bearing clastic rock, which is rich in the Cathaysia flora. The Lower Triassic stratum is composed of clastic rocks and acid volcanic rocks mixed with carbonate rocks, with a thickness of about 3000 m. The Paleozoic stratum and early, Middle Triassic strata constitute the Qingnidong-Haitong overthrust zone which thrusts westward. The lower part of Upper Triassic stratum is composed of red molasse, the middle part is composed of limestone mixed with clastic rocks, and the upper part is composed of extremely thick coal-bearing clastic rocks, which constitute the main body-back-arc foreland basin with the characteristics of foreland basin deformation. Jurassic stratum-Cretaceous stratum are a sequence of red molasses and copper-bearing prunosus clastic rocks mixed with gypsum-salt, with a thickness of nearly 10,000 m. It is dominated by continental facies but has local marine facies. Distributed in strike-slip pull-apart basins, Paleogene stratum and Neogene stratum are a sequence of red clastic rocks mixed with intermediate-acid volcanic rocks, coal streaks and gypsum-salts. Folds of Mesozoic stratum in Yulong-Markam area are distributed in an echelon on the right side, reflecting the dextral strike-slip orogeny of the faults in Qingnidong-Gongjue area. The Lower Paleozoic stratum is characterized by relatively significant deformation but no significant metamorphism, and it is dominated by greenschist facies but has local amphibolite facies. The deformation of Upper Paleozoic stratum is not significant, and only relatively significant folds are found in the Lingzhihe Bridge area to the east of Haitong on the edge of the basin. Only slight metamorphism is found in the Upper Paleozoic stratum to Lower and Middle Triassic stratum, while no metamorphism is found basically in Upper Triassic stratum and above strata.

Due to the opposite subduction between the oceanic plate of Lancang River and oceanic plate of Jinsha River, some subduction arc volcanic rocks such as an assemblage of andesite and dacite in Late Permian (Jiageding in Markam and Lingzhihe Bridge in Haitong) and andesite-dacite-rhyolite in Middle-Late Permian were formed in the landmass and developed on inward rifted basalt in continental plate of Baoshan Block and Zhongza Block and then lost in the same period; the rifted volcanic rock in Ze’e Formation of Late Triassic and trachyte in Lawula Formation in Neogene occurred in Qamdo Block. The oceanic plate of Jinsha River subducted below the Qamdo Block and remained in the mantle may provide the source conditions for the Yulong porphyry zone and porphyry-type copper deposits in Himalayan.

• (3) Zadoi-Dongda Mountain continental margin volcanic arc (II_5–3)

This arc is distributed in the western margin of Qamdo-Markam Block and controlled by the eastward subduction of the north oceanic plate of Lancang River. It is an assemblage of island-type tholeiite, alkaline basalt, andesite, dacite with prismatic jointing developed, rhyolite and corresponding pyroclastic rocks. The collisional dacitic-rhyolitic volcanic rocks in the Middle Triassic are developed in Zhuka-Yanjing areas along the Lancang River.

• (4) Amojiang-Lvchun continental margin volcanic arc (II_5–4)

This arc is located on the southwest side of the ophiolitic melange zone of Ailaoshan and consists of two sequences of volcanic rocks in Amojiang-Lvchun of Western Yunnan. The first sequence is the Late Permian calc-alkalic basaltic andesite-andesite-dacite (and corresponding pyroclastic rocks) assemblage of transitional facies; the second sequence is the Late Triassic collisional dacite-rhyolite assemblage (Gaoshanzhai Formation) with high contents of SiO₂ and Al₂O₃ and distributed in Lvchun-Yuanyang. The intrusive rock batholith is also distributed in the southern Lvchun area, with the isotopic ages of 230–211 Ma. It mainly consists of type I biotite monzogranite and moyite and also contains a few two-mica granites, all of which are products formed in the same collision period. In particular, a basic volcanic zone with a length of about 30 km is distributed along Bulong-Dalongkai-Wusu-Wannianqing area, which closes to the east side of arc volcanic zone in Late Permian. Its lithology is mainly pillow basalt and contains a few acid volcanic lava. This zone is formed in marine arenopelitic flysch strata in Carboniferous, which is obviously the product of extensional environment, but its exact tectonic properties remain to be investigated.

Collisional granitoids formed in Late Indosinian include Renda, Anmeixi, Jiaren, Baimang Snow-capped Mountain, Ludien, Datuan, Xinanzhai, Bade, etc. from north to south, all of which are distributed in strips along both sides of Jinsha River Junction Zone and belong to normal aluminum-super-saturated series rocks, with an initial value for ⁸⁷Sr/⁸⁶Sr of 0.7175.

• (5) Lanping-Pu’er bidirectional back-arc foreland basin (II_5–5)

The exposed strata in this basin are Silurian stratum-Neogene stratum. Silurian stratum-Devonian stratum are composed of flysch and graptolite shale, and Carboniferous stratum-Permian stratum are composed of littoral-neritic rocks and carbonate rocks, with coal-bearing clastic rocks in the upper part. The whole Paleozoic stratum is nearly 10,000 m thick. Carboniferous stratum and subsequent strata are similar to strata in Qamdo area in the north, but lack a Lower Triassic stratum. The volcanic rocks and intermediate-acid intrusive rocks in this zone are not developed.

The Mesozoic stratum is characterized by deformation of the foreland basin. Folds and thrusts are developed on the east and west sides of the basin due to the back-arc thrust and the westward thrust nappe of Diancang Mountain and Ailaoshan Zone, but the folds became wide and gentle toward the center of the basin. Nappe tectonic group and some dome structures are developed in the Lanping-Yunlong area. The nappe consists of the overturned succession of strata from Waigucun Formation of Upper Triassic stratum to Jingxing Formation of Lower Cretaceous stratum, indicating that it is formed by deformation and displacement of the basement of overlying strata in Lanping-Jiangcheng depression zone, such as the detachment, folding, thrust nappe and slip detachment. Dome structure may be formed by plastic flow of detachment bed and uptrusion of diapiric folds. The fracture zone at the basement of nappe (or slip nappe) has become an important ore-bearing space for Lanping super-large lead–zinc deposit.

Due to the northward pushing of the Indian Plate and the high compression stress caused by the blocking of Yangtze Landmass during Himalayan, Deqin-Weixi area in the waist is significantly compressed, and Qamdo Block in the north and Pu’er Block in the south are extruded toward both ends, forming a group of conjugate strike-slip fault systems in the depression zone. A right strike-slip pull-apart basin and a left strike-slip pull-apart basin are formed in Gongjue area on the east side and in Nangqian area on the west side of the northern block, respectively. A left strike-slip pull-apart basin (Qiaohou and Weishan area) and a right strike-slip pull-apart basin (such as Lanping-Yunlong Basin, Zhenyuan Basin and Jiangcheng-Mengla Basin) are formed on the east side and on the west side of the southern block, respectively. Most of these strike-slip pull-apart basins are important metallogenic basins.

• (6) Yun Country-Jinghong continental margin volcanic arc (II_5–6)

This arc is mainly distributed on the east side of Lincang granite belt and spread basically along Lancang River Valley, and it is mainly composed of two sequences of volcanic rock series, which are overlapped: ① Permian andesitic-dacitic-rhyolitic volcanic rocks, mixed with Late Carboniferous intermediate-acid volcanic rocks sporadically exposed, are mainly distributed in the southern part of the volcanic arc and the western margin of Pu'er Basin; ② Triassic collisional volcanic rocks, post-collision arc volcanic rocks and post-collision extensional volcanic rocks, including (from old layer to new layer): Middle Triassic (Manghuai Formation) collision dacite-rhyolite assemblage → Late Triassic (Xiaodingxi Formation) post-collision shoshonite-latite assemblage (northern member in Yun County) → assemblage of basalt-andesite-dacite-rhyolite (southern member) → Late Triassic (Manghuihe Formation) post-collision extensional kalisyenite trachybasalt-rhyolite “bimodal” assemblage.

1.3.2.6 Lancang River Junction Zone (II₆)

Most scholars believe that the southern member of Lancang River Junction Zone is located on the east side of Lincang magmatic arc, extending southward to Jinghong (border) and connecting with the Nan River zone in Thailand. The north member extends from Caojian to the north, passes through Yingpan of Lanping, the west side of Nanzuo, Meri Snow Mountain and Zhayu of Zuogong and then reaches the Qudeng-Jitang fracture zone, which may be the northwest extension zone of Lancang River Junction Zone. It is connected northwestward with the Ulaan-Uul-Northern Lancang River between Northern Qiangtang and Qamdo Block.

This zone is mainly composed of oceanic ridge basalt, mafic–ultramafic cumulate complex, serpentinite and radiolarian siliceous rock, belonging to ophiolitic melange and representing the remnants of the Lancang River Ocean after its closure.

According to the analysis of the temporal and spatial relationship between this rock zone and the collisional-post-collisional magmatic rock zone (P–T₃) of Yun Country-Jinghong arc, Lancang River Ocean was opened in Early Carboniferous, the oceanic crust began to subduct eastward under the Qamdo-Pu’er Block in Early Permian, and the ocean basin was closed and the arc collided with land in the Middle Triassic.

1.3.2.7 Zuogong Block (II₇)

This block is located between Lancang River Junction Zone and Nujiang River Junction Zone and is covered by the nappe of Gaoligong Mountain from the south of Chawalong to the east of Bijiang River. There is only one narrow belt in Zuogong-Riwoqê-Yaanduo area in the north of Chawalong.

The lower part of Jitang Group has high-grade metamorphism and local migmatization, with amphibolite facies. The Lower Paleozoic stratum is a sequence of low-grade metamorphic clastic rocks mixed with carbonate rocks and metamorphic volcanic rocks; Devonian stratum and Permian stratum are the passive margin sedimentary zone on the west side of Lancang River Ocean, which is a sequence of neritic-bathyal fine clastic rocks, siliceous rocks mixed with volcanic rocks and carbonate rocks. Coal-bearing clastic rocks are found in Upper Permian stratum, which belong to residual marine sediments. Jiapila Formation in Upper Triassic stratum was formed by molasse accumulation, and Bolila Formation was formed by the marine carbonate rocks and coal-bearing clastic rocks in Adula Formation and Duogaila Formation and unconformably overlaid on underlying strata. The Paleogene stratum and Neogene stratum are composed of continental coal-bearing clastic rocks.

Biluo Snow Mountain-Chongshan Block is distributed in the east of Puladi Fault (ductile strike-slip shear zone in the Nujiang River Junction Zone) and the west of the ductile shear zone of Biluo Snow Mountain. It is exposed in a narrow strip due to intense extrusion and shear deformation. The block is mainly composed of high-grade metamorphic rock series of Chongshan Group Complex in Proterozoic stratum, Mode Formation Complex of Carboniferous stratum, volcanic-sedimentary rocks and acid magma intrusion of Permian stratum.

Chongshan Group Complex can be roughly divided into two sequences of rock assemblages. The first sequence is biotite plagioclase gneiss, Amphibolite granulite and siliceous biotite garnet gneiss with significant migmatization, with the characteristics of parautochthonous anatectic granite intruding migmatized metamorphic supracrustal rocks, in which vein flow folds, rootless intrafolial folds and hook folds are developed, showing the structural feature of deep plastic flow rheology. The other sequence is composed of biotite quartz schist, amphibole plagioclase granulite, marble and hornblende schist, with obvious foliation transposition; S₂ foliation is characterized by permeability, which is manifested as bedding shear, development of concealed folds; and the rocks generally show high-grade greenschist facies metamorphism.

Mode Formation Complex is a metamorphic body of Carboniferous relict sediments, and its sedimentary formation is significantly different from that of the Carboniferous stratum in the Nujiang River Zone to the west and is dominated by coarse clastic glutenite mixed with mudstone, siliceous rocks, carbonate rocks and a small amount of volcanic rocks. The lithic quartz graywacke and mudstone form a very frequent sedimentary rhythm, which are a sequence of products of the slope environment at the margin of the block due to rapid accumulation of terrigenous clast.

Jidonglong Formation of Permian stratum is composed of clastic rocks, volcanic rocks and pyroclastic rocks inter-bedded with unstable carbonate rocks. Volcanic rocks are a sequence of basalt-andesite-dacite-rhyolite and tuff assemblages. The environment of sedimentary rocks varies greatly, including abyssal turbidite, adlittoral plateau and slump breccia, which is generally a process of environmental evolution of volcanic arc tectonic facies. ⁴⁰Ar/³⁶Ar isochron age of granodiorite in Biluo Snow Mountain-Zhazhuqing area in the block is 221.9 Ma (Liu et al. 1999); in addition, the intrusion of granite in Cretaceous was developed.

1.3.2.8 Lincang Magmatic Arc (II₈)

The main body of the magmatic arc basement is the Lancang Group Complex of Neoproterozoic stratum. Most of Damenglong Group Complex of Mesoproterozoic stratum have been covered by intrusive magmatic rocks and distributed sporadically in a form of massive rock. The rock types mainly include biotite plagioclase granulite mixed with biotite plagioclase gneiss. Lancang Group Complex had been reformed by several tectonic thermal events. So, it is generally dominated by the quartz-mica tectonic schist, mixed with shear lenses such as granulite, phyllite, marble and metamorphic basic rocks. The metamorphic strata involved in Suyi blueschist zone adjacent to Changning-Menglian Junction Zone in the west side are mainly Lancang Group Complex in the Neoproterozoic stratum, and its high-pressure metamorphic mineral assemblage is characterized by subduction type kinetic metamorphism, which obviously forms a pair metamorphic zone with the high-temperature metamorphic assemblage on the east side being represented by Lincang granite and andalusite.

The main body of the magmatic arc is Lincang composite granite batholith, with isotopic age of 290–208 Ma. The batholith is exposed for more than 300 km (covering an area of 1 × 10⁴ km²) and distributed on the east side of Changning-Menglian Junction Zone. It is mainly composed of emplaced tonalite and granodiorite of Permian and is mainly characterized by general gneissic structure and dominated by type “I” granite in its petrological and geochemistry terms. Emplaced granodiorite and monzogranite of Triassic are exposed for large areas and are mainly characterized by complex rock types and dominated by type “S” arc granite in its petrogeochemistry terms.

Most of the magmatic arc caprocks are denuded, and the remaining sedimentary caprock is continental red beds of Huakai Formation of Middle Jurassic stratum. It is worth noting that the thrust nappe of the magmatic arc zone from west to east was restricted by its significant regional inland crust deformation in Paleogene, so the syntectonic monzonitic moyite in Paleogene is developed into vein, lenticular and stock-like emplacement.

1.3.3 Bangong Lake-Shuanghu-Nujiang River-Changning-Menglian Mage-Suture Zone (III)

1.3.3.1 Bangong Lake-Nujiang River Junction Zone (III₁)

The characteristics of the junction zone in the Nujiang River zone south of Chawalong are not obvious in the east member of Bangong Lake-Nujiang River Junction Zone (III₁), and the Gongshan-Gaoligong Mountain Thrust (or Nappe) Zone lies to the west of the Nujiang River Fault. Ophiolitic zone (with abyssal flysch in Late Triassic to Early-Middle Jurassic as matrix) mixed with mafic rocks, ultramafic rocks, limestone, marble, siliceous rocks and abyssal mudstone is found between Dingqing and Chawalong. Relatively complete ophiolite complex is preserved in the Dingqing area, and its ultramafic rocks belong to the magnesian type. Located between Dingqing and Basu, Jiayuqiao Group (III₃) looks like a large tectonic len sandwiched in the junction zone, just like a tectonic terrane.

The ophiolitic zone between Dingqing and Basu is a part of the whole Bangong Lake-Nujiang River Junction Zone, and a relatively complete ophiolite assemblage can be found in Dingqing and its west, in which basalt is of pillow structure and is close to oceanic ridge basalt in composition. Volcanic rocks can be found, and ultramafic rocks (serpentinite) are mainly exposed in the area from the east of Dingqing to Bangda. Ophiolite was formed in the Late Triassic to Early Jurassic. This zone extends southward and then covered by the huge nappe zone of Gaoligong Mountain, so it is formed in the Middle Jurassic stratum and then passes through Chongshan metamorphic deformation zone in the southeast to connect with the Changning-Menglian Junction Zone.

1.3.3.2 Changning-Menglian Junction Zone (III₂)

Changning-Menglian Junction Zone (III₂) starts from Changning and Shuangjiang in the north, passes through Tongchangjie and Laochang to Menglian and extends to Myanmar from the south. Oceanic ridge basalts with N-MORB characteristics were found in Manxin of Menglian and Tongchangjie, and quasi-oceanic ridge basalts were found in Manxin, Yiliu and Tongchangjie, etc. The age of Tongchangjie is Middle Devonian (Zhangqi, isotope age: 385 Ma), while the age of the rest is Early Carboniferous. Associated siliceous rocks are composed of siliceous rocks of pelagic uncompensated basin, in which radiolarias are pelagic abyssal assemblages. This shows that a quite wide Paleo-Tethyan Ocean Basin existed from Carboniferous to Early Permian, with an estimated maximum width of 1367 km. Picrites with similar composition among many beds, lenticular shape and pillow structure are found in oceanic ridge basalts and quasi-oceanic ridge basalts in Manxin and Menglian. They are formed by the condensation of magma with a large number of accumulated olivine crystals ejected from the sea floor due to the rupture of magma chamber under the spreading ridge. In addition, well-developed sheeted dyke swarms were found in areas like Tongchangjie. All these proves the existence of the paleo-oceanographic spreading ridge. Oceanic island basalts in Carboniferous-Permian were exposed in Manxin, Yiliu, Laochang and other areas, located on oceanic ridge and quasi-oceanic ridge basalts in sequence and formed the basalt-limestone assemblage in the ridge together with the limestone strata overlapped on them, with the transitional relationship between them. The discrimination of oceanic island basalt is a difficult problem in the study of rocks and tectonics. It is difficult to distinguish volcanic rocks from enriched basalts formed in other tectonic environments only according to their lithochemistry and geochemistry characteristics, and it is also necessary to combine the analysis data of sedimentary facies. According to the study made by, the limestone in the basalt bed in the areas above does not contain terrigenous clast, but is composed of plateau carbonate rocks far away from the mainland.

In recent years, ophiolite melange and metamorphic rock of high-pressure eclogite-blueschist in Early Paleozoic have been discovered through 1:50,000 regional geological survey and related monographic study. Ophiolitic melange in Early Paleozoic was formed in the Changning-Menglian junction zone in the form of structural “blocks” with various scales and distributed in the areas of Mengyong-Manghong-Nanting River-Ganlongtang-Niujingshan in the nearly north–south direction, mainly composed of serpentinite pyroxenite, serpentinite olivine pyroxenite, metapolycrystal gabbro, metagabbro, metabasalt, plagioclase amphibolite, epidote chlorite actinolite rock, albitite epidote chlorite schist, amygdaloidal basalt, basaltic andesite, siliceous rock and low-grade metamorphic argillaceous siltstone. The U–Pb age of zircon in the cumulate gabbro and gabbro from Nanting River varies from 439.0 to 453.9 Ma, and it can be concluded from geochemistry characteristics of rocks that it was formed in the oceanic ridge (Wang et al. 2013); the U–Pb age of zircon in Niujingshan metagabbro or plagioclase amphibolite (schist) varies from 428.5 to 450.5 Ma, and it can be concluded from geochemistry characteristics of rocks that it was formed in the oceanic ridge (Regional Geological Survey Report on a Scale of 1:50,000 in Shuangjiang County, 2019); the U–Pb age of zircon in the amygdaloidal basaltic andesite in Laonanzhang, Mengyong is 449.3 ± 8.4 Ma, and it is characterized by azores-type oceanic island in geochemistry terms (Sun et al. 2017); the U–Pb age of zircon in metamorphic gabbro of Manxin is 420 Ma (Wang et al. 2018). Newly discovered ophiolite melange in Early Paleozoic and widely distributed ophiolite melange in Late Paleozoic shows the geological history of the continuous evolution of Sanjiang Proto- and Paleo-Tethys; regionally, Sanchahe Formation in Upper Triassic stratum unconformably overlaid on ophiolite melange, which is an important symbol of basin-range transition in Tethyan tectonic zone.

Metamorphic rocks of high-pressure eclogite-blueschis are distributed in the nearly north–south direction for more than 100 km, along which the high-pressure low-temperature blueschist, high-pressure medium-temperature blueschist, high-pressure medium-temperature eclogite, degenerative amphibole eclogite and so on can be found. Among them, eclogite is exposed from Bingdao, Kongjiao, Genhen River and Bangbing in Mengku Town, Shuangjiang, in the north and extends to Nanpen of Jinghong and Mengsongba in the south through Qianmai of Lancang it is formed in the metamorphic rock series of the “Lancang Group and Damenglong Group” in Precambrian stratum in the form of tectonic len with various scales, and the typical minerals include omphacite, jadeite, lawsonite, coesite, phengite, glaucophane, pyrope and rutile. The restored source rocks are mainly tholeiite with similar geochemistry characteristics as E-MORB, followed by alkaline basalt with similar geochemistry characteristics as OIB (Sun et al. 2017). The U–Pb ages of zircon in eclogite are 801.0 ± 9.8 Ma, 227.0 ± 12 Ma, 447.5 ± 3.6 Ma, 291.7 ± 6.3 Ma, 429 ± 2.4 Ma, 231 ± 2.3 Ma, 254 ± 1.4 Ma and 229.0 ± 1.3 Ma (Regional Geological Survey Report on a Scale of 1: 50,000 in Shuangjiang County, 2019; Sun et al. 2018), and 40Ar/39Ar plateau ages of glaucophane are 409.8 ± 23.6 Ma, 279 ± 1.6 Ma, 215 ± 3.3 Ma and 214 ± 0.9 Ma (Zhai 1990; Zhao et al. 1994), and the evolution history of the subduction and accretion, collision orogeny and decomposition of the Proto- and Paleo-Tethys has been recorded.

1.3.3.3 Yuqiao Residual Arc Zone (III₃)

Yuqiao residual arc zone (III₃) is dominated by Jiayuqiao Group (Pz₂), which constitutes a composite anticline in NW–SE direction. The age of Jiayuqiao Group is still controversial up to now, and then it is determined as a sequence of clastic rocks mixed with carbonate rocks with greenschist facies in Late Paleozoic based on the age of fossil collected and lithologic assemblage characteristics in the regional geological survey on a scale of 1:250,000 (Tibet Institute of Geological Survey, 2007). E’xue Group Complex (C-P) is distributed in the E’xuexiong area, Tongka Township, Basu County in NW–SE direction, which is a sequence of clastic rocks with middle-low greenschist facies mixed with carbonate rocks and basic volcanic rocks. The U–Pb age of SHRIMP zircon in hypoamphibole serpentinite is 267 ± 8 Ma (on a scale of 1:250,000 in Qamdo County, 2007). Zhayu area in Zuogong Country (C–P) is called Rongzhong Group Complex, which is mainly a sequence of clastic rocks with middle and low greenschist facies mixed with assemblage of carbonate rocks, metamorphic basalt, metamorphic rhyolite and siliceous rocks. Faults and ductile shear structures exist between the group complexes and formation complexes above, and significantly deformed muscovite quartz schist, amphibole quartz schist and glaucophane schist sandwiched in greenschist are found in the Bangdashegu Reservoir area; with the tectonic deformation being characterized by large-scale bedding ductile shear zone and bedding concealed fold, it is located in the accretionary complex structure-stratigraphic system.

Most of the residual arc blocks in Mesozoic are uplifted, and littoral-neritic clastic rock and carbonate rock assemblage are only deposited in the marginal zone. The seawater receded in the Late Jurassic and then continental molasse deposition developed. In addition to relatively significant basic volcanic activity in Paleozoic, the intermediate-acid and a small amount of basic volcanic rocks causing magmatic activities of this zone are mainly developed in the lower part of Mesozoic stratum. The intermediate-acid intrusive rocks in Jurassic are boron-rich continental crust remelting granites formed in collide orogenic stage, which may provide a formation environment for arc magmatic rocks related to the southward subduction of Bangong Lake-Nujiang River Ocean.

1.3.4 Gangdise-Gaoligong Mountain-Tengchong Arc-Basin System (IV)

1.3.4.1 Baoshan Block (IV₁)

Baoshan Block is located between Nujiang River southern fault and Changning-Menglian Junction Zone, with exposed strata formed in Sinian stratum to Neogene stratum. The Middle and Lower Cambrian stratum in Sinian stratum is composed of flysch sand-slate mixed with volcanic rocks and siliceous rocks, which are characterized by turbidite and relatively active transitional sediment and are gradually developed into sediment of stable block type neritic clastic rocks and carbonate rocks in Late Cambrian to Permian. Coarse clastic rocks and magnesian carbonate rocks are developed in the west of Ordovician stratum to Devonian stratum, and the water body gets gradually deeper while flowing to the east of Devonian stratum, indicating that its western part is close to the provenance and the eastern part is close to the relatively deep basin. Clastic rocks containing glacial boulders and cold-water fauna Eurydesma, etc. as well as eruption of basalt and basaltic andesite were found in the Upper Carboniferous stratum. Carboniferous stratum is lack of Middle stratum, while Permian stratum is lack of Lower stratum. Multi-layered carbonate rocks in Paleozoic are important nonferrous metal ore-hosting formation. Mesozoic stratum unconformably underlaid on the underlying strata of different ages, which is a sequence of clastic rocks mixed with intermediate-basic and intermediate-acid volcanic rocks, with red molasse accumulated at the top. Molasse is mainly distributed on the east and west sides. The Pliocene stratum in Neogene stratum is composed of glutenite and coal-bearing clastic rocks, with limited distribution. Volcanic rocks in this zone mainly include basalt formed in the Late Carboniferous and basalt-rhyolite formed in the Late Triassic. Riebeckite quartz syenite and alkali granite body containing aegirine and riebeckite are exposed in Muchang Township, Zhenkang Country. The tectonic deformation and metamorphism in Baoshan Block are very weak.

Baoshan Block and Yaando-Gengma passive continental margin in Paleozoic to its east side implicitly reflect the continuous evolution process of Paleozoic Tethys Ocean happened in its east. Volcanic rocks are mainly distributed in Baoshan-Zhenkang area and are mainly composed of continental flood tholeiite in Late Carboniferous. They are characterized by the widespread inclusion of quartz standards mineral molecules, and their geochemistry characteristics are similar to those of basalt in Deccan peninsula, reflecting that their mantle primary development regions have some similarities.

1.3.4.2 Shading-Lhorong Fore-Arc-Basin (IV₂)

The depression zone is located in the south of Bangong Lake-Nujiang River Junction Zone and north of Delong-Chongsha Fault, and the exposed strata include Triassic stratum-Neogene stratum. The Triassic stratum is composed of flysch sand-slate (a thickness of nearly 10,000 m) mixed with volcanic rocks and siliceous rocks and the magmatic arc fore-arc accretion sedimentary system of Boshula Ridge in the south of Nujiang River Ocean. For example, areas in the north such as shading area are composed of bathyal-abyssal turbidite and siliceous rocks, while Luolong-Bianba area in the south is composed of littoral-neritic clastic rocks. The lower Jurassic stratum lacks a lower stratum. The lower part of the Middle Jurassic stratum is composed of red, variegated glutenite, sand shale and limestone, partially mixed with basalt; and the upper part is composed of grayish black shale mixed with sandstone and a small amount of limestone, which are formed by fore-arc depression sedimentation. The Cretaceous stratum is composed of coal-bearing clastic rock, which indicates that the depression zone is coming to an end. The Paleogene stratum and Neogene stratum are composed of intermediate-acid and slightly alkaline volcanic rocks and sand shale, which are formed by rift basin sedimentation. Cretaceous stratum, Paleogene stratum and Neogene stratum unconformably contact with underlying strata.

1.3.4.3 Bowo-Tengchong Magmatic Arc (IV₃)

This magmatic arc is located in the west of Dingqing-Basu Ophiolite Melange Zone, which is generally considered as the southeast extension of Gangdise-Nyainqentanglha composite island arc zone, bounded by Delong-Chongshao fault in the north and adjacent to Shading-Luolong fore-arc-basin and separated from Xiachayu-Sudian metamorphic magmatic melange zone in the south by Bowo-Xiachayu-Binglang River fault. The exposed strata extend from Precambrian stratum to Neogene stratum. Gaoligong Mountain Group and Guqin Group of Precambrian stratum (mainly composed of Paleozoic stratum) and “Precambrian stratum” in Ridong area are a sequence of schist, gneiss, marble and migmatite, in which many mylonite zones are developed. Its source rock is mainly composed of pelites mixed with carbonate rocks, basic and intermediate-basic volcanic rocks. Lower Ordovician stratum are composed of carbonate rocks, but are lack of Middle and Upper Ordovician stratum and Silurian stratum. Devonian stratum-Permian stratum are a sequence of clastic rocks mixed with carbonate rocks and basic volcanic rocks. The lower part of the Devonian stratum is composed of continental clastic rocks, which overlays on Lower Ordovician stratum in the form of para-unconformity. Gravel-bearing slates with glacial marine sedimentation were developed in Carboniferous stratum-Permian stratum. A sequence of abyssal turbidite and slump breccia was developed in the Lower Carboniferous stratum in the Laigu and Songzong area, showing the geotectonic pattern of alternating uplift with depression. The intermediate-basic-acid volcanic rock assemblage such as metamorphic basalt, andesite, dacite and rhyolite of Laigu Formation in Late Carboniferous and its geochemistry properties show the characteristics of arc volcanic rocks at the convergent edge, which shall be regarded as the information that the southwest side of Tethys Ocean has been transformed into active margin.

Volcanic rocks formed in the Jurassic-Cretaceous are mainly composed of calc-alkalic rock series, such as basalt-andesite-rhyolite and corresponding pyroclastic rock assemblage, which coincide with the subduction of Nujiang River Ocean Plate and the age determined for ophiolite. This zone extends westward and connects with the volcanic arc in the northern part of Gangdise-Nyainqentanglha. The temporal and spatial relationship between this zone and the Dingqing-Basu ophiolite (melange) zone indicates that the Bowo-Tengchong arc may be a continental margin arc formed under Lhasa Block due to the westward subduction of the Nujiang River Ocean Plate. The Tengchong volcanic group is a typical representative of the super-position of post-collisional potassic volcanic rocks in Cainozoic. Paleogene stratum and Neogene stratum were formed by accumulation of fluviatile-lacustrine glutenite, coal-bearing clastic rocks, basic and intermediate volcanic rocks and pyroclastic rocks.

Granites are well developed in the zone, and the northern part of the granite zone is composed of 4 large linear rock zones distributed in parallel in NW direction, with an age limit from Late Triassic, Jurassic, Early Cretaceous to Neogene, mainly between 140 and 40 Ma. The age decreases gradually from northeast to southwest. Type “I + S” granite of Paleogene in Xiachayu near the Yarlung Zangbo River Junction Zone forms a composite batholith, which is quite different from the typical continental margin arc magmatic zone. Among them, type “S” granite is composed of granite-alkali-feldspar granite, which is the parent rock of Sn and W mineralization in this zone.

1.3.4.4 Xiachayu Magmatic Arc (IV₄)

This magmatic arc zone is located in the southwest of the study area and is mainly a sequence of metamorphic complex and acid intrusive rocks in Himalayan with significant mylonitization and migmatization. The age of this metamorphic complex is unknown. The Rb–Sr age of the migmatitic gneiss in Yingjiang area in the south is between 1102 and 806 Ma. High-grade metamorphic schists and gneiss with unclear contact relationship (which may be Paleozoic stratum or older rock bed) are found below Permian stratum in Chayu area in the north.

Magmatic activity in this zone is very intense, especially the intermediate-acid intrusive rocks. It is generally recognized that Bowo-Tengchong main arc zone turns westward from Bowo and connects with the intermediate-acid rock zone in Gangdise, Tibet. Although both of them were formed from Late Yanshanian to Early Himalayan, the rocks in the two zones are quite different. Gangdise Zone is mainly composed of syntectic type (type I) granites and less of type S granites and is distributed with contemporaneous intermediate-basic volcanic rocks, so it has characteristics of typical continental margin arc magmatic zone. Bowo-Chayu areas are composed of type S, type I and transitional I-S type granites, while this zone in Himalayan is mainly composed of type I granites with primary mantle development characteristics and also includes type S granite formed in collision period. The Tengchong area in the south is mainly composed of type S granite and less of type I granite but is not distributed with contemporaneous volcanic rocks, so it does not have the characteristics of island arc magmatic zone. In addition, some linear batholiths distributed along Gaoligong Mountain have the characteristics of type I-S granite, which may be the products formed in the same collision period.

The tectonic deformation in this zone is relatively significant. The two metamorphic nappes in Gaoligong Mountain and Xiachayu-Sudian area are composed of many small thrust sheets or tectonic slices and are distributed with many mylonitization zones and ductile shear zones. An arc overthrust nappe protruding eastward (Lhari-Chongsha-Gaoligong Mountain-Ruili-Mandalay) forms a left oblique thrust at the north wing of the arc, a right oblique thrust at the south wing, a positive thrust at the top of the arc and a series of extensional (anti-slip) normal faults at its rear margin. A series of basins formed in Cainozoic and parallelly distributed in Tengchong-Yingjiang area forms a tectonic pattern of alternating basin with ridge, and its formation may be related to extensional collapse. Rawu and Laigu areas also have relatively significant fold deformation, but the latter is even more significant than the former, reflecting the difference of the graben-horst zones in tectonic deformation. In the grade of metamorphism, the metamorphism of Preordovician stratum is of high grade, with amphibolite facies generally and significant migmatization; the metamorphism of Ordovician stratum and Upper Paleozoic stratum is of low grade, with low greenschist facies.

Boshula-Gaoligong Mountain nappe was formed in the Jurassic, and Liuku-Mengga foreland depression was formed in the front margin of its south member. During Himalayan, the nappe structure was developed further, the mountain system rose sharply, and extensional collapse occurred in the rear margin under the balance of gravity, which may be related to Cainozoic basin-range structure.

1.3.4.5 Yarlung Zangbo River Junction Zone (IV₅)

Located between Gangdise magmatic arc and the Kangmar-Lhünzê thrust fold zone in the northern Himalayan, the Yarlung Zangbo River Junction Zone is a long and narrow significant tectonic thrust zone. Pelagic-abyssal radiolarian siliceous sediments, mafic volcanic lava, sheeted dyke swarm, cumulate complex and ultramafic rocks, as well as abyssal plains, turbidite fans and a few flysch sedimentary wedges in fore-arc-basins with neritic bed as the main body in Triassic and Cretaceous are developed in this zone. Many ophiolite zones, ophiolitic melange and other plate boundary marker composed of ultrabasic-basic rocks, basic volcanic lava, radiolarian siliceous rocks and flysch are exposed along the Yarlung Zangbo River tectonic zone, representing an important junction zone in Mesozoic Gondwanaland and the traces of the final destruction of the East Tethys Ocean. The junction zone is composed of Yumen ophiolite tectonic thrust sheet in the Triassic, Zedang-Luobusha-Nang County ophiolitic melange tectonic thrust sheet and Langjiexue fore-arc accretionary wedge in the Late Triassic. Yumen ophiolite tectonic thrust sheet in Triassic is the oldest ophiolite in the southern margin of the Yarlung Zangbo River Junction Zone, and it is composed of ultrabasic rocks dominated by lherzolite, basic rocks dominated by altered diabase and basalt and distal turbidite and siliceous rocks. Although the ophiolite in Zedang-Luobusha-Nang County ophiolitic melange tectonic thrust sheet has suffered significant structural damage, the original sequence similar to the typical oceanic crust member of Troodos can be roughly recovered, namely metamorphic peridotite, cumulate complex, gabbro-diabase, pillow basalt, radiolarian siliceous rock and plagioclase granite from bottom to top. Langjiexue fore-arc accretionary wedge in Late Triassic is composed of turbidite fans, abyssal plains and adlittoral basin sediments of Songre Formation in Late Triassic, Jiangxiong Formation in the Norian and Zhangcun Formation in Norian-Rhaetian. From the end of Cretaceous to Oligocene, the ocean basin of Yarlung Zangbo River was closed, Himalayan Landmass collided with Gangdise Landmass in succession, and a large-scale remelting granite intrusion occurred in Gangdise, forming Gangdise syntectonic type S granite zone and related tungsten, tin and pegmatite-type niobium, tantalum and gem minerals.