Discovery of ∼4.0 Ga detrital zircons in the Changdu Block, North Qiangtang, Tibetan Plateau

Using in situ zircon dating by LA-ICP-MS and MC-ICP-MS, detrital zircon of 3981±9 Ma age was found in metamorphic rocks of the Ningduo Rock Group, Changdu Block of Northern Qiangtang. This is the oldest age record that has been found in the Qiangtang area. This finding also constitutes the third zircon locality in China with an age older than 3.9 Ga. Thus, the discovery provides new information for the study of Hadean crust. In addition, we found 3.51–3.13 Ga, ∼2440 Ma, ∼1532 Ma, ∼982 Ma and ∼618 Ma age peaks from 100 test spots. The younger ages of ∼982 Ma and ∼618 Ma correspond to the formation of the Rodinian super-continent and the Pan-African event, respectively. These findings suggest a close relationship between these zircons and the Gondwanan super-continent. The age of ∼618 Ma defines the lower limit on the deposit time of the protolith for the garnet-mica-quartz schist in the Ningduo Rock Group. Zircons with an age of ∼982 Ma generally display a negative ɛHf(t) and a two-stage Hf model with concentrated ages around 1933–2553 Ma. This pattern indicates that the source area of the Ningduo Rock Group underwent a significant separation of depleted mantle into the crust during the Paleoproterozoic Era. However, zircons with ages of 2854–3505 Ma also show a negative ɛHf(t) and a two-stage Hf model with a concentration of ages around 3784–4316 Ma. These results demonstrate that the source area of the Ningduo Rock Group contains a residual amount of ancient (Hadean) crustal materials. This paper provides new information on the relationship between the basement of the Qiangtang area and the Paleoproterozoic basements of the Gangdese and Himalayan regions, which constrains the northern boundary of Gondwana.

Exploring the formation and early evolution of the Earth has been an important part of Earth science research. This type of research has involved a series of important scientific studies [1], including identification of when the first continent formed, the nature of this material, and whether recycling between crust and mantle existed during the early evolution of the Earth. Complex and diverse geological processes over time have resulted in a limited distribution of rocks formed during the Hadean (4.56-3.85 Ga) [2,3]. Until now, crustal rocks with age of more than 3.8 Ga only have been discovered in Greenland [4,5], Canada [6,7], Anshan in China [8][9][10] and Eastern Antarctica [11][12][13]. The oldest rocks that have been found on Earth are from the Acasta gneiss of the Wopmay orogenic belt in Canada, and they have been dated to ~4016 Ma [14]. Although there are limited records of rocks surviving from early Earth, abundant ancient zircon residues have been found within younger geologic bodies [15]. After a long exploration, a series of detrital zircons with ages of ≥3.8 Ga have been discovered in the northeastern Bavaria region in Germany [16], northern Wyoming, USA [17], the Yilgarn craton of Western Australia [18], the San Francisco craton in Brazil [19], the

Regional geological setting
The study area is located in the Changdu Block of northern Qiangtang in Tibet, and lies between the Xijinwulan-Jinshajiang and Bangonghu-Nujiang suture zones in western Yushu County, Qinghai Province ( Figure 1). The Changdu Block is a narrow, NE-trending belt situated along the south of the Bayankala orogenic belt. Because of the Mesozoic-Cenozoic cover and the fractured dislocation of Precambrian basement rocks toward the NW-SE, they are scattered as faulted blocks, mainly consisting of the Ningduo and the Xiongsong rock groups. The "Southwest Regional Stratigraphy" project has provided a revision of Precambrian stratigraphic sequences of this region, and concluded that the Xiongsong and Ningduo rock groups are synonymous [29], such that the "Xiongsong Rock Group" (named in Mangkang County-Batang County) now belongs in the Ningduo Rock Group. Consequently, the basement of the Changdu Block is composed of only the Ningduo Rock Group, which is distributed mainly in Duocai, Zhiduo County, Qinghai Province; Longbao and Xiaosumang, Yushu County; the Jinshajiang region in western Gongjue County-Mangkang County; and eastern Jiangda County-Batang County on the border between Tibet and Sichuan.
This paper focuses on the Ningduo Rock Group in Longbao (Zanie), Yushu County, Qinghai Province. The Ningduo Rock Group in this area appears as a narrow belt located between the Xiewu Ophiolite mélange zone and the Longbao ophiolite mélange zone, and mainly consists of high-grade greenschist to low-grade amphibolite facies metamorphic rocks, such as biotite plagioclase gneiss, garnet-mica quartz schist, quartzite, granulite with amphibolite schist, and chlorite-amphibole schist. The occurrence of these rock types is at 170°∠62°. The protolith mainly consists of sedimentary clastic rocks, basic-intermediate volcanic rocks, and carbonate rock formations with high maturity. The age of the Ningduo Rock Group has long been disputed. Previous surveys have classified the Qinghai Province into the upper Paleozoic. The 1:200000 regional geological survey [30,31] inferred that the Ningduo Rock Group should be placed in the Paleo-Mesoproterozoic. This suggestion is based on zircon U-Pb ages of 2200±140 Ma from two-mica plagioclase gneiss in Xiariduo, 1870±280 Ma from biotite plagioclase gneiss in the Changkeqing area, and 1780±150 Ma and 1680±309 Ma from gneissic biotite granite in Dasumang and Xiaosumang. However, the 1:250000 regional geological survey of Zhiduo and Qurougaka obtained a single zircon with ages of 709±66 Ma [32] and 835-915 Ma [33], which are considered to be the metamorphic age of the Ningduo Rock Group. Based on the above isotopic ages, rock associations, metamorphism, nonlayered deformation characteristics, and regional stratigraphic correlations, we tentatively placed the Ningduo Rock Group in the Meso-Neoproterozoic [34].
Two ophiolite mélange zones extend along the NW-SE portion of the study region. Both are dismembered from the Tongtianhe ophiolite mélange zone in the 1:250000 regional geological survey of Yushu [35]. Regionally, these are equivalent to part of the Xijinwulan-Jinshajiang suture zone. Following later tectonic events, the Ningduo Rock Group was strongly fractured and foliated, and integrated into a mélange belt of the Longbao ophiolite in form of rock blocks. The northwest-trending faults are well-developed in this region, and constitute boundaries among different stratal units. A great number of diabase-babbro dykes are found in the northwest. They intruded the epimetamorphic clastic rocks with medium basic to medium acidic volcanic lava and volcaniclastic rocks of the Yiji Formation in the early Devonian.

Analytical methods
The selection of a single mineral zircon was accomplished in the Langfang Laboratory, Hebei Bureau of Geology and Mineral Resources, Minister of Land and Resources. The sample (07ND-1) was crushed to pass through a 20−30 mesh. About 180 grains of zircon were obtained after flotation and magnetic separation. A total of 120 zircons with no cracks or inclusions were selected under a binocular microscope for further tests. These samples were embedded into epoxy and polished to expose the internal structures of zircon grains. Before zircon U-Pb dating with LA-ICP-MS and MC-ICP-MS, reflected and transmitted light microscopy and cathodoluminescence (CL) image analyses were undertaken.
Zircon CL images were taken at the State Key laboratory of Continental Dynamics, Northwest University. The micrographs of the internal structures of zircons were produced with a Cathode-fluorescent spectrometer (Mono CL3+) manufactured by Gatan in the United States.
The zircon U-Pb and Lu-Hf isotopic compositions were analyzed at the State Key Laboratory of Continental Dynamics, Northwest University. The test spots from 07ND-1-01 to 07ND-1-60 were undertaken by LA-ICP-MS, using 30 μm laser beam ablation. However, the spots from 07ND-1-1b to 07ND-1-68b were tested by MC-ICP-MS, with 44 μm laser beam ablation. The isotope ratios and element contents were calculated with Glitter software (Ver4.0, Macquarie University). According to the method of Andersen [36], isotopic ratio corrections were conducted to eliminate the effects of common lead. Based on the average crustal Pb isotopic composition given by Stacey, we corrected the isotopic ratio and common lead. Concordia age calculations and mapping were achieved with Isoplot (Ver3.0) [37]. Additional detailed equipment characteristics and analytical procedures are found in [38][39][40].

Zircons
Most zircons collected from the garnet-mica quartz schist (07ND-1) in the Ningduo Rock Group were rounded (some of the defects may have been caused by mechanical damage during sample grinding), with great difference in size (60-180 μm) and color variations (from light brown, light smoky gray, light yellow, light pink to colorless). These variations indicate a multi-source for these detrital zircons. From the CL images, it is clear that the majority of the zircons appeared rounded and had developed internal zonation ( Figure 2), a characteristic of origination from a magma source. A few zircons (14b, 26b, 27b, 68b) developed fuzzy and wide internal zonation, which may have been caused by later alterations. Individual zircons (2b, 42b, 66b) had mottled structures, and some (13, 50b, 44b) had a kernel. Some zircons with ages >900 Ma (5, 25, 26, 47, 50, 44b, 66b) were surrounded by dark overgrowths, revealing that they never underwent strong tectono-thermal events. The No. 49 (Figure 2(a)) zircon appeared rounded and showed a welldeveloped core, mantle and rim structure, which indicates multi-growth.
There were a few ancient detrital zircons in the garnet-mica quartz schist of the Ningduo Rock Group (07ND-1), and the oldest zircon (No. 49) appeared with a coremantle-rim structure. The 207 Pb/ 206 Pb age of 3981±9 Ma was tested in the core. The test spot was located below and close to the concordia line in the graph (Figure 3 The zircon U-Pb concordia diagram (Figure 3(a)) and the chart of age frequency distribution (Figure 3(b)) showed that the concordia ages concentrate in four groups: 1200-850 Ma, with a peak age of about 982 Ma. Most of the test spots in this group were located in or adjacent to the concordia line, and most of the zircons in this group were rounded, and had developed internal oscillatory zonation ( Figure 2(b)), with higher Th/U ratios (generally in 0. 31-4.67). This information suggests that these zircons came from a magma source that experienced the Grenville tec-tono-magmatic events. Another 206 Pb/ 238 U age group was 700-530 Ma, and the peak age was ~618 Ma. These samples had rounded to angular zircons with developing zonation ( Figure 2(b)), and higher Th/U ratios (0. 38-5.92). In addition, the 207 Pb/ 206 Pb ages were found to be around 2600-2300 Ma and 1700-1400 Ma, and the age peaks were ~2440 Ma and ~1532 Ma. These zircons were rounded, had wide developing internal zonation, and higher Th/U ratios (0.33-1.23 and 0.23-2.50). From these results, it can be concluded that these zircons came from a source that experienced tectono-magmatic events occurring about 618, 2440 and 1532 Ma BP.

Hf isotopes
There were 68 test spots obtained for the Lu-Hf isotope test on zircons from the garnet-mica quartz schist of the Ningduo Rock Group. A total of 18 spots (3b, 5b, 6b, 13b, 17b, 23b, 24b, 28b, 29b, 32b, 33b, 39b, 41b, 46b, 51b, 53b, 54b, 67b) were rejected because of low confidence in the data caused by low Hf contents. Thus, 50 valid test spots remained ( Table 2). The low 176 Lu/ 177 Hf ratio (mostly <0.002) in all zircons showed that there was lower accumulation of radiogenic Hf after their crystallization. The data on the 176 Hf/ 177 Hf ratios can be divided into four groups ( Figure  4

Discussion
The U-Pb ages of the garnet-mica quartz schist in the Ningduo Rock Group of the Longbao area concentrated in four groups -2600-2300 Ma, 1700-1400 Ma, 1200-850 Ma and 700-530 Ma -and their peak ages were ~2440 Ma, ~1532 Ma, ~982 Ma and ~618 Ma, respectively. The first peak age of ~982 Ma was consistent with the Grenville orogeny (1190-980 Ma) [41][42][43] caused by convergence of Rodinia. The second peak age of ~618 Ma was the youngest age of the erosion source area, and was close to the Pan-African Movement (roughly between 600-550 Ma) [44,45]. These results indicate that some of the zircons of the Ningduo Rock Group came from a source that experienced the Pan-African event. These zircons with young ages also constrained the lower limit of the deposition time of this stratum. In other words, these zircons likely formed in the late Neoproterozoic. The zircons with age peaks of ~2440 Ma and ~1532 Ma were rounded, developed wide internal zonation, and had high Th/U ratios. This indicates that they came from a source that experienced tectonomagmatic events that occurred about 2440 Ma and 1532 Ma ago. In addition, there were a few ancient detrital zircons with ages older than 3100 Ma, among which is the oldest one with a 207 Pb/ 206 Pb age of 3981±9 Ma, as well as others with 207 Pb/ 206 Pb ages of 3505±18 Ma, 3450±8 Ma, 3211±20 Ma and 3127±10 Ma. For zircons with ages of 3981±9 Ma, this is the oldest record of the crust that has been discovered in the Qiangtang area, and it also is the third oldest zircon that has been discovered in China, after that in Tibet and Qinling. These results indicate that there residual Hadean signatures are preserved in Northern Qiangtang. This information is crucial to studies of crustal rocks and evolution of early Earth.
Zircon U-Pb dating and Hf isotopic analyses have provided important information regarding the study of the growth and recycling of continental crust [46,47]. Hf model ages indicate that the age of the magma source in the Qiantang area is the growth age of the crust, at the time it separated from the depleted mantle. However, most zircons did not form directly via mantle differentiation from parent magma. Thus, in this case, the two-stage model of ages is more closely related to the growth age of the crust. The Hf isotopic results indicate that zircons with ages of ~982 Ma produced a negative ε Hf (t), and the two-stage model of ages yielded an age of about 1933-2553 Ma, which verifies that there may have been initial crust in the Paleoproterozoic in the source area of the Ningduo Rock Group. A few zircons with ages around 2854-3505 Ma also showed a negative ε Hf (t), and the T DM2 yielded ages within 3784-4316 Ma. This information indicates that the source area of the Ningduo Rock Group was much older (Hadean) crust than the residue, which is consistent with the zircon age of 3981±9 Ma. These results probably indicate that there were crustal materials of the initial formation stage of the Earth in the Changdu Block of Northern Qiangtang. It is difficult to confirm from the source area of these zircons, but based on the two-stage model of ages and the negative ε Hf (t), their source may have been TTG rocks that intruded into the crust after separating from the depleted mantle during the Hadean.
Presently, there is great controversy regarding whether Precambrian basement existed in Qiangtang. Most researchers have believed that the basement of the Qiangtang consists of two-layered Proterozoic metamorphic rocks [48][49][50][51]. Nonetheless, Wang et al. [52] indicated that there probably was an Archean continental core in the Qiangtang area. Based on combined data sets of zircon U-Pb and Rb-Sr isotopic ages (318-384 Ma) and a field survey of the Qiangtang area, Li [53] suggested that the main time of formation of the epimetamorphic rocks in central Qiangtang was that of the late Carboniferous. However, the 1:250000 regional geological survey obtained some ages for Jiningian in Eastern Qiangtang. In recent years, SHRIMP U-Pb ages of cumulate gabbros have revealed age clusterings around 438±11 Ma [54], which is useful for determination of the Ordovician ophiolite in the hinterland of Qiangtang. Based on the youngest age of ~618 Ma and regional geological features, we suggest that the Ningduo Rock Group formed in the late Neoproterozoic to Palaeozoic.
It is worth mentioning that detrital zircons with ages of 4.0-4.1 Ga were discovered in Pulan County and Western Nepal in the Himalayas, and a number of ages consistent with Pan-African events were reported [55][56][57]; whereas, the Himalayan-Gangdese region has been considered to belong to East Gondwana. Based on U-Pb ages and Lu-Hf isotopic studies on metamorphic rocks of the late Neoproterozoic in the Cathaysia Block, Yu et al. [58] obtained many detrital zircon ages for the Grenville (~1.0 Ga), Neoarchean, and a few ages in the Eoarchean (~3.8 Ga), Paleo-Mesoar-chean (3.3-3.0 Ga), Paleoproterozic (1.7-1.4 Ga), Mesoproterozoic (1.7-1.4 Ga) and Pan-Africa (0.70-0.54 Ga) intervals. They believed that the source was mainly Eastern India-Eastern Antarctic, and suggested that the southern margin of the South China Block was close to India and Eastern Antarctic in the period of the Rodinian divergence and Gondwanan convergence. Other than the zircon age of 3.98 Ga, many ages in the Grenville (~980 Ma) and Pan-Africa (~618 Ma) intervals were obtained. We speculate that there were similarities between the Changdu Block and Gondwana. Thus, it is important to study the relationship between the basements of Qiangtang and that of the Gangdese and Himalayan areas to further confine the northern boundary of Gondwana.

Conclusions
(1) For detrital zircons collected from garnet-mica quartz schist of the Ningduo Rock Group in Changdu Block, northern Qiangtang, ages of 3981±9 Ma and 3505±18 to 3127±10 Ma tested by LA-ICP-MS revealed that there were ancient crustal residues in the Changdu Block.
(2) Detrital zircon ages of the Ningduo Rock Group mainly clustered in four groups --2600-2300 Ma, 1700-1400 Ma, 1200-850 Ma and 700-530 Ma --with age peaks of ~2440 Ma, ~1532 Ma, ~982 Ma and ~618 Ma, respectively. The first peak at ~982 Ma is consistent with the Grenville orogeny caused by convergence of Rodinia, and the second peak of~618 Ma is roughly consistent with the Pan-African Movement, which resulted from convergence of Gondwana. These data indicate that there was a relationship between the Changdu Block and Gondwana. This new evidence is of great importance with respect to understanding the relationship between the basement of the Qiangtang and that of Gangese and Himalayan.
(3) The youngest detrital zircon age was ~618 Ma in the Ningduo Rock Group, which may constrain the lower boundary of the stratum from late Neoproterozoic to early Paleozoic.
(4) Detrital zircons with ages of ~982 Ma were negative in ε Hf (t), and the two-stage Hf model of ages yielded about 1933-2553 Ma. These results suggest that the source area of the Ningduo Rock Group was separated into crust from the depleted mantle during the Paleoproterozoic. In other words, there were crustal growth events in the Paleoproterozoic. However, zircons with ages of 2854-3505 Ma showed a negative ε Hf (t) and their two-stage Hf models of ages were around 3784-4316 Ma, which demonstrates that there still were much older (Hadean) crustal residues in the source area of the Ningduo Rock Group.