J. Cent. South Univ. (2019) 26: 3470-3487

DOI: https://doi.org/10.1007/s11771-019-4267-4

Origin of Early Creceouscalc-alkaline granite, Taxkorgan:Implications for evolution of Tethys evolution in central Pamir

LI Rui-hua(李睿华)^{1, 2}, PENG Bo(彭勃)¹, ZHAO Cai-sheng(赵财胜)³,

YU Miao(于淼)^{1, 2, 4}, SONG Lin-shan(宋林山)⁵, ZHANG Han(张晗)^{1, 6}

1. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources,CAGS, Beijing 100037, China;

2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;

3.Technology and International Cooperation Division, Ministry of Nature Resources of China (MLR),Beijing 100034, China;

4. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment

Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;

5. Geological Research Academy of Xinjiang, Urumqi 830011, China;

6. Yanduzhongshi Geological Analysis Laboratories Ltd., Beijing 100094, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The Pamir plateau may have been a westward continuation of Tibet plateau. Meanwhile, the Rushan-Pshart suture is correlative to the Bangong-Nujiang suture of Tibet, and the Central Pamir is the lateral equivalent of the Qiangtang Block. We present the first detailed LA-ICPMS zircon U-Pb chronology, major and trace element, and Lu-Hf isotope geochemistry of Taxkorgan two-mica monzogranite to illuminate the Tethys evolution in central Pamir. LA-ICPMS zircon U-Pb dating shows that two-mica monzogranite is emplaced in the Cretaceous (118 Ma). Its geochemical features are similar to S-type granite, with enrichment in LREEs and negative Ba, Sr, Zr and Ti anomalies. All the samples show negative zircon εHf(t) values ranging from –17.0 to –12.5 (mean –14.5), corresponding to crustal Hf model (T_DM2) ages of 1906 to 2169 Ma. It is inferred that these granitoids are derived from partial melting of peliticmetasedimentary rocks analogous to the Paleoproterozoic Bulunkuole Group, predominantly with muscovite schists component. Based on the petrological and geochemical data presented above, together with the regional geology, this work provides new insights that Bangong–Nujiang Ocean closed in Early Cretaceous(120-114 Ma).

Key words: Tethys ocean; Pamir plateau; S-type granite; Early Cretaceous tectono-magmatism; Geochronology and petrogenesis

Cite this article as: LI Rui-hua, PENG Bo, ZHAO Cai-sheng, YU Miao, SONG Lin-shan, ZHANG Han. Origin of Early Creceouscalc-alkaline granite, Taxkorgan: Implications for evolution of Tethys evolution in central Pamir [J]. Journal of Central South University, 2019, 26(12): 3470-3487. DOI: https://doi.org/10.1007/s11771-019- 4267-4.

1 Introduction

Pamir tectonic salient located at the junction between the Pan-Asia and the Tethyan tectonic domain (Figure 1(a)). Its tectonic evolution was closely related to the Paleo-Tethys and Neo-Tethys evolution [1-14]. Many scholars study on volcaniclastic strata, magmatic rocks and tectonic evolution of Pamir tectonic salient, showing that the Pamir plateau may have been a westward continuation of Tibet plateau [1, 6, 15-21]. The Tanymas suture of the central and northern Pamirs is part of the Jinsha suture [22], the Rushan-Pshart suture is correlative to the Bangong-Nujiang suture of Tibet and the Central Pamir is the lateral equivalent of the Qiangtang Block [19, 22-25], the Indus-Yalu suture of Tibet is correlative to the Shyok zone in the southern Pamirs [19, 22, 26]. (Figure 1(a)).

In Neo-Tethys tectonic domain, northward subduction of Neo-Tethyan oceanic lithosphere and related magmatism in Cretaceous have been widely discussed in Lhasa and Qiangtang terrane in the past two decades [27-33]. However, Cretaceous magmatism of Pamir has a sparse database. It is uncertain whether they have similar petrogenesis to Qiangtang terrane and tectonic evolution background. Moreover, the closure time of the Rushan ocean between the Central and Southern Pamir remains controversial, ranging from Middle Jurassic [32, 34-37] to Early or Middle Cretaceous [32, 37-43]. We present the first detailed LA-ICPMS zircon U–Pb chronology, major and trace element and Lu-Hf isotope geochemical investigations of Early Cretaceous granite in Taxkorgan. These new data allow us to explore the origin of the two-mica monzogranite and their relationship to the Neo-Tethys evolution in Pamir tectonic salient. Meanwhile, these data can help understand the closure time of western part of Bangong-Nujiang suture, which will bring new insights into the tectonic history of Tethys evolution.

2 Geological setting and geology of granite

2.1 Geological setting

The Pamir plateau forms a prominent tectonic salient along the western end of the Tibet-Tarim margin, comprising east-west trending Mesozoic– Paleozoic terranes and intervening sutures broadly correlative with units exposed to the east, in Tibet (Figure 1(b)) [1, 6, 20, 22, 44-46]. Pamir tectonic salient is divided into northern, central and southern Pamir by Akbaytal-Tanymas and Rushan-Pshart suture, respectively [1]. The northern Pamir is a Paleozoic arc and subduction-accretion complex like the Kunlun and HohXil–Songpan- Ganziterranes of northern Tibet [19, 22, 47]. The central Pamir comprises Cambrian–Jurassic platform rocks correlative with the north Qiangtang block [15, 48]. In Central Pamir, the sequence of rock reveals a Paleozoic history culminating with unconformable deposition of late Carboniferous and early Permian sandstone, limestone, and marl [1]. The southern Pamir consists of Proterozoic gneiss, Paleozoic–Mesozoic metasedimentary rock, and Cretaceous–Paleogenegranitoid equivalent to the south Qiangtangblock in Tibet [44, 48].

The Precambrian strata mainly outcrop between Karakorum fault and West Kunlun suture, such as Taxkorgan, Dabuda, Resikamu, Arkarz and Kudi, consisting mainly of gneiss and amphibolite [49, 50]. Ordovician strata mainly consisted in northern part of West Kunlun suture, including sandstone and limestone. Carboniferous strata mainly scattered throughout most of the study area, consisting mainly of limestone, slate, silty mudstone and shale. Permian strata can be seen nearby the Karakorum fault and the West Kunlun suture in region map of southwestern Xinjiang, consisting mainly of intermediate-acid volcanic rocks and neritic facies sedimentary rock. Jurassic-Cretaceous strata mainly outcrop nearby the Karakorum fault, the Jinsha suture and the West Kunlun suture, which consisted of the limestone, slate and silty mudstone (Figure 1(c)). In Western Kunlun suture, there are Ordovician-Permian intermediate-acid intrusive rocks, Triassic granites consisted in Jinsha suture, and Jurassic-Cretaceous granites consisted on both sides of Karakorum fault. Meanwhile, these rocks are controlled by northwest trend faults, such as Karakorum fault and Taxkorgan fault. From West Kunlun suture to Karakorum fault, magmatite emplacement ages become younger; this phenomenon can be further supported by closure time of different sutures (Figure 1(c)).

2.2 Geology of granite

The study area strata are consisited of Triassic Limestone, Permian Carbonaceous Slate and Paleoproterozoic Buluokuole group amphibole gneiss. The study area also developed lots of magmatites, which are northwest-trending planar distribution, and these magmatitesare mainly consisted of intermediate-acid rocks from Paleozoic to Cenozoic, especially Mesozoic and Cenozoic magmatic activities are the strongest and the largest scale. Mesozoic rock mainly consisted of two-mica monzogranite, which intruded into the lower Permian strata (mainly siltstone and sandstone). Meanwhile, two-mica monzogranite also was intruded by the Cenozoic porphyritic granite [57] (Figure 2).

Figure 1 Tectonic map of Central Asia (modified after Refs. [46, 52-55]) (a) and sketch map of geology in Southwestern Xinjiang (b)

The two-mica monzogranite (PM-7) is fine- grained and locally shows flesh pink (Figure 3(a)). It consists mainly of K-feldspar (25 vol%-30 vol%), quartz (25 vol%-30 vol%), plagioclase (20 vol%-25 vol%), muscovite (5 vol%-10 vol%) and biotite (5 vol%-8 vol%), with accessory zircon, apatite, and epidote (Figure 3(b)). Plagioclase (1-1.7 mm) is mainly anhedral to subhedral, with polysynthetic twinning or oscillatory zoning. K-feldspar (1-1.5 mm) is anhedral with tartan twinning. Some K-feldspar develops slight kaolinization alteration. Quartz is anhedral with a diameter of 1-2.2 mm. Muscovite (0.3-1.2 mm) is protogenesis.

Figure 2 Simplified geological map of Taxkorgan [56]

Figure 3 Photograph (a) and photomicrograph (b) of two-mica monzogranite (Kfs-K-feldspar; Mus-muscovite; Pl- plagioclase; Qz-quartz; Bi-biotite)

3 Sampling and analytic methods

Zircon U-Pb dating of PM-7 from 81°04′39″E, 37°38′28″N is shown in Figure 4. We collected four samples from surface exposures of two-mica monzogranite, which is nearby to zircon sample. All samples were crushed to 75 μm using an agate mill for whole-rock geochemical analysis.

U-Pb dating of zircons was conducted synchronously by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as a description by Refs. [58-60]. Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50 s of data acquisition from the sample. The Agilent Chemstation was utilized for the acquisition of each analysis. Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were performed by ICPMSDataCal [58, 59], detailed experiment progress and data processing method also can be seen in Refs. [58-60].

Hafnium isotopic ratios of zircon were conducted by LA-MC-ICP-MS at Nanjing FocuMS Technology Co. Ltd., China. Teledyne Cetac Technologies Analyte Excite laser-ablation system (Bozeman, Montana, USA) and Nu Instruments Nu Plasma II MC-ICP-MS (Wrexham, Wales, UK) were combined for the experiments. The 193 nm ArFexcimer laser, homogenized by a set of beam delivery systems, was focused on zircon surface with a fluence of 6.0 J/cm². Ablation protocol employed a spot diameter of 50 μm at 8 Hz repetition rate for 40 s (equating to 320 pulses). Helium was applied as a carrier gas to efficiently transport aerosol to MC-ICP-MS. Two standard zircons were treated as quality control every ten unknown samples [61, 62].

The major and trace element analysis of the bulk rock samples was mainly carried out in the Yanduzhongshi Geological Analysis Laboratories Ltd. The bulk rock major elements were analyzed using X-ray fluorescence spectrometry techniques (Zetium, PANalytical). The analytical errors for major elements were better than 1%. Trace elements of those samples were analyzed by inductively coupled mass spectrometry (ICP-MS). The analytical uncertainty of the elements examined here was better than 5% for ICP-MS analysis, except for a few samples with low contents of trace elements for which the uncertainty was about 10%. The obtained values of trace elements in the GSR-2 standard are all consistent with their recommended values.

4 Results

4.1 LA-ICPMS zircon U-Pb dating

LA-ICPMS zircon U–Pb ages have been determined for PM-7. The results are summarized in Table 1, and zircon CL images and U–Pb Concordia plots are illustrated in Figure 4. All zircons, with the length mostly between 100 and 250 μm, show regular oscillatory magmatic zoning (Figure 4(c)), consistent with a magmatic origin. Twenty analyses from PM-7 on the Concordia curve yield weighted mean ²⁰⁶Pb/²³⁸U ages of (118±0.86) Ma (Figures 4(a) and (b)), belonging to Early Cretaceous, which is the best estimation of magma crystallization.

4.2 Whole-rock geochemistry

Whole-rock major and trace element data for the two-mica monzogranite (PM-7) from Taxkorgan is shown in Table 2. The samples from PM-7 have SiO₂ of 72.95 wt%-74.58 wt%, and plot in the field of granite in total alkalis (Na₂O+K₂O) versus SiO₂ (TAS) diagram (Figure 5(a)). They are all peraluminous with high A/CNK (molar ratio of Al₂O₃/(CaO+Na₂O+K₂O))>1.1 (Table 2 and Figure 5(c)). These host rocks are Subalkaline (Figure 5(a)) with high K₂O in the field of high-K calc-alkaline series (Figure 5(b)). The REE contents are relatively low (32×10^-6-49×10^-6). Chondrite- normalized REE patterns display light REE enrichment with strong negative Eu anomalies (Eu/Eu*=0.44-0.62) and (La/Yb)_N of 4.15–8.0. The REE patterns of PM-7 display a similar trend to the S-type granite of North and South Qiangtang, besides, collisional granites of Bangong-Nujiang suture display more strong negative Eu anomalies trend than samples of PM-7 (Figure 6(a)). The four samples exhibit similar trace element patterns that they have similar negative anomalies in Ba, Sr, Ti and positive anomalies in Rb, Th, U, K as shown in the primitive mantle-normalized trace-element diagrams. Comparing to the S-type granite of North and South Qiangtang, PM-7 also has the similar trend, besides, the contents of Ba, Sr, P from collisional granites of Bangong-Nujiang suture have more strong negative anomalies (Figure 6(b)).

4.3 Hf isotopes in zircon

In situ Hf isotope analyses have been carried out on zircons at the same spots used for LA-ICPMS zircon U–Pb dating. The results for the mean zircon Hf isotopic composition for each sample is presented in Table 3 and illustrated in Figure 7. The initial ε_Hf(t) isotopic compositions are calculated at t=118 Ma. All analyzed grains show that initial ¹⁷⁶Hf/¹⁷⁷Hf ranges from 0.282223 to 0.282347 and negative ε_Hf(t) values ranges from -17.0 to -12.5 (mean -14.5). The two-stage Hf model ages range from 1.97 to 2.25 Ga, and the T_DM ages is from 1264 to 1471 Ma.

Table 1 LA-ICP-MS U-Pb zircon analytical data of two-mica monzogranite from Taxkorgan

Figure 4 Zircon U-Pb Concordia diagram (a), weighted mean age diagram (b) and CL image of zircons (c) for two-mica monzogranite (n-number of measuring points)

Table 2 Major and trace element compositions of two-mica monzogranite from Taxkorgan

Continued

Figure 5 Alkali versus silica (TAS) diagram [63] (alkaline-sub-alkaline curve from [64]) (a), SiO₂-K₂O diagram [65] (b) and A/CNK-A/NK diagram [66] (c) for two-mica monzogranite

Figure 6 Chondrite-normalized REE patterns (after Ref. [67]) (a), and primitive mantle-normalized trace element spider diagrams (b) of Taxkorgan Two-mica monzogranite (after Ref. [68]) (S-type granite of north Qiangtang from Ref. [69]; S-type granite of South Qiangtang from Ref. [70]; collisional granites of Bangong-Nujiang suture from Ref. [71])

Table 3 In-situ zircon Hf isotopic data of two-mica monzogranite from Taxkorgan

Figure 7 Hf isotope compositions versus U-Pb ages of zircon of granite porphyry from Taxkorgan two-mica monzogranites (data of Amdo after Ref. [71] and A’ranbaotai after Ref. [70])

5 Discussion

5.1 Petrogenesis of Taxkorgan two-mica monzogranite

The Taxkorgan two-mica monzogranite with characteristics of high-K calc-alkaline (Figure 5(b)), strongly peraluminous (Figure 5(c)), both REE patterns and spider diagrams are similar to S-type granite from North and South Qiangtang(Figures 6(a) and (b)), meanwhile, Al₂O₃ (2.36-2.6) can be found in CIPW standard mineral analysis, which is also plotted in the field of S-type granite on the ACF diagram (Figure 8). All of these lines of evidence indicate that the Taxkorgan two-mica monzogranite can be classified as S-type [72-75]. As known, strongly peraluminous granites (S-type granite) are divided into two groups: the muscovite- bearing peraluminous granitoids (MPGs, unusually two-mica monzogranite to leucogranite) and the biotite-rich, cordierite-bearing peraluminous granitoids (CPGs, unusually tonalite, granodiorite and monzogranite) [76]. These two types of granites can probably be produced during the same geodynamic event and possibly from the same source, but they are not different members of the same magmatic suite [76], PM-7 enriches primary muscovite without cordierite, belonging to the MPGs. Previous studies on experimental petrology and geochemistry have documented that peraluminous granitoids (including strongly peraluminous or S-type granites) are mainly generated by partial melting of metasedimentary rocks (metapelites and metagreywackes) under water-undersaturated conditions, possibly added with metaigneous rocks [77-81].

Figure 8 Plot of An(Al-Na-K)-C(Ca)-F(Mg+Fe²⁺) of Taxkorgan two-mica monzogranites (after [84])

The CaO/Na₂O ratios are probably a better measure of the fraction of argillaceous material in the sedimentary sources of strongly peraluminous granites, because CaO/Na₂O ratios are affected by each of these variables (temperature, pressure, H₂O activity, and protolith composition) as well, but the dominant control is the plagioclase/clay ratio of the source. Strongly peraluminous granite melts produced from plagioclase-poor, clay-rich sources will tend to have lower CaO/Na₂O ratios than melts derived from sources which are plagioclase-rich and clay-poor [82]. In this work, Taxkorgan two-mica monzogranites have a low CaO/Na₂O ratios (<0.3) with higher SiO₂ contents and lower (TFeO+MgO+ TiO₂) contents, which means that the sources of metasedimentary rocks are pelite (Figure 9(a)), Rb/Sr and Rb/Ba ratios mean the source is clay-rich (Figure 9(b)) and the major elements are plotted in the field of felsic pelites (Figures 9(c) and (d)). It is recognized that the compositions of melts of two-mica granites are produced by dehydration- melting of muscoviteschists, namely felsic metapelites [83].

Zircon as early crystallization mineral from the magma can retain its original isotopic composition and thus was widely used to trace petrogenetic process [85]. Zircons of Taxkorgan two-mica monzogranites with crystallization ages at 118 Ma show ε_Hf (t) values ranging from -17.0 to -12.5 (mean -14.5) (Table 3, Figure 7), which means the source from crust melting. As mentioned before, the exposed basement in the study area is predominantly the Buluokuole Group, which was traditionally regarded as slightly metamorphosed Paleoproterozoic strata [12, 46, 86], which also can be supported by Paleoproterozoic gneiss (about 2242 Ma) (unpublished data). Besides, the T_DM2 ages are 1906 to 2169 Ma; this work reckons that Taxkorgan two-mica monzogranites maybe come from Paleoproterozoic pelitic metasedimentary (muscovite schists).

S-type granites can be divided into high temperature and high pressure by Al₂O₃/TiO₂ ratios, low Al₂O₃/TiO₂ ratios (<100) will have been derived at higher temperatures (875-1000 °C). However, high Al₂O₃/TiO₂ ratios (>100) will have been derived at high pressure (<875 °C) [82]. Zr saturated temperature (T_Zr) provides a useful estimate of initial magma temperature at the source [87]. The Al₂O₃/TiO₂ ratios of PM-7 are 147 to 164 and calculated T_Zr for the samples of PM-7 are 672-703°C (Table 1). The PM-7 can be classified as high pressure type (Figure 10(a)), which belongs to “cold” grantoids, suggesting pelitic source with abundant biotite, muscovite or Ca-amphibole [87, 88], this type of granites is relatively crystal-rich and unlikely to erupt [87].

Figure 9 Source discrimination diagrams for Taxkorgantwo-mica monzogranites ((a) and (b) from Ref. [82];(c) and (d) from Ref. [83])

5.2 Tectonic background implications

Many authors have proposed relating granitoid types to tectonic settings [92-94], S-type granites are commonly regarded as syn- or post-collisional [75, 94-96]. In R2-R1 discrimination diagrams (Figure 11), sample plot in the field of syn-collision, likewise, Y versus Nb and Y+Nb versus Rb tectonic background discrimination diagrams (Figure 12) also plotted in the field of syn-collision. Besides, REE patterns and spider diagrams also show a similar trend to collisional granites of Bangong- Nujiang suture (Figures 6(a) and (b)), except for some element contents have more strong negative anomalies. As a result, we inferred the tectonic background of TaxkorganMPGs is syn-collision. It is reckoned that the MPGs were likely to generate where the thickened crust is affected by major crustal shears or overthrust structures during continental collision, through ‘‘wet’’ anatexis of crustal rocks and crystal fractionation of the magmas [92]. Based on magmatic temperature, we defined it as cold granites, and all of the cold granites were emplaced in environments of crustal thickening [87]. In w(TiO₂)-temperature diagram (Figure 10(b)), Taxkorgan MPGs all plot in the field of Himalayan leucogranites, which further supports that the tectonic background of TaxkorganMPGs is collision and crustal thickening.

Figure 10 CaO/Na₂O vs Al₂O₃/TiO₂ diagram for granites (a) and TiO₂ vs temperature for granites (b) and data for Himalayan leucogranites from Refs. [78, 79, 89-91] (b)

Figure 11 R₁-R₂ discrimination diagrams for Taxkorgan two-mica monzogranites

In addition, the Cretaceous stratum of Central Pamir and South Qiangtang Bolck is consisted of the Late Cretaceous terrestrial facies coarse clastic rocks [97], which covers the middle Jurassic marine carbonate in angular unconformity above the Jurassic formation, meanwhile, the Central Pamir and South Qiangtang Bolck absents the Early Cretaceous strata and develops the Early Cretaceous postcollisional granites [98-100]. These geologicalfacts can further support that the tectonic background of study zone in Early Cretaceous is collision tectonic environment.

Figure 12 Tectonic background discrimination diagrams for Taxkorgan two-mica monzogranites (after [94]):

Previous work about opening time and closing time of the Bangong–Nujiang Tethyan Ocean (BNTO) had some controversy. Some scholars believe that the opening time of BNTO has been documented before Middle Triassic [99, 100] others reckon that the opening time of BNTO is the Late Triassic [6, 101]. Based on closing time of the BNTO, there are also two kinds of views, the Late Jurassic–Cretaceous [6, 34, 91, 102-105] or Late Cretaceous [99, 106, 107]. The Cretaceous magmatic rocks of the Bangong–Nujiang suture offer an opportunity to understand the magmatic processes that operated during the closure of the Bangong–Nujiang Tethyan Ocean and associated continental collision. Indeed, syn-collisional two-mica granites near Amdo were studied [71], indicating the eastern part of the Bangong-Nujiang suture closed in Early Cretaceous (115-114 Ma). Besides, the red layer, located within Duolong region of the southern margin of the South Qiangtang terrane, is expected to have been related to the closure of the Bangong–Nujiang Neo-Tethys Ocean [31]. The basis of the red layer also found the youngest detrital zircon U–Pb age of 116 Ma, suggesting that it is the product of collisional orogeny, closely followed by closure of the Bangong–Nujiang Neo-Tethys Ocean [31]. In our study area, we reckoned that 118 Ma is the closure time in Taxkorgan region of Bangong-Nujiang suture. Meanwhile, Ref. [22] studied west side of the study area in Pamir, the Early Cretaceous magmatism activity has a peak at 120 Ma, which may reflect collision events of central and southern Pamir. From west to east, the closure time of Bangong-Nujiang Neo-Tethys Ocean is gradually later. It is suggested that the northern margin of the Lhasa terrane remained at latitude about 20°± 4°N during ca. 110-50 Ma, the paleomagnetic data also support closure time of Bangong-Nujiang ocean [108]. So we can conclude that Bangong–Nujiang Neo-Tethys Ocean closed in Early Cretaceous (120-114 Ma).

Based on the petrological and geochemical data presented above, together with the regional geology, this work proposes an integrated model for the origin of Early Cretaceous granitoids from central Pamir tectonic salient, as shown in Figure 13. We provide new insights into the tectonic history of Bangong–Nujiang Neo-Tethys Ocean in central Pamir region.

6 Conclusions

A comprehensive study of zircon U–Pb dating, Hf isotope and geochemical data of the Early Cretaceous two-mica monzogranites in Taxkorgan county of central Pamir allow us to draw the following conclusions.

1) The two-mica monzogranites from Taxkorgan are emplaced in the Early Cretaceous (118 Ma);

2) The pluton has strongly negative Eu anomalies and strongly peraluminous characteristic, showing geochemical affinity with S-type granites;

Figure 13 Schematic illustrations showing closure of Meso-Tethyan (a) and Bangong Ocean (b)

3) The rocks are generated by partial melting of Paleoproterozoic pelitic metasedimentary (muscovite schists), induced by thickened lower- crust of cold muscovite-enrichment magmas in asyn-collision tectonic setting;

4) Based on our new data and previous data, we suggested that the closure time of Bangong– Nujiang Neo-Tethys Ocean is the Early Cretaceous (120-114 Ma).

Acknowledgments

The authors would like to thank the project partners of Xikaimining company and Geological Research Academy of Xinjiang for their valuable support during the fieldwork.

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(Edited by FANG Jing-hua)

中文导读

塔什库尔干早白垩世钙碱性花岗岩成因：对中帕米尔特提斯洋演化的启示

摘要：帕米尔高原作为青藏高原的西延，Rushan-Pshart缝合带对应青藏高原的班公湖-怒江缝合带，中帕米尔地块即为羌塘板块的西向延伸部分。本文对塔什库尔干二云母二长花岗岩进行了详细的LA-ICPMS锆石U-Pb年代学, 岩石地球化学和Lu-Hf同位素分析，阐明了中帕米尔构造结的特提斯演化。 LA-ICPMS锆石U-Pb定年结果显示，二云母二长花岗岩形成于早白垩世(118 Ma)。地球化学特征与S型花岗岩相似，富集LREEs，亏损Ba，Sr，Zr，Ti。锆石样品全部显示负的ε_Hf(t)，范围介于-17.0~-12.5(平均值-14.5)，对应的二阶段模式年龄(T_DM2)为1906~2169 Ma。上述地球化学特征表明该花岗岩形成于部分熔融的古元古代布伦阔勒群泥质变质碎屑岩，主要由白云母片岩组成。鉴于上述岩石成因与地球化学数据，综合区域地质，本文进一步限定了中帕米尔地区班公湖-怒江洋闭合时代为早白垩世(120~114 Ma)。

关键词：特提斯洋；帕米尔高原；S型花岗岩；早白垩世构造岩浆作用；年代学和岩石成因

Foundation item: Project(41802103) supported by the National Natural Science Foundation of China; Project(2017YFC0601403) supported by the National Key R&D Program of China

Received date: 2019-01-10; Accepted date: 2019-03-18

Corresponding author: PENG Bo, PhD, Assistant Researcher; Tel: +86-10-68999064; E-mail: p.engbo@163.com; ORCID: 0000-0002- 9944-1425