Geochemistry, zircon U–Pb geochronology and Hf isotopes of granitic rocks in the Xitieshan area, North Qaidam, Northwest China: Implications for Neoproterozoic geodynamic evolutions of North Qaidam

Precambrian Research 264 (2015) 11–29

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Geochemistry, zircon U–Pb geochronology and Hf isotopes of granitic rocks in the Xitieshan area, North Qaidam, Northwest China: Implications for Neoproterozoic geodynamic evolutions of North Qaidam Jiangang Fu a,b , Xinquan Liang a,∗ , Yun Zhou c , Ce Wang a,b , Ying Jiang a,b , Yongsheng Zhong d a State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Wushan, Guangzhou 510640, China b University of the Chinese Academy of Sciences, 19 Yuquan Street, Shijingshan District, Beijing, China c Guilin University of Technology, No. 12 Jiangan Road, Guilin 541004, China d Western Mining Co. Ltd., Xining 810001, China

a r t i c l e

i n f o

Article history: Received 22 November 2014 Received in revised form 21 March 2015 Accepted 2 April 2015 Available online 11 April 2015 Keywords: Geochemistry Zircon U–Pb age Zircon Hf isotope Granitic rocks Xitieshan North Qaidam

a b s t r a c t Neoproterozoic granitic rocks in the Dakendaban Group are widely distributed throughout the Xitieshan area as the old basement in North Qaidam, NW China. The granitic rocks are composed predominantly of K-feldspar, plagioclase, quartz, muscovite and biotite, with subordinate zircon, garnet, titanite and sillimanite. Two granitic rock samples yielded ages of 930 ± 6 Ma and 918 ± 6 Ma using LA-ICPMS zircon U–Pb dating, which are interpreted as the protolith formation age of the granitic rocks, and reflecting an important Neoproterozoic magmatic event in the Xitieshan area. Geochemical data suggest that the granitic rocks are characterized by high SiO2 , K2 O, and CaO/Na2 O ratios (0.48–1.04), with LREE enrichment and strong Eu negative anomalies (Eu/Eu* = 0.44–0.51), and negative anomalies of Nb, Ta, Ti, Zr, and Hf. These rocks have consistent Nd isotope compositions with εNd (t) values ranging from −4.83 to −4.27, while the sample 2011ZJ18-3 yielded the negative εHf (t) values ranging from −6.7 to −1.3, and tDM2 (Hf) model ages ranging from 1.9 to 2.2 Ga. These geochemical and zircon Lu–Hf isotopic features suggest that the granitic rocks in the Xitieshan area belong to the S-type granite, which were mainly derived from the partial melting of the Paleoproterozoic continental crust material (1.9–2.2 Ga) that were predominantly composed of metagreywackes and subordinate metapelitic sources. Based on the regional tectonic reconstruction, we propose that the studied rocks occurred as syn-orogeny on an active continental margin in a compressional environment, which resulted from the Neoproterozoic arc-continent collision and subsequently continent–continent collision, leading to the significant continental growth. The Early Neoproterozoic tectonothermal event in North Qaidam shares many affinities with that of the Tarim block and South China block (Yangtze block) that were associated with the final consolidation of block at the Jinning orogeny, showing an agreement with the formation of Rodinia. The Early Neoproterozoic S-type granitic magmatism in NW China, including the Qaidam and adjacent blocks, is an important episode of continental crust reworking. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Present address: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Wushan, Guangzhou 510640, Guangdong Province, China. Tel.: +86 2085290113/13828489329; fax: +86 2085290151. E-mail address: [email protected] (X. Liang). http://dx.doi.org/10.1016/j.precamres.2015.04.006 0301-9268/© 2015 Elsevier B.V. All rights reserved.

North Qaidam, one of the most important tectonic metallogenic belts in NW China (Fig. 1A), is characterized by the exposure of the ophiolitic melange belt, an ultra-high pressure (UHP) metamorphic zone, and the arc-volcanic rocks of the Tanjianshan Group (Shi et al., 2006; Yang et al., 2006, 2001; Yu et al., 2012; Zhang et al., 2005b). The Xitieshan terrane is located in the central part

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J. Fu et al. / Precambrian Research 264 (2015) 11–29

Fig. 1. (A) Geologic setting of the Qaidam-Qilian blocks in the northwestern China. (B) Local geology of the Xitieshan terrane and study area. Map modified after Zhang et al. (2009a,b) and Xu et al. (2006). The sources of the age data are given with the Arabic numbers in brackets: [1] Lin et al. (2006); [2] Zhang et al. (2006); [3] Zhang et al. (2012b); [4] Yu et al. (2013a,b).

J. Fu et al. / Precambrian Research 264 (2015) 11–29

of North Qaidam, has been known for both the giant Xitieshan Pb–Zn deposit which is hosted in the volcanic sedimentary rocks of the Ordovician Tanjianshan Group, and the UHP metamorphic rocks hosted by the Proterozoic gneisses of the Dakendaban Group (Fig. 1B). Previous studies on the Xitieshan focused on the stratigraphy (Li et al., 2007; Liang et al., 2014; Lin et al., 2006; Wu et al., 1987), tectonic context (Wu et al., 2010), geochronology (Fu et al., 2014; Liang et al., 2014), geochemistry (Gao et al., 2011; Song et al., 2011; Wang et al., 2003), genesis of the deposit (Feng et al., 2010; Wu et al., 2008, 1987; Zhang et al., 2005a), the genesis and exploration significance of chemical sedimentary rocks, and the source of ore-forming fluid (Wang et al., 2009, 2008; Zhu et al., 2010). Furthermore, advancements in understanding the geological footprint of the eclogite forming the UHP metamorphic belt have been made through detailed petrography, whole-rock lithogeochemistry, Sr–Nd isotopes, zircon geochronology, and P-T-t paths (Lu et al., 1999; Song et al., 2009a,b, 2003; Wan et al., 2006; Yu et al., 2013b; Zhang et al., 2013, 2009a, 2005b). However, there are very few researches in the host gneisses of the eclogite (Cheng et al., 2007; Li et al., 2008; Wan et al., 2006), and especially the absence of the integrated studies which are essential for understanding the genetic relationships between the North Qaidam magmatism and its geodynamic environment. In this paper, we present detailed petrography, U–Pb zircon ages, whole-rock lithogeochemistry, Sr–Nd isotopic data, and zircon Hf isotope data of the granitic rocks in the Dakendaban Group. The objectives of this study are: (1) to provide a complete lithogeochemical database for the granitic rocks in the Xitieshan area, and for a comparison with the other parts of North Qaidam and adjacent blocks; (2) to provide temporal constraints on the protolith formation of the granitic rocks; (3) to elucidate tectonic settings for protolith emplacement of the granitic rocks; (4) to determine the petrogenesis of the granitic rocks in the Xitieshan area. 2. Geologic setting The North Qaidam tectonic belt is located at the northeastern margin of the Tibet Plateau in NW China (Fig. 1A), and it is characterized by a sequence of volcanic rocks of island arc affinity, the UHP metamorphic rocks, and ophiolitic complexes (Shi et al., 2006; Xu et al., 2006; Yang et al., 2006; Zhang et al., 2009b, 2007). The belt extends for about 400 km in a NNW direction and is bounded by the Qaidam block to the southwest along the Qaidam fault, and the Qilian block to the northeast along the Oulongbuluke-Maoniushan fault; it is offset by the sinistral Altyn Tagh fault in the northwest (Fig. 1A), and is cut by the Elashan fault to the east. To the south, the Qaidam basin is considered as a Cenozoic intra-continental sedimentary basin underlain mainly by a Precambrian crystalline basement and a Paleozoic fold belt (part of the Qaidam block). To the north, the Qilian block consists predominantly of the Paleozoic sedimentary rocks conformably overlying the Precambrian granitic gneiss, pelitic gneiss, schist and marble (Song et al., 2012; Tung et al., 2013). Further to the north, the North Qilian is characterized by blueschist, eclogite, and ophiolite complex that occur between the Alxa block and the Qilian block (Song et al., 2009a). The North Qaidam tectonic belt consists mainly of the Dakendaban Group and Tanjianshan Group. The Dakendaban Group is composed predominantly of the Precambrian granitic gneiss and paragneiss, amphibolite, mafic granulite and marble intercalated with minor slices of eclogite and garnet peridotite (Zhang et al., 2008). The granitic gneisses occur as a belt along the northern edge of the block, and extend approximately 300 km from Dulan in the southeast to Yuqia in the northwest (Wan et al., 2006). They are in fault contact with the Tanjianshan Group and intruded by the Early Silurian granite (ca. 428 Ma) (Meng et al., 2005) in the Xitieshan area (Fig. 1B). The Tanjianshan Group is exposed discontinuously

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in the Saishitengshan, Lvliangshan, Xitieshan, and Dulan regions from northwest to southeast (Fig. 1A), and is composed predominantly of sedimentary and volcanic rocks that have been deformed and metamorphosed from lower greenschist to amphibolite facies (Wu et al., 1987) (Fig. 2). The stratigraphic sequence in the Xitieshan region consists mainly of the Dakendaban Group in the northeast and the Tanjianshan Group in the southwest. Further to the southwest, the Xitieshan area is covered by the Cenozoic sediments (Fig. 2). The Dakendaban Group is laterally extensive and mainly composed of the Neoproterozoic granitic gneiss and paragneiss (Fig. 3), with minor amphibolite, mafic granulite, marble, and locally eclogite and garnet peridotite (Lin et al., 2006; Zhang et al., 2009a). The granitic rocks are gneissic in structure, and are dominantly composed of plagioclase, K-feldspar, quartz, biotite and muscovite (Fig. 4). The metasedimentary rocks have been metamorphosed to granulite facies, and consist of biotite, muscovite, sillimanite, garnet, quartz, and feldspar, and they are the host rocks of the eclogites (Fig. 3C). The Tanjianshan Group hosts the Xitieshan massive sulfide deposits, and is divided into four informal units from the northeast to the southwest, they are: (1) unit a, or the lower volcanic-sedimentary rocks, comprises bimodal volcanic rocks (unit a-1) and sedimentary rocks (unit a-2); (2) unit b, or intermediate-basic volcaniclastic rocks; (3) unit c, purplish red sandy conglomerates; (4) unit d, or basic volcanic rocks, from base to up, comprises the lower basic volcaniclastic rocks (unit d-1), middle clastic sedimentary rocks (unit d-2), upper basic volcaniclastic rocks (unit d-3), and uppermost basic lava (unit d-4) (Fig. 2) (Wu et al., 1987; Zhang et al., 2005a). The Xitieshan mine area was strongly deformed during the Late Ordovician to Early Devonian leading to the present geometry of the camp (Guo, 2000; Wang et al., 2000). Two deformational events have been recognized in the area: an early ductile deformation regionally defined as NW shear zone and a late brittle-ductile deformation defined by a NE trend. The early deformation event was associated with the oblique continent–continent collision between the Qaidam and Qilian blocks whereas the second deformation was resulted from the horizontal compressed collision (Guo, 2000). 3. Sample petrology Fourteen representative samples were collected from the Xitieshan area for this study. Their locations are outlined in Fig. 2. These granitic rock samples are characterized by the gray white in color, medium- to coarse-grained texture, and gneissic structure (Fig. 3A, B and D). These rocks comprise predominantly K-feldspar (30%), plagioclase (20–25%), quartz (25–30%), muscovite (10–15%) and biotite (5%), with subordinate zircon, garnet, titanite and sillimanite. K-feldspar grains are generally flesh pink in color, ranging in size from 10 to 15 mm, with the feature of the augen structure, and formed the S-C fabric during the later structural deformation (Fig. 3D), which indicated the dextral shear direction as well. The quartz grains are anhedral and granular, ranging from 2 to 5 mm in size, and show a feature of clearly undulatory extinction (Fig. 4A and B). Some small and recrystallized grains are always closely associated with feldspar grains and mica flakes, and forming the orientated arrangement (Fig. 4C). Muscovites show flaky structures, ranging in size from 2 to 8 mm, and they are locally warped during the later deformation (Fig. 4D). 4. Analytical techniques 4.1. Zircon U–Pb dating Two samples from the Dakendaban Group in the Xitieshan area were selected for zircon U–Pb geochronology: samples 2010STS-01

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J. Fu et al. / Precambrian Research 264 (2015) 11–29

Fig. 2. Geologic map of the Xitieshan deposit area, show the sampling location of granitic rocks.

and 2011ZJ18-3 were collected from Xitieshangou and Zhongjiangou respectively (Fig. 2). Each sample weighed approximately 5 kg for analyses. The samples were washed, then crushed in a jaw crusher and pulverized to a fine powder. The powder was panned using a Wilfley table to produce 100–200 ml of heavy mineral concentrate. The concentrate was passed through heavy liquids and a Frantz magnetic separator in a sequence of steps designed to separate minerals according to their density and magnetic susceptibility. The high quality zircons were from the least magnetic high-density fractions (Romeo et al., 2006), and were selected for analysis based on their morphology (Fig. 5). The entire analyses were performed at the State Key Laboratory of Isotope Geochemistry located in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). Using a combination of cathodoluminescence (CL) and optical microscopy, the clearest, least fractured rims of the zircon crystals were selected as suitable targets for laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis (Fig. 5). The methodology for LA-ICPMS was described by Tu et al. (2011). Sample mounts were placed in the two-volume sample cell flushed with Ar and He. Laser ablation was operated at a constant energy 80 mJ and at 8 Hz, with a spot diameter of 31.0 ␮m. The ablated material was carried by the He gas to an Aglient 7500a ICP-MS. Element corrections were made for mass bias drift, which was evaluated by reference to standard glass NIST 610. Temora standard was used as the age standard (206 Pb/238 U = 416.8 Ma) (Black et al., 2003). Traceelement concentrations were obtained by normalizing count rates for each analyzed element to those for Si, and assuming SiO2 to be stoichiometric in zircon (Tu et al., 2011). Results of isotopic measurement on zircons (all errors are given at the 1 level) are listed in Table 1 and are shown in conventional U–Pb concordia plots in Fig. 6. Age calculations were made using the method of Liu et al.

(2008). Errors associated with individual analyses were calculated using the numerical error propagation method of Ludwig (2003), and are quoted at the 95% confidence level. Decay constants used are those recommended by Steiger and Jäger (1977), and compositions for initial common Pb were taken from the model of Stacey and Kramer (1975). 4.2. Whole-rock geochemical analysis Fresh samples (each one weights about 0.5 kg) were selected for major- and trace-element analysis. Samples were crushed initially in a jaw crusher. A handpicked subsample of chips free of vein material and vesicles was powdered in an agate mortar. Major elements were determined using X-ray fluorescence spectrometry (XRF) with relative standard deviations within 5% using standard fused bead and pressed pellet technique with a ratio of 1:8 for sample to Li2 B4 O7 flux. The loss on ignition (LOI) was determined by a pre-ignition method before major element analyses. More detailed analytical procedures would be found in Goto and Tatsumi (1996) and Li et al. (2006a,b). Trace elements, including the rare earth elements (REE) and high field strength elements (HFSE), were analyzed using inductively coupled mass spectrometry (ICPMS) at the State Key Laboratory of Isotope Geochemistry in GIGCAS, following the protocol of Jenner et al. (1990) and Longerich et al. (1990), with data reduction following the procedure of Jenner et al. (1990). Thirty-seven trace elements including HFSE and REE were determined using an HF–HNO3 –HClO4 dissolution. The dissolving process for ICPMS analysis is presented by Chen et al. (2010). An Rh (ppb) internal standard solution was used to correct for monitor drift in mass response. USGS reference standards (BHVO-2, AGV-1, GSR-1, GSR-2, GSR-3, GSR-4, W-2, SY4, and SARM4) were also chosen as external calibration standards for calculating the element concentrations. The total procedure blank was treated

Table 1 Analytical data of zircon LA-ICPMS U–Pb dating on the granitic rocks in the Xitieshan area, North Qaidam. Serial no.

Spot

Th232

U238

Th/U

207

Pb/206 Pb

207

Pb/235 U

206

Pb/238 U

207

Pb/206 Pb

Ratio

1

Ratio

1

Ratio

1

Age (Ma)

207

1

Pb/235 U

206

Age

1

Pb/238 U

Age

Concordance (%) 1

58 154 268 67 54 90 76 46 52 68 1 71 70 34 74 62 60 77 73 60 76 70 71 67 128

114 320 293 204 216 213 207 238 135 153 164 230 169 152 150 137 429 167 138 317 144 697 193 295 126

0.5 0.48 0.92 0.33 0.25 0.42 0.37 0.19 0.39 0.45 0.01 0.31 0.41 0.23 0.49 0.45 0.14 0.46 0.53 0.19 0.53 0.1 0.37 0.23 1.01

0.1067 0.0585 0.0712 0.0669 0.0679 0.0646 0.0622 0.0646 0.0728 0.0705 0.0552 0.0716 0.073 0.0658 0.0686 0.0659 0.0704 0.0722 0.0784 0.0691 0.0693 0.0675 0.0676 0.0693 0.0902

0.0095 0.0022 0.0038 0.0043 0.005 0.006 0.0057 0.0049 0.0044 0.004 0.0037 0.0042 0.0038 0.004 0.0071 0.0048 0.0037 0.0044 0.0042 0.0034 0.0062 0.0022 0.003 0.0023 0.0049

2.7639 0.9153 1.4987 1.4462 1.474 1.4172 1.4593 1.3953 1.5775 1.5139 0.4935 1.5729 1.5133 1.4027 1.4325 1.4038 1.5234 1.6269 1.5808 1.4719 1.4804 1.4514 1.4342 1.5164 3.1504

0.2723 0.0364 0.0729 0.091 0.1065 0.128 0.1215 0.1044 0.0993 0.087 0.0331 0.1058 0.0769 0.0869 0.1414 0.1019 0.0914 0.1029 0.0957 0.0763 0.1354 0.0581 0.0635 0.0477 0.1651

0.184 0.1433 0.1549 0.1548 0.1552 0.1558 0.1556 0.1551 0.1552 0.155 0.0649 0.1554 0.155 0.1553 0.1547 0.1553 0.1554 0.1559 0.1554 0.1552 0.1558 0.1559 0.1547 0.1549 0.2502

0.0035 0.0017 0.0019 0.0024 0.0026 0.0031 0.0028 0.0026 0.0025 0.0026 0.0012 0.0037 0.0022 0.0028 0.0043 0.003 0.0038 0.0034 0.0029 0.0022 0.0031 0.0035 0.0029 0.002 0.0034

1744.1 550 962.7 835.2 864.8 762.7 683.3 761.1 1007.1 942.6 420.4 973.8 1013 798.2 887 1200 940.4 992.3 1166.7 901.9 905.6 853.7 857.4 909.3 1431.5

163.1 83.3 141.7 135.2 155.6 196.3 195.2 165.7 124.1 110.2 151.8 127.8 106.9 129.6 214.8 153.7 113.1 107.4 101.4 107.6 184.4 67.4 91.5 68.5 103.7

1345.8 659.9 929.9 908.3 919.8 896.2 913.7 886.9 961.4 936 407.3 959.6 935.8 890.1 902.6 890.5 939.8 980.7 962.7 918.9 922.4 910.5 903.3 937 1445.1

73.6 19.3 29.7 37.8 43.7 53.8 52.3 44.3 39.1 35.1 22.5 38.8 31.1 36.7 59.1 43.1 36.8 38.1 36.2 31.3 55.5 24.1 26.5 20.1 40.4

1089 863.6 928.3 927.9 929.9 933.2 932.1 929.5 930 929 405.1 931.2 929 930.8 927 930.4 931.1 933.7 931.3 930.2 933.2 934 927.3 928.3 1439.4

19 9.8 10.5 13.6 14.3 17.2 15.4 14.3 14.1 14.5 7.2 20.7 12.5 15.5 23.9 16.5 21.3 19.1 16.1 12.5 17.5 19.6 16.4 11.2 17.7

81 131 100 102 101 104 102 105 97 99 99 97 99 105 103 104 99 95 97 101 101 103 103 99 100

Sample 2011ZJ18-3 1 2011ZJ18-3-04 2011ZJ18-3-13 2 2011ZJ18-3-15 3 2011ZJ18-3-01 4 5 2011ZJ18-3-02 6 2011ZJ18-3-03 2011ZJ18-3-05 7 2011ZJ18-3-06 8 2011ZJ18-3-07 9 2011ZJ18-3-08 10 2011ZJ18-3-09 11 2011ZJ18-3-10 12 2011ZJ18-3-11 13 2011ZJ18-3-12 14 2011ZJ18-3-14 15 2011ZJ18-3-16 16 2011ZJ18-3-17 17 2011ZJ18-3-18 18 2011ZJ18-3-19 19 2011ZJ18-3-20 20

549 57 57 59 75 64 70 52 68 50 138 64 50 98 66 88 75 245 71 69

1261 1173 888 742 571 638 775 517 701 668 694 639 523 744 638 790 662 658 757 677

0.44 0.05 0.06 0.08 0.13 0.1 0.09 0.1 0.1 0.08 0.2 0.1 0.1 0.13 0.1 0.11 0.11 0.37 0.09 0.1

0.0848 0.0836 0.0762 0.0704 0.0706 0.0719 0.0772 0.0689 0.0682 0.0796 0.0769 0.0693 0.0705 0.0689 0.0702 0.0684 0.0726 0.0693 0.07 0.0694

0.0027 0.0026 0.0024 0.0022 0.002 0.002 0.0029 0.0025 0.0022 0.0023 0.0025 0.002 0.0019 0.0017 0.002 0.0021 0.0021 0.002 0.0021 0.0022

1.1842 1.0569 1.4476 1.4905 1.4928 1.5222 1.5132 1.4559 1.4341 1.5026 1.5118 1.4534 1.4815 1.4464 1.474 1.4375 1.524 1.4626 1.4747 1.4709

0.036 0.0345 0.0489 0.0478 0.045 0.043 0.0537 0.0557 0.0488 0.0444 0.0502 0.0462 0.0441 0.0411 0.052 0.0472 0.0465 0.0422 0.0446 0.048

0.1014 0.0942 0.1383 0.1528 0.1527 0.1528 0.1536 0.1525 0.1526 0.1539 0.153 0.1528 0.1532 0.1531 0.1528 0.1527 0.1527 0.1527 0.1528 0.1529

0.0017 0.0028 0.0018 0.0024 0.0026 0.0019 0.002 0.0024 0.0027 0.0018 0.0017 0.0026 0.0022 0.0023 0.0033 0.0022 0.0028 0.0024 0.0032 0.0031

1310.2 1283.3 1099.1 938.9 946.3 983.3 1127.8 898.2 872.2 1188 1120.4 909.3 944.1 894.4 1000 879.6 1011.1 909.3 927.8 909.3

58.3 61.1 62.5 67.6 62 57.4 73.6 75.9 66.7 56 64.8 58.2 56.3 51.9 59.3 63 59.3 57.4 61.1 66.7

793.3 732.3 908.9 926.5 927.4 939.4 935.7 912.3 903.3 931.4 935.2 911.3 922.9 908.4 919.8 904.7 940.1 915.1 920.1 918.5

16.7 17 20.3 19.5 18.3 17.3 21.7 23 20.4 18 20.3 19.1 18 17.1 21.3 19.7 18.7 17.4 18.3 19.7

622.4 580.4 835 916.6 916.1 916.9 921.1 915.1 915.3 922.8 917.7 916.8 918.7 918.5 916.9 916.3 916.3 916.2 916.9 916.9

9.9 16.7 10.1 13.3 14.4 10.9 11.1 13.6 14.9 10.2 9.7 14.6 12.5 12.9 18.6 12.1 15.5 13.6 17.9 17.2

78 79 92 99 99 98 98 100 101 99 98 101 100 101 100 101 97 100 100 100

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Sample 2010STS-01 1 2010STS-01-14 2010STS-01-23 2 3 2010STS-01-01 4 2010STS-01-03 2010STS-01-04 5 6 2010STS-01-05 2010STS-01-06 7 2010STS-01-07 8 9 2010STS-01-08 2010STS-01-09 10 11 2010STS-01-10 12 2010STS-01-11 13 2010STS-01-12 14 2010STS-01-13 15 2010STS-01-15 2010STS-01-16 16 2010STS-01-17 17 2010STS-01-18 18 2010STS-01-19 19 2010STS-01-20 20 2010STS-01-21 21 2010STS-01-22 22 23 2010STS-01-24 2010STS-01-25 24 25 2010STS-01-02

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Fig. 3. Field outcrop photograph of granitic rocks of the Dakendaban Group in the Xitieshan area. (A) Granitic gneiss from Zhongjiangou, phenocrysts occur as the directional alignment, the red pen is 13 cm in length. (B) Gneissic structure of granitic rocks from Xitieshangou, the hammer is 27 cm in length. (C) Outcrop photograph of eclogite, which are hosted by gneiss of the Dakendaban Group, and always occur as lenticle, from Zhongjiangou, the hammer is 27 cm in length. (D) S-C fabric of granitic rocks from Zhongjiangou, the cap of black pen is 5 cm in length.

in the same way as the samples, and was used to correct for all the samples and reference standards. Analytical precision for most elements was better than 5%. The analysis results are presented in Table 2.

0.513024, respectively. The analytical data and calculated results are listed in Table 3.

4.4. Zircon Hf isotope analysis 4.3. Sr and Nd isotope analysis To determine Sr and Nd isotopes, six granitic gneiss samples were selected (the least-altered samples), and were crushed and ground in an agate ring mill. All the analytical procedures were analyzed by the micromass isoprobe multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) at the State Key Laboratory of Isotope Geochemistry located at the GIGCAS. For each sample, about 70–140 mg of rock powders was digested with distilled HF–HNO3 in screw-top PFA beakers at 120 ◦ C for 5 days. Sr and REE were separated by using cation columns, and subsequently separated Nd from the REE fraction through using HDEHP columns. All Sr and Nd isotopic data were analyzed by MC-ICPMS: the 87 Sr/86 Sr value for NBS987 standard is 0.710260 ± 0.000005 (2), and the 143 Nd/144 Nd value for JNdi-1 standard is 0.512098 ± 0.000004 (2). All measured 87 Sr/86 Sr values were fractionation corrected to 86 Sr/88 Sr = 0.1194, while the 146 Nd/144 Nd value of 0.7219 was used to correct all the 143 Nd/144 Nd values. For comparison, two standard samples (i.e. BHVO-2 and JB-3) were analyzed by using the same chemical separation procedure and test methods as that described above. The 87 Sr/86 Sr values for BHVO-2 and JB-3 are 0.703545 and 0.703468, respectively, while 143 Nd/144 Nd values are 0.512950 and

Zircon Lu–Hf isotopic analysis was carried out at the State Key Laboratory of Isotope Geochemistry located at the GIGCAS. A Resolution M-50 laser-ablation system was attached to a Neptune MC-ICPMS. A normal single spot analysis consists of 30 s gas blank collection and 30 s laser ablation. The integration time was 0.131 s and 200 cycles of data were collected. The laser parameters were 30 s for 200 cycles, with an 8 Hz repetition rate, and a laser power of 80 mJ/pulse. Helium was used as carrier gas and a small flow of nitrogen was added in gas line to enhance the sample signal. Typical ablation time Lu–Hf isotopic analyses were performed at the GIGCAS using a beam diameter of 45 ␮m and a frequency of 10 Hz. More details about the analytical techniques are given by Hou et al. (2007) and Xu et al. (2004). The zircon Lu–Hf analytical data and calculated results are listed in Table 4. The present 176 Hf/177 Hf and 176 Lu/177 Hf values of chondrite and depleted mantle are 0.282772 and 0.0332, and 0.28325 and 0.0384, respectively (Rudnick, 1995; Teixeira et al., 2011). A decay constant for the 176 Lu value of 1.865 × 10−11 year−1 was adopted (Wan et al., 2003). Zircon Hf model ages are interpreted following the convention that adopts single-stage model (TDM1 ) relative to the depleted mantle when εHf (t) values are positive, while two-stage model (TDM2 ) relative to average continental crust when εHf (t) values are negative (Altherr et al., 2000). Penglai

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Table 2 Whole rock geochemical compositions of the granitic rocks from the Xitieshan area, North Qaidam. X01-1 SiO2 (wt %) Al2 O3 CaO Fe2 O3 T FeOT FeO K2 O MgO MnO Na2 O P2 O5 TiO2 L.O.I Total Sc (ppm) Ti V Cr Mn Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U Na2 O + K2 O K2 O/Na2 O CaO/Na2 O A/CNK A/NK Rb/Sr Rb/Ba TZr FeOT /(FeOT + MgO) Zr + Nb + Ce + Y (Na2 O + K2 O)/CaO FeO/MgO AFM CFM Rb/Nd (La/Yb)N Eu*

69.02 14.80 1.29 3.48 3.13 2.66 4.79 0.92 0.04 2.42 0.16 0.49 2.28 99.70

X01-2 67.22 13.99 2.34 4.39 3.95 3.36 4.26 1.10 0.07 2.27 0.17 0.58 3.37 99.74

9.553 2740.2 37 21.37 289.7 3.847 7.299 8.215 46.79 20.13 2.568 189.7 102.3 27.87 152.3 9.764 4.506 545.3 30.99 67.1 8.131 30.42 6.652 1.008 6.009 0.966 5.507 1.074 2.784 0.41 2.522 0.374 4.604 0.995 32.59 13.7 2.464 7.21 1.98 0.53 1.29 1.61 1.85 0.35 797.61 0.77 257.03 5.61 3.42 3.66 0.32 6.24 8.81 0.49

6.53 1.88 1.03 1.11 1.68

0.78 0.00 2.79 3.58 2.77 0.46

X01-3 68.52 15.07 1.44 4.29 3.86 3.28 3.81 1.10 0.05 2.34 0.19 0.61 2.28 99.69

X01-4 70.60 14.31 3.00 3.65 3.29 2.79 1.11 1.30 0.04 3.56 0.14 0.54 1.41 99.68

X01-5 69.60 14.38 1.13 4.22 3.80 3.23 3.77 1.07 0.04 2.34 0.16 0.56 2.43 99.70

11.17 1980.8 31.48 17.54 268.6 4.62 5.565 8.424 56.01 18.09 1.693 113.5 77.65 29.4 166.4 10.26 5.164 359.6 32.42 68.73 8.566 32.01 7.035 0.964 6.344 1.021 5.803 1.129 2.999 0.438 2.843 0.402 4.753 0.876 24 13.78 2.588

6.414 3099.3 49.87 24.73 285.2 7.762 8.403 9.112 44.74 16.86 2.35 59.34 152.1 28.12 158.5 9.475 5.797 189.4 33.32 70.97 8.594 32.42 6.955 1.091 6.257 0.976 5.49 1.08 2.864 0.439 2.882 0.42 4.659 1.039 24.29 15.02 3.408

12.45 3239 43.21 22.26 310.8 5.831 8.112 13.46 76.54 21.01 2.142 151.3 108.2 26.7 195.1 11.3 4.097 449.4 35.64 77.33 9.446 35.04 7.599 1.085 6.632 1.026 5.597 1.065 2.791 0.398 2.561 0.378 5.712 1.047 46.42 15.85 2.72

6.15 1.62 0.62 1.42 1.89 1.46 0.32 813.61 0.78 274.79 4.26 3.51 3.04 0.29 3.55 8.18 0.44

4.67 0.31 0.84 1.14 2.03 0.39 0.31 791.14 0.72 267.07 1.55 2.53 3.12 0.65 1.83 8.29 0.51

6.10 1.61 0.48 1.44 1.82 1.40 0.34 831.20 0.78 310.43 5.41 3.55 2.96 0.23 4.32 9.98 0.47

X01-6 67.16 14.79 2.20 3.99 3.59 3.05 4.11 1.04 0.06 2.36 0.16 0.51 3.35 99.74

6.48 1.74 0.93 1.20 1.77

0.78 0.00 2.95 3.45 3.20 0.47

X01-7 68.04 14.49 1.89 3.98 3.58 3.04 3.74 1.09 0.05 2.56 0.18 0.55 3.15 99.71

Z01-1 67.70 14.59 2.00 4.55 4.10 3.48 3.26 0.81 0.04 2.19 0.20 0.72 3.71 99.77

Z01-2 68.60 14.89 1.52 3.87 3.49 2.96 4.12 0.71 0.03 2.12 0.18 0.61 3.07 99.73

Z01-3 67.23 14.26 2.40 4.29 3.86 3.28 2.99 0.82 0.04 3.61 0.16 0.57 3.36 99.74

13.32 3183.6 45.28 22.96 392.7 6.107 8.226 13.64 55.07 21.47 2.533 172.1 107.3 29.64 164.2 11.04 4.211 487.8 32.21 69.82 8.596 31.65 6.928 1.125 6.341 1.035 5.734 1.115 2.936 0.436 2.698 0.401 4.813 1.148 21.81 14.57 2.948

14.87 4275.5 55.28 34.77 338 7.709 9.068 17.53 88.53 23.24 2.693 134.4 98.92 34.89 239.2 13.09 16.11 462 47.23 107.7 12.81 47.98 10.61 1.582 9.256 1.44 7.49 1.343 3.353 0.46 2.8 0.409 6.879 1.233 19.73 21.91 4.439

13.2 3644.1 48.66 29.55 236.8 5.675 8.638 11.4 69.62 22.6 2.543 178.8 99.69 26.78 194.1 11.7 11.24 498.2 39.96 86.74 10.71 39.68 8.592 1.269 7.349 1.106 5.733 1.004 2.465 0.341 2.153 0.327 5.64 1.042 21.24 18.87 3.587

11.88 3310.8 44.58 22.86 344.9 6.744 6.808 10.78 54.73 19.67 2.203 128.4 132.6 28.13 172 10.17 6.106 409.4 33.29 72.62 8.998 33.91 7.444 1.115 6.706 1.063 5.72 1.078 2.773 0.387 2.306 0.346 5.096 0.995 17.51 15.24 3.497

6.30 1.47 0.74 1.24 1.76 1.60 0.35 800.32 0.77 274.70 3.34 3.29 3.11 0.40 5.44 8.56 0.52

5.45 1.49 0.92 1.36 2.05 1.36 0.29 843.94 0.83 394.88 2.72 5.04 2.97 0.41 2.80 12.10 0.49

6.24 1.94 0.72 1.39 1.87 1.79 0.36 827.10 0.83 319.32 4.11 4.90 3.55 0.36 4.51 13.31 0.49

6.61 0.83 0.67 1.05 1.55 0.97 0.31 785.89 0.83 282.92 2.75 4.71 3.05 0.51 3.79 10.36 0.48

A/CNK = (Al2 O3/101.94)/((CaO/56.08) + (Na2 O/61.982) + (K2 O/94.2)), A/NK = (Al2 O3 /101.94)/((Na2 O/61.982) + (K2 O/94.2)). Eu/Eu* = EuN /(SmN *GdN )1/2 ; capital subscript N: normalized to Chondrites (Sun and McDonough, 1989). TZr : zircon saturation temperatures in Celsius degree. AFM = Al2 O3 /(MgO + FeOT ), CFM = CaO/(MgO + FeOT ), mol.%.

Z01-4 69.68 14.38 1.74 3.31 2.98 2.53 4.05 0.72 0.03 2.17 0.17 0.58 2.88 99.72

Z01-5 68.09 14.33 2.02 4.41 3.97 3.37 3.83 0.81 0.04 1.95 0.20 0.69 3.39 99.74 13.37 3812 50.52 28.97 279 6.816 10.83 14.32 62.25 22.51 2.317 150.5 89.21 32.43 206.7 12.26 11.67 480.2 43.46 99.94 11.73 43.61 9.582 1.449 8.558 1.314 6.79 1.227 3.125 0.441 2.689 0.391 5.96 1.108 17.35 20.11 4.03

6.23 1.87 0.80 1.29 1.81

0.80 0.00 3.58 4.11 3.89 0.47

5.78 1.96 1.04 1.30 1.95 1.69 0.31 826.63 0.83 351.33 2.86 4.93 3.00 0.42 3.45 11.59 0.49

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J. Fu et al. / Precambrian Research 264 (2015) 11–29

Fig. 4. Photomicrographs of granitic rocks of the Dakendaban Group in the Xitieshan area. (A) The augen structure and wavy extinction of quartz phenocrysts in granitic rocks from Xitieshangou, sample X01-5. (B) Typical granitic rocks from Zhongjiangou, sample Z01-1. (C) The directional alignment of crystal formed the banded and also strong alteration at the surface of phenocrysts from Xitieshangou, sample X01-3. (D) The contorted structure of mica in granitic rocks, which is the characteristics of the ductile shear deformation from Xitieshangou, sample X01-5.

zircon was used as the reference standard (Li et al., 2010b). Analyses of standard Penglai Zircon over the measurement period provided 176 Hf/177 Hf = 0.282924 ± 0.000008.

5. Results 5.1. Zircon U–Pb isotopes

Fig. 5. CL images of representative zircon grains, showing internal structures, analyzed locations, and calculated apparent 206 Pb/238 Pb ages, white solid circles denote U–Pb dating spots, and numbers in circles are equivalent to spot analyses given in Table 1.

5.1.1. Sample 2010STS-01 Zircon grains from sample 2010STS-01 are euhedral, paleyellow in color and long or short prismatic in form, ∼100 ␮m in length with aspect ratio ranging from 1 to 3. Twenty-five zircon grains were chosen for the U–Pb analyzing. Oscillatory zonings are common and clear in most crystals (Fig. 5A), suggesting that they are magmatic in origin. Some of them exhibit a narrow white light zone at the edge of the grains, which resulted from the late hydrothermal modification. The zircons have U and Th contents ranging from 114 to 697 ppm and from 1.1 to 154.5 ppm, respectively, with Th/U ratios of 0.1–1.0. Except for two plots that fall out the concordia plot, and one younger zircon age of 405 Ma (spot 2010STS-01.10) and one older zircon age of 1431 Ma (spot 2010STS01.02), the other 206 U/238 Pb ages are almost consistent with each other, yielding a weighted mean age of 930 ± 6 Ma (Fig. 6A and B), which was interpreted as the crystallization age of the granite. Spot 2010STS-01.10 is a metamorphic zircon with a Th/U ratio of 0.01, and yields a late metamorphic U–Pb age of 405 Ma (Table 1), which

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Fig. 6. The Concordia diagrams and weighted average age dating diagrams of granitic rocks samples 2010STS-01 and 2011ZJ18-3 (A–D) respectively. MSWD: mean-squared weighted deviates.

is consistent with the exhumation age of the UHP belt (Xu et al., 2006). 5.1.2. Sample 2011ZJ18-3 Zircon grains from sample 2011ZJ18-3 are euhedral, elliptical and elongated, with a typical feature of the oscillatory zonings (Fig. 5B). Twenty U–Pb analyses were performed using LA-ICPMS method. The U and Th contents of the zircon grains range from 516 to 790 ppm and from 52 to 245 ppm, respectively, with Th/U ratios of 0.1–0.37. Except for three zircon plots that fall out the Concordia plot, the 206 U/238 Pb ages for most grains are yielded with a consistently weighted mean age of 918 ± 6 Ma (Fig. 6C and D), and this age is interpreted to represent the crystallization age of the granitic gneiss protolith. Spot 2011ZJ18-3-13 yielded a younger age of 580 Ma with a Th/U ratio of 0.05, which probably related to the Pb-loss. 5.2. Major and trace element compositions Granitic rock samples fall in the granodiorite field in the TAS classification plot (Fig. 7A). As a whole, these samples have high contents of SiO2 (67.1–70.6%), K2 O (3.3–4.8%) with an exception of sample X01-4 (K2 O of 1.1%), and A/CNK (1.1–1.4). Most samples are plotted in the high potassium calc-alkaline series with one sample in the tholeiite series (Fig. 7B), and fall in the field of Mg-rich granite (Fig. 7C) and the peraluminous domain of the Shand’s index diagrams (Fig. 7D). These samples from the Xitieshan area have high loss on ignition (LOI) contents, ranging from 1.4 to 3.7% (Table 2), suggesting that they suffered different degrees of post-magmatic alteration.

All samples are characterized by similar REE signatures, with light rare earth elements (LREE) enrichment and strong Eu negative anomalies with Eu/Eu* ratios between 0.44 and 0.51 on the chondrite-normalized REE patterns (Fig. 8A), and the presence of significant Nb, Ta, Ti, Zr, and Hf negative anomalies on the primitive mantle-normalized patterns (Fig. 8B). 5.3. Whole-rock Sr–Nd isotopes The granitic rocks from the Xitieshan area have Rb contents ranging from 59.3 to 189.7 ppm and Sr contents ranging 98.9–152.1 ppm, with 87 Rb/86 Sr ratios ranging from 0.39 to 1.85. They also exhibit 147 Sm/147 Nd ratios of 0.129676–0.133669 and 143 Nd/144 Nd ratios of 0.511995–0.512034. The ε (t) values are Nd very consistent, ranging from −4.83 to −4.27 (Table 3), which were calculated at 918 Ma, suggesting that these granitic rocks were mainly derived from the continental crust. Sr–Nd isotopic data are shown in Table 3, and plotted in Fig. 9. Their single-stage Nd model ages (TDM1 ) range from 2.08 to 2.15 Ga and two stage-model ages (TDM2 ) range from 1.93 to 1.98 Ga (Table 3). 5.4. Zircon Hf isotopes The zircons from sample 2011ZJ18-3 were also analyzed for their Lu–Hf isotopes at the same spots which were previously analyzed for U–Pb ages, and the analyzed data are presented in Table 4. Fourteen spots were analyzed for zircon Lu–Hf isotopes in sample 2011ZJ18-3. The 176 Hf/177 Hf ratios vary from 0.282032 to 0.282135, and the initial Hf isotope ratios (176 Hf/177 Hf) I range from 0.282011 to 0.282150. The 176 Yb/177 Hf and 176 Lu/177 Hf ratios range from

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Table 3 Sr and Nd isotope data for the granitic rocks in the Xitieshan area, North Qaidam. Sample

Age (Ga)

Rb (ppm)

Sr (ppm)

Rb/Sr

87

X01-1 X01-4 X01-7 Z01-1 Z01-2 Z01-3

0.918 0.918 0.918 0.918 0.918 0.918

189.7 59.34 172.1 134.4 178.8 128.4

102.3 152.1 107.3 98.92 99.69 132.6

1.9 0.4 1.6 1.4 1.8 1.0

5.407305 1.132676 4.670544 3.952854 5.222713 2.804586

Sample

147

X01-1 X01-4 X01-7 Z01-1 Z01-2 Z01-3

0.132181 0.129676 0.132315 0.133669 0.130887 0.132695

Sm/144 Nd

143

Nd/144 Nd

0.512017 0.512003 0.512007 0.512018 0.511995 0.512034

Rb/86 Sr

87

Sr/86 Sr

0.787749 0.742736 0.773458 0.764208 0.773316 0.718198

(87 Sr/86 Sr)i

(87 Sr/86 Sr)CHUR

Sm (ppm)

Nd (ppm)

0.716800 0.727874 0.712176 0.712343 0.704789 0.681400

0.703429 0.703429 0.703429 0.703429 0.703429 0.703429

6.7 7.0 6.9 10.6 8.6 7.4

30.4 32.4 31.7 48.0 39.7 33.9

fSm/Nd

TCHUR (Ga)

TDM1 (Ga)

TDM2 (Ga)

(143 Nd/144 Nd)i

(143 Nd/144 Nd)CHUR

εNd (t)

−0.328008 −0.340744 −0.327326 −0.320442 −0.334587 −0.325392

1.46 1.44 1.49 1.50 1.49 1.44

2.12 2.08 2.14 2.15 2.12 2.10

1.96 1.95 1.97 1.97 1.98 1.93

0.5112210 0.5112218 0.5112101 0.5112130 0.5112065 0.5112350

0.5114535 0.5114535 0.5114535 0.5114535 0.5114535 0.5114535

−4.55 −4.53 −4.76 −4.70 −4.83 −4.27

Fig. 7. (A) TAS diagram (Middlemost, 1994) for the granitic rocks from the Xitieshan area. (B) K2 O versus SiO2 diagram (Rickwood, 1989). (C) FeOT /(FeOT + MgO) versus SiO2 diagram showing the boundary between ferroan and magnesian granitoids (Frost et al., 2001). (D) Shand’s index Al/(Na + K) versus Al/(Ca + Na + K) plot (Maniar and Piccoli, 1989).

0.032583 to 0.057368 and from 0.000732 to 0.001286, respectively (Table 4). The negative εHf (t) values range from −6.7 to −1.3, and are consistent with the crustal source origin. The signal-stage Hf model ages (TDM ) range from 1509 to 1730 Ma, and two stage-model ages (TDM2 ) range from 1865 to 2202 Ma (Table 4) (Fig. 10). 6. Discussion 6.1. The protolith formation age of granitic rocks in the Xitieshan area The granitic rocks of the Dakendaban Group in the whole North Qaidam are extensively distributed, and occur as a zone with about

300 km from Dulan in the southeast to Yuqia in the northwest, there are many controversies about this gneiss zone of the Dakendaban Group from different parts of North Qaidam, such as their different lithological classification, petrogenesis, and especially the protolith formation ages which range from 850 Ma to 1030 Ma. According to their petrologic features, tectonic setting and the formation age, Lu et al. (2002) suggested that the original Dakendaban Group should be divided into four types of lithological assemblages, they are: (1) the Early Paleoproterozoic Delingha complex rocks; (2) the Late Paleoproterozoic to Mesoproterozoic Dakendaban Group; (3) the Neoproterozoic granitic rocks (the focus of this study); and (4) the Shaliuhe (Yuqia) Group. They further reported

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Table 4 LA-MC-ICPMS zircon Lu–Hf isotope data for granitic rocks in the Xitieshan area, North Qaidam. Sample spot

Age (Ma)

176

Yb/177 Hf

ZJ18-3-01 ZJ18-3-02 ZJ18-3-03 ZJ18-3-06 ZJ18-3-07 ZJ18-3-10 ZJ18-3-11 ZJ18-3-12 ZJ18-3-14 ZJ18-3-16 ZJ18-3-17 ZJ18-3-18 ZJ18-3-19 ZJ18-3-20

917 916 916 916 915 917 916 917 917 916 916 916 917 917

0.044870 0.042691 0.046649 0.046786 0.035602 0.055401 0.032583 0.050256 0.056519 0.051374 0.057368 0.050140 0.039932 0.038705

176

Lu/177 Hf

0.001046 0.000967 0.001021 0.001078 0.000782 0.001266 0.000732 0.001089 0.001286 0.001162 0.001238 0.001138 0.000899 0.000893

176

Hf/177 Hf

0.282096 0.282101 0.282094 0.282104 0.282177 0.282032 0.282163 0.282092 0.282072 0.282089 0.282084 0.282078 0.282128 0.282135

2

176

Hf/177 Hf i

0.000007 0.000006 0.000006 0.000006 0.000005 0.000008 0.000006 0.000007 0.000006 0.000006 0.000006 0.000007 0.000006 0.000006

0.282078 0.282084 0.282076 0.282085 0.282163 0.282011 0.282150 0.282073 0.282050 0.282069 0.282063 0.282059 0.282112 0.282119

␧Hf (0)

εHf (t)

2

TDM (Hf)

TDM2 (Hf)

fLu/Hf

−24.53 −24.32 −24.61 −24.28 −21.53 −26.93 −21.98 −24.72 −25.53 −24.86 −25.08 −25.23 −23.32 −23.08

−4.29 −4.09 −4.38 −4.06 −1.32 −6.70 −1.75 −4.49 −5.29 −4.63 −4.85 −5.01 −3.08 −2.83

0.25 0.22 0.22 0.21 0.19 0.30 0.20 0.24 0.22 0.20 0.22 0.23 0.21 0.22

1631 1621 1633 1622 1509 1730 1525 1639 1675 1646 1656 1660 1581 1571

2052 2039 2057 2037 1865 2202 1892 2064 2114 2073 2086 2096 1976 1961

−0.97 −0.97 −0.97 −0.97 −0.98 −0.96 −0.98 −0.97 −0.96 −0.96 −0.96 −0.97 −0.97 −0.97

samples 2010STS-01 and 2011ZJ18-3, respectively, and they are in agreement with the previously reported 900–1100 Ma granitic rock ages from North Qaidam, suggesting that Early Neoproterozoic magmatism also occurred in the Xitieshan area at ca. 920 Ma. Neoproterozoic metamorphic events have been reported in the North Qaidam and adjacent blocks, including the 890 ± 14 Ma garnet-sillimanite paragneiss and 945 Ma paragneiss from the Xitieshan area (Zhang et al., 2012b, 2008), the 920 Ma micaschist from the Yuqia area (Yu et al., 2013a), and the 910–928 Ma pelitic rocks from the Qaidam-Qilian block (Song et al., 2012). These metamorphic ages are consistent with the Neoproterozoic magmatic ages, and suggesting they occurred during the same tectono-thermal event. 6.2. The nature of granitic rocks and their tectonic environment

Fig. 8. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the granitic rocks (A and B) respectively, and the chondrite and primitive mantle values are after Sun and McDonough (1989).

a zircon U–Pb age of 1020 ± 41 Ma from trondhjemite in the Yuqia area, an 803 ± 7 Ma from granodiorite in the Lvliangshan area and a 744 ± 28 Ma from K-feldspar granite in the Xitieshan area. Zhang et al. (2006) obtained an upper intercept age of 952 ± 13 Ma from the gneiss in the Xitieshan area, and this age is within-error of the protolith age of the granitic gneiss (952 ± 19 Ma) from the Yuqia area (Lin et al., 2006), and interpreted as the formation age of the protolith rocks. Cheng et al. (2007) reported the ages of 910 + 10/−17 Ma and 920 + 30/−17 Ma in the Quanjihe and Shaliuhe areas, respectively, and suggested a protracted magmatism for a minimum of 40 m.y. in North Qaidam ranging from ca. 950 Ma to 910 Ma. Subsequently, garnet biotite gneisses in the Xitieshan area yielded a monazite age of 938 ± 23 Ma and a zircon SHRIMP age of 945 ± 7 Ma (Zhang et al., 2012b). Yu et al. (2013a,b) obtained an age of 923 ± 12 Ma on a granitoid from the Dulan area. Two samples in this study yield the ages of 930 ± 6 Ma and 918 ± 6 Ma from

The granitic rocks from the Xitieshan area are easily identified at the field outcrop, with the characteristic of augen-like and/or sheared phenocrysts of whitish K-feldspar, and enrichment of muscovite. They underwent the late strong ductile shear deformation, and exhibited the characteristics of mylonitization (Figs. 3 and 4). These rocks are characterized by the high contents of K2 O, calcalkaline and peraluminous series, with a strong fractionation of LREE and HREE (La/YbN = 8.2–13.3), strong Eu negative anomalies with Eu/Eu* ratios of 0.44–0.51, and obviously Nb, Ta and Ti negative anomalies on the primitive mantle-normalized pattern, which represented typical characteristics of the subduction related rocks, and likely resulted from the chemical differentiation of arc-derived magmas (Taylor and McLennan, 1995). In the discrimination diagrams (Fig. 11), the granitic rocks from the Xitieshan area are generally plotted in the field of I and S-types granite (Fig. 11A and B), except for sample Z01-1 plotting in the field of A-type granite, however, the εNd (t) value of −4.7 suggest that it clearly lacks of evidence for mantle-derived sources, so not a A-type granite (Li et al., 2003b). These granitic rocks have generally low CaO (1.13–3.00%), relatively higher CaO/Na2 O ratios (0.48–1.04) and K2 O/Na2 O ratios (1.47–1.98), except samples Z01-1 and Z01-3 with K2 O/Na2 O = 0.31 and 0.83, respectively. These petrographic and geochemical features suggest that the Xitieshan granitic rocks are S-type granitoids. In the CaO–FeOT and Y–Rb plots (Fig. 11C and D), they further exhibit the characteristic of the S-type granite. Zircon saturation temperatures range from 786 to 844 ◦ C (Table 2), which are interpreted as the protolith crystallization temperatures, and are in agreement with that of S-type granite (Miller et al., 2003). In order to better constrain the petrogenesis of the magmatic precursors of the granitic rocks in the Xitieshan area, incompatible (CI ) and compatible (CC ) elements, high and moderately incompatible elements (CH and CM ) are chosen to determine the different magmatic processes, including the partial melting, fractional

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Fig. 9. (A) 143 Nd/144 Nd versus 87 Sr/86 Sr diagram. (B) εNd (t) versus 87 Sr/86 Sr diagram. C. 143 Nd/144 Nd versus 147 Sm/144 Nd diagram. (D) εNd (t) versus 143 Nd/144 Nd diagram. Regional data are from Wan et al. (2006).

Fig. 10. εHf (t) versus age (Ma) diagram.

crystallization and mixing process (Peccerillo et al., 2003; Schiano et al., 2010). The Xitieshan granitic rocks are calc-alkaline, and the formation of these granitic magma has been ascribed to two ways: (1) fractional crystallization of mantle-derived basaltic magma (Barth et al., 1995; Singer et al., 1992) and (2) partial melting of mafic to intermediate igneous sources (Drummond and Defant, 1990; Rapp and Watson, 1995). Both εNd (t) values ranging from −4.83 to −4.27 and εHf (t) values ranging from −6.7 to −1.3

preclude a mantle-derived origin of the Xitieshan granitic rocks (Teixeira et al., 2011). In Rb versus Rb/V (Fig. 12A) and Rb/Nd versus Rb (Fig. 12C) discrimination plots, granitic rock samples form an oblique straight line that shows a partial melting trend, which is also presented in the La versus Rb discrimination plot (Fig. 12B). These plots and isotopic characteristics indicate that the magmatic precursors of granitic rocks were derived from the partial melting of ancient crustal materials. On one hand, the high-K granitic magmas in convergent settings are generally caused by two processes: (1) in continental arc settings, parent mantle melts with enriching the slab-derived fluids are contaminated with crustal material during the ascent (Hildreth and Moorbath, 1988; Schiano et al., 2010) and (2) in syn- to post-collisional settings, melting of crustal source rocks probably resulted from the decompression following delamination of the lithospheric root or slab break off (Roberts and Clemens, 1993). On the other hand, compositional differences of magmas are always controlled by the partial melting of different source rocks (Whalen et al., 1987), normally including amphibolites, tonalitic gneisses, metagreywackes and metapelites. Clay-rich, plagioclase-poor (<5%) pelite-derived strongly peraluminous granites normally have lower CaO/Na2 O ratios (<0.3) than their clay-poor, plagioclase-rich (<25%) psammite-derived counterparts (Sylvester, 1998). The Xitieshan strongly peraluminous granites have high CaO/Na2 O (0.48–1.04, Table 2), and suggest that their sources are predominantly plagioclase-rich, clay-poor psammitic rocks. Tung et al. (2013) reported the analogous and contemporaneous granitioids in Haiyan and Gahai of the Qilian block, and further indicated that partial melting of the psammitic rock sources produced S-type granite under the conditions of pressures above 10 kbar and

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Fig. 11. (A and B) Zr + Nb + Ce + Y versus FeOT /MgO and (K2 O + Na2 O)/CaO plots showing A-type granites and fields for fractionated felsic granites (FG) and unfractionated M-, I- and S-type granites (OGT), coordinates of these fields are X = 350, Y = 4 and 16 (A) and 7 and 28 (B) (Whalen et al., 1987). (C) CaO versus FeOT diagram (Chappell and White, 2001). (D) Y versus Rb diagram (Chappell, 1999).

temperatures below 1000 ◦ C (Patino Douce and Beard, 1995; Tung et al., 2013). In the AFM versus CFM experimental discrimination diagram (Fig. 12D), most samples fall in the field of the partial melting from metagreywackes and nearly to the partial melting from metapelitic sources, suggesting that clastic sedimentary rocks are the protolith of the partial melting process. Moreover, the source rocks for these S-type granitoids mostly occurred 40–50% in degree of partial melting in the Rb/Ba versus Rb/Sr diagram (Sylvester, 1998) (Fig. 13D), and also were clay-poor and calculated psammite-derived. High-K calc-alkaline magmatism probably occurs at either active continental margins or post-collisional stages (Barbarin, 1999; Liégeois et al., 1998). The Xitieshan granitoids show pronounced enrichment in LILE Th and Rb, and depletion in HFSE, including Nb, Ta and Ti (Fig. 8B), which are typical features of subduction-related (Gorton and Schandl, 2000; Kessel et al., 2005; Münker et al., 2004). These trace elements geochemical characteristics are similar to those of modern island arc rocks (Smithies et al., 2005). In the tectonic environment discrimination diagrams, the Xitieshan granitic rocks are plotted in the volcanic arc field (VAG) (Fig. 13A–C). Moreover, in the Rb/Ba versus Rb/Sr diagram, most samples are plotted in the fields of shale source and calculated psammite-derived melt (Fig. 13D), these clay-poor sedimentary rocks are probably the major source rocks of S-type granitic magma in the Xitieshan area. The original sedimentary material of these meta-sedimentary rocks was mainly derived from a early continental basement and was probably formed in an active continental margin environment (Wan et al., 2006). Similar tectonic

environment was also reported in North Qaidam and adjacent blocks, e.g. mafic–ultramafic rocks (ca. 910 Ma) in the Qilian block also formed in an arc environment (Tung et al., 2012). Guo et al. (1999) and Lu et al. (2008) suggested that the granitic rocks from North Qaidam were syn-collisional granite. The Maxianshan granitoids (930–940 Ma) in the Qilian block were interpreted as products of continent–continent collision (Wan et al., 2003). The Xitieshan granitic rocks yielded the εHf (t) values ranging from −6.7 to −1.3, the εNd (t) values ranging from −4.83 to −4.27 and tDM2 (Hf) model ages ranging from 1.9 to 2.2 Ga, they are consistent with the crustal source origin, suggesting that the granitic rocks were derived from partial melting of the Paleoproterozoic crustal source between 1.9 and 2.2 Ga. There is little evidence of a mantle component in these subduction-related volcanic arc rocks, thus it is likely that they formed at the early stage of subduction in the Jinning Period. Subsequently, arc-continent and continent–continent collisions happened, and resulted in the continental growth (Rudnick, 1995). Therefore, the Neoproterozoic Xitieshan granitoids occurred as syn-orogeny on an active continental margin in a compressional environment. These progresses probably occurred during the formation of the Rodinia super-continent (Lu et al., 2008; Wang et al., 2013; Yu et al., 2013a). 6.3. The Neoproterozoic tectonothermal events and regional correlation The Neoproterozoic North Qaidam underwent the late strong tectonic-thermal activities, especially Ordovician UHP

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Fig. 12. (A) Plot of Rb versus Rb/V for the granitic rocks from the Xitieshan area, with partial melting curve (Schiano et al., 2010). (B) Plot of La versus Rb for the granitic rocks, with partial melting curve (Schiano et al., 2010). CH1 versus CH2 diagram (with H 1 and H 2 being two highly incompatible elements) with curves showing calculated melt compositions produced by fractional crystallization, partial melting and mixing processes. (C) Plot of Rb/Nd versus Rb for the granitic rocks from the Xitieshan area, with partial melting curve (Schiano et al., 2010). CH and CM are the concentrations of H, a highly incompatible element, and M, a moderately incompatible element, respectively, showing theoretical correlation curves during fractional crystallization, partial melting and mixing processes. (D) Plot of AFM versus CFM for the granitic rocks from the Xitieshan area (Altherr et al., 2000). AFM, Al2 O3 /(MgO + FeOT ) and CFM, CaO/(MgO + FeOT ).

metamorphism, which resulted in the rare evidences for the Neoproterozoic tectonic evolution in North Qaidam. It is therefore necessary to make a larger region for illustrating the Late Mesoproterozoic to Early Neoproterozoic tectonothermal events. Previous researches have shown that there are many similarities among North Qaidam, Qilian, Altyn Tagh, East Kunlun, and Altun in age of magmatism and metamorphism (Wan et al., 2006; Wang et al., 2013; Yu et al., 2013a; Zhang et al., 2008), and further suggested that Altun-Qilian-North Qaidam (AQNQ) was a part of the Tarim block in the early Neoproterozoic, and they occurred as a single block (Yu et al., 2013a). There are also some geological evidences about island arc volcanism, syn-tectonic granite magmatism and glaucophane-schist metamorphism in the Tarim Craton and surrounding continental fragments, including Qilian block, Qaidam block, North Qaidam, West and East Kunlun (Lu et al., 2008; Zhang et al., 2008). These evidences are very important for understanding the Neoproterozoic tectonothermal events in North Qaidam, northwest China. The Wandonggou Group occurred on the passive continental margin in Mesoproterozoic, which was composed predominately argillaceous clastic rocks, volcanic rocks and carbonate rocks, and suggesting the formation of the proto-Qinkun ocean (Hao et al., 2004). In the subsequent subduction setting (Hao et al., 2004; Li et al., 1999a,b), island arc volcanic rocks formed. Although there

is no evidence for the Mesoproterozic island arc volcanic rocks in North Qaidam so far, these island arc volcanic rocks in West Kunlun have been reported (Zhang et al., 2003). The Mesoproterozoic island arc volcanic rocks of the Ailiankate Group are composed predominantly of andesites and minor rhyolites which yielded a Sm–Nd isochron age of 1200 ± 82 Ma (Zhang et al., 2003), representing the formation age of island arc volcanic rocks (Fig. 14A). Meanwhile, Zhang et al. (2003) reported Ar–Ar ages of 1050 ± 1 Ma and 1021 ± 1 Ma from an amphibolite of the Ailiankate Group, and were interpreted as the age of the later tectonothermal event. These island arc volcanic rocks provided a very favorable tectonic setting for the subsequent granite. With increasing subduction and collision, the Neoproterozoic syn-orogenic granitic rocks extensively occurred in North Qaidam, and ranging in age from ca. 900 Ma to 1000 Ma, these ages were interpreted as a Mesoproterozoic to Early Neoproterozoic synorogenic event (Fig. 14B) (Wan et al., 2001; Zhang et al., 2006). Several syn-collision events have been also reported in North Qaidam and surrounding continental fragments, including 930 Ma Altyn Tagh garnet granite (Lu et al., 2006), 917 Ma Shaliuhe granite (Lu et al., 2006), 928 Ma Luofengpo dimicaceous granite (Lu et al., 2006), 906–924 Ma granitic rocks in the Danshuiquan and Bashiwake areas from the Altyn Complex (Wang et al., 2013). In addition, Lu et al. (2002) studied the 1020 ± 41 Ma Yuqiahe trondhjemite

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Fig. 13. (A) Rb versus Ta + Yb diagram (Pearce et al., 1984). (B) Ta versus Yb diagram (Pearce et al., 1984), showing granitic rocks samples follow the active continental margin signature. (C) Sr/Y versus Y diagram showing the normal arc magmas feature. (D) Rb/Sr versus Rb/Ba plots of the granitic rocks in the Xitieshan area (Sylvester, 1998).

and 1020 ± 20 Ma Shaliuhe granite gneiss from North Qaidam. The coesite-bearing eclogites of the Yuqiahe Group in North Qaidam were mostly hosted in the Neoproterozoic granitic rocks, and this eclogite-granitic gneiss zone was interpreted as a symbol of the continent–continent collision (Li et al., 1999b). Therefore, the arccontinent and continent–continent collisions are major tectonic processes for the formation of the granitic rocks (Fig. 14B). These tectono-thermal events are consistent with the Grenville Orogenic event in North America and the assembly of Rodinia (Lu et al., 2008). After the formation of Rodinia supercontinent, there were many geological records related to rifting events in NW China, ranging from 850 Ma to 740 Ma, including mafic dyke swarms, alkaline granites and bimodal volcanic rocks (Lu, 2001; Lu et al., 2008; Song et al., 2010). Song et al. (2010) reported the 850 Ma continental flood basalts in the North Qaidam UHPM belt, and suggested they were derived from a mantle plume in a continental rift stage. The later than 850 Ma basaltic lava flows and tuffs of the Xinglongshan Group also formed in an extensional environment (Wan et al., 2000). Magmatism with an age of 850 Ma was interpreted as the onset of the rifting of the Rodinia supercontinent (Li et al., 2010a, 2006a; Tung et al., 2013). The ca. 800 Ma granitoids in Qilian block and West Kunlun were rift-type, and were interpreted as the major episode of the rifting of the Rodinia supercontinent (Li et al., 2003b; Zhang et al., 2004). These rifting events resulted in the formation of rift basins along the northern margin of the Tarim block and within the Quanji massif (Lu et al., 2008). Meanwhile, volcanic rocks of the Quanji Group yielded a zircon U–Pb age of 740 Ma (Lu, 2002), and thick sedimentary rocks of the Quanji Group occurred as typical records of Rodinia supercontinent breakup in

the Middle Neoproterozoic, which are relatively preserved in North Qaidam. This Group is characterized by the conglomerates and pebbly sub-arkoses, sandstones and volcanic rocks, and thick marine carbonates with abundant stromatolites from the bottom up in order (Li et al., 2003a,c). There are three major cratons in China before Phanerozoic orogenic processes: the Tarim, North China and Yangtze cratons (TC, NCC, and YC, respectively) (Lu et al., 2008; Zheng et al., 2013) (Fig. 15A). Combined with the geochemistry and age of the granitic rocks, and comparison with the adjacent Qilian block in composition, it is clear that Early Neoproterozoic granitic plutons in North Qaidam are very similar to those in the Qilian block (Gehrels et al., 2003; Guo et al., 1999). Both terranes underwent the same tectonic evolution and formed a coherent crustal province during the Jinning orogeny (Wan et al., 2006). In the larger regional scale, the Early Neoproterozoic tectonothermal event in North Qaidam exhibits many affinities with that of the Tarim block, and is further interpreted as a part of the Tarim block, and they occurred as a single block (Lu et al., 2008; Wan et al., 2006; Yu et al., 2013a). On the other hand, the final cratonization of the North China craton happened after the Luliangian Orogeny (ca. 1.85 Ga) (Peng et al., 2014; Zhang and Zheng, 2013; Zhao and Zhai, 2013). Whereas the Tarim craton begun to activate as an unconformity between the sedimentary cover and metamorphosed basement during the Tarimian Orogeny (ca. 1.05–0.90 Ga), which exhibited many similarities to the Jingning Orogeny in the Yangtze craton (Wang and Mo, 1995), such as the same timing (ca. 0.9 Ga), same collision-rifting history and mechanism (both underwent Rodinia break-up during the Nanhua Period at 0.8–0.7 Ga), and same glacial deposits in the

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Fig. 14. Sketch model for Neoproterozoic tectonic evolution in North Qaidam. (A) Subduction of oceanic crust and island-arc magmatism between ca. 1200 and 1020 Ma. (B) Arc-continent collision and syn-collisional magmatism between ca. 1000 Ma and 900 Ma, the formation age of the granitic rocks from the Xitieshan area focused at ca. 920 Ma.

Fig. 15. Cartoons showing Neoproterozoic locations of Australia, Tarim, Yangtze, Siberia and North China craton, and the assembly of Rodinia supercontinent modified after Li et al. (2008) and Lu et al. (2008).

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Nanhua System (He et al., 2012; Li et al., 2003b,c, 2002a,b, 1999c, 2008; Zhang et al., 2012a,c,d). Generally, the Neoproterozoic North Qaidam, as a part of the Tarim craton, has many similarities to the Yangtze craton, but much difference from the North China craton. It is a reasonable explanation that the Tarim block was closed to the Yangtze block, or they have formed an integral block named as the Yangtze–Tarim craton (Lu et al., 2003) or “South-West China United Continent (SWCUC)” (Song et al., 2012) during the Neoproterozoic period, which is corresponding to the global formation of Rodinia supercontinent (Fig. 15B). To better understand the Neoproterozoic tectonic evolution in NW China, it is obviously needed to do more researches about the Neoproterozoic granitic rocks in North Qaidam. This paper just serves as a modest spur to induce more researchers’ attention on the systematic research of the granitic rocks in the different place in North Qaidam, NW China. 7. Conclusions Based upon petrological, geochronological, geochemical, and Rb–Sr, Sm–Nd and Lu–Hf isotopic data from the Neoproterozoic granitic rocks in the Xitieshan area of North Qaidam, several conclusions can be presented as follows: (1) The granitic rocks from the Xitieshan area mainly consist of K-feldspar, plagioclase, quartz, muscovite and biotite. Two samples yielded the ages ranging from 930 Ma to 918 Ma, with a focus on ca. 920 Ma, which was interpreted as the crystallization age of magmatism at Xitieshan. (2) Geochemical data suggest that the protoliths of granitic rocks in the Xitieshan area belong to S-type granites, and they were derived from the partial melting of ancient crustal materials, with the features of the subduction related arc rocks. (3) A subduction-related tectonic environment along the ancient edge of a continental block is a potential tectonic setting that is capable of producing arc-like rocks. Acknowledgements This contribution was financially supported by the National Natural Science Foundation of China (Grant Nos. 41173066). Additional financial assistance came from Western Mining Ltd, Qinghai Province, China. The authors wish to express their sincere appreciation to Shengmei Duan of Western Mining Ltd., and Houyou Li and Heng Ou of 217 Team of Nonferrous Geological Prospecting Bureau, Hunan Province for access to Xitieshan drill core and outcrop, and providing geologic data in the form of drill logs, maps, plan sections, and unpublished reports. We thank Jean-Luc Pilote for his reviews and suggestions, which have greatly improved the manuscript. The editor Guochun Zhao and two anonymous reviewers are thanked for their constructive comments and suggestions. References Altherr, R., Holl, A., Hegner, E., Langer, C., Kreuzer, H., 2000. High-potassium, calcalkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany). Lithos 50, 51–73. Barbarin, B., 1999. A review of the relationships between granitoid types, their origins and their geodynamic environments. Lithos 46, 605–626. Barth, A.P., Wooden, J.L., Tosdal, R.M., Morrison, J., 1995. Crustal contamination in the petrogenesis of a calc-alkalic rock series: Josephine Mountain intrusion, California. Geol. Soc. Am. Bull. 107, 201–212. Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.N., Davis, D.W., Korsch, R.J., Foudoulis, C., 2003. TEMORA 1: a new zircon standard for Phanerozoic U–Pb geochronology. Chem. Geol. 200, 155–170. Chappell, B.W., 1999. Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites. Lithos 46, 535–551. Chappell, B.W., White, A.J.R., 2001. Two contrasting granite types: 25 years later. Aust. J. Earth Sci. 48, 489–499.

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