Geochemistry and Geochronology of the Amphibolite Blocks in Ophiolitic Me´Langes Along Bangong-Nujiang Suture, Central Tibet

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Journal of Asian Earth Sciences 33 (2008) 122–138 www.elsevier.com/locate/jaes

Geochemistry and geochronology of the amphibolite blocks in ophiolitic me´langes along Bangong-Nujiang suture, central Tibet

Wei-Liang Wang a,*, J.C. Aitchison a, Ching-Hua Lo b, Qing-Gao Zeng c

a Tibet Research Group, Department of Earth Sciences, University of Hong Kong, James Lee Building, Pokfulam Road, Hong Kong SAR, China b Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei 106, Taiwan c Geological Survey Bureau of Tibet Autonomous Region, Lhasa 851400, China

Received 10 April 2007; received in revised form 18 October 2007; accepted 19 October 2007

Abstract

Amphibolites occur as blocks within serpentinite-matrix me´lange at Dong Tso and Lagkor Tso in the Gertse area of central Tibet, where they are associated with the Bangong-Nujiang suture (BNS). Geochemical data shows amphibolites have similar MORB or arc- like MORB geochemical characteristics. Lagkor Tso amphibolites show greater arc aﬃnity, whereas those from Dong Tso amphibolites are more MORB-like. These characteristics are similar to basalts from continental back-arc basin settings. Hornblendes from the amphibolites dated by the 40Ar/39Ar single-grain laser fusion technique show similar metamorphic ages 170–177 Ma, which is interpreted to represent the timing of the main metamorphic event experienced in this region. We suggest that the western and middle sectors of the BNS contain the remnants of a short-lived back-arc basin (the Bangong-Nujiang ocean), which partly opened and closed in the Middle Jurassic. 2007 Elsevier Ltd. All rights reserved.

Keywords: Central Tibet; Bangong-Nujiang suture zone; Ophiolites; Amphibolites; Geochemistry; Geochronology; Back-arc basin

1. Introduction 1984; Zhou et al., 1997; Wang, 2000; Pan et al., 2006), the orientation of subduction (Cheng and Xu, 1986; Pearce The Bangong-Nujiang suture (BNS) between the Lhasa and Deng, 1988; Zhu et al., 2006), and the spatial and tem- and Qiangtang terranes is an east–west trending belt that poral relationships of ophiolitic remnants (Tang and extends >2500-km across central Tibet. It contains isolated Wang, 1984; Guo et al., 1991; Yin and Harrison, 2000; occurrences of ophiolitic rocks that represent remnants of Qiu et al., 2006). Various geophysical surveys to identify oceanic crust (Girardeau et al., 1984; Chang et al., 1986; mantle and crust structure have been conducted across Yin et al., 1988; Wang, 2000)(Fig. 1A). Although some the suture (Hirn et al., 1984; Kong et al., 1996; Zhao geological investigations have been carried out along the et al., 2001, 2004; Wittlinger et al., 2004), although they BNS, especially in recent years (Taylor et al., 2003; Shi reveal conﬂicting results regarding what lies beneath the et al., 2004; Zhang, 2004; Kapp et al., 2005b; Solon BNS. et al., 2005; Guynn et al., 2006; Zhu et al., 2006; Pan The Tibet Research Group (TRG) of the University of et al., 2006; Qiu et al., 2006), due to its inaccessibility Hong Kong and the Geological Survey Bureau of Tibet and physical diﬃculties, knowledge of this suture is limited. Autonomous Region of the People’s Republic of China Several issues regarding the tectonic evolution of the belt have undertaken mapping and geological studies in the remain controversial including: the timing of opening and Gertse area during 2002–2006. One of the studies examines closure of the Meso-Tethyan ocean (Girardeau et al., metamorphic rocks associated with ophiolitic me´lange at Dong Tso and Lagkor Tso in order to elucidate the tec- * Corresponding author. Tel.: +852 9121 0390; fax: +852 2517 6912. tonic evolution of the BNS. We describe the geochemical E-mail address: [email protected] (W.-L. Wang). and geochronological characteristics of amphibolite-facies

Fig. 1. (A) Map showing major blocks of Tibet, approximate locations of intervening suture zone, and the main occurrences of the ophiolites along the Bangong-Nujiang suture zone. (B) Simplified geological map of Gertse area, central Tibet (modified after Cheng and Xu, 1986; Tapponnier et al., 2001; Pan and Ding, 2004; Kapp et al., 2005b). MFT, Main Frontal Thrust; MBT, Main Central Thrust; YTS, Yarlung Tsangpo Suture; KF, Karakoram Fault; ATF, Altyn Tagh Fault; BNS, Bangong-Nujiang Suture; CQMB, Central Qiangtang Metamorphic Belt; JS, Jinsha Suture; AKMS, Anyemagen Kunlun Muztag Suture. metamorphic rocks within ophiolitic me´lange at each local- et al., 2005; Qiu et al., 2004a,b, 2006), Lagkor Tso (Cheng ity and discuss their implications. and Xu, 1986; Aitchison et al., 2005, 2006; Pan et al., 2006), Bangong Lake (Wang et al., 1985; Guo et al., 1991; Matte 2. Geology et al., 1996; Shi et al., 2004)(Fig. 1A). Based on a summary (Table 1) of the distribution, ages 2.1. Bangong-Nujiang suture and tectonic characteristics of ophiolitic complexes along the BNS, it appears that most of the ophiolitic units formed This zone is marked by a sub-linear series of ophiolitic above a subduction zone. However, some characteristics of fragments between the Lhasa and Qiangtang terranes. It the ophiolites along the BNS are difficult to reconcile with is arbitrarily divided into three sectors from west to east, a typical or simple ophiolitic unit (Dilek and Robinson, the Bangong Lake-Gertse (western sector), the Dongqiao- 2003). For example, details of pre- and post-collision defor- Amdo (middle sector) and the Dingqing-Nujiang (eastern mation are poorly known although the ophiolitic units them- sector) (Bureau of Geology and Mineral Resources of Xiz- selves are strongly dismembered and few outcrops (Table 1) ang Autonomous Region (BGMRXAR), 1993). Well-pre- show MORB-like characteristics based on previous research served ophiolitic fragments occur scattered along the (Yang et al., 1991; Ye et al., 2004a). Published data suggested suture near Dingqing (Li, 1988; You, 1998; Wang, 2000), that the formation age of ophiolites along the BNS occurred Suo and Baxoi (Zhou, 1996), Dongqiao (Girardeau et al., between the Late Triassic to Early Jurassic and the emplace- 1984; Deng and Wang, 1987; Zhou et al., 1997), Naqu ment of oceanic crust began in the Middle Jurassic. (Pearce and Deng, 1988), Gyanco (Deng and Wang, 1987; Pearce and Deng, 1988; BGMRXAR Team 2, 2.2. Ophiolites in the Gertse area 1993), Xainza (Girardeau et al., 1985; Yang et al., 2003), Amdo (Pearce and Deng, 1988; Pearce and Mei, 1988; Two occurrences of ophiolitic rocks are present in the Lai and Liu, 2003), Dong Tso (Bao et al., 1996; Aitchison study area. The Dong Tso ophiolite crops out to the east 124 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

Table 1 Summary of the distribution, ages, and tectonic characteristics of the ophiolitic units along the Bangong-Nujiang suture Region Ophiolities Formation age (Ma) Emplacement age (Ma) Remarks Eastern Dingqing 197.3 ± 3.3 (40Ar/39Ar)1, Middle–Late Jurassic 6 Fore arc, SSZ7,8,9 Late Triassic 2,3,4,5 Suo ? ? Harzburgite, dunite, chromite 10,11,12 Baxoi Middle Dongqiao 150? (Os/Os )15, Late Triassic16,17 179 (K/Ar)13 , 180 ± 3 and 175 ± 3 SSZ14,18,19,20, back arc26, MORB24 (40Ar/39 Ar)14, Late Jurassic18,19,20,21, before Middle Jurassic22,23 Amdo Late Triassic – Early Jurassic18,27 165 (40Ar/39Ar)25 Arc-basin28, island arc26, SSZ18 Naqu Late Triassic – Early Jurassic18,21,27,26 ? Fore arc16,26 Xainza Late Triassic – Early Jurassic18 Middle Jurassic31 Fore arc 26, back arc29,30,32, SSZ31 Gyanco Before Middle Jurassic Middle Jurassic Fore arc26,32 Western Dong Tso 191 ± 2.2 (Sm/Nd)34 Before Jurassic33, 140 ± 4.07 (K/Ar) and SSZ34,35,36,37,38 152.30 ± 3.60 (K/Ar)34, 141 (K/Ar)36 Lagkor Tso Middle Jurassic38,39 166 ± 2.5 (U/Pb)39 MORB39, SSZ35 Bangong Lake Before Middle Jurassic42 Middle–Late Jurassic43,44,45 SSZ40,41,46,35, back arc43, MORB45 1You (1998); 2Wang et al. (2002a); 3Wang et al. (2002b); 4Li (1988); 5Wang (2000); 6Wang et al. (2002c); 7Zhang and Yang (1986); 8Liu et al. (2002); 9Zhang et al. (2003); 10BGMRXAR (1993); 11BGMRXAR Team 2 (1993); 12Zhou (1996); 13Cheng (1994); 14Zhou et al. (1997); 15Zhi et al. (2005); 16Schneider et al. (2004); 17Chen et al. (2004a); 18Girardeau et al. (1984); 19Marcoux et al. (1987); 20Qu et al. (2003); 21Chen et al. (2004b); 22Lu et al. (2003); 23Lu et al. (2004); 24Ye et al. (2004a); 25Guynn et al. (2003); 26Pearce and Deng (1988); 27Wang and Tang (1984); 28Lai and Liu (2003); 29Yang et al. (2003); 30Ye et al. (2004a,b); 31Girardeau et al. (1985); 32Deng and Wang (1987); 33Bao et al. (1996); 34Qiu et al. (2004a); 35Qiu et al. (2006); 36Xia (1993); 37Zeng et al. (2005); 38Aitchison et al. (2005); 39Zhang et al. (2007); 40Shi et al. (2004); 41Shi (2007); 42Shi and Yang (2005); 43Guo et al. (1991); 44Matte et al. (1996); 45Yang et al. (1991); 46Kapp et al. (2003). of Gertse north of Dong Tso; The Lagkor Tso ophiolite from gabbro collected north of Dong Tso has yielded a occurs south of Gertse on the northern shores of Lagkor Middle Jurassic age ( Aitchison, unpublished data). Bao Tso (‘Tso’ means lake in Tibetan) (Fig. 1B). Although et al. (1996) suggested that the ophiolite represents the some geological investigations have been carried out in this early stage of a Middle Jurassic ocean basin. area, few workers have concentrated on the ophiolites (Bao Metamorphic rocks, especially amphibolites, occur as et al., 1996; Aitchison et al., 2005, 2006; Qiu et al., 2004a,b, blocks in serpentinite-matrix me´lange at the base of the 2006) and no previous study has focused on the metamor- ophiolite where they are associated with ultramafic litholo- phic rocks associated with the ophiolitic units. gies. The amphibolites are dark green or greenish grey to light colored and exhibit a strong foliation defined by the 2.2.1. Dong Tso ophiolite sub-parallel alignment of amphibole-plagioclase crystallo- The ophiolite, which is preserved in a series of imbricate blasts. Quartz and calcite are rare, occurring in veins as sec- thrust slices, is characterized by a complete ophiolitic suite ondary mineralization products. The amphibolites show emplaced southwards onto the Lhasa terrane (Fig. 2). It variable mineral assemblages and textures and can be includes ultramafic rocks (metaperidotites and harzburg- divided into four groups. (1) Medium-grained garnet ites), serpentinites, isotropic and layered gabbros, a well- amphibolites containing the assemblages (a) Hbl+Pl+ preserved sheeted dyke complex, pillow basalts and minor Qtz+Grt+Ilm, (b) Hbl+Pl+Qtz+Grt+Cpx and (c) chert. Amphibolite-facies metamorphic blocks occur within Hbl+Pl+Qtz+Cpx+Chl. (2) Fine to medium-grained serpentinite-matrix me´lange at the base of the ophiolite. banded amphibolites containing the assemblages, (d) Locally, the serpentinites have experienced significant sil- Hbl+Qtz+Pl+Ilm, (e) Hbl+Cpx+Qtz+Pl+Ilm and (f) ica-carbonate alteration particularly along major fault Hbl+Qtz+Pl+Chl. (3) Coarse to fine-grained massive zones. Felsic stocks locally intrude harzburgitic ultramafic amphibolites containing the assemblages, (g) Hbl+Cpx+ rocks. Chert is locally present although it is typically Pl+Qtz±Chl and (h) Hbl+Qtz+Pl. (4) Coarse-grained altered, and Bao et al. (1996) reported, but did not figure, meta-gabbros containing, (i) Hbl+Cpx+Qtz±Ep±Chl. Jurassic radiolarians from east of Dong Tso on the north- Garnet schists also occur in the serpentinite matrix and ern shores of Zhaxi Tso (Fig. 1B). Pillow basalts are in are composed of Grt+Chl+Qtz±Ms±Bt. fault contact with the Triassic–Jurassic limestone along the north margin of the ophiolitic unit (Aitchison et al., 2.2.2. Lagkor Tso ophiolite 2005) and Cretaceous volcaniclastic rocks overlap both The Lagkor Tso ophiolite (Fig. 3) is dominated by ser- sides of the unit (Xia, 1993). Qiu et al. (2004a, 2006) stud- pentinite-matrix me´lange and lies in faulted contact with ied the geochemistry of peridotites at Dong Tso and Permian limestone on its southern side. Aptian/Albian reported that they have supra subduction zone (SSZ) char- orbitolinid-bearing limestone depositionally overlies the acteristics. They also reported a Sm–Nd age for gabbro northern side of the ophiolite and places constraints on sample of 191 ± 22 Ma. U/Pb SHRIMP dating of zircons the timing of its obduction. The me´lange is marked by a W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 125

Fig. 2. Geological map of Dong Tso ophiolites based on geological maps of Cheng and Xu (1986), Pan and Ding (2004), and our observations. disrupted ophiolitic suite, which includes metaperidotite, the assemblages, (c) Hbl+Pl+Chl±Ep; the metagabbros isotropic and layered gabbros, pillow lavas, cherts and contain, (d) Hbl+Pl+Cpx+Qtz+Ilm and (e) metamorphic rocks. Minor volcaniclastic rocks, diorite, Hbl+Pl+Cpx±Ol±Ilm. The estimated P–T conditions for granodiorite and tonalite also occur in the me´lange. the amphibolite-facies metamorphism are 5–7 kbar and Almost all ophiolitic lithologies can be found in the me´l- 555–655 C (Wang, unpublished data). No reverse meta- ange, although primary contacts are rarely preserved. Var- morphic zones are observed in this area (Fig. 3), and the ious lithologies are intercalated within the me´lange matrix metamorphism has no obvious relationship to obduction. and crop out as meter- to hectometer-sized blocks. Little Based on the occurrence of coeval arc-related granodiorites previous work has focused on the tectonic setting in which in the same region, it may simply be that these rocks have the ophiolite was generated. Our investigations indicate experienced elevated T and P conditions in a back-arc that the radiolarian cherts contain the Middle to Late intra-continental rift setting where there was elevated heat Jurassic assemblages (Aitchison and Davis, 2006) and the ﬂow (Aitchison, unpublished data). Some small (few m3, U/Pb SHRIMP dating of zircons from rocks associated Fig. 3) blocks of metamorphic rocks crop out along the with the ophiolitic me´lange give Middle Jurassic ages north side of the me´lange zone and include amphibolites, (Aitchison, unpublished data). greenshist and metagabbros, which preserve a strong folia- An extensive E–W oriented knocker (>2 · 0.5 km2)of tion typically oriented 105/76S. metamorphic rocks occurs at the NE end of Lagkor Tso (Fig. 3). The metamorphic rocks are dominantly amphibo- 3. Geochemistry lites and metagabbros, together with minor lenses and irregular mixtures of schists, metacherts, quartz dikes, dio- 3.1. Analytical method rite and shale. The overall structural trend in the me´lange trends to WNW–ESE and dips steeply NE. Amphibolite Fresh amphibolites were collected from the least-altered blocks and their foliations are generally aligned sub-paral- metamorphic units at Dong Tso and Lagkor Tso judged lel to the overall trend. Local variation in dip is observed both from outcrop appearances and from thin section where the largest amphibolite knocker is cut by a N–S ori- examinations. Samples were cut with a diamond-impreg- ented strike-slip fault. The amphibolites can be divided into nated brass blade, crushed in a steel jaw crusher that was three groups – banded amphibolites, massive amphibolites brushed and cleaned with de-ionized water between sam- and metagabbros. The banded amphibolites contain the ples, and pulverized in agate mortars to minimize potential assemblages (a) Hbl+Pl+Qtz+Ilm±Grt and (b) contamination. Major-element determinations for the bulk Hbl+Pl+Chl+Ilm±Bt; the massive amphibolites contain rocks were performed by wavelength-dispersive X-ray ﬂuo- 126 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

Fig. 3. (A) Map showing the ophiolitic me´lange of Lagkor Tso area. (B) Geological map of the main amphibolitic block in Lagkor Tso. rescence spectrometry (WD-XRFS), after fusion with lith- episode, some elements, especially alkalis and large ion ium tetraborate using a Philips PW2400 at the University lithophile elements (LIL), may have suffered significant ele- of Hong Kong. The precision was better than ±2% (rela- ment mobility. Therefore, a better classification needs to be tive) for a concentration of 0.5 wt.%. Trace elements were applied to determine the alkaline character of the igneous analyzed by inductively coupled plasma mass spectrometry protolith of the amphibolites using elements that are rela- (ICP-MS) of nebulized solutions using a VG Plasma-Quad tively fluid-immobile during a metamorphic event. The Excell ICP-MS after a 2-day closed beaker digestion using P2O5–Zr diagram for basalts on which all samples plot in a mixture of HF and HNO3 acids in high-pressure bomb. the tholeiitic fields clearly reveals their tholeiitic affinity Pure element standard solutions were used for external cal- (Winchester and Floyd, 1976; Fig. 4B). ibration and BHVO-1 and SY-4 were used as reference Most of samples are characterized by moderate to high materials. The precision of ICP-MS was better than ±5% Al2O3 (12.736–18.445%), CaO (5.940–15.017%) and Na2O (relative) for most elements. Detailed descriptions of these (1.706–5.322%) contents, and variable with P2O5 (0.018– analytical procedures are presented elsewhere (Qi and 0.189%) contents, although they may have been more or Gregoire, 2000; Zhou et al., 2004) and the data is given less modified through amphibolite-facies metamorphism. in Table 2. The P2O5, TiO2, MnO and Na2O concentrations positively correlative with the Zr values, whereas negative correla- 3.2. Geochemical characteristics and classifications tions can be observed between CaO, MgO and Zr concentrations (Appendix Fig. A1). These trends are Determining the original chemistry of metamorphic understandable in terms of fractional crystallization of rocks is rendered possible with the combined use of both basaltic magma. The negative correlations with Zr are con- major and trace elements. On a volatile-free basis, the pro- sistent with early crystallization of phases, such as olivine, toliths of most amphibolites can be classified as basalts pyroxene and Ca-plagioclase during magma differentiation. (Dong Tso) and basaltic andesites (Lagkor Tso), with sub The positive correlations between P2O5,TiO2 and Zr values alkaline affinity by using the total alkalis-silica diagram may represent an ilmenite and/or Ti-rich amphibole frac- (TAS) (Le Bas et al., 1992; Fig. 4A). However, as the tionation. The remaining oxides show a wide scatter and amphibolites have experienced at least one metamorphic poorly defined trends that suggest heterogeneity of parent W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 127

Table 2 Major oxides and trace element abundances of Gertse amphibolites of ophiolitic units, central Tibet Sample Dong Tso amphibolites WI05-062 092 I4W4 WI05-067 093 102 104 106 080 120 123 Major oxides (wt. %)

SiO2 46.86 47.57 47.42 43.53 46.52 48.86 46.39 48.97 48.44 49.00 48.86 TiO2 0.58 1.00 0.92 1.49 1.38 1.72 1.97 1.71 1.61 1.88 1.90 Al2O3 15.24 14.55 18.45 12.74 14.08 14.56 15.07 14.62 13.96 13.37 13.55 Fe2O3 9.24 10.60 6.11 13.19 13.48 13.43 14.09 13.38 13.23 14.66 15.16 MnO 0.14 0.16 0.10 0.15 0.21 0.17 0.19 0.17 0.20 0.19 0.22 MgO 9.94 8.96 9.00 10.13 7.91 6.43 7.46 6.58 7.23 6.16 7.30 CaO 12.02 12.47 15.02 11.63 10.93 9.11 9.64 9.13 9.62 9.21 8.07

Na2O 3.78 2.25 1.76 3.44 2.51 3.39 2.95 3.04 3.33 3.79 3.45 K2O 0.20 0.42 0.26 0.31 0.09 0.81 0.84 0.70 0.78 0.24 0.53 P2O5 0.04 0.05 0.03 0.08 0.10 0.18 0.17 0.17 0.16 0.17 0.19 LOI 2.53 1.69 0.83 2.99 2.39 0.90 1.06 0.93 1.15 1.67 1.15 Total 100.57 99.69 99.89 99.68 99.60 99.56 99.82 99.40 99.71 100.34 100.37 Trace elements (ppm) Sc 33.80 33.75 20.02 42.41 38.60 17.67 34.25 33.36 31.11 33.92 32.59 V 188.05 245.30 164.15 304.44 375.76 259.25 265.86 282.47 271.41 377.61 379.55 Cr 215.66 302.67 1003.91 215.92 165.87 181.92 188.20 181.61 181.59 76.18 98.12 Ni 120.66 107.26 274.51 91.80 111.99 694.02 80.56 80.36 67.01 80.03 65.99 Cu 4.02 11.26 4.66 14.73 56.07 33.55 49.83 54.87 71.61 14.23 44.29 Zn 33.00 39.26 37.02 47.28 93.46 106.78 122.03 102.14 102.66 46.83 77.45 Ga 13.33 13.30 13.20 15.67 16.51 17.04 17.95 18.41 17.76 18.24 17.22 Rb 4.29 14.03 5.66 5.64 1.12 3.49 12.69 7.52 14.56 5.20 12.85 Sr 192.74 146.38 367.64 163.03 172.58 192.47 251.89 303.77 160.35 129.23 173.95 Y 17.68 21.97 27.32 39.34 40.72 24.00 43.19 41.83 36.59 51.06 45.21 Zr 17.83 21.30 16.44 36.07 50.87 139.09 92.18 90.49 74.25 60.76 89.37 Nb 0.80 1.34 0.99 2.74 4.16 18.07 8.93 7.53 6.56 7.30 8.78 Ba 70.98 38.35 79.65 65.18 23.64 17.82 54.70 47.42 130.14 111.66 209.64 La 0.94 1.50 0.78 1.79 3.59 13.87 6.48 7.90 6.86 6.44 8.03 Ce 2.90 4.41 2.72 7.24 9.38 36.11 17.93 19.04 17.63 15.87 19.99 Pr 0.56 0.83 0.55 1.44 1.66 5.21 2.92 3.15 2.71 2.67 2.99 Nd 2.87 4.23 3.09 6.92 8.64 20.25 12.38 12.84 11.31 12.20 10.73 Sm 1.32 1.71 1.62 3.04 2.65 4.85 4.29 4.11 3.69 3.55 3.90 Eu 0.53 0.66 0.68 1.03 0.83 1.46 1.28 1.31 1.11 1.13 1.25 Gd 2.03 2.38 2.58 3.67 2.87 4.28 4.38 4.22 3.93 3.59 4.29 Tb 0.36 0.43 0.49 0.79 0.65 0.70 0.99 0.88 0.79 0.77 0.87 Dy 2.99 3.56 3.99 6.35 4.76 4.48 7.20 6.79 6.06 5.77 6.59 Ho 0.62 0.73 0.79 1.28 1.05 0.80 1.69 1.29 1.47 1.35 1.53 Er 1.80 2.11 2.45 3.54 2.65 1.87 3.89 3.90 3.42 3.09 3.75 Tm 0.25 0.31 0.34 0.51 0.41 0.26 0.60 0.59 0.52 0.45 0.55 Yb 1.81 2.07 2.31 3.52 2.77 1.52 3.94 3.87 3.37 3.20 3.50 Lu 0.26 0.30 0.37 0.51 0.37 0.20 0.55 0.56 0.51 0.47 0.57 Hf 0.70 0.86 0.90 1.32 1.04 3.43 2.73 2.65 2.15 1.53 2.15 Ta 0.08 0.14 0.05 0.24 0.22 1.32 0.87 0.74 0.61 0.49 0.61 Pb 0.79 0.13 0.18 0.79 0.19 0.77 0.53 0.89 0.66 0.22 0.26 Th 0.08 0.13 0.26 0.25 0.21 0.88 0.32 0.37 0.43 0.25 0.30 Sample Lagkor Tso amphibolites Wl05-006 008 Wl05-023 WI05-031 046 038 037 041 043 024 004 014 016 Major oxides (wt.%)

SiO2 55.61 52.69 50.60 50.66 51.38 51.64 47.68 52.19 50.81 50.99 53.93 48.03 49.08 TiO2 0.74 1.17 0.78 0.98 1.37 0.35 1.32 0.56 0.67 0.77 0.63 0.63 0.58 Al2O3 18.43 16.18 16.92 15.72 17.80 10.11 16.20 15.86 14.33 16.99 16.21 18.40 17.29 * Fe2O3 8.37 11.61 10.29 10.11 9.35 10.22 13.50 8.34 10.35 9.68 10.69 9.48 9.27 MnO 0.16 0.21 0.23 0.24 0.17 0.19 0.19 0.19 0.19 0.17 0.18 0.16 0.16 MgO 3.23 4.96 7.51 8.63 3.88 12.42 6.46 8.57 8.00 6.97 5.10 7.94 8.05 CaO 5.94 5.23 7.10 7.27 7.66 11.42 9.41 9.55 8.79 7.54 6.80 7.86 7.37

Na2O 5.22 5.32 4.08 3.75 4.76 1.77 3.05 2.74 3.38 3.88 3.40 2.96 3.27 K2O 0.62 0.28 0.32 0.67 0.33 0.15 0.31 0.29 0.37 0.84 0.34 1.22 1.25 P2O5 0.17 0.09 0.15 0.06 0.10 0.02 0.06 0.03 0.03 0.02 0.03 0.02 0.02 LOI 1.01 1.69 1.62 1.75 1.65 1.25 1.35 1.66 1.93 1.28 1.93 2.59 2.73 Total 99.49 99.43 99.60 99.83 98.45 99.54 99.52 99.98 98.84 99.12 99.22 99.27 99.07 (continued on next page) 128 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

Table 2 (continued) Wl05-006 008 Wl05-023 WI05-031 046 038 037 041 043 024 004 014 016 Trace elements (ppm) Sc 15.36 28.79 3.27 35.89 23.25 47.75 33.91 29.36 35.91 29.22 34.87 27.03 30.04 V 231.19 235.08 202.28 200.48 296.32 175.25 555.89 200.13 309.75 245.33 300.27 286.14 243.47 Cr 13.83 9.42 11.92 173.06 2.70 320.78 48.69 245.04 61.60 71.96 58.20 84.16 81.96 Ni 9.88 8.95 65.44 62.21 4.52 96.55 24.73 94.69 37.88 37.00 36.61 113.52 29.17 Cu 76.34 16.53 153.72 27.49 24.38 49.25 26.51 143.87 21.27 137.07 69.86 15.22 39.39 Zn 59.41 88.76 40.71 130.40 38.75 66.15 57.88 48.19 50.20 70.85 78.98 90.35 57.44 Ga 20.29 15.80 8.09 14.52 15.37 12.77 16.07 12.32 12.68 14.76 14.11 15.98 13.01 Rb 11.09 4.18 7.75 13.12 6.48 1.98 4.34 6.10 4.01 15.15 6.92 26.52 24.29 Sr 403.87 157.33 84.25 139.77 181.70 44.25 158.85 109.08 105.76 170.72 91.57 202.35 151.90 Y 24.20 23.35 24.73 25.49 27.66 11.62 10.04 12.71 11.26 11.47 15.95 10.75 11.07 Zr 56.16 51.22 104.45 62.28 35.67 12.82 22.10 24.55 27.32 31.44 27.39 29.53 22.74 Nb 2.23 1.24 2.55 0.84 1.92 0.42 0.77 0.63 0.68 0.78 0.92 0.74 0.62 Ba 140.96 35.20 43.21 74.57 44.88 16.48 32.11 34.60 29.15 185.69 165.24 326.95 307.53 La 10.14 3.82 10.89 2.45 5.69 1.10 2.22 1.78 2.03 2.22 1.88 1.85 1.81 Ce 22.96 9.66 20.03 7.77 14.74 2.36 5.19 4.31 5.22 6.19 4.73 5.10 4.66 Pr 3.20 1.53 2.75 1.34 2.37 0.38 0.73 0.68 0.73 0.79 0.56 0.71 0.67 Nd 11.50 5.96 10.30 5.18 10.61 2.48 3.64 2.63 2.48 3.01 3.89 3.51 3.86 Sm 3.31 2.15 2.44 2.04 3.31 0.71 0.99 1.04 0.97 1.02 1.04 0.93 1.01 Eu 1.02 0.81 0.76 0.75 1.17 0.30 0.43 0.42 0.38 0.51 0.48 0.40 0.45 Gd 3.03 2.28 2.09 2.27 3.68 1.04 1.30 1.36 1.29 1.45 1.61 1.24 1.24 Tb 0.56 0.49 0.42 0.48 0.65 0.23 0.27 0.29 0.24 0.28 0.32 0.25 0.25 Dy 3.76 3.74 3.05 3.86 4.56 1.90 1.99 2.10 1.90 2.06 2.60 1.72 1.92 Ho 0.86 0.83 0.68 0.72 0.83 0.41 0.37 0.40 0.35 0.43 0.56 0.46 0.42 Er 2.22 2.00 1.79 2.33 2.44 1.12 0.96 1.26 1.03 1.09 1.57 0.88 1.12 Tm 0.32 0.32 0.25 0.36 0.34 0.17 0.14 0.19 0.17 0.18 0.25 0.18 0.17 Yb 2.07 2.21 1.96 2.46 2.09 1.29 1.06 1.28 1.08 1.32 1.78 0.92 1.19 Lu 0.30 0.32 0.29 0.37 0.31 0.18 0.17 0.19 0.16 0.17 0.26 0.14 0.17 Hf 1.43 1.50 2.17 1.64 1.03 0.53 0.55 0.79 0.82 0.81 0.72 0.61 0.74 Ta 0.14 0.12 0.13 0.11 0.18 0.07 0.08 0.09 0.06 0.08 0.07 0.06 0.07 Pb 1.46 0.91 1.57 1.77 0.26 0.17 0.08 0.20 0.72 0.90 0.89 Th 0.33 0.49 0.68 0.44 0.40 0.25 0.27 0.25 0.33 0.23 0.17 0.16 0.26 Fe2O3 as total Fe. magma composition or secondary disturbance during have been derived from a common mantle source. How- metamorphism. ever, the samples from Dong Tso have systematically higher MREE to HREE abundances than the Lagkor 3.3. Trace and REE elements geochemistry Tso samples (8.29 6 SmN 6 22.01, 11.03 6 YbN 6 18.85 in Dong Tso samples; 2.52 6 SmN 6 16.99, As trace and REE elements may be affected by sea-floor 4.04 6 YbN 6 11.76 in Lagkor Tso samples; where N indi- alteration and metamorphism, plotting elemental abun- cates chondrite-normalized). This suggests that Dong Tso dance against Zr is one reliable method to detect changes amphibolites suffered a lesser degree of magmatic differen- associated with alteration and metamorphism. Although, tiation than those at Lagkor Tso and is in accord with there are no obvious correlations between most elements major element patterns. and Zr, some trace elements (Y, Hf, Nb, V) and REE ele- On a MORB-normalized incompatible trace element ments against Zr have the better linear correlations, which diagram (Fig. 5C and D), the patterns of samples from indicates they are little changed (Appendix Fig. A2). the two areas almost overlap exhibiting broad similarity The chondrite-normalized REE patterns for most Dong with arc volcanic rocks. Most amphibolites show enrich- Tso and Lagkor Tso amphibolites are characterized by rel- ments in large ion lithophile elements (LILE) (e.g., Rb, atively flat middle REE (MREE) to heavy REE (HREE), Ba) and relative depletions in high field strength elements and slightly enriched or depleted light REE (LREE), which (HFSE) (i.e., Nb, Ta, Zr, Ti). MORB-normalized patterns are similar to, but higher in abundance than, those of aver- are similar to those of Okinawa Trough-Ryukyu arc bas- age N-MORB, indicating they may be derived from a frac- alts, which were formed in an intra-continental back-arc tionated asthenospheric mantle (Fig. 5A and B). However, basin (Fig. 5C and D) (Shinjo et al., 1999; Shinjo, 1999). a few samples from Lagkor Tso (Wl05-006, Wl05-023 and There are, however, noteworthy differences between the Wl05-046) show more enrichment in LREE indicative of an amphibolites and other arc volcanic rocks for the highly arc basalt signature. It also must be noted that the REE incompatible elements, of which Sc is highly enriched, espe- patterns of the amphibolites from the two blocks signifi- cially in Lagkor Tso amphibolites. This may be due to var- cantly overlap indicating that all the amphibolites may iable components in the parental magma, different degrees W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 129

16 Dong Tso amphibolites have more of a MORB signature. On a plot of Ti vs. V (Shervais, 1982), most samples lie Alkaline series Phonolite 14 in MORB and back-arc basin/MORB ﬁelds (Fig. 6A). Sev- Tephri- 12 phonolite eral factors indicate that the amphibolites diﬀer from typi- Trachyte cal MORB. (1) Some samples plot as island arc tholeiites 10 Phono- (IAT). (2) The amphibolites have OIB characteristics on O tephrite 2 Foidite Trachy- Trachydacite 8 andesite Rhyolite N-MORB normalized diagrams (Fig. 5) and are similar O+K

2 Basaltic Tephrite trachy- to the Okinawa back-arc basin basalts (Shinjo, 1999; Shin- Na 6 basanite Trachy- andesite basalt jo et al., 1999). (3) Although the samples have MORB-like Dacite REE patterns, their abundances are higher than for N- 4 Andesite MORB. (4) On a Y–La–Nb diagram (Cabains and Lecolle, 2 Tholeiite series 1989), samples plot close to, or within, the ﬁeld of conti-

Basalt Basaltic

Picro- andesite basalt nental back-arc tholeiites (Fig. 6B) and plot similar to Oki- 0 35 40 45 50 55 60 65 70 75 nawa back-arc basin basalts (Shinjo, 1999; Shinjo et al., SiO 1999). 2 In summary, trace and REE geochemistry suggests that 1.0 most amphibolites from the two blocks were derived from Dong Tso amphibolites highly depleted asthenospheric mantle in a back-arc set- Lagkor Tso amphibolites ting. Some samples from Lagkor Tso, however, may repre-

Alkali basalt sent products of an associated island arc.

4. Geochronology 5 O 2 0.5 P 4.1. Analytical method Tholeiitic basalt Hornblendes separated from amphibolites were analyzed using 40Ar/39Ar single-grain laser fusion techniques. They were enriched by magnetic and gravimetric means 0.0 and handpicked under a binocular microscope to purify 0 200 400 the hornblende grains in the range of 140–250 lm. The Zr grains were irradiated together with the standard at the Fig. 4. (A) Geological classification (TAS) diagram (after Bas et al., 1992) THOR reactor at Nuclear Science and Technology Devel- for the samples studied. The dashed line that separates alkaline from opment Center (NSTDC) of National Tsing-Hua Univer- subalkaline series is from Irvine and Baragar (1971). (B) P2O5 vs. Zr sity, Taiwan, for 30 h. After irradiation, standards and discrimination diagram (after Winchester and Floyd, 1976); in these samples were totally fused using a Nd–YAG laser ablation diagrams most of samples plot in the tholeiitic basalt field. system with gas measured using a VG3600 mass spectrom- of magmatic differentiation, or to mobilization of the most eter at the 40Ar/39Ar geochronology laboratory, the incompatible element during the metamorphic event. On Department of Geosciences, National Taiwan University, the other hand, the amphibolites show positive Sr anoma- Taiwan. Relevant analytical details can be found in Lo lies that may reflect plagioclase accumulation in some sam- et al. (2001, 2002). Ages were calculated from argon isoto- ples and/or may represent the arc-type signature of the pic ratios measured after correlations made for mass dis- parental melts. crimination, interfering nuclear reactions, procedural blanks and atmospheric argon contaminations. 3.4. Petrogenesis 4.2. Analytical results Interpretation of the tectonic setting of a metamorphosed and deformed terrane is difficult. The chemistry Detailed experimental results are given in the Appendix of some elements, in particular, the LIL elements (such (Table A1); data are plotted on isotope correlation dia- as K, Ba, Rb, Sr), has been affected by metamorphism. grams and Cumulative Gaussian plus histogram diagrams Therefore, elements, such as Ti, Zr, Y, La and Nb, which in Fig. 7. All errors are quoted at the 1r level of uncer- are effectively immobile during metamorphism (Appendix tainty. Four samples were selected for 40Ar/39Ar dating. Fig. A2), should be put to use in interpretation of the tec- Sample I4W4 was collected 1.2 m from the contact between tonic setting of the Gertse amphibolites. metamorphic and ultramafic rocks, and sample Wl05-067 Geochemical data shows the two amphibolite blocks was collected from another amphibolite block at Dong have similar characteristics, with both MORB-like REE Tso (Fig. 2). Wl05-006 and Wl05-011 were collected from and arc-like MORB-normalized patterns. However, Lag- a Knocker at Lagkor Tso, and are also located near the kor Tso amphibolites show strong arc affinity, whereas contact between metamorphic and ultramafic rocks 130 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

A B

100 100

Dong Tso

10 Lagkor Tso 10 Rock/Chondrites Rock/Chondrites

1 1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1000 1000 C D

100 100 Dong Tso Lagkor Tso

Okinawa BABB

10 10 Okinawa BABB Rock/NMORB Rock/NMORB 1 1

.1 .1 Cs Rb Ba Th Nb K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy YYbLu Cs Rb Ba Th Nb K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu

Fig. 5. Chondrite-normalized REE patterns and spider plots for the amphibolites in Gertse area (normalization after Sun and McDonough, 1998), and comparison among Dong Tso amphibolites, Lagkor Tso amphibolites and Okinawa back-arc basin basalts.

(Fig. 3). Samples I4w4, Wl05-011 and Wl05-067, are dom- for Wl05-011). All the ages were also evaluated using inated by hornblende and plagioclase (>90%), and also the cumulative probability plots and histograms with the contain chlorite, quartz and/or clinopyroxene. The straight program IsoPlot 3.0 (Ludwig, 2003) to summarize the grain boundaries between hornblende and plagioclase indi- probability of distributions of the age data with their nor- cate textural equilibrium. They also show minor late miner- mally distributed errors (Fig. 7). In summary, the amphib- alization (<3%) during which plagioclase and/or chlorite olite samples yield ages 170–177 Ma which is interpreted developed in veins. The late mineralization maybe related to represent the timing of the main metamorphic event to hydrothermal alternation process. The mineralogical experienced in this area. assemblage of Wl05-006 is hornblende, plagioclase, quartz and ilmenite. The ages and simple descriptions of the sam- 5. Discussion and conclusions ples are summarized in Table 3. The single grain laser fusion age yielded intercept ages The amphibolites, along the BNS at Dong Tso and Lag- of 177.6 ± 3.3 Ma (I4W4) and a slightly diﬀerent age of kor Tso, occur as blocks within the serpentinite-matrix 167.0 ± 7.2 Ma for Wl05-067. The samples have reason- me´lange indicating that the amphibolite-facies metamor- able 40Ar/36Ar initial ratios (294 ± 2.0 for I4W4 and phism occurred prior to the formation of the me´lange. 304.6 ± 16.1 for Wl05-067) and mean square of the They experienced an amphibolite-facies metamorphic event weighted deviation values (MSWD) (0.826 for I4W4 and during the early Middle Jurassic. The metamorphic age is 0.823 for Wl06-067). Samples Wl05-006 and Wl05-011 constrained by 40Ar/39Ar dating on hornblende (170– gave apparent dates in the ranges 167.1–179.7 Ma 177 Ma). It is coeval with both the timing of ophiolite crys- (n = 18) and 166.9–179.9 Ma (n = 17), respectively, with tallization determined from U/Pb SHRIMP dating of zir- total gas ages 176.4 ± 0.4 Ma (Wl05-006) and cons in gabbro and formation of arc-related 173.9 ± 0.6 Ma (Wl05-011). They yield intercept ages of granodiorites present in the me´lange (Aitchison, unpub- 177.6 ± 3.4 Ma for Wl05-006 and 176.0 ± 3.9 Ma for lished data). A Middle Jurassic amphibolite-facies meta- Wl05-011, with reasonable 40Ar/36Ar initial ratios morphic event is also recorded by Dongqiao ophiolites (287.5 ± 4.8 for Wl05-006 and 290.8 ± 13.6 for Wl05- and the Amdo basement further eastwards along the 011) and MSWD values (2.253 for Wl05-006 and 2.352 BNS. The Dongqiao amphibolites have been interpreted W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 131

low basalts and represent truly oceanic sediments are lim- 600 10 ARC<20>OFB ited to the Middle to Late Jurassic although some

Low-Ti IAT Triassic radiolarian assemblages have been reported from 500 BON Okinawa BABB IAT the eastern sector of the BNS (Wang and Tang, 1984; MORB 50 Wang, 2000; Shi and Yang, 2005; Aitchison and Davis, 400 BAB MORB 2006). This suggests that oceanic crust developed over a V 300 short period and was possibly not extensive at least in the middle and western sectors of the BNS. (3) The strong Alkaline 100 200 faunal affinity between the Lhasa and southern Qiangtang terranes suggests that the existence of a wide ocean here is 100 questionable, whereas paleontological data suggests that the central Qiangtang metamorphic belt (CQMB or Shu- 0 0101520255 anghu suture) represents a distinct boundary between Cat- Ti/1000 haysian and Gondwanan faunas (Wang and Mu, 1983; Li, 1987; Li et al., 1995; Zhang, 2001; Yin and Grant-Mackie, Y/15 2005; Zhang et al., 2006c). (4) Blueschists and eclogites characteristic of the subduction-related metamorphism in NMORB * Back arc basin convergent margin setting where anomalously high P/T Okinawa BABB gradients exist crop out along the CQMB rather than along VAT the BNS (Cheng and Xu, 1986; Li, 1987; Li et al., 1995; * Zhang et al., 2006a,b). This, together with the juxtaposition of faunal provinces, has led many Chinese workers to con- EMORB sider the CQMB to represent a major suture. We note that this interpretation remains controversial and some researchers favor an interpretation where development of the CQMB is linked to flat southward-directed subduction Alkaline of the Songpan-Ganzi oceanic crust under the Qiangtang Calc-alkali Cont. Intercontinental rifts terrane (Kapp et al., 2000, 2003). If it was not a major oceanic basin, then the nature of La/10 Nb/8 the Bangong-Nujiang ocean remains unclear. One model Fig. 6. (A) Ti vs. V discrimination diagram (Shervais, 1982) and (B) Y– claims the ocean is opened in response to subduction of La–Nb discrimination diagram (Cabains and Lecolle, 1989). Comparison Paleo-Tethyan oceanic crust along the Jinsha suture (Liu among Dong Tso amphibolites, Lagkor Tso amphibolites and Okinawa et al., 1992; Haines et al., 2003). However, this is not rea- back arc basin basalts. sonable because the Paleo-Tethyan ocean closed during the Late Triassic to Early Jurassic (Alle`gre et al., 1984; as the metamorphic sole of a SSZ ophiolite (Zhou et al., Chang et al., 1986; Dewey et al., 1988; Yin and Harrison, 1997), whereas the amphibolite-facies metamorphism at 2000). It has also been suggested that low-angle northward Amdo is interpreted to be related to a continental arc subduction of the Neo-Tethyan ocean played an important (Guynn et al., 2006). All the amphibolite-facies metamor- role in the evolution of the Bangong-Nujiang back-arc phism can be related to events associated with formation basin (Wang et al., 1987; Liu et al., 1990). This model of the Bangong-Nujiang ocean in the Early–Middle has become more popular in recent years based on the Jurassic. studies of Jurassic magmatism in the Lhasa terrane (Kapp The BNS, between the Qiangtang and Lhasa terranes in et al., 2005a; Kapp et al., 2005b; Chu et al., 2006; Dong central Tibet, is classically interpreted as the remnants of a et al., 2006; Zhu et al., 2006). Many recent studies suggest major oceanic basin that opened before the Triassic and that low-angle northward subduction of a Neo-Tethyan closed by northward subduction beneath the Qiangtang ocean slab had reached the areas beneath the northern terrane in the Middle to Late Jurassic (Girardeau et al., Lhasa terrane and may have affected the collision between 1984; Tang and Wang, 1984; Pearce and Deng, 1988). the Lhasa and Qiangtang terranes in the Jurassic. However, there is little evidence to indicate that at least Our results indicate that the amphibolites within the the western and middle sectors of the BNS are part of a ophiolitic me´lange at Dong Tso and Lagkor Tso along classic suture like the more extensively studied Yarlung the BNS, in central Tibet, have MORB or arc-like MORB Tsangpo suture to its south in Tibet. (1) With the notable geochemical characteristics. Lagkor Tso amphibolites exception of recent reports by Guynn et al. (2006), little evi- show greater arc affinity, whereas Dong Tso amphibolites dence of any Jurassic magmatic arc or major Middle Meso- have a strong MORB signature. These characteristics are zoic tectonism has been reported from either side of the similar to basalts from a back-arc basin setting. The meta- BNS (Coward et al., 1988; Schneider et al., 2004). (2) morphic ages of amphibolites are 170–177 Ma, which is The ages of radiolarian cherts that are associated with pil- interpreted to represent the timing of a significant meta- 132 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

9 4 8

7 3 6 5 2 4 Number 3 Number .004 2 .004 1 1 0 0 110 130 150 170 190 210 230 250 270 120 140 160 180 200 220 240 .003 .003

Age (Ma) Age (Ma)

40 /

/ .002

.002 I4w4 Ar 40 36 Ar Wl05-067 36 ( Ar/ Ar) i = 294.3 ± 2.0 36 = 0.826 (40Ar/36Ar) i = 304.6 ± 16.1 MSWD Age (Ma) = 177.6 ± 3.3 Ma MSWD = 0.823 .001 .001 Age (Ma) = 167.0 ± 7.2 Ma A B 0 0 0 .003 .006 .009 .012 0 .005 .01 .015 .02 .025 39Ar / 40Ar 39Ar / 40Ar

3 7 6 2 5 4 .004 3 .004 Number

Number 1 2 1 .003 .003 0 0 150 160 170 180 190 200 210 158 162 166 170 174 178 182 186 190

Ar Age (Ma)

Age (Ma) Ar

40 / .002 / .002 Wl05-011

Wl05-006 40 36 Ar Ar ( Ar/ Ar) i = 290.8 ± 13.6 40 36

36 36 ( Ar/ Ar) i = 287.5 ± 4.8 MSWD = 2.352

MSWD = 2.253 Age (Ma) = 176.0 ± 3.9 Ma .001 Age (Ma) = 177.6 ± 3.4 Ma .001 C D 0 0 0 .008 .016 .024 .032 0 .007 .014 .021 .028 39Ar / 40Ar 39Ar / 40Ar

Fig. 7. Isotope correlation diagrams and Cumulative Gaussian plus histogram diagrams for the ages of hornblendes, Gertse amphibolites based on 40Ar/39Ar single-grain laser fusion techniques. Two samples (I4W4 and Wl05-067) come from Dong Tso and The other two (Wl05-006 and Wl05-011) come from Lagkor Tso.

Table 3 Gertse amphibolites dated by 40Ar/39Ar single-grain laser fusion techniques Sample Location Lithology Mineralogy Age Wl05-006 N 32 05.0730 E84 Amphibolite Hbl (I)+Pl (I)+Qtz (I)+Ilm (I). Dated hornblende grains are 0.1–0.2 mm and 177.6 ± 3.4 Ma 10.3320 well oriented. Wl05-011 N 32 05.1030 E84 Amphibolite Hbl (I)+Pl (I, II)+Cpx (I)+Chl (I, II)+Qtz (I). Dated hornblende grains are 176.0 ± 3.9 Ma 10.5490 0.5–0.8 mm in size. I4w4 N 32 19.4050 E84 Amphibolite Hbl (I)+Pl (I, II)+Chl (I, II)+Qtz (I). The hornblendes are well orientated 177.6 ± 3.3 Ma 44.8400 parallel to the foliation. Wl05-067 N 32 20.7880 E84 Amphibolite Hbl (I)+Pl (I)+Cpx (I)+Chl (I, II) 169.5 ± 4.8 Ma 39.2500 (I) Presenting main mineral assemblage (>95%); (II) indicating secondary mineral development (in veins, <3%). morphic event that was widely experienced in this area. tors of the BNS contain the remnants of a short-lived Based on the results of this present study and the literature back-arc basin (the Bangong-Nujiang ocean), which partly data, we suggest that at least the western and middle sec- opened and closed in the Middle Jurassic. W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 133

Acknowledgements 03) awarded to Aitchison. We thank the reviewers Paul Kapp and journal editor J.G. Liou for constructive com- We thank members of the Tibetan Geological Survey ments that improved the manuscript. and the Tibetan Geological Society, whose eﬀorts have helped to make this research possible. F. Xiao is thanked for her help for XRF analysis. This research was supported Appendix by grant from the Research Grant Council of Hong Kong Special Administrative Region, China (project HKU7001/ See Table A1, Figs. A1 and A2.

Table A1 Detailed data for Gertse amphibolites dated by 40Ar/39Ar single-grain laser fusion techniques Grain Atm. % 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Date (Ma) ± (Ma) Sample: I4W4 (HbL), J-value = 0.00380929, Total Gas Age = 173.2 ± 2.6 Ma, Mean age = 173.8 ± 5.2 Ma 2403 86.94 6.05E-01 1.65E+01 1.75E-01 2.04E+02 3.38E+02 176.4 17 2404 68.63 2.10E-01 1.38E+01 8.64E-02 8.90E+01 4.23E+02 183.8 3.9 2405 71.13 2.16E-01 1.07E+01 1.08E-01 8.85E+01 4.10E+02 168.7 5.9 2406 89.1 7.42E-01 1.16E+01 2.54E-01 2.45E+02 3.30E+02 176.1 9.8 2407 82.45 4.34E-01 2.05E+01 1.52E-01 1.54E+02 3.54E+02 178.8 22.2 2408 82.73 4.20E-01 1.38E+01 1.35E-01 1.49E+02 3.54E+02 170 7.8 2409 84.98 4.74E-01 1.47E+01 1.57E-01 1.63E+02 3.45E+02 162.8 14.2 2410 77.05 3.00E-01 7.33E+00 9.32E-02 1.14E+02 3.81E+02 172.5 4.2 2411 77.29 2.90E-01 1.08E+01 9.96E-02 1.10E+02 3.79E+02 164.9 6.7 2501 81.13 3.90E-01 1.73E+01 1.13E-01 1.41E+02 3.60E+02 175.5 8.9 2502 83.74 4.56E-01 1.43E+01 1.37E-01 1.59E+02 3.50E+02 171.4 17.5 2503 83.06 4.35E-01 1.05E+01 1.42E-01 1.54E+02 3.54E+02 171.7 7.6 2504 86.97 6.10E-01 1.64E+01 1.68E-01 2.06E+02 3.37E+02 177.2 7.2 2505 84.53 4.96E-01 1.11E+01 1.24E-01 1.72E+02 3.48E+02 175.7 7.7 2506 74.26 2.64E-01 1.35E+01 9.71E-02 1.04E+02 3.93E+02 176.2 4.5 2507 81.84 4.11E-01 1.99E+01 2.07E-01 1.47E+02 3.57E+02 176.4 13 2508 71.25 2.32E-01 1.33E+01 1.04E-01 9.48E+01 4.09E+02 179.6 13.4 2509 79.12 3.36E-01 1.02E+01 1.27E-01 1.25E+02 3.71E+02 171.6 10.6

Sample: Wl05-006 (HbL), J-value = 0.00380929, Total Gas Age = 176.4 ± 0.7 Ma, Mean age = 174.4 ± 2.9 Ma 708 40.96 6.65E-02 1.08E+01 2.84E-02 4.60E+01 6.92E+02 178.7 2.4 709 46.23 8.04E-02 1.04E+01 3.74E-02 4.97E+01 6.19E+02 176.1 1.5 710 33.07 4.78E-02 9.92E+00 4.23E-02 4.05E+01 8.47E+02 178.3 3.2 711 31.73 4.48E-02 1.12E+01 3.77E-02 3.91E+01 8.72E+02 175.6 2.7 712 33.91 5.01E-02 1.08E+01 2.82E-02 4.13E+01 8.24E+02 179.6 3.4 713 31.62 4.48E-02 1.22E+01 4.10E-02 3.90E+01 8.70E+02 175.8 2.7 714 34.88 5.18E-02 1.11E+01 4.00E-02 4.15E+01 8.01E+02 177.7 2.9 715 41 6.20E-02 9.85E+00 3.81E-02 4.29E+01 6.92E+02 167.1 1.1 801 31.68 4.51E-02 1.10E+01 3.62E-02 3.94E+01 8.75E+02 177.4 3.1 802 42.56 6.92E-02 1.07E+01 3.59E-02 4.62E+01 6.67E+02 174.8 3.1 803 32.75 4.65E-02 1.08E+01 3.84E-02 3.95E+01 8.49E+02 175 1 804 31.38 4.51E-02 1.10E+01 3.69E-02 3.98E+01 8.83E+02 179.7 1.5 805 31.68 4.53E-02 1.12E+01 3.54E-02 3.96E+01 8.75E+02 178.3 1.3 806 30.77 4.27E-02 1.14E+01 3.11E-02 3.83E+01 8.96E+02 174.6 0.9 807 30.77 4.34E-02 1.08E+01 3.70E-02 3.91E+01 9.00E+02 178 1.7 808 43.08 7.05E-02 1.06E+01 6.25E-02 4.65E+01 6.60E+02 174.4 6.6 809 55.56 1.17E-01 1.09E+01 5.87E-02 6.06E+01 5.19E+02 177.4 4.5 810 52.29 1.03E-01 9.33E+00 7.30E-02 5.66E+01 5.52E+02 177.6 7.3

Sample: Wl05-011 (HbL), J-value = 0.00380929, Total Gas Age = 173.9 ± 0.6 Ma, Mean age = 174.0 ± 4.0 Ma 814 30.25 4.21E-02 1.04E+01 2.72E-02 3.85E+01 9.16E+02 176.8 1.5 815 28.09 3.87E-02 1.24E+01 3.68E-02 3.74E+01 9.67E+02 177.4 2.7 816 27.6 3.62E-02 1.10E+01 4.65E-02 3.57E+01 9.88E+02 170.7 3.5 817 27.8 3.81E-02 1.09E+01 3.99E-02 3.75E+01 9.86E+02 178.3 2.9 818 30.11 4.08E-02 1.07E+01 3.74E-02 3.74E+01 9.16E+02 172.3 2.9 819 29.06 4.03E-02 1.05E+01 2.93E-02 3.83E+01 9.50E+02 178.5 2.8 901 24.99 3.25E-02 1.15E+01 3.52E-02 3.50E+01 1.08E+03 173 1.6 902 20.98 2.73E-02 1.10E+01 2.96E-02 3.45E+01 1.26E+03 179.3 1.4 903 21.77 2.77E-02 9.86E+00 3.53E-02 3.42E+01 1.24E+03 175.8 1.7 904 18.02 2.29E-02 1.02E+01 3.50E-02 3.34E+01 1.46E+03 179.9 1.3 905 22.92 2.95E-02 1.11E+01 3.68E-02 3.44E+01 1.17E+03 174.7 2.3 906 26.47 3.47E-02 1.15E+01 4.27E-02 3.55E+01 1.02E+03 172.1 2.5 (continued on next page) 134 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138

Table A1 (continued) Grain Atm. % 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Date (Ma) ± (Ma) 907 24.18 3.13E-02 1.12E+01 2.97E-02 3.48E+01 1.11E+03 173.7 1.8 908 25.41 3.24E-02 1.09E+01 3.77E-02 3.45E+01 1.06E+03 169.8 3.7 909 27.23 3.51E-02 1.12E+01 4.44E-02 3.50E+01 9.98E+02 168.2 2.2 910 24.61 3.05E-02 1.00E+01 3.74E-02 3.36E+01 1.10E+03 166.9 2.3 911 24.68 3.16E-02 1.17E+01 3.76E-02 3.43E+01 1.08E+03 170.2 1.5

Sample: Wl05-067 (HbL), J-value = 0.00380929, Total Gas Age = 174.9 ± 2.6 Ma, Mean age = 175.0 ± 3.5 Ma 1801 45.63 7.58E-02 1.28E+01 6.48E-02 4.70E+01 6.20E+02 169 2 1802 51.36 9.66E-02 1.23E+01 7.62E-02 5.38E+01 5.57E+02 172.7 6.8 1803 47.97 8.93E-02 1.32E+01 6.86E-02 5.30E+01 5.93E+02 181.4 8.3 1804 47.41 8.28E-02 1.08E+01 6.78E-02 4.99E+01 6.03E+02 173 5.6 1805 46.03 8.01E-02 9.43E+00 7.31E-02 4.99E+01 6.23E+02 177.3 9.8 1806 38.59 6.00E-02 1.16E+01 5.69E-02 4.37E+01 7.28E+02 176.9 6.4 1807 56.7 1.19E-01 8.91E+00 6.80E-02 6.11E+01 5.12E+02 174.2 4.8 1808 45.95 7.92E-02 9.32E+00 6.30E-02 4.94E+01 6.24E+02 175.8 7.9 1809 35.73 5.33E-02 1.32E+01 6.73E-02 4.13E+01 7.76E+02 175.3 7.1

3 0.3 wt.% TiO2 P2O5 wt.%

2 0.2

1 0.1

0 0.0

0.4 7 Na O wt.% MnO wt.% 2 6

5 0.3 4

3 0.2 2

0.1 0

20 20 CaO wt.% MgO wt.%

10 10

Dong Tso amphibolites Lagkor Tso amphibolites

0 0 0 100 200 300 400 0 100 200 300 400 Zr (ppm) Zr (ppm)

Fig. A1. Selected major elements (TiO2,P2O5, MnO, Na2O, CaO and MgO) bivariate plot for the amphibolites using Zr as diﬀerentiation index. W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 135

80 Y (ppm) 10 Hf (ppm) 60 8

40 6

4 20 2

0 0

600 80

V (ppm) La (ppm) 500 60

400 40 300

20 200

100 0

6 20 Yb (ppm) Sm (ppm) 5

3 10

Dong Tso amphibolites 1 Lagkor Tso amphibolites

0 0 0 100 200 300 400 0 100 200 300 400

Zr (ppm) Zr (ppm)

Fig. A2. Selected trace elements (Y, Hf and V) and REE elements (La, Yb and Sm) vs. Zr for Dong Tso and Lagkor Tso amphibolites.

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