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

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Geochemistry and Geochronology of the Amphibolite Blocks in Ophiolitic Me´Langes Along Bangong-Nujiang Suture, Central Tibet Available online at www.sciencedirect.com 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 inter- preted 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. Ó 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 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.10.022 W.-L. Wang et al. / Journal of Asian Earth Sciences 33 (2008) 122–138 123 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.
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