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Discovery of the Early Jurassic Gajia Mélange in the Bangong–Nujiang

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Discovery of the Early Jurassic Gajia Mélange in the Bangong–Nujiang Int J Earth Sci (Geol Rundsch) (2017) 106:1277–1288 DOI 10.1007/s00531-016-1405-1 ORIGINAL PAPER Discovery of the early Jurassic Gajia mélange in the Bangong– Nujiang suture zone: Southward subduction of the Bangong– Nujiang Ocean? Wen Lai1 · Xiumian Hu1 · Dicheng Zhu2 · Wei An3 · Anlin Ma1 Received: 13 April 2016 / Accepted: 21 September 2016 / Published online: 4 October 2016 © Springer-Verlag Berlin Heidelberg 2016 Abstract Mélange records a series of geological processes sandstone blocks is 177 2.4 Ma, suggesting an Early ± associated with oceanic subduction and continental colli- Jurassic depositional age for the sandstones within the sion. This paper reports for the frst time the presence of Gajia mélange. The Gajia mélange likely records the south- Early Jurassic mélange from NW Nagqu in the southern ward subduction of the Bangong–Nujiang Ocean during margin of the Bangong–Nujiang suture zone, termed as the the Early Jurassic. Gajia mélange. It shows typically blocks-in-matrix struc- ture with matrix of black shale and siliceous mudstone, Keywords Gajia mélange · Early Jurassic · Provenance and several centimeters to several meters sized blocks of analysis · Bangong–Nujiang suture zone sandstone, silicalite, limestone and basalt. The sandstone blocks consist of homologous sandstone and two types of exotic sandstone, with different modal compositions. The Introduction Group 1 of exotic sandstone blocks consists of mainly of feldspar and quartz, whereas the Group 2 is rich in volcanic The mélange is composed of the matrix, homologous detritus. The Group 3 of homologous sandstone blocks is blocks and exotic blocks, which are different in composi- rich in feldspar and volcanic detritus with rare occurrence tions, ages and sources (Harris et al. 1998; Hsü 1974), and of quartz. U–Pb age data and in situ Hf isotopic composi- is mainly formed in the tectonic setting of oceanic subduc- tions of detrital zircons from sandstone blocks are similar tion and continental collision (e.g., Chang et al. 2001; Har- to those from the Lhasa terrane, suggesting that the sand- ris et al. 1998; Wang et al. 1988; An et al. 2016). stone blocks in the Gajia mélange most probably came Wide distribution of the Jurassic ophiolite and oceanic from the Lhasa terrane. The YC1σ(2 ) age of homologous sediments in the Bangong–Nujiang suture zone suggests + that the Bangong–Nujiang Ocean did exist between the Lhasa and Qiangtang terranes (Allègre et al. 1984; Dewey Electronic supplementary material The online version of this et al. 1988). However, the subduction polarity remains in article (doi:10.1007/s00531-016-1405-1) contains supplementary dispute. One traditional view is that the Bangong–Nujiang material, which is available to authorized users. Ocean subducted northward beneath the Qiangtang terrane * Xiumian Hu (Allègre et al. 1984; Chen et al. 2012; Guynn et al. 2006; [email protected] Leier et al. 2007a; Yin and Harrison 2000). Alternatively, others have argued for southward subduction beneath the 1 State Key Laboratory of Mineral Deposits Research, School Lhasa terrane (Hsü et al. 1995; Kang et al. 2010; Zhu et al. of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China 2009a, 2011b) or a double-sided subduction zone involv- ing both northward subduction beneath the Qiangtang ter- 2 State Key Laboratory of Geological Process and Mineral Resources, School of Earth Sciences and Resources, China rane and southward subduction beneath the Lhasa terrane University of Geosciences, Beijing 100083, China (Deng et al. 2014; Hao et al. 2016; Pan et al. 2012; Zhu 3 School of Resources and Environmental Engineering, Hefei et al. 2013, 2016). Studying the geologic record of oceanic University of Technology, Hefei 230009, China subduction such as mélange is fundamental to reconstruct 1 3 1278 Int J Earth Sci (Geol Rundsch) (2017) 106:1277–1288 the details of subduction process during the destruction his- 2.5, respectively (Li et al. 2014b, 2015; Liu et al. 2015; + tory of the Bangong–Nujiang oceanic lithosphere. Wang et al. 2015; Yang et al. 2011; Zhai et al. 2013). The aim of the present study is to unravel the mélange To the south of the study area, the Lhasa terrane is discovered near the Gajia village in the southern margin of bounded by the Indus–Yarlung Zangbo and the Bangong– Bangong–Nujiang suture zone. We present detailed petro- Nujiang suture zones (Allègre et al. 1984; Dewey et al. logical, detrital zircon U–Pb geochronology and Hf isotope 1988; Yin and Harrison 2000). The Lower Cretaceous mar- data on different types of sandstone blocks contained in ginal marine and deltaic clastic sediments interbedded with the Gajia mélange of the Bangong–Nujiang suture zone in volcanic tuffs (Leier et al. 2007a; Zhang et al. 2012), mid- central Tibet (Fig. 1b). The results allowed us to determine Cretaceous Orbitolina-bearing Langshan limestone (Rao the provenance and depositional age of the mélange and to et al. 2015; XZBGM 1993), and the Upper Cretaceous demonstrate that the Bangong–Nujiang oceanic lithosphere to Cenozoic nonmarine conglomerate (Kapp et al. 2005, subducted southward beneath the Lhasa terrane at the time 2007b) are extensively exposed on the northern Lhasa when the Gajia mélange formed. block. The Cretaceous magmatic rocks are also widespread on the northern Lhasa block (Zhu et al. 2009a, 2011b). The central Lhasa terrane is mainly represented by Carbonifer- Geological setting ous metasediments, Permian limestone and Jurassic silici- clastic successions (XZBGM 1993; Yin et al. 1988). And The study area is at the southern margin of Bangong– the southern Lhasa block is characterized by the Late Tri- Nujiang suture zone (Fig. 1a) which continues for at least assic-Early Tertiary Gangdese batholiths and Tertiary Lin- 1200 km east–west along the strike and is dominated by zizong volcanic succession (Chu et al. 2006; Ji et al. 2009; Jurassic deep water turbidites, mélange and ophiolite frag- Zhu et al. 2011b) and the Cretaceous Xigaze forearc basin ments (Dewey et al. 1988; Kapp et al. 2005). The Bangong- (An et al. 2014; Wu et al. 2010). Zhu et al. (2011a) dem- Nujiang Ocean possibly opened in the Paleozoic according onstrated that the detrital zircons of Lhasa terrane defne a the discovery of the Paleozoic ophiolite fragments (Zhu distinctive age population of ca. 1170 Ma, which is differ- et al. 2013) and the Late Triassic turbidites unconformably ent from both the southern Qiangtang and Amdo terrane. on the ophiolite (Chen et al. 2005) in the suture zone, and In addition, obviously different from the southern Qiang- closed during Late Jurassic–Early Cretaceous time (Baxter tang and Amdo terrane, the εHf(t) values of igneous zir- et al. 2009; Dewey et al. 1988; Ding et al. 2005; Zhu et al. cons from Lhasa terrane are <2.0 in Ordovician to Middle 2016). Triassic and 5.0 to 20.0 in Middle Triassic to Jurassic − + The Amdo terrane, located in the northeast of the Ban- (Chu et al. 2006; Dong et al. 2014; Ji et al. 2009; Zhu et al. gong–Nujiang suture zone, is dominated by Precambrian 2009b, 2011b). gneiss and metasedimentary rocks, Mesozoic granitoids Located in the Gajia village of the Nagqu county and Cenozoic sedimentary rocks (Guynn et al. 2006; Kidd (Fig. 1b), the Gajia mélange consists of limestone, basalt, et al. 1988; Xu et al. 1985; XZBGM 1993). Guynn et al. turbidite sandstone and silicalite blocks and was consid- (2012) reported bimodal distribution of Neoproterozoic ered as normal sedimentary strata in the Upper Jurassic to (920–820 Ma) and Cambro–Ordovician (540–460 Ma) Lower Cretaceous (Kidd et al. 1988) or Middle Triassic crystallization ages of the orthogeneses in Amdo ter- (Nimaciren and Xie 2005). rane. Meanwhile, the Mesozoic granitoids in Amdo have bimodal distribution of 185–170 and 110–120 Ma crystal- lization ages, with ε (t) values of 21.7 ~ 0.6 (Liu et al. Methods Hf − + 2015; Zhu et al. 2011b). Between the Bangong–Nujiang suture zone in the south The probable depositional environments and deformation and the Longmu–Shuanghu suture zone in the north, the characters of Gajia mélange were investigated and distin- southern Qiangtang terrane is mainly represented by Trias- guished based on lithofacies and sedimentary features in sic–Jurassic shallow marine deposition, with some Later the feld. Sandstone blocks from the Gajia mélange were Cretaceous and Cenozoic nonmarine sedimentary rocks sampled systematically for further analysis. (XZBGM 1993). The Jurassic (150–170 Ma) acidic igne- Seven sandstones from the Gajia mélange were selected ous rocks, Triassic (200–230 Ma) acidic igneous rocks, to do modal framework-grain analysis on thin sections. Permian (280–290 Ma) basic igneous rocks and Ordovician Over 350 larger than 62.5 μm grains were counted follow- (450–500 Ma) acidic igneous rocks are extensively exposed ing the Gazzi–Dickinson method (Dickinson and Suczek on the southern Qiangtang terrane, and the εHf(t) values of 1979; Ingersoll et al. 1984). the Later Triassic to Jurassic, Ordovician to Middle Triassic Accessory minerals were separated from seven sand- igneous zircons mainly range from 4.2 to 17.7, 19.4 to stone samples by elutriation and magnetic separation. − 1 3 Int J Earth Sci (Geol Rundsch) (2017) 106:1277–1288 1279 82 84 (a) 86 88 90 92 94 N 80 JSMZ 35 W E 353 Hongjishan Northern 34 S 208 Qiangtang 280-300 350-365 225 34 Rutog Southern 208 217 Qiangtang 374 210 BNMZ 340-350 230-300 33 275-285 210 220 210 275-290 Rongma 33 Aweng 210 Co 150-160 Shuanghu Gar Yanhu LSMZ Gerze 32 Amdo Xungba
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