Origin and pre-Cenozoic evolution of the south Qiangtang basement, Central Tibet

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<ul><li><p>ou</p><p>anTb</p><p>ajing</p><p>Keywords:TibetQiangtangHigh-pressure rocks</p><p>tranewar. We show that the Qiangtang metamorphic belt can be separated in a Paleozoic</p><p>(e.g. Freymueller, 2011; Gehrels et al., 2011; Haines et al., 2003; 2012; Zhu et al., 2012). It is dened by a broad and discontinuous belt</p><p>e geological evolution</p><p>into low-metamorphiclastic metasediments</p><p>Tectonophysics 623 (2014) 5266</p><p>Contents lists available at ScienceDirect</p><p>Tectonop</p><p>.e lQiangtang terrane, with the Jinsha Suture Zone (JSZ) in between (Yinand Harrison, 2000) (Fig. 1a). This suture zone represents the Paleo-Tethys (or a branch of it) between Eurasia and the Qiangtang terrane</p><p>and subduction-related mlange with high-pressure rocks, includingeclogites (Liu et al., 2011), in the center of the terrane. PermianJurassicshallow-marine limestones, interbedded with minor sand-mudstones,Flysch Complex, and the Qiangtang and Lhasa terranes. These terranesare separated by EW striking suture zones of Paleozoic to Mesozoicage (Fig. 1a) (Yin and Harrison, 2000).</p><p>The SongpanGanzi Flysch Complex lies to the north of the</p><p>Qiangtang terrane is important to understand thof the Tibetan Plateau.</p><p>The Qiangtang terrane is traditionally dividedgrade Paleozoic (?CarboniferousPermian) cTapponnier et al., 2001). However, due to its high elevation, remotenessand complex geology, open questions remain on the pre-Cenozoicevolution of the plateau, particularly its interior, where the Qiangtangterrane is located in Central Tibet. From north to south, the CentralTibetan Plateau is formed by the Qaidam terrane, the SongpanGanzi</p><p>of ophiolite fragments and mlange. It is assumed to be the result ofclosure of the Meso-Tethys Ocean and subsequent collision of theLhasa and Qiangtang terranes during the Early Cretaceous (Deweyet al., 1988; Guynn et al., 2006, 2012; Kapp et al., 2003a, 2007). Consid-ering this complex setting, better knowledge of the geology of thethat was consumed by southward subduction</p><p> Corresponding author at: Department of GeoscienTbingen, Wilhelmstrasse 56, D-72074 Tbingen, German</p><p>E-mail address: zhongbao.zhao@ifg.uni-tuebingen.de</p><p>http://dx.doi.org/10.1016/j.tecto.2014.03.0160040-1951/ 2014 Elsevier B.V. All rights reserved.ature on Earth, has beenophysical investigations</p><p>Nujiang Suture Zone (BNSZ) separates the Qiangtang terrane from theLhasa terrane in the south (Fig. 1a) (Dewey et al., 1988; Guynn et al.,The Tibetan Plateau, the largest orogenic fethe focus of many recent geological and gePaleo-TethysObductionSubduction mlange</p><p>1. Introductionhigh-pressure rocks and Permian sediments. Detrital zircon ages (youngest peak at 591 Ma) and an ~470 Magranite intrusion age constrain the age of the Qiangtang Basement to be between the Late Precambrian toMiddleOrdovician. This is in agreement with the observed unconformity between basement and overlying MidLateOrdovician strata. This unconformity and the zircon age spectra are comparable to the Himalaya area. TheQiangtang terrane must thus have been part of the Gondwana supercontinent during the Early Paleozoic.The occurrence of the Late Triassic eclogite and glaucophane-bearing schists in the Central Qiangtang terrane in-dicates the existence of a suture zone between the North and South Qiangtang terranes before the Late Triassic.This suture zone resulted from closure of the Paleo-Tethys between theNorth and SouthQiangtang terranes. LateTriassic syn-collisional granites and rapidly exhumed high-pressure rocks resulted from the closure of this oceanand nal amalgamation of the re-united Qiangtang terranes with Eurasia. Collision of the Lhasa and Qiangtangterranes is correlated with north dipping reverse faults in the south Qiangtang terrane, which were probablyrelated to exhumation of the Qiangtang basement and formation of the Qiangtang Culmination.</p><p> 2014 Elsevier B.V. All rights reserved.</p><p>terrane in Late Triassic to Early Jurassic times (Dewey et al., 1988;Kapp et al., 2000; Nie et al., 1994; Yin and Nie, 1996). The BangongAccepted 18 March 2014Available online 24 March 2014</p><p>autochthonous basement and an overlying allochthonous thrust stack of subduction mlange that containsReceived in revised form 17 March 2014 relatively little is known so fOrigin and pre-Cenozoic evolution of the sCentral Tibet</p><p>Zhongbao Zhao a,b,, Paul D. Bons a, Genhou Wang b, Ya Department of Geosciences, Eberhard Karls University Tbingen, Wilhelmstrasse 56, D-72074b School of Earth Science and Resources, China University of Geosciences, Beijing 100083, Chinc State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Bei</p><p>a b s t r a c ta r t i c l e i n f o</p><p>Article history:Received 9 October 2013</p><p>The Qiangtang terrane in CenPaleo-Tethys. We present a</p><p>j ourna l homepage: wwwbeneath the Qiangtang</p><p>ces, Eberhard Karls Universityy. Tel.: +49 7071 2976469.(Z. Zhao).th Qiangtang basement,</p><p>Liu c, Yilong Zheng b</p><p>ingen, Germany</p><p>100037, China</p><p>l Tibet, with its high-pressure rocks, is a key area to unravel the evolution of the1:50,000 map of the Rongma area in the central Qiangtang terrane, of which</p><p>hysics</p><p>sev ie r .com/ locate / tectooverlie the southern and northern anks of the Qiangtang terrane.Some authors subdivided the Qiangtang terrane into two parts, theNorth Qiangtang terrane and the South Qiangtang terrane (sometimesreferred to as East and West Qiangtang, respectively), because the high-pressure rocks are found in the center of the Qiangtang terrane (Fig. 1b)</p></li><li><p>53Z. Zhao et al. / Tectonophysics 623 (2014) 5266(Li et al., 2006; Zhai et al., 2011). The boundary between these twoterranes is the Longmu CoShuanhu Suture Zone, situated north of themlange and high-pressure rocks.</p><p>The Qiangtang terrane is considered to have been a contiguous partof the Cimmerian super terrane along the Tethys margin of Gondwana(Guynn et al., 2012), because the terrane exposes Cambrian gneiss inAmdo (Guynn et al., 2006; Kapp et al., 2000; Xu et al., 1985), extensivelate Carboniferousearly Permian glaciomarine deposits in the centralQiangtang terrane (Li et al., 1995), and yields similar UPb detritalzircon age distributions for Upper Paleozoic strata within the southQiangtang terrane (Dong et al., 2011; Gehrels et al., 2011; Kapp et al.,2003b; Leier et al., 2007; Pullen et al., 2008).</p><p>Within the Qiangtang terrane, eclogite-bearing blueschist-faciesrocks have been found in Gangmu Co, Rongma etc., but their origin isunder debate (Kapp et al., 2000, 2003b; Li et al., 2006; Liang et al.,2012; Liu et al., 2011; Zhang et al., 2006a). Two basic, but radicallydifferent models have been proposed for the origin of these high-pressure rocks (Fig. 2). The intra-Qiangtang suture model envis-ages northward subduction of the Paleo-Tethys along the LongmuCoShuanhu Suture Zone (LSSZ) that separates the North and</p><p>Fig. 1. Tectonic sketch of central Tibet, showing the several terranes and suture zones. (a) StruLSSZ Longmu CoShuanghu Suture Zone; BNSZ BangongNujiang Suture Zone; and IYTibet. The middle part is the Qiangtang culmination, with Mesozoic to Neozoic sedimentsModied after Deng et al. (2005), Kapp et al. (2003b), Li (2006) and Zeng et al. (2002, 20South Qiangtang terranes, and thrusting of the mlange over theSouth Qiangtang terrane (Fig. 2a) (Li et al., 2009; Zhai et al., 2011). Ac-cording to the alternative underthrustmodel, earlyMesozoicmlangewas thrust under the Qiangtang terrane from the Jinsha Suture Zone(JSZ) that bounds the Qiangtang terrane 200 km to the north (Fig. 2b)(Kapp et al., 2000, 2003b; Pullen et al., 2011). In thismodel themlangewas underplated to theQiangtang terrane and exhumed by detachmentfaulting. In the intra-Qiangtang suture model, allochthonousmlange isunderlain by autochthonous Paleozoic basement, while the underthrustmodel predicts that the deeper crust in large parts of northern Tibet ismlange. Knowing the exact nature of the Paleozoic metamorphicrocks is therefore crucial to advance our understanding of the crustalstructure of the Qiangtang terrane. The two very dissimilar modelsshow that whether the Paleozoic low-grade metamorphic rocks areautochthonous basement or not has far-reaching consequences, notonly for the pre-Cenozoic evolution of the Qiangtang terrane, but alsofor the whole Tibetan plateau and the Indo-Asian collision (Kappet al., 2003b). Unfortunately, the presence of basement in theQiangtangterrane (Dong et al., 2011; Pullen et al., 2011; Wang and Wang, 2001)and the origin of the high-pressure rocks (Kapp et al., 2003b; Li et al.,</p><p>ctural sketch of Tibet. From north to south, the sutures are JSZ Jinsha Suture Zone;SZ IndusYarlung Suture Zone. (b) The main units of the Qiangtang terrane, centralmostly found on its the northern and southern anks.06).</p></li><li><p>zoic</p><p>54 Z. Zhao et al. / Tectonophysics 623 (2014) 52662009) still remain controversial. The main aim of this paper is to clarifythe relationship between the various units in the Qiangtang terrane,</p><p>Fig. 2.Competing tectonicmodels for the central Qiangtangmetamorphic belt in earlyMeso(Kapp et al., 2003b).using detailed mapping and geochronology. We show that the low-grade metamorphic rocks are indeed autochthonous basement rocksof Early Paleozoic age and Gondwana afnity.</p><p>2. Geology of the Rongma area</p><p>Due to the remoteness and difcult eld conditions, the Qiangtangterrane has not yet been mapped and studied in great detail. To gainbetter understanding of Paleozoic Tibet and the Paleo-Tethys, theRongma areawasmapped at a scale of 1:50,000 (Fig. 3). Paleozoicmeta-morphic rocks, subduction mlange, including eclogites, and Mesozoicstrata are exposed in the area (Liu et al., 2011; Zhai et al., 2007).Below we present and discuss the lithostratigraphy and structureseparately.</p><p>2.1. Lithostratigraphy</p><p>Rocks that are exposed in the area vary greatly in their age, origin,metamorphic facies and deformation (Kapp et al., 2000, 2003b).Based on this, they can be divided into four main units. These are(1) Qiangtang basement (QB), consisting of greenschist-facies Paleozoicstrata with pervasive folding and cleavage, (2) nautiloids-bearingMiddle to Late Ordovician slate and ammonoid-bearing Carboniferoussandstone and siltstone (OC), (3) strongly deformed subductionmlange (SM) including high-pressure metamorphic rocks (HP) and(4) the uppermost cover unit that consists of Permian to Jurassicshallow marine deposits (Fig. 3). Units 13 were intruded by the 210 5 Ma Gangtang Co Granite, NW of the village of Rongma (Kapp et al.2003b).</p><p>Aswewill showbelow, theQB forms the structurally lowest unit andis exposed over large areas north of Rongma. Cheng and Xu (1986) andKapp et al. (2003b) interpreted the fossil-free metamorphic rocks asCarboniferous in age, and assigned them to the Zhanjin Formation. Inthe mapping area, the unit is mainly of clastic sedimentary origin,</p><p>times: (a) the intra-Qiangtang suturemodel (Li et al., 1995) and (b) theunderthrustmodelmetamorphosed to quartzite, phyllite, meta-arkose and minor marble(Li et al., 2006, 2007). Some mac lenses are found, oriented parallelto the main foliation. The unit was intruded by Early Permian dykeswarmswith gabbro, diabase andminor ultramac rocks. The supposedCarboniferous age (Cheng andXu, 1986)will be discussed and correctedbelow.</p><p>Middle to Late Ordovician strata of the Tashishan Formation areexposed at two locations, where they unconformably overlie the QB.One is ~20 km north of Gangtang Co and the other is near Rongmavillage (Figs. 3, 4a). The Tashishan formation is characterized bysiltstone and slates, interbedded with volcanoclastics and calcareousslates that are rich in crinoids, nautiloids and graptolites. Previously,these outcrops of Ordovician strata were interpreted as klippen(Li, 2006). The Ordovician layers, however, unconformably overlie theQB. Where the unconformity is exposed, a basal conglomerate isfound, consisting of cmdm size quartzite and marble clasts in acalcareous matrix (Fig. 5a,b).</p><p>Carboniferous sediments, cm to b1 m thick sandstone and siltstonebeds, are found a few kilometers west of the Ordovician outcrop nearRongma village (Fig. 4a). The beds form an open km-scale synclinebounded by a normal fault in the east. Carboniferous beds are not asstrongly folded as the underlying basement rocks (QB) and beds areparallel to the contact with these underlying rocks. In the absence ofany distinct faulting or shearing, the contact between QB and Carbonif-erous is interpreted as an unconformity.</p><p>The subduction mlange (SM) is mainly found in the middle of themapping area (Fig. 3). It is characterized by strongly deformed marinesedimentary rocks that contain less-deformed lenses. The marinesediment consists of pelites, siltstones, sandstones and minor lime-stones and cherts. These lithologies form the matrix of the mlange,which contains blocks and lenses, tens of meters up to kilometers insize, of more competent rocks, such as pillow basalts, minor ultramacs,</p></li><li><p>Fig. 3. (a) Detailed geological map of the Qiangtang basement, subductionmlange and overlying sediments of the Rongma area based on ourmapping. Locations of samples discussed inthis paper are shown. Locations of Fig. 4a and b are shown by the yellow frames.</p><p>55Z. Zhao et al. / Tectonophysics 623 (2014) 5266</p></li><li><p>56 Z. Zhao et al. / Tectonophysics 623 (2014) 5266gabbros, diabase, etc. (Cheng and Xu, 1986; Kapp et al., 2003b).Blueschist and eclogite-facies metamorphic slices (HP) are regarded aspart of the subduction mlange. They occur in roughly NS trendingbelts and consist of eclogites, glaucophane-bearing schist, garnet-bearing phengitequartz schist andmarbles. The eclogites are generallyfound to be enclosed as lenses in garnet-bearing phengitequartz schist.The protoliths of HP rocks have an oceanic afnity, and include basalt,gabbro and pelagic sediments (Zhang et al., 2006b). The subductionmlange was interpreted as an accretionary mlange formed at a</p><p>Fig. 4. (a) 3-D reconstruction of relationships between basement (QB), Ordovician slates, COrdovician slates and Carboniferous sandstone unconformably overlie the basement,while the sare shown in lower-hemisphere, equal area stereoplots for ve locations (I to V) in the block diamlange, the Permian sediments and high-pressure rocks form an allochthonous stack of thrustCogranite. Locations of diagrams are shown in Fig. 3. ReconstructionsweremadewithMove b(Db2): bedding in QB at a small angle to the shortening direction is folded, while bedding in ththerefore stretched.convergent-plate margin (Zhai et al., 2004). Both the basement units(QB and OC) and the subduction mlange are intruded by the LateTriassic Gangtang Co granite (Figs. 3, 4b).</p><p>The Late Permian to Jurassic shallow-marine deposits are dividedinto three types. In the north, Late Permian to Late Triassic shallow-marine limestones, interbedded with minor siltstone and coral layers,contain Eurasian warm-water faunas (Cathaysian afnity; similar tothe South China block) (Li et al., 1995). In the middle part of themapping area, Permian shallow-marine limestones of the Longge</p><p>arboniferous sandstones (O...</p></li></ul>

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