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Age and Anatomy of the Gongga Shan Batholith, Eastern Tibetan Plateau, and Its Relationship to the Active Xianshui-He Fault

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Age and Anatomy of the Gongga Shan Batholith, Eastern Tibetan Plateau, and Its Relationship to the Active Xianshui-He Fault This is a repository copy of Age and Anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/101886/ Version: Accepted Version Article: Searle, MP, Roberts, NMW, Chung, S-L et al. (8 more authors) (2016) Age and Anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault. Geosphere, 12 (3). pp. 948-970. ISSN 1553-040X https://doi.org/10.1130/GES01244.1 © 2016, Geological Society of America. This is an author produced version of a paper published in Geosphere. Uploaded in accordance with the publisher's self-archiving policy. Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ 1 Age and Anatomy of the Gongga Shan batholith, Eastern Tibetan Plateau and its 2 relationship to the active Xianshui-he fault. 3 4 5 M.P. Searle1, N.M.W. Roberts2, S-L. Chung3,4, Y-H. Lee5, K.L. Cook3,6, J.R. Elliott1, 6 O.M. Weller7, M.R. St-Onge7, X. Xu8, X. Tan8, and K. Li8. 7 8 1. Department of Earth Sciences, Oxford University, South Parks Road, Oxford 9 OX1 3AN, UK. 10 2. NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, 11 Nottingham, UK. 12 3. Department of Geosciences, National Taiwan University (NTU), Taipei 10617, 13 Taiwan. 14 4. Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan. 15 5. Department of Earth and Environmental Sciences, National Chung-Cheng 16 University, Chiayi, Taiwan. 17 6. Now at: GFZ German Research Center for Geosciences, Telegafenberg, 14473 18 Potsdam, Germany. 19 7. Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, 20 Canada. 21 8. Institute of Geology, China Earthquake Administration, Huayanli A1, 22 Chaoyang district, Beijing 100029, China. 23 24 25 1 26 Abstract 27 The Gongga Shan batholith of eastern Tibet, previously documented as a ~32 – 12.8 28 Ma granite pluton, shows some of the youngest U-Pb granite crystallisation ages 29 recorded from the Tibetan plateau, with major implications for the tectonothermal 30 history of the region. Field observations indicate that the batholith is composite, with 31 some localities showing at least seven cross-cutting phases of granitoids that range in 32 composition from diorite to leucocratic monzogranite. In this study we present U-Pb 33 ages of zircon and allanite dated by LA-ICPMS on seven samples, to further investigate 34 the chronology of the batholith. The age data constrain two striking tectonic-plutonic 35 events: a complex Triassic-Jurassic (ca. 215-159 Ma) record of biotite-hornblende 36 granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a 37 Miocene (ca. 14-5 Ma) record of monzonite-leucogranite emplacement. The former age 38 range is attributed to widespread ‘Indosinian’ tectonism, related to Paleo-Tethyan 39 subduction zone magmatism along the western Yangtze block of South China. The 40 younger component may be related to localised partial melting (muscovite- 41 dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, 42 and are amongst the youngest crustal melt granites exposed on the Tibetan Plateau. 43 Zircon and allanite ages reflect multiple crustal re-melting events, with the youngest at 44 ca. 5 Ma resulting in dissolution and crystallization of zircons and growth/resetting of 45 allanites. The young garnet, muscovite and biotite leucogranites occur mainly in the 46 central part of the batholith and adjacent to the eastern margin of the batholith at 47 Kangding where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault 48 is the most seismically active strike-slip fault in Tibet and thought to record the 49 eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts 50 all granites of the Gongga Shan massif and has a major transpressional component in 51 the Kangding – Moxi region. The course of the Xianshui Jiang river is offset by ~62 52 km along the Xianshui-he fault and in the Kangding area granites as young as ~5 Ma 53 are cut by the fault. Our new geochronological data show that only a part of the Gongga 54 Shan granite batholith is composed of young (Miocene) melt, and we surmise that as 55 most of eastern Tibet is composed of Precambrian – Triassic Indosinian rocks there is 56 no geological evidence to support regional Cenozoic internal thickening or 57 metamorphism and no evidence for eastward directed lower crustal flow away from 58 Tibet. Instead we suggest that underthrusting of Indian lower crust north as far as the 59 Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau. 2 60 61 62 INTRODUCTION 63 64 The Tibetan Plateau (Fig. 1) is the world’s largest area of high elevation (~5 km 65 average) and thick crust (70-85 km thick), and the timing of its rise is important not 66 only for tectonics but also for understanding the influence of topography on climate and 67 the erosional flux of sedimentary detritus into rivers and oceans. Geological evidence 68 for crustal thickening and topographic uplift includes the timing of compressional 69 deformation, regional metamorphism and magmatism. Earlier notions of a Late 70 Cenozoic thickening and rise of the Tibetan plateau (e.g. Molnar et al., 1993) following 71 the Early Eocene collision of India with Asia and the closing of the intervening Neo- 72 Tethys ocean at 50.5 Ma (Green et al., 2008) have since been challenged by more recent 73 geological investigations. Chung et al. (2005), Kapp et al. (2007) and Searle et al. 74 (2011) noted the widespread occurrence of Andean-type subduction-related Late 75 Jurassic – Early Eocene granites (Ladakh-Gangdese batholith) and calc-alkaline 76 volcanic rocks across the Lhasa block strongly suggesting an Andean-type topography 77 with similar crustal thickness during the period prior to the India-Asia collision. Soon 78 after India-Asia collision, calc-alkaline Gangdese-type magmatism ended (St-Onge et 79 al., 2010) and volumetrically minor, sporadic but widespread adakitic magmatism 80 occurred across the plateau since 50 Ma (Chung et al., 2005). 81 The present-day structure of eastern Tibet is characterised by a high, flat plateau, 82 with a few exceptional topographic anomalies, such as the Gongga Shan (7556 m) 83 massif, and a steep eastern margin along the LongMen Shan, showing an abrupt 84 shallowing of the Moho from depths of 60-80 km beneath the plateau to depths of 35- 85 40 km beneath the Sichuan basin (Zhang et al., 2010). There is an almost complete lack 86 of Cenozoic shortening structures, with the exception of steep west-dipping faults 87 associated with the M7.9 Wenchuan earthquake along the LongMen Shan margin 88 (Hubbard and Shaw, 2009). Most of the deformation in eastern Tibet is Indosinian 89 (Triassic-Jurassic) in age (Harrowfield and Wilson, 2005; Wilson et al., 2006; Roger et 90 al., 2008). The thick Triassic Songpan-Ganze ‘flysch’ sedimentary rocks are tightly 91 folded about upright fold axes and lie above a major horizontal detachment above 92 Palaeozoic basement (Harrowfield and Wilson, 2005). Most granites that have been 93 dated intruded during the period 220-188 Ma (Roger et al., 2004, 2008; Zhang et al., 3 94 2006). A complete Barrovian-type metamorphic sequence is present in the structurally 95 deeper Danba dome, where peak sillimanite grade metamorphism has been dated at 96 179.4 ± 1.6 Ma using in situ U-Pb monazite analysis(Weller et al., 2013). There is no 97 record of any Cenozoic metamorphism anywhere in north, east or central Tibet. The 98 old ages of deformation, metamorphism and magmatism argue strongly against the 99 Miocene-recent homogeneous crustal shortening models for Tibet (Dewey and Burke, 100 1973; England and Molnar, 1979). 101 Cenozoic structures in central and eastern Tibet are represented by large-scale 102 strike-slip faults (Fig. 2). The Ganzi (Yushu) and Xianshui-he left-lateral strike-slip 103 faults cut across all the geology of the eastern plateau and have diverted river courses 104 in the upper Yangtse and Jinsha river systems. These faults curve around the Eastern 105 Himalayan syntaxis and are thought to be responsible for southeastward extrusion of 106 the south Tibetan crust (Molnar and Tapponnier, 1975; Peltzer and Tapponnier, 1988; 107 Peltzer et al., 1989; Tapponnier et al., 2001) and clockwise rotations caused by the 108 northward indentation of India (England and Molnar, 1990). 109 Along the Xianshui-he fault a granitic batholith, the Gongga Shan massif, crops 110 out for about 200 km along the southwestern margin (Fig. 3). This forms a major 111 topographic high, with the Gongga Shan massif reaching 7756 meters. Roger et al.
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