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Age and Anatomy of the Gongga Shan Batholith, Eastern Tibetan Plateau

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Age and Anatomy of the Gongga Shan Batholith, Eastern Tibetan Plateau Research Paper GEOSPHERE Age and anatomy of the Gongga Shan batholith, eastern Tibetan GEOSPHERE; v. 12, no. 3 Plateau, and its relationship to the active Xianshui-he fault doi:10.1130/GES01244.1 Michael P. Searle1, Nick M.W. Roberts2, Sun-Lin Chung3,4, Yuan-Hsi Lee5, Kristen L. Cook3,*, John R. Elliott1, Owen M. Weller6, Marc R. St-Onge6, Xi-Wei Xu7, Xi-Bin Tan7, and Kang Li7 17 figures; 1 supplemental file 1Department of Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, UK 2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, UK CORRESPONDENCE: mike .searle@ earth .ox .ac.uk 3Department of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan 4Institute of Earth Sciences, Academia Sinica, No.128, Sec. 2, Academia Road, Nangang, Taipei 11529, Taiwan 5Department of Earth and Environmental Sciences, National Chung-Cheng University, 168 University Road, Min-Hsiung, Chiayi, Taiwan CITATION: Searle, M.P., Roberts, N.M.W., Chung, 6Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada S-L., Lee, Y-H., Cook, K.L., Elliott, J.R., Weller, O.M., 7Institute of Geology, China Earthquake Administration, Huayanli A1, Chaoyang district, Beijing 100029, China St-Onge, M.R., Xu, X-W., Tan, X-B., and Li, K., 2016, Age and anatomy of the Gongga Shan batholith, east- ern Tibetan Plateau, and its relationship to the active Xianshui-he fault: Geosphere, v. 12, no. 3, p. 948– 970, doi:10.1130/GES01244.1. ABSTRACT granites of the Gongga Shan massif and has a major transpressional compo- nent in the Kangding-Moxi region. The course of the Xianshui Jiang river is Received 29 July 2015 The Gongga Shan batholith of eastern Tibet, previously documented as offset by ~62 km along the Xianshui-he fault and in the Kangding area granites Revision received 7 March 2016 a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite as young as ca. 5 Ma are cut by the fault. Our new geochronological data show Accepted 8 April 2016 crystallization ages recorded from the Tibetan Plateau, with major implica- that only a part of the Gongga Shan granite batholith is composed of young Published online 5 May 2016 tions for the tectonothermal history of the region. Field observations indicate (Miocene) melt, and we surmise that as most of eastern Tibet is composed that the batholith is composite; some localities show at least seven cross- of Precambrian–Triassic Indosinian rocks, there is no geological evidence to cutting phases of granitoids that range in composition from diorite to leuco- support regional Cenozoic internal thickening or metamorphism and no evi- cratic monzogranite. In this study we present U-Pb ages of zircon and allanite dence for eastward-directed lower crustal flow away from Tibet. We suggest dated by laser ablation–inductively coupled plasma–mass spectrometry on that underthrusting of Indian lower crust north as far as the Xianshui-he fault seven samples, to further investigate the chronology of the batholith. The resulted in Cenozoic uplift of the eastern plateau. age data constrain two striking tectonic-plutonic events: a complex Triassic– Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) INTRODUCTION record of monzonite-leucogranite emplacement. The former age range is at- tributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduc- The Tibetan Plateau (Fig. 1) is the world’s largest area of high elevation tion zone magmatism along the western Yangtze block of south China. The (~5 km average) and thick crust (70–85 km thick), and the timing of its rise is younger component may be related to localized partial melting (muscovite de- important for tectonics and for understanding the influence of topography on hydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze climate and the erosional flux of sedimentary detritus into rivers and oceans. terrane, and are among the youngest crustal melt granites exposed on the Geological evidence for crustal thickening and topographic uplift includes the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting timing of compressional deformation, regional metamorphism, and magma- events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of tism. Earlier notions of a late Cenozoic thickening and rise of the Tibetan Pla- zircons and growth and/or resetting of allanites. The young garnet, muscovite, teau (e.g., Molnar et al., 1993) following the early Eocene collision of India with and biotite leucogranites occur mainly in the central part of the batholith and Asia and the closing of the intervening Neotethys Ocean at 50.5 Ma (Green et adjacent to the eastern margin of the batholith at Kangding, where they are al., 2008) have been challenged by recent geological investigations. In Chung cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seis- et al. (2005), Kapp et al. (2007a, 2007b), and Searle et al. (2011), the widespread mically active strike-slip fault in Tibet and is thought to record the eastward occurrence of Andean-type subduction-related Late Jurassic–early Eocene extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites (Ladakh-Gangdese batholith) and calc-alkaline volcanic rocks across For permission to copy, contact Copyright the Lhasa block was noted, strongly suggesting an Andean-type topography Permissions, GSA, or [email protected]. *Now at GFZ German Research Center for Geosciences, Telegafenberg, 14473 Potsdam, Germany with similar crustal thickness during the period prior to the India-Asia collision. © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 3 Searle et al. | Gongga Shan batholith, Tibet Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/948/4092719/948.pdf 948 by guest on 02 October 2021 Research Paper 70° 75° 80° 85° 90° 95° 100° 105° 110° 40° Tien Shan 40° Tarim Basin Qaidam Basin Haiyuan Fault Pamirs Ordos Altyn Tagh Fault Kunlun Fault Karakoram Fault 35° Hindu Kush KUNLUN 35° Jinsha suture SONGPAN−GANZE Xianshui−he Fault Garze−Yushu Faul Fig. 2 Bangong−Nujiang Suture Fig. 17 QIANGTANG Jiali Fault t LHASA Main Frontal Thrust 30° Sichuan 30° Indus−Yalu Suture Fig. 4 Basin Gongga Shan Xiaojiang Fault 60°80° 100° 120° India Fig. 3a China KAZAKHSTAN MONGOLIA 25° 25° Red−River Fault 40° Sa Ganga Delta gaing Fault IRAN CHINA Strike Slip Normal INDIA Thrust 20° Suture Bay of Bengal Indo−China Gulf of Tonkin 20° 20° 70° 75° 80° 85° 90° 95° 100° 105° 110° Figure 1. Digital elevation map of the Tibetan Plateau showing the main active faults, sutures, and terranes (after Taylor and Yin, 2009). The location of Gongga Shan peak (7556 m) is shown by the red triangle. Soon after the India-Asia collision, calc-alkaline Gangdese-type magmatism the exception of steep west-dipping faults associated with the M7.9 Wenchuan ended (St-Onge et al., 2010) and volumetrically minor, sporadic, but widespread earthquake along the Longmen Shan margin (Hubbard and Shaw, 2009). Most adakitic magmatism occurred across the plateau from 50 Ma (Chung et al., 2005). of the deformation in eastern Tibet is Indosinian (Triassic–Jurassic) in age The present-day structure of eastern Tibet is characterized by a high, flat (Harrow field and Wilson, 2005; Wilson et al., 2006; Roger et al., 2003, 2004, plateau, with a few exceptional topographic anomalies, such as the Gongga 2010). The thick Triassic Songpan-Ganze flysch sedimentary rocks are tightly Shan (7556 m) massif, and a steep eastern margin along the Longmen Shan, folded about upright fold axes and are above a major horizontal detachment showing an abrupt shallowing of the Moho from depths of 60–80 km beneath above Paleozoic basement (Harrowfield and Wilson, 2005). Most granites that the plateau to depths of 35–40 km beneath the Sichuan Basin (Z. Zhang et al., have been dated intruded during the period 220–188 Ma (Roger et al., 2004, 2010). There is an almost complete lack of Cenozoic shortening structures, with 2010). A complete Barrovian-type metamorphic sequence is present in the GEOSPHERE | Volume 12 | Number 3 Searle et al. | Gongga Shan batholith, Tibet Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/948/4092719/948.pdf 949 by guest on 02 October 2021 Research Paper 34° Fig. 3a Kunlun Fault structurally deeper Danba dome, where peak sillimanite-grade metamorphism Tibetan has been dated as 179.4 ± 1.6 Ma using in situ U-Pb monazite analysis (Weller Plateau Fig. 17 et al., 2013). There is no record of any Cenozoic metamorphism anywhere in 33° north, east, or central Tibet. The old ages of deformation, metamorphism, and Ganzi (Garze−Yushu) Fault magmatism argue strongly against the Miocene–recent homogeneous crustal East shortening models for Tibet (Dewey and Burke, 1973; Houseman et al., 1981). Cenozoic structures in central and eastern Tibet are represented by large- Jinsha suture Tibet Xianshui−he Fault 32° scale strike-slip faults (Fig. 2). The Ganzi (Yushu) and Xianshui-he left-lateral strike-slip faults cut across all the geology of the eastern plateau and have Longriba Fault diverted river courses in the upper Yangtse and Jinsha river systems. These faults curve around the eastern Himalayan syntaxis and are thought to be re- Litang Fault 31° Bangong Nu sponsible for southeastward extrusion of the south Tibetan crust (Molnar and Fig. 4 Longmen Shan Thrust Belt Tapponnier, 1975; Peltzer and Tapponnier, 1988; Peltzer et al., 1989; Tapponnier Chengdu et al., 2001) and clockwise rotations caused by the northward indentation of India (England and Molnar, 1990). jiang suture 30° Along the Xianshui-he fault a granitic batholith, the Gongga Shan massif, Indus Yalu suture Gongga Shan crops out for ~200 km along the southwestern margin (Fig.
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