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Gangdese Magmatism in Southern Tibet and India–Asia Convergence Since 120 Ma

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Gangdese Magmatism in Southern Tibet and India–Asia Convergence Since 120 Ma Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018 Gangdese magmatism in southern Tibet and India–Asia convergence since 120 Ma DI-CHENG ZHU1,2*, QING WANG1, SUN-LIN CHUNG3,4, PETER A. CAWOOD5,6 & ZHI-DAN ZHAO1 1State Key Laboratory of Geological Processes and Mineral Resources, and Institute of Earth Sciences, China University of Geosciences, Beijing 100083, China 2CAS Centre for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China 3Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 4Department of Geosciences, National Taiwan University, Taipei, Taiwan 5School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC 3800, Australia 6Department of Earth Sciences, University of St Andrews, North Street, St Andrews KY16 9AL, UK *Correspondence: [email protected] Abstract: A compilation of 290 zircon U–Pb ages of intrusive rocks indicates that the Gangdese Batholith in southern Tibet was emplaced from c. 210 Ma to c. 10 Ma. Two intense magmatic pulses within the batholith occur at: (1) 90 ± 5 Ma, which is restricted to 89–94° E in the eastern segment of the southern Lhasa subterrane; and (2) 50 ± 3 Ma, which is widespread across the entire southern Lhasa subterrane. The latter pulse was fol- lowed by a phase of widespread but volumetrically small, dominantly felsic adakitic intrusive rocks at 16 ± 2 Ma. The Linzizong volcanism in the Linzhou Basin was active from 60.2 to 52.3 Ma, rather than 69– 44 Ma as previously estimated. During 120–75 Ma, Gangdese Batholith magmatism migrated from south to north, arguing against rollback of the downgoing, north-dipping Neo-Tethyan oceanic lithosphere for the gen- eration of the 90 ± 5 Ma magmatic pulse. Petrological, geochemical and metamorphic data indicate that this pulse was likely to have been generated through subduction of the Neo-Tethyan oceanic ridge lithosphere. Sub- sequent Gangdese Batholith magmatism propagated both south and north during 70–45 Ma, and finally concen- trated at the southern margin of the Lhasa Terrane at 45–30 Ma. The enhanced mafic magmatism since c. 70 Ma, magmatic flare-up with compositional diversity at c. 51 Ma and increased magmatic temperature at 52–50 Ma are interpreted as the consequences of slab rollback from c. 70 Ma and slab breakoff of the Neo-Tethyan oceanic lithosphere that began at c. 53 Ma. The India–Asia convergence was driven by Neo-Tethyan subduction with a normal rate of convergence at 120–95 Ma, ridge subduction at 95–85 Ma, then subduction of a young and buoy- ant oceanic lithosphere after ridge subduction with rate deceleration at 84–67 Ma, Deccan plume activity and slab rollback with rate acceleration at 67–51 Ma, slab breakoff for sudden drop of the convergence rate at c. 51 Ma, and finally the descent of the high-density Indian continental lithosphere beneath Asia since c. 50 Ma. Supplementary material: U–Pb age data of detrital zircons from the uppermost Shexing Formation sandstones in Maxiang are available at https://doi.org/10.6084/m9.figshare.c.4267850 Convergence between the Indian and Asian conti- magmatic belt) (Fig. 1a)(Allègre et al. 1984; Zhu nents is one of the major tectonic events that has et al. 2013), and subsequent India–Asia collision shaped the Earth since the Cretaceous. It involved (Zhu et al. 2015), which resulted in the formation two main stages of contrasting tectonic processes, of the Tibetan Plateau (Yin & Harrison 2000; Searle including subduction of the Neo-Tethyan oceanic et al. 2011). Reconstructions of the convergence lithosphere, which is expressed in the development history have received much attention over the past of a continental arc (termed the Gangdese arc in several decades (e.g. Dewey & Burke 1973; Allègre southern Tibet, which is part of the Gangdese et al. 1984; Yin & Harrison 2000; Zhu et al. 2013; From:TRELOAR,P.J.&SEARLE, M. P. (eds) Himalayan Tectonics: A Modern Synthesis. Geological Society, London, Special Publications, 483, https://doi.org/10.1144/SP483.14 © 2018 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018 D.-C. ZHU ET AL. Tarim Linzizong volcanic succession Ophiolitic melange zone (a) Qaidam (62-52 Ma in Linzhou basin) Gangdese Batholith Xigaze forearc basin (123-76 Ma) Qiangtang Ladakh JSSZ Mesozoic granitoids in central Lhasa 1 IYZSZ Lhasa (210-110 Ma) Great Counter thrust (19-10 Ma) BNSZ Early Cretaceous volcanic succession in central 2 Gangdese thrust (30-24 Ma) Lhasa (142-110 Ma) Kohistan IYZSZ = Indus-Yarlung Zangbo suture zone Gangdese 5 cm a–1 80e SNMZ = Shiquan River-Nam Tso mélange zone magmatic belt LMF = Luobadui-Milashan Fault Rutog 82e BNSZ = Bangong-Nujiang suture zone JSSZ = Jinsha suture zone India 33e 84e 86e 88e 92e 94e 90e Amdo 32 BNSZ e 80e 2 SNMZ Northern Lhasa 31e Nyainqentanglha Mountains LMF Central Lhasa IYZSZ (b) 82e Southern Lhasa 30e N Lhasa WE 84e 2 Xigaze 2 Saga 1 29 S 0 50 100 km e Fig. c Nang 86e 88e 90e 92e 94e 86 87 92 85e e e 88e 89e 90e 91e e 94e N = 166 (c) 65 Luoza 52 30e 54 53 61 50 52 66 48 49 70 61 42 52 61 48 73 83 65 67 65 66 70 71 70 50 45 Bayi 51 56 55 61 70 65 44 67 67 66 47 60 65 55 51 51 55 49 51 61 51 49 Lhasa 81 48 Rhyolite 62 44 51 54 47 53 44 29e30Ą 52 49 86 89 95 42 64 46 38 56 51 61 63 57 50 94 51 87 47 49 75 68 52 48 50 109 51 93 60 42 103 73 100 52 91 85 50 45 89 Saga 44 44 45 54 42 44 50 81 77 93 88 50 93 60 119 Xigaze 102 91 42 30 79 94 IYZSZ 82 51 122 87 Chongmuda 94 38 87 92 45 Langshan 82 0 50 100 km 29e Serpentinitized peridotite was Nang 95 84 80 98 29e intruded by diabase vein of ca. 50 Ma 80 Fig. 1. Tectonic framework of the Lhasa Terrane and location of the Gangdese magmatic belt. (a) Showing the Gangdese magmatic belt in the context of the Tibetan Plateau. (b) The distribution of the Gangdese Batholith and the Linzizong volcanic succession (adapted from Pan et al. 2004). (c) The distribution of intrusive rocks in the Gangdese Batholith with host-rock crystallization ages (Zhu et al. 2015) showing the present-day variation of the Gangdese magmatism in time and space, and the locations of the Great Counter Thrust and the Gangdese Thrust (Yin et al. 1994, 1999). In this contribution, the Gangdese magmatic belt consists of the Gangdese Batholith and associated volcanic successions with ages of c. 240–10 Ma, and the Gangdese arc includes the Gangdese Batholith and associated volcanic successions with ages of c. 140–10 Ma. Hu et al. 2016; van Hinsbergen et al. 2018). How- important new insights into mantle dynamics during ever, the details of mantle dynamics driving this the India–Asia convergence, and further constrain- evolving convergence (e.g. Chung et al. 2005; ing the timing of the initial India–Asia collision. Kapp et al. 2007; Zhu et al. 2013), as well as the tim- ing of the initial India–Asia collision (see Hu et al. 2016 for a review) are still intensely debated. Definition of the Gangdese arc The Gangdese arc in southern Tibet (Fig. 1a), which records the subduction of the Neo-Tethyan The magmatism in the southern Lhasa subterrane oceanic lithosphere and subsequent India–Asia colli- (i.e. the Gangdese magmatic belt) (Fig. 1a) was sion (Allègre et al. 1984; Yin & Harrison 2000; active from the Middle Triassic to the middle Mio- Chung et al. 2005; Ji et al. 2009a; Zhu et al. 2011, cene (c. 240–10 Ma), and consists mainly of the 2015), provides an unprecedented opportunity to voluminous Gangdese Batholith (Fig. 1b) and coeval address these scientific problems. This contribution volcanic successions that include the Lower–Middle updates the comprehensive dataset on the age and Jurassic, the Cretaceous and the Paleocene–Eocene geochemistry of magmatic rocks from the Gangdese Linzizong volcanosedimentary sequences (Allègre arc, revealing spatial and temporal variations in the et al. 1984; Yin & Harrison 2000; Pan et al. 2004; composition of the magmatic activity, providing Chung et al. 2005; Ji et al. 2009a; Zhu et al. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018 GANGDESE MAGMATISM IN SOUTHERN TIBET 2009c, 2011, 2013, 2015; C. Wang et al. 2016). The The Gangdese arc and its equivalent in NW India Triassic–Jurassic magmatism is unlikely to be (i.e. the Ladakh–Kohistan arc) extend for over related to the northwards subduction of the Neo- 2500 km across the SW Tibetan Plateau (Fig. 1a). Tethyan oceanic lithosphere as new palaeomagnetic The southern margin of the arc is marked by the data suggest that the Lhasa Terrane drifted away Indus–Yarlung Zangbo suture (Fig. 1b). The north- from Gondwana in the Late Triassic (Li et al. ern limit is equivocal and has been previously 2016), post-dating the oldest volcanic rocks docu- extended to include the extensive Mesozoic gran- mented in the southern Lhasa subterrane (237 ± itoids of the Nyainqentanglha Mountains in the cen- 1 Ma: C. Wang et al. 2016). Instead, the Triassic– tral Lhasa subterrane (e.g. Harris et al. 1988a, b; Jurassic magmatism is most likely to be associated Kapp et al. 2007). However, these rocks should with the southwards subduction of the Bangong– not be included in the Gangdese arc as they are Nujiang Tethyan oceanic lithosphere (Zhu et al.
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