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

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 followed 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 generation 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 concentrated 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 buoyant 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 timing 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. more likely to be associated with the southwards 2011). This is because the central Lhasa subterrane subduction of the Bangong–Nujiang Tethyan oce- was once an ancient microcontinent (e.g. part of anic lithosphere (Zhu et al. 2011; Li et al. 2018). the Cimmerian continent) with juvenile crust not In this contribution, the northern limit of the arc is only in its southern portion but also in its northern approximately defined as the Luobadui–Milashan edge, with the latter indicating development of a Fault (LMF) (Fig. 1b) which separates the southern south-dipping subduction zone (Zhu et al. 2011). and central Lhasa subterranes (Zhu et al. 2011). This is similar to the inference of Sengör (1979), This suggests that the magmatism associated with who proposed that the Neo-Tethyan Ocean may the development of the Gangdese arc is mostly have opened as a back-arc basin in response to the located in the southern Lhasa subterrane (Fig. 1b). southwards subduction of the Paleo-Tethyan oceanic This contribution focuses on the Gangdese arc mag- lithosphere north of the Cimmerian continent. matism younger than 120 Ma as the information The timing of north-directed subduction initia- about the 140–120 Ma magmatic record is largely tion of the Neo-Tethyan oceanic lithosphere beneath limited to that retrieved from detrital zircons (e.g. the southern margin of the Lhasa Terrane remains Wu et al. 2010) with only a sparse igneous record unclear. Previous studies suggested that when micro- (e.g. Zhu et al. 2009c). continents (or terranes or oceanic plateaus) collide with a continent, subduction can jump from one side of the terrane (or oceanic plateau) to the other Gangdese arc magmatism in time and space (Niu et al. 2003; Nair & Chacko 2008; Cawood The Gangdese Batholith et al. 2009). This mechanism has been invoked by Zhu et al. (2011, 2013) to explain the subduction ini- The Gangdese Batholith (also termed the Trans- tiation of the Neo-Tethyan oceanic lithosphere, Himalayan Batholith) is linked with the Ladakh which may have been triggered by the Lhasa–Qiang- and Kohistan batholiths to the west (Allègre et al. tang collision at c. 140–130 Ma; the timing of which 1984; Schärer et al. 1984; Harris et al. 1988a, b; is constrained by sedimentary, metamorphic, mag- Pan et al. 2004; Ji et al. 2009a) and the Lohit Bath- matic and structural geology data (Zhu et al. olith to the east (Lin et al. 2013), extending over 2016a). This interpretation is supported by the pres- 1500 km along strike, and varying between 10 and ence of subduction-related high-Mg adakite-like 80 km in width along the southern Lhasa subterrane rocks (c. 137 Ma) in the Gangdese arc, which can (Fig. 1b, c). The batholith intrudes lower Paleozoic be interpreted as having been derived from the partial metasedimentary rocks (Dong et al. 2010), early Car- melting of the Neo-Tethyan oceanic lithosphere at boniferous gneissic granites, Triassic–Cretaceous depths of 70–85 km (Zhu et al. 2009c). volcanosedimentary rocks and the Paleocene– Therefore, the magmatic record in the southern Eocene Linzizong volcanic succession (Pan et al. Lhasa subterrane can be divided into two separate 2004; Zhu et al. 2013). stages: the first is Triassic−Jurassic magmatism A growing body of zircon U–Pb age data indi- (>140 Ma) that is likely to be related to the south- cates that the Gangdese Batholith was emplaced wards subduction of the Bangong–Nujiang Tethyan from the Late Triassic (c. 210 Ma) to the late Mio- oceanic lithosphere; and the second is Cretaceous– cene (c. 10 Ma) (Fig. 2a), with two intense pulses Cenozoic magmatism (<140 Ma) that is related to of magmatic activity at 90 ± 5 and 50 ± 3 Ma (peak the northwards subduction of the Neo-Tethyan oce- at c. 51 Ma), and a phase of widespread but volumet- anic lithosphere and subsequent India–Asia collision. rically small, dominantly felsic adakitic, plutonism at In this contribution, the Gangdese arc in southern 16 ± 2 Ma (Fig. 2b). The 90 ± 5 Ma magmatism is Tibet is defined sensu stricto as recording the mag- restricted to longitude 89–94° E, whereas the 50 ± matic activities (including the Gangdese Batholith 3 Ma magmatism is widespread along the entire and associated volcanic successions) that occurred southern Lhasa subterrane (Fig. 2a). As first docu- in the southern Lhasa subterrane since c. 140 Ma. mented by Zhu et al. (2015), the >72 Ma magmatism Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

120 110 (a) N = 290 90 ± 5 Ma 100 90 80 70 60 50 ± 3 Ma 50 40 30 16 ± 2 Ma

Zircon U-Pb age (Ma) 20 10 0 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 Longitude (°E) 60 200 (b)(50 ± 3 Ma N = 290 c) N = 290 50 (peak at ca. 51)

40 150

30 Number 20 16 ± 2 Ma 90 ± 5 Ma 100

10 Distance to the trench (km)

0 50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 0 102030405060708090100110120130 Zircon U-Pb age (Ma) Zircon U-Pb age (Ma)

Fig. 2. Zircon U–Pb ages of the Gangdese Batholith. (a) Plot of zircon U–Pb age v. longitude (°E) showing the spatial variations of the Gangdese Batholith magmatism. (b) Histogram of crystallization ages of the Gangdese Batholith showing two intense pulses of magmatic activity at 90 ± 5 and 50 ± 3 Ma (peak at c. 51 Ma). (c) Plot of zircon U–Pb age v. original distance to the trench (i.e. the present-day distance between samples and ophiolites plus the amount of tectonic shortening during the Cenozoic between them) showing the spatial migration of the Gangdese Batholith magmatism. Data sources for the estimates of the Cenozoic tectonic shortening that occurred between the Yarlung–Zangbo ophiolite and the Gangdese arc are Einsele et al. (1994), Yin et al. (1999), Murphy & Yin (2003), Quidelleur et al. (1997) and Harrison et al. (2000). from longitude 85° E to 95° E of the batholith is con- Cenozoic tectonic shortening allow us to quantita- ﬁned to a narrow belt in the south (blue dashed line tively reconstruct the migration of the Gangdese in Fig. 1c), shifting northwards at 72–65 Ma (red Batholith magmatism in time and space, as shown dashed line in Fig. 1c), then south at 64–48 Ma to in Figure 2c. It is evident that the 120–95 Ma mag- spread over a broader area than the earlier activity matism was restricted to a narrow belt (c. 70– and, ﬁnally, in the period 47–38 Ma it is largely 60 km distance to the suture, which is inferred to restricted to the southern edge of the Lhasa Terrane. approximate the position of the trench), slightly Such spatial and temporal migration of the Gangdese migrated northwards to c. 90 km at 95–70 Ma, and Batholith magmatism is further corroborated by then propagated towards both to the north and restoring the Cenozoic tectonic shortening that south (c. 170–65 km) at 70–45 Ma, followed by a occurred between the Yarlung Zangbo ophiolite and progressively southwards migration from c. 130 to the Gangdese arc. 65 km at 45–26 Ma (Fig. 2c). With the exception Structural and stratigraphic studies indicate that of the 70–45 Ma magmatism that extends over a rel- the original width of the Xigaze forearc basin was atively wider area (c. 170–65 km), the majority of c. 65 km (Einsele et al. 1994), and that the Gangdese the Gangdese arc magmatism since 120 Ma was pri- Thrust and Great Counter Thrust (Fig. 1b) have been marily emplaced at a distance of c. 90– 60 km (Fig. shortened by c. 66 km at longitude 88° E (e.g. Yin 2c) from the suture and inferred trench. This is closer et al. 1994, 1999; Murphy & Yin 2003) and c. to the trench than typical arc–trench distances today, 62 km at longitude 92° E (Quidelleur et al. 1997; which are of the order of 160 ± 60 km (Gill 1981; Harrison et al. 2000). These restorations of the Stern 2002). Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

The pre-Late Cretaceous and Miocene intrusive as these rocks are Early Cretaceous in age (142–110 rocks occur as small relicts of varying size, whereas Ma) (Zhu et al. 2009a, 2011; L.Y. Wang et al. 2016). the Late Cretaceous (90 ± 5 Ma) and early Paleogene The Linzizong volcanic succession is composed, (60–45 Ma) rocks crop out as batholiths within the from bottom to top, of the Dianzhong, Nianbo and Gangdese Batholith. The widespread intrusive Pana formations (Liu 1993) that are typically rocks that were emplaced at 50 ± 5 Ma contain abun- exposed in Linzhou Basin (Fig. 4b, c). The Dia- dant mafic dykes and enclaves of similar age nzhong Formation is dominated by thick andesitic (Fig. 3a–d) (e.g. Ji et al. 2009a; Zhu et al. 2011, rocks (Fig. 4b), which unconformably overlie 2013, 2015). Geochemically, the 90 ± 5 Ma magma- strongly folded Upper Cretaceous Shexing Forma- tism is dominated by intermediate to gabbroic rocks tion siltstone and mudstone (Fig. 4c, d). The andesitic with the presence of coeval ultramafic cumulates breccias from the uppermost part of the Dianzhong (including abundant hornblendite, and minor wehr- Formation are unconformably overlain by the lite and dunite) and highly fractionated leucogranite Nianbo Formation consisting of siltstone, marl and dykes (Fig. 3e). This phase of magmatism is followed limestone (Fig. 4b), interbedded with andesitic by felsic rocks at c. 82–72 Ma (Fig. 3e). Subsequent rocks, and intruded by diabase dykes dated at 52.9 magmatism is characterized by the presence of sig- ± 0.4 Ma (Yue & Ding 2006)(Fig. 4e, f). The Pana nificant mafic magmatic rocks at 70–43 Ma, with Formation is characterized by the presence of thick notable compositional diversity from mafic to felsic rhyolite and rhyolitic ignimbrite (Fig. 4b) with rocks at 50 ± 3 Ma (Fig. 3e). Younger rocks (43– columnar jointing (Fig. 4g). These varying litholo- 10 Ma) are mostly felsic with sparse mafic enclaves gies from different formations of the Linzizong vol- (c. 30 Ma) from Chongmuda (Figs 1c & 3e). canic succession in the Linzhou Basin serve as the reference for stratigraphic division and correlation of the Linzizong volcanic succession elsewhere The Linzizong volcanic succession along the southern Lhasa subterrane (Fig. 1a). The geochronological framework of the Linzi- The Linzizong volcanic succession extends for more zong volcanic succession in the Linzhou Basin has than 1000 km along the southern Lhasa subterrane received much attention over the past decade (e.g. (Figs 1b & 4a). Some volcanic rocks in the central Zhou et al. 2004; He et al. 2007; Huang et al. Lhasa subterrane (Fig. 1b) that were previously 2015; Chen et al. 2016). Early Ar–Ar dating sug- included in the Linzizong volcanic succession (Yin gested that the volcanic rocks of the Dianzhong, & Harrison 2000) are part of an older succession, Nianbo and Pana formations were 64–60, 54 and

(a) 12DJ15-1 (b) (c) 11NM01-1 (d) 11LS01-1

(e)

Fig. 3. (a)–(d) Field occurrences of the Gangdese Batholith showing abundant maﬁc dykes and enclaves in granitoids. (e) Plot of zircon U–Pb age v. SiO2 content of the Gangdese Batholith showing the compositional changes of the Gangdese Batholith with time. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

84 (a) 82e e Qiangtang Terrane (b) Linzizong stratigraphic column in Linzhou Basin 86 32e BNSZ e 90 52.3 13LZ04-1 88e e 92e 32e 52.6 Pana Formation 13LZ05-1 Rhyolite and rhyolitic ignimbrite with 80e N columnar jointing (1950 m) 0 100 km (E2p) W E Lhasa Terrane 52.3 12LZ25-1 31e 31e S Nianbo Fm 52.6 12LZ27-1 Andesite and volcanic breccia (345 m) (E2n) Limestone, marl, tuffaceous siltstone 55.4 12LZ29-1 interbedded with mudstone (500 m) 82e Pangduo 30e 30e Dianzhong Fm Andesite interbedded with minor 60 (E1d) dacitic tuff (2351 m) Pana Formation Linzhou Nianbo Formation 84e Maxiang Lhasa 60.1 13LZ01-1 and 13LZ17-1 Dianzhong Formation IYZSZ Shexing Fm Siltstone and mudstone India 29e (K2sh)

Method 91°10′ Zircon SIMS U-Pb Zircon LA-ICPMS 40Ar/39Ar dating P2l Unconformity Strata (c) dating ( 60.2 ) U-Pb dating ( 68.7 ) ( 64.4 ) 48.7 P2l Thrust Pana Fm 52.5 - 52.3 54~50 53.5~43.9 91°05′ 2 Gulu E2p Nianbo Fm 55.4 - 52.5 56~54 56.5 13LZ04-1 47.4 52.3 Dianzhong Fm 60.2 - 58.3 66~59 64.4~60.6 51.8 47.1 E2n3 52.6 2 2 Rhyolitic ignimbrite & Rhyolitic ignimbrite 30°00′ 13LZ05-1 E2n E2p volcaniclastic rock E2p1 and rhyolite 43.9 50.5 50.2 E2n1 Andesite and BD-114 13LZ06-1 3 2 Siltstone interbedded 52 E2n volcanic breccia E2n with mudstone 1 12LZ23-1 49.7 53.5 1 Pana E2p E2p K2sh Andesite interbedded 1 Limestone and marl 3 52 E2n E1d with dacitic tuff 51.1 12LZ25-1 52.3 48.7 52.7 Andesite interbedded Siltstone and 13LZ16-1 E1d2 K2sh 53.5 53.9 54.1 with dacitic tuff mudstone 13LZ14-1 54.5 Luobadui Formation Zircon SIMS U-Pb P2l 60.2 Nianbo 54.4 13LZ13-1 limestone (Zhu et al., 2015) 52.6 13LZ08-1 12LZ27-1 56.5 58.3 E2n1 Xiagunba 55.4 12LZ29-1 E1d2 (h) Maxiang E2n2 55.7 56.8 61.5 59.1 1 63.4 E2n SH530022 2 Chongga 60.6 Andesite in the 3 E1d E1d 68.7 Dianzhong Fm (E1d) 60.2 12LZ06-1 58.5 66 D-3 64.4 64.9 13LZ17-1 Angular 90.6 2 65.8 60.2 unconformity E1d 62.6 K2sh Granite 13LZ01-1 62.5 Dianzhong 0 2km Folded Upper Cretaceous 29°54′ siltstone in the Shexing Fm (K2s) 91°05′ 91°10′

Ignimbrite with columnar jointing 11LZ02-1 Ignimbrite 13LZ16-1 13LZ01-1 (d) (e) (f) Pana Fm (E2p) (g) Pana Fm (E2p) 52.7 ± 1.9 Ma 60.2 ± 0.6 Ma Diabase dyke (Hornblende Ar-Ar dating 13LZ05-1 52.9 ± 0.4 Ma) Tuf Andesitic volcanic 52.6 ± 0.4 Ma si lt fa breccia Dianzhong Fm stoneceous (E N ianbo Fm (E (E1d2)

2n ) Andesite Shexing Fm (K2sh) Tuffaceous 2 siltstone n) Dianzhong N.Chongga NE.Xiagunba S.Gulu N29.952e, E91.198e N29.970e, E91.149e N29.985e, E91.187e N30.002e, E91.148e

Fig. 4. Distributions and ﬁeld occurrences of the Linzizong volcanic succession in southern Tibet. (a) Distributions of different formations of the Linzizong volcanic succession. (b) The stratigraphic column of the Linzizong volcanic succession in the Linzhou Basin (adapted from Mo et al. 2007). (c) Distributions of the Linzizong volcanic succession in the Linzhou Basin (adapted from Dong et al. 2005). Sources of the age data are Zhou et al. (2004), He et al. (2007), Zhu et al. (2015) and Chen et al. (2016).(d)–(h) Field occurrences of the Linzizong volcanic succession in the Linzhou Basin and Maxiang.

50–44 Ma, respectively (Fig. 4c)(Zhou et al. 2004). hence, this age should be used with caution in con- However, the presence of excess argon in the plagio- straining the base of the Linzizong volcanic succes- clase from the lowermost Dianzhong Formation sion (He et al. 2007). Recently, the duration of the andesite (64.4 ± 0.6 Ma) (Zhou et al. 2004), and Dianzhong, Nianbo and Pana formations in the Linz- the alteration observed in the youngest Pana Forma- hou Basin have been precisely calibrated to be 60.2– tion shoshonitic rock (43.9 ± 0.4 Ma), indicates that 58.3, 55.4–52.6 and 52.6–52.3 Ma, respectively, by these ages are questionable. Subsequently, the laser secondary ion mass spectrometry (SIMS) zircon ablation inductively coupled plasma mass spectrom- U–Pb dating of volcanic rocks from the lower and etry (LA-ICPMS) zircon U–Pb dating for a rhyolite upper stratigraphic boundaries of each formation at the base of the Dianzhong Formation (Fig. 4c) (Zhu et al. 2015). Such high-precision calibration yielded an age of 68.7 ± 2.4 Ma. However, this age indicates that: (1) the duration of volcanism in the data display a large mean square weighted deviation Linzhou Basin is some 8 myr (i.e. from 60 to (MSWD) of 3.6 (He et al. 2007), suggesting that the 52 Ma), differing signiﬁcantly from the previous zircons were not from a single age population and, estimates of up to 25 myr (i.e. from 69 to 44 Ma) Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

14 (a)(41 b) Detrital zircon sample from Maxiang 52 E2n 88 ± 1 Ma (< 160 Ma, n = 68 analyses) 47 65 12 0.0142 K2sh 90 To Yangbajing 10 0.0138 85 ± 1 Ma

E1d U 0.0134 86

8 238 E1d 0.0130

Pb/ 82

68.9 ± 0.6 Ma 206 0.0126 Fig. 4g 6 n = 7 Number 85 ± 1 Ma 0.0122 MSWD = 0.01 E2n 4 0.0118 0.02 0.04 0.06 0.08 0.10 0.12 0.14 K2sh 207 235 Maxiang 70 Pb/ U K2sh 2 Granite 0 Dated andesite sample from lowermost Dianzhong Formation Detrital zircon sample from uppermost Shexing Formation (Fig. 5b) 70 80 90 100 110 120 130 140 150 160 Angular unconformity viewed from Maxiang (Fig. 4h) Zircon U-Pb age (Ma)

Fig. 5. (a) Geological map in Maxiang (adapted from Pan et al. 2004) and (b) age distribution of detrital zircons from the uppermost Shexing Formation sandstones (see the Supplementary material).

(Fig. 4c)(Zhou et al. 2004; He et al. 2007; Chen reveal a signiﬁcant increase in the melting tempera- et al. 2016); and (2) the tectonic event documented ture of the magma-source region at 52–50 Ma from by the angular unconformity between the Nianbo the early Dianzhong and Nianbo formations to the Formation and the underlying Dianzhong Formation late Pana Formation (Fig. 6c, d)(Zhu et al. 2015; lasted for c. 3 myr. Liu et al. 2018). The duration of the angular unconformity between the Dianzhong Formation and the underlying strongly folded Upper Cretaceous Shexing For- Geodynamic processes related to the mation is not well constrained. Previously, this India–Asia convergence unconformity was proposed to have developed between 90 and 69 Ma based on the age of the youn- Continental convergence is the natural consequence gest Shexing Formation red beds (c. 90 Ma) and the of plate tectonics involving the closing of ocean oldest Dianzhong Formation volcanic rocks (68.7 ± basins and subsequent continent–continent colli- 2.4 Ma, He et al. 2007) in the Linzhou Basin (Kapp sion, and is essentially driven by a variety of forces, et al. 2007). However, this duration should be cali- most notably the descent of oceanic lithosphere at brated to 90–60 Ma in the Linzhou Basin as the low- subduction zones (i.e. slab pull) (Forsyth & Uyeda ermost Dianzhong Formation andesite was erupted 1975; Conrad & Lithgow-Bertelloni 2002). It is at c. 60.2 Ma (Zhu et al. 2015). A further c. 45 km commonly accompanied by a series of geodynamic west to the Linzhou Basin at Maxiang (Figs 4h & processes, including mid-oceanic ridge subduction, 5a), andesite from the lowermost Dianzhong Forma- slab rollback and the breakoff of the subducting tion has yielded an age of 68.9 ± 0.6 Ma (Zhu et al. oceanic plate (Davies & von Blanckenburg 1995; 2017b) and the youngest age group of detrital zircons Coulon et al. 2002; van Hunen & Miller 2015). from the uppermost Shexing Formation sandstone is Outlined below is an analysis of the potential dated at 85 ± 1 Ma (Fig. 5b), suggesting that the role of these processes in the evolution of the strong folding and deformation of the Upper Creta- Gangdese arc. ceous Shexing Formation at Maxiang took place between 85 and 69 Ma. 90 ± 5 Ma magmatism by slab rollback or Geochemical data available for the Linzizong oceanic ridge subduction volcanic succession in Linzhou Basin (Yue & Ding 2006; Mo et al. 2007, 2008; Lee et al. 2012; Zhu This phase of extensive magmatism is followed et al. 2015; Liu et al. 2018) suggest the presence by the development of the angular unconformity of a calc-alkaline andesite–dacite association in the (85–69 Ma) between the Dianzhong and Shexing Dianzhong Formation and shoshonitic bimodal vol- formations, as seen in Maxiang (Figs 4h & 5a). canic suite in both the Nianbo and Pana formations Two popular hypotheses have been proposed to (Fig. 6a, b). Calculations of both the whole-rock zir- explain the generation of the extensive 90 ± 5 Ma con saturation temperature (Watson & Harrison magmatism: slab rollback (Ma et al. 2013a, b; Xu 1983) and clinopyroxene geothermometry (Putirka et al. 2015) and oceanic ridge subduction (Zhang et al. 2003) for the Linzizong volcanic succession et al. 2010, 2011; Guo et al. 2013; Zhu et al. 2013). Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

85 1000 (a)(N = 56 c) 2 80 950 SiO > 56 wt% (N = 46) 75 900 70 Rhyolite 850 65 Dacite

(wt%) 800 2 Gap 60 Andesite 750 SiO 55 Basaltic andesite Pana 700 50 Basalt Nianbo Dianzhong 45 Diabase dyke 650 Pana Nianbo Dianzhong 40 Zircon saturation temperature (°C) 600 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 Zircon U-Pb age (Ma) Zircon U-Pb age (Ma)

100 1200 Shoshonitic (b) 1180 (d) N = 119 1160

1140 Pana 1120

10 Calc-alkaline 1100

Ce/Yb 1080

Diabase dyke 1060 Pana Formation Tholeiitic 1040 Pana Nianbo Formation Nianbo Dianzhong Formation 1020 Dianzhong 1 1000 0.01 0.1 1 10 Clinopyroxene geothermometer (°C) 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 Ta/Yb Zircon U-Pb age (Ma)

Fig. 6. Changes in magmatic compositions and temperature with time for the Linzizong volcanic succession in Linzhou and Pangduo. (a) Plot of zircon U–Pb age v. SiO2 content (Zhu et al. 2015) showing a compositional gap in the late Nianbo and Pana formations. (b) Plot of Ce/Yb v. Ta/Yb (Pearce 1982) showing the calc-alkaline nature of the Dianzhong Formation, and the shoshonitic nature of the Nianbo and Pana formations. (c) Plot of zircon U–Pb age v. zircon saturation temperature (°C) for the Linzizong volcanic succession. Zircon saturation temperatures were calculated from whole-rock compositions with SiO2 >56 wt% following the method of Watson & Harrison (1983). (d) Plot of zircon U–Pb age v. clinopyroxene crystallization temperature (°C) (Putirka et al. 2003) for the basaltic rocks from the Linzizong volcanic succession in the Linzhou and Pangduo formations (Liu et al. 2018).

Slab rollback hypothesis. Arguments in support of Second, the presence of 137–100 Ma magmatic the slab rollback hypothesis include: (1) the south- rocks in the southern Lhasa subterrane (including wards migration of the Cretaceous magmatism in c. 137 Ma high-Mg adakitic andesites and c. the Lhasa Terrane, which was active earlier in 122 Ma high-Mg diorites) (Zhu et al. 2009c, 2013; the central and northern Lhasa subterranes (140– Wang et al. 2013; Li et al. 2018) and of the abundant 110 Ma), and later in the southern Lhasa subterrane 130–100 Ma Gangdese arc-derived detrital zircons (100–80 Ma); and (2) the minor occurrence of the (peak at c. 110 Ma) with positive εHf(t) values Early Cretaceous magmatic rocks in the present-day from the Xigaze forearc basin sedimentary rocks southern Lhasa subterrane (Ma et al. 2013a, b). (Wu et al. 2010; An et al. 2014) indicate that Early However, this hypothesis is not consistent with the Cretaceous magmatism is more extensive in the following lines of evidence. southern Lhasa subterrane than previously docu- First, restoration of the c. 50% Late Cretaceous mented (Coulon et al. 1986; Kapp et al. 2007). and Cenozoic shortening in the Lhasa Terrane Third, recent petrological, geochemical and (Burg et al. 1983; Kapp et al. 2007) suggests that palaeomagnetic studies of the Xigaze ophiolites the entire terrane and the incorporated igneous reveal an extensional setting that may be associated rocks were >600 km wide during the Early Creta- with high-angle (rather than low-angle) subduction ceous (Zhu et al. 2009a; Li et al. 2018), which is of the Neo-Tethyan oceanic lithosphere during the broader than the arcs above the present-day low- Early Cretaceous (Dai et al. 2013; Mafﬁone et al. angle subduction zone in the Andes (c. 420 km) 2015; Xiong et al. 2016). Therefore, the Early Creta- (Trumbull et al. 2006). ceous magmatic rocks in the central and northern Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

Lhasa subterranes cannot be linked with the low- adakitic granodiorites with low MgO (<1.0 wt%) angle subduction of the Neo-Tethyan oceanic litho- and Mg# (37–31) observed in the Gangdese Batho- sphere that forms the basis for the slab rollback lith (Wen et al. 2008). hypothesis (Ma et al. 2013a, b; Xu et al. 2015). Generally, if the oceanic ridge is parallel or sub- Rather, these magmatic rocks are most likely to parallel to the trench, then there would be no slab be associated with the subduction of the Bangong– pull to maintain ongoing subduction because the Nujiang oceanic lithosphere and subsequent Lhasa– oceanic ridge lithosphere is young and buoyant Qiangtang collision (Zhu et al. 2009a, 2011, (Thorkelson 1996; Niu 2017). However, this may 2016a, b; Li et al. 2018). not have been the case for the oceanic ridge of the Our compilation of age data reveals that the Neo-Tethys as its continued subduction can be 120–75 Ma magmatic activity in the Gangdese arc driven by: (1) the slab pull from the active north- did not migrate southwards but remained stationary wards subduction of the same neighbouring Neo- through this time period (Fig. 2c), suggesting that Tethyan oceanic plate beneath the southern margin the extensive 90 ± 5 Ma magmatism cannot be of Eurasia (Seton et al. 2012); and (2) the ridge attributed to the slab rollback of the Neo-Tethyan push from the South Indian Ocean spreading due oceanic lithosphere. to the complete separation of the Indian Plate from Australia–Antarctica supercontinent at c. 102 Ma Oceanic ridge subduction hypothesis. The presence (Hu et al. 2010). of c. 122 Ma oceanic crust-derived adakitic high-Mg diorites (Wang et al. 2013) and other 122–100 Ma 50 ± 3 Ma magmatic flare-up by slab breakoff magmatic rocks in the Gangdese arc (see Zhu et al. 2013; Li et al. 2018 for reviews) indicates that the Did the slab breakoff occur in the India–Asia colli- Neo-Tethyan oceanic lithosphere was subducting sion zone? Slab breakoff is defined from the Euro- northwards beneath the southern Lhasa subterrane pean Alps as ‘the buoyancy-driven detachment of during this period (Fig. 7a). Given that the Neo- subducted oceanic lithosphere from the light conti- Tethyan Ocean was c. 6900 km wide during the nental lithosphere that follows it during continental Early Cretaceous (Ma et al. 2016) and that the collision’ by von Blanckenburg & Davies (1995, India–Asia collision commenced at 60–55 Ma (see p. 120). It has received much attention over the last Zhu et al. 2015; Hu et al. 2016 a for review), subduc- two decades (see Niu 2017; Garzanti et al. 2018 for tion of the Neo-Tethyan oceanic ridge is inevitable. reviews) as the favoured mechanism to account Petrological, geochemical and metamorphic studies for the generation of syncollisional magmatism and reveal the occurrence of the 90 ± 5 Ma non-adakitic the exhumation of high-pressure metamorphosed and adakitic rocks, the high-temperature charnock- rocks in collision zones. However, this hypothesis ites with high-density and dry CO2-rich fluid inclu- has recently been challenged in the European Alps sions, and the high-temperature granulite-facies based on high-resolution seismic data that demon- metamorphism of the country metamorphic rocks strate the continuity of the Alpine slab (Zhao et al. of charnockites, all of which point to an anomalously 2016), arguing against evidence for slab breakoff. high heat supply that can be linked with ridge sub- As a result, this raises a question of the validity of duction (Zhang et al. 2010, 2011; Guo et al. 2013). invoking the slab breakoff hypothesis to interpret Assuming that the Neo-Tethyan spreading ridge the generation of a syncollisional magmatism and was orientated approximately parallel or subparallel the exhumation of high-pressure metamorphosed to the trench, as suggested by the palaeogeographical rocks in the India–Asia collision zone in southern reconstructions of Seton et al. (2012), then its sub- Tibet. duction would have been likely to have resulted in Tomographical images beneath southern and the opening of a slab-window at the ridge axis central Tibet have revealed the Indian Plate as either (Fig. 7b). Thus, the oceanic crust north of the spread- continuous (Zheng et al. 2007) or discontinuous ing ridge could have been detached and continued to (Replumaz et al. 2010; Liang et al. 2016). These sink into the mantle due to slab pull (negative buoy- present-day seismic anomalies are interpreted as evi- ancy), whereas the young and hot (thus buoyant) dence of the subducting Indian continental litho- oceanic crust immediately south of the spreading sphere (Liang et al. 2016; Zhao et al. 2017) rather ridge would have led to increased coupling (Marti- than the Neo-Tethyan oceanic lithosphere. There- nod et al. 2010) with the overriding Lhasa litho- fore, such tomography, regardless of its continuity, sphere (Fig. 7c). Such coupling could explain the cannot be used as evidence arguing against the significant folding and deformation of the Upper occurrence of slab breakoff during the early stage Cretaceous strata (Fig. 4h) in the southern Lhasa sub- of the India–Asia collision. As the initial India– terrane between 85 and 69 Ma (Fig. 5a, b). Partial Asia collision is most likely to have occurred at melting of the young and hot oceanic crust after 60–55 Ma (Zhu et al. 2015; Hu et al. 2016) and 21 ridge subduction can account for the c. 80 Ma ±4°N(Lippert et al. 2014), and the collision zone Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

Ocean-continent subduction (120-55 Ma) Continent-continent collision (55-45 Ma)

(a). Normal northwards subduction (120-95 Ma) (e). Initial India-Asia collision (c. 55 Ma) N 1). Adakitic high-Mg diorite of c. 120 Ma 1) First contact between Indian and Asian Late Dianzhong andesitic 2). Eroded magmatic rocks of c. 130-100 Ma Gangdese arc continent edges volcanism followed by erosion Underplated basaltic magma related to the Comei plume 2) Oceanic subduction remains active and Remnant sea India Newly failed Neo-Tethyan spreading ridge Lhasa is dominated by andesitic magmatism 3) Erosion and development of the angular CC CC unconformity around Paleocene-Eocene boundary (ca. 56 Ma) in the southern SCLM Lhasa subterrane Slab pull SCLM Slab pull 4) Magmatism migrates southwards Young and hot 5) Remnant sea is present (thus buoyant) seafloor Rollback (f). Ongoing collision (55-53 Ma) Early Nianbo sedimentation (b). Ridge subduction (95-85 Ma) 1) Weakened volcanism 2) Southwards migration of magmatism due 1). High magmatic to slab rollback productivity 3) Dominated by terrestrial sedimentation 2). Adakitic charnockite 3). Low-H2O and dry CO2-rich fluids Slab pull 4). High-temperature granulite-facies metamorphism Newly failed Rollback ridge Late Nianbo Slab pull volcanism (c). Flattening northwards subduction (85-70 Ma) (g). Tectonic transition due to slab breakoff (c. 53 Ma) 1). Northwards migration of felsic dominated magmatism 1) Termination of oceanic subduction 2). Folding and deformation of the 2) Partial loss of the slab-pull force Upper Cretaceous strata 3) A short period of extension 3). Partial melting of the young 4) Bimodal magmatism Slab necking and hot seafloor to generate Slab pull and breakoff adakitic magmatism with low Mg# Inflow of hot 4). Decrease of the convergence rate of the Indian Plate asthenosphere Slab pull Slab breakoff-induced events (52-45 Ma) Pa’na Young and hot (thus buoyant) seafloor (h). volcanism 1) Complete loss of the slab-pull force 2) Sudden decrease of the convergence rate (d). Late northwards subduction (70-55 Ma) Dianzhong 3) Rapid eruption of rhyolite and rhyolitic andesitic volcanism ignimbrite and dramatically increased 1) Oceanic subduction remains active and India Neo-Tethys zircon saturation temperature andesitic magmatism dominates CC 2) Magmatism migrates northwards 4) Widely developed mafic enclaves and dykes in the Gangdese Batholith SCLM 5) Diabase dykes intrude at ca. 50 Ma into the Yarlung–Zangbo Suture Zone Slab pull 6) Intense magmatic activity with compositional Inflow of hot Detached UC = Upper Crust diversity asthenosphere slab 7) Granulite-facies metamorphism LC = Lower Crust Rollback CC = Continental Crust 8) Exhumation of ultra high-pressure rocks SCLM = Subcontinental Lithospheric Mantle

Intracontinental convergence Latitude (°N) 19° 20° 21° 22° 23° 24° 25° 26° 27° 28° 29° 30°

(i) c. 52 Ma Late Nianbo and Pana (j) c. 45 Ma (k) c. 25 Ma volcanism Langshan OIB India High Sr/Y granite India 0 UC km LC SCLM Delamination of the Indian continental lithospheric mantle Eclogitized Slab breakoff Slab breakoff lower crust

500 Stacking Stacking

22° 32° GPS: 29.1328°N, 90.1629°E, 4269 m 12° 42° Tethyan Himalaya Stacking km

1000 Geologists

Porphyritic two-mica gneissic granite dyke Present (44 ± 1 Ma)

1500 TH = Tethyan oceanic slab; IN = Indian continental slab

Fig. 7. Geodynamic processes driving the India–Asia convergence since 120 Ma (adapted from Zhu et al. 2015). A slab-stacking hypothesis for the subducted slab of the Neo-Tethyan Oceanic lithosphere and the subducting slab of the Indian Plate is proposed here. See the text for details. The seismic image (inset) is taken from Replumaz et al. (2010). is nowadays located at c. 29° N, if the slab breakoff mantle sections beneath the present-day Indian did occur then the broken slab fragments of volumet- Plate. Seismic tomography across the Pamir–Hindu ric signiﬁcance should be seismically detected in Kush in the western Himalayan syntax has detected Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET high-velocity anomalies at depths of between 1100 is generally younger than the zircon U–Pb age due and 1600 km under the Indian Plate (van der Voo to its lower closure temperature relative to zircon. et al. 1999). These anomalies are interpreted as the • c. 50 Ma – the inferred low-angle subduction of remnants of subducted Tethyan oceanic lithospheric the Neo-Tethyan oceanic lithosphere during the slabs that have sunk into the lower mantle and subse- Cretaceous (Coulon et al. 1986) and the system- quently were overridden by the northwards-moving atic results of whole-rock Ar–Ar dating led sev- Indian Plate (van der Voo et al. 1999; Replumaz eral studies to propose that the magmatic activity et al. 2004). These anomalies are absent at depths migrated southwards during the Early Cenozoic shallower than c. 1000 km (van der Voo et al. and that the extensive magmatic activity with 1999), consistent with the occurrence of slab break- diverse compositions was the result of the breakoff (Replumaz et al. 2010). off of the Neo-Tethyan oceanic lithosphere at c. 50 Ma (Yue & Ding 2006; Lee et al. 2009, – When did the slab breakoff occur in the India Asia 2012). Subsequent studies have also suggested collision zone? Previous studies speculated that that the extensive magmatism recorded in the the breakoff of the Neo-Tethyan oceanic lithosphere Gangdese Batholith at 51–46 Ma was the product – occurred at 50 40 Ma (Davies & von Blanckenburg of the breakoff of the Neo-Tethyan oceanic litho- 1998), and that the Linzizong volcanic succession sphere (Ma et al. 2014). and the coeval Gangdese Batholith are likely to be • c. 53 Ma – as shown in Figure 2b, zircon U–Pb age the products of oceanic lithosphere breakoff (Davies data from the Gangdese Batholith indicates the & von Blanckenburg 1998; Yin & Harrison 2000). presence of a magmatic flare-up at c. 51 Ma. This Subsequently, the timing of the slab breakoff of the event is coeval with the Pana Formation ignimbrite Neo-Tethyan oceanic lithosphere has been inten- and dacite (52–50 Ma) with an increased mag- sively investigated over the last 20 years with sug- matic temperature (Fig. 4c, d) in Linzhou Basin gested timings presented below: and Pangduo (Zhu et al. 2015; Liu et al. 2018). • c. 45 Ma – according to the ages of eclogite from Similar observations, including the increase in the Western Himalaya (c. 45 Ma) and the K-rich magmatic temperature and the heterogeneity of magmatism from SE Tibet (inferred to be magma composition at 53–50 Ma, have also been c. 40 Ma), Kohn & Parkinson (2002) inferred identified in the Gangdese Batholith (Wang et al. that the breakoff of the Neo-Tethyan oceanic lith- 2015). If there was no change in the mechanisms osphere occurred at c. 45 Ma. Although this age of of magma generation (e.g. adding fluids, increas- eclogite is subsequently supported by the age data ing temperature and decreasing pressure) triggered of zircons from eclogites at Tso Morari and the by distinct deep processws, the steady-state sub- Kaghan Valley in Western Himalaya (Donaldson duction of the Neo-Tethyan oceanic lithosphere et al. 2013), the Eocene K-rich magmatism in SE cannot explain the magmatic flare-up and the in- Tibet inferred by Kohn & Parkinson (2002) is crease in the magmatic temperature at 52–50 Ma. actually Cretaceous–Early Tertiary in age (Chung In this case, any hypothesis for interpreting the et al. 2003). Chung et al. (2005) also argued that generation of the extensive 70–45 Ma magmatism the breakoff occurred at c. 45 Ma based on the should address the distinct observations elaborated timing of metamorphic events from the Hima- above. Considering that the magmatic activity layas (Kohn & Parkinson 2002) and the timing propagated both to the south and north at 70– of the onset of the ‘hard’ India –Asia collision 45 Ma, and finally concentrated on the southern (c. 44 Ma: Lee & Lawver 1995). Negredo et al. margin of the batholith at 45–30 Ma (Fig. 2c), we (2007) estimated that the breakoff in the Western argue that the enhanced mafic magmatism since c. Himalaya is likely to have occurred at c. 48– 70 Ma (Fig. 3e) and the magmatic flare-up with 44 Ma by measuring the tomographically inferred increased magmatic temperature at 52–50 Ma length of the Indian continental lithospheric plate (Fig. 4c, d) can readily be interpreted as the result under the Hindu Kush region and by comparison of slab rollback (Fig. 7d–f) since c. 70 Ma, fol- with palaeomagnetic reconstructions. Recently, lowed by slab breakoff (Fig. 7g, h)(Zhu et al. Ji et al. (2016) proposed that the breakoff of the 2015). Given that the magmatic flare-up with com- Neo-Tethyan oceanic lithosphere occurred at c. positional diversity and the increase in the mag- 45 Ma according to the identification of ocean matic temperature at 52–50 Ma are related to the island basalt (OIB)-type gabbro that was dated complete breakoff of the Neo-Tethyan oceanic at c. 45 Ma by titanite U–Pb dating from Lang- lithosphere, it is reasonable that the breakoff is shan on the northern margin of the Tethyan Hima- likely to have initiated at c. 53 Ma (Zhu et al. laya (Fig. 1c). However, it is more reasonable to 2015). This is because numerical modelling of slab conclude that the slab breakoff indicated by the breakoff indicates a duration of less than 2 myr Langshan OIB-type gabbro should happen a little from the onset of thinning to complete breakoff bit earlier (>c. 45 Ma) as the U–Pb age of titanite at depth of 35– 200 km (Duretz et al. 2012). Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

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A c. 53 Ma time for slab breakoff of the Neo- It can be envisaged that, at 84–67 Ma (Fig. 7c), Tethyan oceanic lithosphere can also account for subduction of the young and hot Neo-Tethyan litho- other tectonomagmatic events in the India–Asia col- sphere was followed by progressively older oceanic lision zone. This is because the mantle-derived mafic slab. This change may have been accompanied magmatism associated with slab breakoff will last for by progressive steepening and rollback of the several million years, whereas crust-derived felsic descending slab (Fig. 7d–f), which is suggested by magmatism can proceed over a considerably longer the southwards propagation of the Gangdese mag- period (van de Zedde & Wortel 2001). These events matism from 70–65 to 65–45 Ma (Fig. 2c), and include: (1) the occurrence of abundant 52–47 Ma which is likely to have contributed to the prominent mafic magmatism (including enclaves and dykes) increase in the net convergence rate from 67 to (Fig. 3a–d) that suggests significantly increased 51 Ma. contributions from the mantle; (2) the presence of Existing reconstructions have collectively c. 52.5 Ma bimodal volcanic rocks (Fig. 6a) that revealed a significant slowdown of the Indian Plate points to the partial melting of enriched metasomatic at c. 51–50 Ma (Patriat & Achache 1984; Besse layers within lithospheric mantle and to crustal melt- et al. 1984) from 150 to 100 mm a−1 (van Hinsber- ing caused by thermotectonic effects as a result of gen et al. 2011). Such a slowdown is traditionally slab breakoff (Davies & von Blanckenburg 1995); attributed to the increased resistance to subduction and (3) the identification of c. 50 Ma diabase dykes and is interpreted to be a result of the initial India– that intrude into serpentinitized peridotite within Asia collision (Patriat & Achache 1984; Besse the Yarlung–Zangbo Suture Zone (Fig. 1c) and the et al. 1984). This interpretation is, however, ques- occurrence of the c. 45 Ma Langshan OIB-type gab- tionable as under such an initial collisional regime bro (Ji et al. 2016), both of which can be attributed to no appropriate mechanisms (e.g. adding fluids, the decompression melting of asthenosphere after increasing temperature and decreasing pressure) slab breakoff. can be invoked to cause the magmatic flare-up (Fig. 2b) and the increase in the temperatures of mag- The India–Asia convergence since 120 Ma matism during emplacement (Fig. 6c, d)at52– 50 Ma as documented in the Gangdese arc. We Marine geophysical data from the Indian Ocean note that the sudden drop in the convergence rate indicate an increase in the India–Asia conver- at c. 51 Ma is coincident with the rapid eruption of gence rate from c. 50 mm a−1 during 120–90 Ma to thick rhyolitic ignimbrite (c. 52.5 Ma) in the Linzi- c. 140 mm a−1 during 90–84 Ma, which has been zong volcanic succession and intense magmatism attributed to the Morondova mantle plume activity (c. 51 Ma) in the Gangdese Batholith, suggesting (van Hinsbergen et al. 2011). The convergence rate that the slowdown of the Indian Plate is largely the between India and Asia between 110 and 50 Ma consequence of slab breakoff (Fig. 7h). This is can be directly interpreted as the subduction velocity because slab breakoff will result in the loss of the of the Neo-Tethyan oceanic lithosphere. This is slab-pull force (Davies & von Blanckenburg 1995; because the palaeolatitude of the southern margin Duretz et al. 2012), which exerts a dominant influ- of the Lhasa Terrane (21 ± 4° N) did not change ence on the velocity of the subducting plate and from 110 to 50 Ma (Lippert et al. 2014) and the will cause a drastic change in plate motion (Forsyth ridge spreading of the Neo-Tethyan Ocean may & Uyeda 1975; Conrad & Lithgow-Bertelloni 2002; have terminated at c. 120 Ma, as indicated by the Austermann et al. 2011). lack of ophiolites younger than c. 120 Ma within Inferred complete breakoff of the Neo-Tethyan the Yarlung–Zangbo Suture Zone (Zhu et al. oceanic lithosphere at c. 51 Ma together with the 2016a). In this case, the decrease in convergence relatively high rate of India–Asia convergence rate − rate of the Indian Plate from c. 140 mm a−1 at 90– at 51–45 Ma (80–100 mm a 1: van Hinsbergen 84 Ma to 80–90 mm a−1 at 84–67 Ma (van Hinsber- et al. 2011), raises the question of why didn’t the gen et al. 2011) can be linked with the subduction subduction of the c. 600 km-wide continental litho- of a young and buoyant oceanic crust after ridge sphere of the Indian Plate prevent the generation of subduction, due to its increased coupling with the the widespread magmatism observed in the Gangd- overriding Lhasa lithosphere (Fig. 7c). ese arc at 51–45 Ma? 2D numerical modelling indi- As noticed by van Hinsbergen et al. (2011), the cates that when the lower crust of the subducting 67–51 Ma convergence rate (150–160 mm a−1) plate has a moderate viscosity, both delamination was too high to have been driven solely by the Dec- of the continental lithospheric mantle from the con- can plume head. 3D laboratory experiments indicate tinental crust and breakoff of the oceanic plate can that a long history of prior subduction, steep dip occur in continental collision zones (Magni et al. angle and old age of the subducted slab can cause 2013). In the case of the India–Asia collision an increase in slab-pull force and thus accelerate zone, Greater India is located on the northern mar- plate motion at subduction zones (Schellart 2004). gin of the Indian Plate and thus its lower crust Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET should have a relatively high viscosity. However, Dating the initial India–Asia collision such a high viscosity may have been decreased through the magmatic record due to the extensive underplating of the Comei mantle plume-type basaltic magma during the Early Cre- Estimates of the timing of initial India–Asia collision taceous (Zhu et al. 2009b). As a result, the viscosity range from 70 to 20 Ma (Molnar & Tapponnier of the lower crust is most likely to have been mod- 1975; Garzanti et al. 1987; Rowley 1996; Yin & erate, and thus would facilitate the breakoff of the Harrison 2000; Aitchison et al. 2007; van Hinsber- Neo-Tethyan oceanic lithosphere and the delamina- gen et al. 2011; Hu et al. 2015, 2016). These uncer- tion of the continental lithospheric mantle of Greater tainties reflect, in part, the differing approaches used India from the lower crust (Fig. 7h–j)(Magni et al. to define collision. 2013). A combination of these deep processes provides space, materials and heat for the mantle, and Previous estimates based on the magmatic crustal melting to trigger the extensive magmatism record with the compositional diversity observed in the Gangdese arc at 51–45 Ma (Fig. 7h, i). These pro- The youngest calc-alkaline magmatic rocks (c. cesses are also capable of causing the decompres- 40 Ma) in the Gangdese Batholith have been used sion melting of the asthenosphere to generate the to argue that the India–Asia collision did not c. 45 Ma Langshan OIB-type gabbro (Ji et al. 2016) occur until 40 Ma (Allègre et al. 1984; Aitchison and the partial melting of the eclogitized lower crust et al. 2007). However, new geochronological and of Greater India to produce the c. 43 Ma high Sr/Y geochemical data indicate that the calc-alkaline granite (Zeng et al. 2011)(Fig. 7j) in the Tethyan magmatism in the Gangdese Batholith also occur- Himalaya. red from 21 to 10 Ma (Fig. 8a) and displayed arc The broken slab of the Neo-Tethyan Ocean will signatures on the commonly used tectonic discrimi- continue sinking through a fixed slot into (and thus nation diagrams (e.g. Fig. 8b). These ‘arc-type’ stacking within) the mantle beneath the Indian Plate calc-alkaline rocks are younger than the youngest due to negative buoyancy (Fig. 7i) as the locus of proposed age of the India–Asia collision (c. the trench is relatively fixed at 21 ± 4° N from 20 Ma: van Hinsbergen et al. 2012), indicating that 110 and 50 Ma (Lippert et al. 2014). Such a slab- the termination of arc-type calc-alkaline magmatism stacking hypothesis can be invoked to explain the cannot be used to constrain the timing of the colli- present-day seismic anomalies in the mantle at sion (Harrison et al. 2000; Chung et al. 2005; Ji depths of between 1100 and 1600 km (van der et al. 2009b). Voo et al. 1999; Replumaz et al. 2004, 2010). St-Onge et al. (2010) proposed that the India– Both the thickened lower crust and the delaminated Asia collision occurred in the early Eocene (c. lithospheric mantle of Greater India continued to 49 Ma) based on the observations that the I-type subduct and sink into (and thus stacking within) granodiorite (c. 57.7 Ma) is intruded by an S-type the mantle (Fig. 7j)(Ingalls et al. 2016), providing leucogranite dyke (i.e. the Chumatang dyke; c. a significant driving force to drag the Indian Plate 47.1 Ma) in the Ladakh Batholith. All of these gran- northwards (Capitanio et al. 2010) since c. 50 Ma. ites are truncated by the unconformity along the base The northwards movement of the Indian Plate of the Indus molasse basin, providing a minimum since c. 45 Ma can probably explain the horizontal age constraint on the initial India–Asia collision at compression of the Lhasa Terrane as expressed by this locality (Searle 2018). However, White et al. the Gangdese Thrust that was active at 30–24 Ma (2011) suggested that the presence of amphibole (Yin et al. 1999) and the weakening magmatism and magnetite in the Chumatang leucogranite dyke in the Gangdese arc during 40–25 Ma (Fig. 2b). indicates that this dyke has I-type affinities and The subducting continental lithosphere of Greater thus cannot be used to indicate the termination of India was likely to have been overturned (Fig. 7j, k) subduction-type magmatism. Instead, White et al. and ultimately broken off at c. 25 Ma (Mahéo et al. (2012) proposed that the subduction of the Neo- 2002; Replumaz et al. 2010) due to the north- Tethyan oceanic lithosphere continued until at least wards dragging of the Indian Plate (Fig. 7k). These 31 Ma, and that the collision between India and interpretations are seismically supported by the Asia is constrained by the transition from amphibole- south-dipping anomaly extending from the top of bearing I-type granodiorite (c. 31.4 Ma) to S-type the transition zone to the uppermost lower mantle granite (c. 18 Ma) identified in the Karakorum Bath- beneath India (Replumaz et al. 2010)(Fig. 7k) olith. However, we consider that this is less reliable and the presence of a 15 km-thick strong velocity– as the presence of amphibole in the granite is primar- depth gradient layer above the Moho in the south- ily related to melting conditions (e.g. water content) ern Lhasa subterrane that indicates the underplating and magmatic processes, and cannot be used as an of lower Indian crust developed since 25–20 Ma effective discriminant of the tectonic setting of (Nábeleǩ et al. 2009). magma generation. For example, some of the Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

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90 Rb/30 (a)(b) Subduction: 80-66 Ma 65-56 Ma 80 hiFe SiO2 > 63wt% miFe DI = 40-85 Ongoing collision (syncollision): rc + MgO) 55-47 Ma miFe 70 Tholeiitic total loFe Post-collision: Calc-alkai Volcanic a Syn- 47-37 Ma collisional 35-26 Ma /(FeO 60 SiO2 > 63wt% 21-9 Ma total DI = 40-85 Late and postcollisional 50 80-66 Ma 47-37 Ma

100×FeO 65-56 Ma 35-26 Ma 55-47 Ma 21-9 Ma Within plate 40 55 60 65 70 75 Hf 3×Ta Volatile-free SiO2 (wt%)

Fig. 8. Discrimination diagrams for rock series and tectonic setting of granitic rocks (<80 Ma; SiO2 >63 wt%) from the Gangdese Batholith (Zhu et al. 2017b). (a) Plot of volatile-free SiO2 (wt%) v. SiO2–FeOtotal/(FeOtotal + MgO) (Gill 2010) showing that almost all of the samples are calc-alkaline. (b) Plot of Rb/30 v. Hf v. 3 × Ta (Harris et al. 1986) indicating that the samples generated during syncollision and post-collision still fall into the ﬁeld of the volcanic arc. It follows that this diagram cannot be taken as an effective method to discriminate the tectonic settings of granitic rocks. Samples that experienced magmatic differentiation (differentiation index >85) and accumulation (differentiation index >40) have been excluded in both ﬁgures as their compositions cannot be used to represent the compositions of primary magma.

21–9 Ma granites in the Gangdese Batholith (e.g. the New magmatic approach to date the initial granodiorite of c. 17 Ma: Xu et al. 2016; Yang et al. India–Asia collision 2016) also contain amphibole, but these granites are definitely post-collisional. In an ocean–continent convergence zone, with con- Several studies have shown that if there is a sig- tinuous subduction of the oceanic plate, initial colli- nificant difference in isotopic compositions between sion is defined as commencing when the lithosphere the overriding plate (juvenile and, thus, depleted) of the lower-plate passive continental margin arrives and the underlying subducting plate (ancient and at the trench and contacts directly with the continen- thus enriched) in collision zones, the timing of colli- tal crust of the overriding-plate active continental sion can be backdated by constraining the age of the margin. But collision is an ongoing process and change in the isotopic composition of the overlying can be broken into three phases, with associated plate from depleted to enriched (Elburg & Foden magmatic activity: 1998; Elburg et al. 2002). In this case, the decrease in zircon εHf(t) values at c. 50 Ma (Ji et al. 2009a) • Initial collision – in which a remnant sea may be or c. 55 Ma (Chu et al. 2011) documented in the present between the opposing continental blocks, Gangdese Batholith can be interpreted to be the notably in continental margin re-entrants, and the result of the involvement of crustal materials from oceanic plate is still subducting. Normal the Indian continent, indicating that the India–Asia calc-alkaline andesitic magmatism, induced by collision occurred prior to c. 50 Ma or c. 55 Ma. the dehydration of the subducting oceanic plate, However, this interpretation does not take into is still continuing (Fig. 9a). account the influence of ancient basement-derived • Ongoing collision – which includes two cases. In materials in the central Lhasa subterrane (Zhu et al. the first, the oceanic plate continues to sink, fol- 2011, 2013) indicated by variations in the isotope lowed by rollback due to the presence of the slab- compositions. Likewise, the negative changes in pull force (Niu 2014), which enhances mantle the whole-rock εNd(t) values and zircon εHf(t) values convection in the wedge mantle (➊ in Fig. 9b) at 50.2 ± 1.5 Ma documented in the Kohistan– and leads to the propagation of the dehydration Ladakh Batholith are interpreted to indicate that the zone toward the trench. As a result, the magma- Kohistan–Ladakh intra-oceanic arc collided with tism intensifies and migrates toward the trench. the Indian continent at c. 50 Ma (Bouilhol et al. The second case develops as a result of crustal 2013). However, this change to more negative values shear heating (Harris et al. 1986) and convective is based on only one granodiorite sample, and heat from small-scale mantle flow (Magni et al. whether it represents the earliest record of a compo- 2012)(➋ in Fig. 9b), and results in only small sitional change requires further validation. crust-derived peraluminous magmatism. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

Passive (a)(Active b)(Terrigenous Passive margin c) Active margin Passive margin margin Oceanic Remnant margin Active margin sediment sea sediments CC CC CC CC CC CC 2 SCLM SCLM SCLM Gravity rebound 1 Subduction Releasing channel Thin continental w Slab fluids Slab pull CC = Continental Crust lithosphere Slab indow Direction of breakoff SCLM = Subcontinental Rising mantle flow Lithospheric Mantle Slab rollback asthenosphere

Collisional process: Initial collision Ongoing collision Tectonic tranisition a. Intense magmatism with compositional diversity Magmatic Dominated by normal Calc-alkaline magmatism migrating towards the calc-alkaline andesitic trench or only minor crust-derived peraluminous b. Mafic magmatism showing within-plate basalt geochemistry Response: c. Bimodal magmatism magmatism magmatism d. A2-type felsic magmatism Adding fluids through the Adding fluids through the dehydration of the Primary mecha- Increasing the temperature of the lithosphere and dehydration of the subducting subducting oceanic lithosphere or shear heating, nism of magma decreasing the pressure of the asthenosphere generation: oceanic lithosphere convective heat from small-scale mantle flow

Fig. 9. Schematic sequence of the relationship between collisional processes and magmatic responses in collision zones (Zhu et al. 2017b). ➊ and ➋ in (b) indicate the two likely mechanisms for magma generation during the ongoing collision (syncollision): ➊ Slab rollback enhances the mantle convection in the wedge mantle and leads to the propagation of the dehydration zone towards the trench, resulting in the intensiﬁcation of magmatism that migrates towards the trench; and ➋ crustal shear heating and convective heating from small-scale mantle ﬂow result in the development of a small crust-derived peraluminous magmatism.

• Tectonic transition – during which extensive mag- depth of the ultrahigh-pressure metamorphism (cf. matic activity with diverse compositions develops Leech et al. 2005; Donaldson et al. 2013) with the as a result of slab breakoff (Fig. 9c). After slab age and depth of slab breakoff, and proposed the breakoff, oceanic subduction weakens rapidly revised equation (1) to calculate the timing of the ini- (even stops), the convergence rate decreases rap- tial collision. The angle of subduction is assumed to idly and the collisional zone develops into an vary from 30° to 60°, corresponding to a slab rollback intracontinental zone as the slab-pull force on process. This provides a method of using magmatic the subducting oceanic plate disappears (Duretz data to quantitatively date the initial continental col- et al. 2012). lision: According to this sequence of collisional events, if the timing of oceanic slab breakoff can be con- age of initial collision strained, the convergence rate, depth of slab breakoff = depth of slab breakoff and subduction angle can be used to calculate the × ( ) timing of the initial collision. The age and depth of convergence rate sin subduction angle ultrahigh-pressure metamorphism have previously + age of slab breakoff. (1) been used to calculate the timing of the initial collision (Leech et al. 2005; Donaldson et al. 2013). However, ultrahigh-pressure rocks do not necessar- Application of a new magmatic approach to the ily represent the fragment of the continent that initial India–Asia collision subducted to the greatest depths because the shallow- est subducted rocks generally exhume to the surface Timing of the initial India–Asia collision using mag- first, while the most deeply subducted rocks are gen- matic data from southern Tibet. Calculations of the erally the last to exhume to the surface (Zheng et al. geochemical parameters indicate that the Gangdese 2015). Consequently, the timing of collision that is crust is likely to have thickened to c. 60 km at 60– calculated using the age and depth of ultrahigh- 45 Ma due to magma underplating (Zhu et al. pressure metamorphism is most likely to be less 2017a). As a result, the lithospheric mantle that than the actual timing of the initial collision. In con- was isostatic with this eclogitized crust must also trast, the collision age that is calculated from the have thickened (Lustrino 2005; McKenzie & Priest- age and depth of slab breakoff is likely to be closer ley 2008). Therefore, the depth of slab breakoff (c. to the actual initial collision age. This is because 130 km: Leech et al. 2005) is likely not to have not slab breakoff generally occurs preferentially at the significantly exceeded the bottom of the lithosphere, location with the largest difference in density in making it possible to generate extensive magmatism the ocean–continent transition zone, which repre- after slab breakoff (van de Zedde & Wortel 2001). sents the deepest subducted continental lithosphere. Clearly, the younger ages of slab breakoff at c. 45 Therefore, Zhu et al. (2015) replaced the age and or c. 50 Ma cannot explain the magmatic flare-up Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL. and the increase in temperature calculated for mag- likely to be the tectonic signature of the initial matic activity that is documented for the Gangdese India–Asia collision. In this case, the new tectonic Batholith and the Linzizong volcanic rocks at around interpretation for the Linzizong volcanic succession 52–51 Ma. We therefore adopt the hypothesis that is that the Dianzhong Formation calc-alkaline volca- the slab breakoff began at c. 53 Ma to backdate the nic rocks are likely to have formed from the late-stage timing of the initial India–Asia continental collision. subduction of the Neo-Tethyan oceanic lithosphere Assuming that the intermediate depth of slab break- to the initial stage of the India–Asia collision. The off (c. 130 km; Leech et al. 2005) did not exceed the lower part of the of the overlying Nianbo Formation, bottom of the lithosphere, the subduction angle which is dominated by terrestrial sediments, was increased from 30° to 60° in response to slab rollback deposited during the ongoing India–Asia collision, and the convergence rate was high (150 mm a−1: van and the shoshonitic volcanic rocks in the upper Hinsbergen et al. 2011), the initial India–Asia colli- Nianbo and Pana formations are the result of slab sion can be quantitatively constrained at 54.7– breakoff (Zhu et al. 2015). 54.0 Ma using equation (1) assuming that the lower- plate margin was parallel to the upper-plate margin. This timing of the initial India–Asia collision is in Concluding remarks and future work good agreement with multidisciplinary estimates on collision ages (see Hu et al. 2016 for a review), indi- The Gangdese Batholith in southern Tibet was cating the applicability of the method. The timing emplaced from c. 210 to c. 10 Ma and shows two suggests that the slab breakoff of the Neo-Tethyan intense pulses of magmatic activity. The earlier oceanic lithosphere is likely to have occurred soon pulse at 90 ± 5 Ma is confined to the east of longi- after (less than 2 myr) the initial India–Asia colli- tude 89° E. The second pulse at 50 ± 3 Ma (peak at sion, rather than later as implied by geodynamic sim- c. 51 Ma) extends for more than 1500 km (longitude ulations (30–5 Ma: see van Hunen & Allen 2011 and 80–95° E) along the southern Lhasa Terrane. The references therein). A key reason for this difference Linzizong volcanic succession in the Linzhou − is that the convergence rate is too fast (110 mm a 1: Basin was active during 60–52 Ma, rather than 69– − Molnar & Stock 2009; 150 mm a 1: van Hinsbergen 44 Ma as previously interpreted. The Gangdese et al. 2011). Batholith magmatism was collectively restricted to the southern portion of the southern Lhasa subter- New tectonic interpretation of the Linzizong volcanic rane from 120 to 70 Ma (although it migrated succession. Existing studies have proposed three slightly northwards at 95–70 Ma), and then propa- tectonic interpretations for the Linzizong volcanic gated both south and north with enhanced mantle succession: (1) arc-type magmatism at the active con- input at 70–45 Ma, before finally being restricted tinental margin, which predates the initial India–Asia to the southern edge of the southern Lhasa subter- collision that occurred at 40 Ma or later (Allègre rane at 45–26 Ma. Normal northwards subduction et al. 1984; Xu et al. 1985; Coulon et al. 1986; Harris of the Neo-Tethyan oceanic lithosphere at 120– et al. 1988a, b; Pearce & Mei 1988; Aitchison et al. 95 Ma, ridge subduction at 95–85 Ma, subduction 2007); (2) syncollision magmatism assuming that the of a young and buoyant oceanic lithosphere at 84– initial India–Asia collision occurred at 70–65 Ma 67 Ma, Deccan plume activity and slab rollback at (Mo et al. 2003, 2008); and (3) the result of a steep- 67–51 Ma, complete slab breakoff at c. 51 Ma, and ened subduction angle at 70–50 Ma and the breakoff descent of the high-density Indian continental litho- at c. 50 Ma of the Neo-Tethyan oceanic lithosphere sphere since 50 Ma are suggested to have operated (Ding et al. 2003; Chung et al. 2005, 2009; Yue & and impacted on the India–Asia convergence since Ding 2006; Lee et al. 2009, 2012). 120 Ma. The significant deceleration of the conver- Clearly, these different tectonic interpretations of gence rate at 51–50 Ma is likely to have been the the Linzizong volcanic succession depend heavily on result of slab breakoff rather than initiation of the the timing of the initial India–Asia collision. A time India–Asia collision. of c. 55 Ma (Zhu et al. 2015), as discussed above, Reasons why the 90 ± 5 Ma magmatic rocks are is consistent with the development of the angular confined to longitude 89–94° E (Fig. 2a) remains unconformity between the Nianbo Formation and unclear − is it simply a function of sampling bias or the underlying Dianzhong Formation in the Linzhou does it reflect a geodynamic control? These rocks Basin (Liu 1993; Dong et al. 2005; Huang et al. are tentatively linked with the ridge subduction of 2015) (58 –55 Ma: Zhu et al. 2015) and the termina- the Neo-Tethyan oceanic lithosphere, but its validity tion of the Xigaze forearc deposition (58–54 Ma: needs further evaluation, especially data from west of Orme et al. 2015). Therefore, it is possible that the longitude 89° E. Other possibilities, including the angular unconformity between the Nianbo Forma- subduction of a Neo-Tethyan oceanic plateau or a tion and the Dianzhong Formation (although the distinct subduction geometry and kinematics (Thor- scale is minor), which lasted for only c. 3 myr, is kelson 1996), should also be tested in future work. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

It is worthwhile to reiterate that the widely used of the Xigaze Ophiolite. Geological Society of America tectonic discrimination diagrams for magmatic rocks Bulletin, 126, 1595–1613, https://doi.org/10.1130/ are ineffective, or even misleading, in constraining b31020.1 the setting of the Gangdese arc, exemplifying that AUSTERMANN, J., BEN-AVRAHAM, Z., BIRD, P., HEIDBACH, O., geochemical data of magmatic rocks should be SCHUBERT,G.&STOCK, J.M. 2011. Quantifying the forces needed for the rapid change of Pacific plate used with caution in discriminating tectonic settings motion at 6 Ma. Earth and Planetary Science Letters, in the ancient rock record. The timing of the initial 307, 289–297. – India Asia collision (c. 55 Ma) obtained using our BESSE, J., COURTILLOT, V., POZZI, J.P., WESTPHAL,M.& proposed petrological approach is consistent with ZHOU, Y.X. 1984. Paleomagnetic estimates of crustal the collision ages obtained by other disciplines (see shortening in the Himalayas thrusts and Zangbo suture. Hu et al. 2016 for a review). However, our approach Nature, 311, 621–626. is proposed on the basis of the identification of slab BOUILHOL, P., JAGOUTZ, O., HANCHAR, J.M. & DUDAS, F.O. – breakoff; and whether slab breakoff occurred, either 2013. Dating the India Eurasia collision through arc Earth and Planetary Science Letters here or in other collision zones, remains a topic of magmatic records. , 366, 163–175, https://doi.org/10.1016/j.epsl.2013. discussion (see Niu 2017; Garzanti et al. 2018 for 01.023 reviews). If slab breakoff does occur and our model BURG, J.-P., PROUST, F., TAPPONNIER,P.&MING, C.G. 1983. is correct, then the relationships documented for the Deformation phases and tectonic evolution of the Lhasa India–Asia collision zone provides a solution for block (southern Tibet, China). Eclogae Geologicae determining the occurrence of slab breakoff in Helvetiae, 76, 643–665. other zones. Our approach depends primarily on a CAPITANIO, F.A., MORRA, G., GOES, S., WEINBERG, R.F. & combination of reconstructing the spatial and tempo- MORESI, L. 2010. India–Asia convergence driven by the subduction of the Greater Indian continent. Nature ral variation of magmatic records through geochro- – nological and geochemical methods, as well as Geoscience, 3, 136 139. CAWOOD, P.A., KRÖNER, A., COLLINS, W.J., KUSKY, T.M., detecting the detached oceanic lithosphere in deeper MOONEY, W.D. & WINDLEY, B.F. 2009. Accretionary mantle using high-resolution seismic tomography. orogens through Earth history. In:CAWOOD, P.A. & KRÖNER, A. (eds) Earth Accretionary Systems in Space This contribution is dedicated and Time. Geological Society, London, Special Publi- Acknowledgements – // / / to Professor Xuan-Xue Mo in recognition of his 80th birth- cations, 318,1 36, https: doi.org 10.1144 SP318.1 – day. We thank Mike Searle for inviting this contribution, CHEN, B.B., DING, L., XU, Q., YUE, Y.H. & XIE, J. 2016. U and Douwe van Hinsbergen, Jonathan Aitchison, Mike Pb age framework of the Linzizong volcanic rocks from Searle and Andrew Mitchell for helpful and critical com- the Linzhou Basin, Tibet. Quaternary Sciences, 36, – ments that improved the manuscript. We also thank 1037 1054 [in Chinese with English abstract]. ’ An-Lin Liu for her assistance with the preparation of the CHU, M.F., CHUNG, S.L. ET AL. 2011. India s hidden inputs to manuscript. This is CUGB petrogeochemical contribution Tibetan orogeny revealed by Hf isotopes of Transhima- No. PGC2015-0032. layan zircons and host rocks. Earth and Planetary Sci- ence Letters, 307, 479–486, https://doi.org/10.1016/j. epsl.2011.05.020 Funding This research was financially co-supported CHUNG, S.L., LO, C.H. & LEE, T.Y. 2003. Petrologic case by the Chinese National Natural Science Foundation for Eocene slab breakoff during the Indo-Asian colli- – (grant Nos 91755207, 41872060 and and 41225006), sion: comment. Geology, 31,e7 e8. the Programme of Introducing Talents of Discipline to CHUNG, S.L., CHU, M.F. ET AL. 2005. Tibetan tectonic evo- Universities (i.e. the 111 project) (B18048), and the MOST lution inferred from spatial and temporal variations in Special Fund from the State Key Laboratory of Geological post-collisional magmatism. Earth-Science Reviews, – // / / Processes and Mineral Resources, China University of 68,173 196, https: doi.org 10.1016 j.earscirev.2004. Geosciences (grant No. MSFGPMR201802). P.A. Cawood 05.001 acknowledges support from the Australian Research CHUNG, S.L., CHU, M.F. ET AL. 2009. The nature and timing Council (grant No. FL16 0100168). of crustal thickening in Southern Tibet: geochemical and zircon Hf isotopic constraints from postcollisional adakites. Tectonophysics, 477,36–48. References CONRAD, C.P. & LITHGOW-BERTELLONI, C. 2002. How mantle slabs drive plate tectonics. Science, 298, 207–209, AITCHISON, J.C., ALI, J.R. & DAVIS, A.M. 2007. When and https://doi.org/10.1126/science.1074161 where did India and Asia collide? Journal of Geophys- COULON, C., MALUSKI, H., BOLLINGER,C.&WANG, S. 1986. ical Research: Solid Earth, 112, B05423, https://doi. Mesozoic and Cenozoic volcanic rocks from central org/10.1029/2006JB004706 and southern Tibet: 39Ar–40Ar dating, petrological ALLÈGRE, C.J., COURTILLOT,V.ET AL. 1984. Structure and characteristics and geodynamical significance. Earth evolution of the Himalaya–Tibet orogenic belt. Nature, and Planetary Science Letters, 79, 281–302. 307,17–22. COULON, C., MEGARTSI,M.ET AL. 2002. Post-collisional tran- AN, W., HU, X.M., GARZANTI, E., BOUDAGHER-FADEL, M.K., sition from calc-alkaline to alkaline volcanism during WANG, J.G. & SUN, G.Y. 2014. Xigaze forearc basin the Neogene in Oranie (Algeria): magmatic expression revisited (South Tibet): provenance changes and origin of a slab breakoff. Lithos, 62,87–110. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

DAI, J., WANG, C.S., POLAT, A., SANTOSH, M., LI,Y.&GE, applied in space and time. Earth-Science Reviews, Y. 2013. Rapid forearc spreading between 130 and 177, 303–319. 120 Ma: evidence from geochronology and geochemis- GILL, J.B. 1981. Orogenic Andesites and Plate Tectonics. try of the Xigaze ophiolite, southern Tibet. Lithos, Springer, Berlin. 172–173,1–16, https://doi.org/10.1016/j.lithos. GILL, R. 2010. Igneous Rocks and Processes: A Practical 2013.03.011 Guide. Wiley-Blackwell, Chichester, UK. DAVIES, J.H. & VON BLANCKENBURG, F. 1995. Slab breakoff: GUO, L., ZHANG, H.F., HARRIS, N., PAN, F.B. & XU, W.C. a model of lithosphere detachment and its test in the 2013. Late Cretaceous (c. 81 Ma) high-temperature magmatism and deformation of collisional orogens. metamorphism in the southeastern Lhasa terrane: impli- Earth and Planetary Science Letters, 129,85–102, cation for the Neo-Tethys ocean ridge subduction. Tec- https://doi.org/10.1016/0012-821X(94)00237-S tonophysics, 608, 112–126. DAVIES, J.H. & VON BLANCKENBURG, F. 1998. Thermal con- HARRIS, N.B.W., PEARCE, J.A. & TINDLE, A.G. 1986. Geo- trols on slab breakoff and the rise of high-pressure rocks chemical characteristics of collision-zone magmatism. during continental collisions. In:HACKER, B.R. & LIOU, In:COWARD, M.P. & RIES, A.C. (eds) Collision Tecton- J.G. (eds) When Continents Collide: Geodynamics and ics. Geological Society, London, Special Publications, Geochemistry of Ultrahigh-Pressure Rocks. Springer, 19,67–81, https://doi.org/10.1144/GSL.SP.1986. Dordrecht, The Netherlands, 97–115. 019.01.04 DEWEY, J.F. & BURKE, K.C.A. 1973. Tibetan, Variscan and HARRIS, N.B.W., XU, R.H., LEWIS, C.L., HAWKESWORTH, C.J. Precambrian reactivation: products of continental colli- &ZHANG, Y.Q. 1988a. Isotope geochemistry of the sion. Journal of Geology, 81, 683–692. 1985 Tibet Geotraverse: Lhasa to Golmud. Philosoph- DING, L., KAPP,P.&DENG, W.M. 2003. Cenozoic volca- ical Transactions of the Royal Society of London A: nism in Tibet: evidence for a transition from oceanic Mathematical, Physical and Engineering Sciences, to continental subduction. Journal of Petrology, 44, 327, 263–285. 1833–1865. HARRIS, N.B.W., XU, R.H., LEWIS, C.L. & JIN, C.W. 1988b. DONALDSON, D.G., WEBB, A.A.G., MENOLD, C.A., KYLANDER- Plutonic rocks of the 1985 Tibet Geotraverse, Lhasa to CLARK,A.R.C.&HACKER, B.R. 2013. Petrochronology Golmud. Philosophical Transactions of the Royal Soci- of Himalayan ultrahigh-pressure eclogite. Geology, 41, ety of London A: Mathematical, Physical and Engineer- 835–838, https://doi.org/10.1130/G33699.1 ing Sciences, 327, 145–168. DONG, G.C., MO, X.X., WANG,L.&ZHOU, S. 2005. A HARRISON, T.M., YIN, A., GROVE, M., LOVERA, O.M., RYER- new understanding of the stratigraphic successions SON, F.J. & ZHOU, X.H. 2000. The Zedong window: a of the Linzizong volcanic rocks in the Linzhou record of superposed Tertiary convergence in south- basin, northern Lhasa, Tibet, China. Geological Bul- eastern Tibet. Journal of Geophysical Research: Solid letin of China, 24, 549–557 [in Chinese with English Earth, 105, 19 211–19 230. abstract]. HE, S., KAPP, P., DECELLES, P.G., GEHRELS, G.E. & HEIZLER, DONG, X., ZHANG, Z.M. & SANTOSH, M. 2010. Zircon U– M. 2007. Cretaceous–Tertiary geology of the Gangdese Pb chronology of the Nyingtri Group, southern Lhasa Arc in the Linzhou area, southern Tibet. Tectonophy- Terrane, Tibetan Plateau: implications for Grenvillian sics, 433,15–37. and Pan-African provenance and Mesozoic–Cenozoic HU, X.M., JANSA, L., CHEN, L., GRIFFIN, W.L., O’REILLY, metamorphism. The Journal of Geology, 118, S.Y. & WANG, J.G. 2010. Provenance of Lower Creta- 677–690, https://doi.org/10.1086/656355 ceous Wölong Volcaniclastics in the Tibetan Tethyan DURETZ, T., SCHMALHOLZ, S.M. & GERYA, T.V. 2012. Himalaya: implications for the ﬁnal breakup of Eastern Dynamics of slab detachment. Geochemistry, Geophys- Gondwana. Sedimentary Geology, 223, 193–205. ics, Geosystems, 13, Q03020, https://doi.org/10. HU, X.M., GARZANTI, E., MOORE,T.&RAFFI, I. 2015. Direct 1029/2011GC004024 stratigraphic dating of India–Asia collision onset at the EINSELE, G., LIU,B.ET AL. 1994. The Xigaze forearc basin: Selandian (middle Paleocene, 59 ± 1 Ma). Geology, 43, evolution and facies architecture (Cretaceous, Tibet). 859–862. Sedimentary Geology, 90,1–32. HU, X.M., GARZANTI, E., WANG, J.G., HUANG, W.T., AN,W. ELBURG,M.&FODEN, J. 1998. Temporal changes in arc &WEBB, A. 2016. The timing of India–Asia collision magma geochemistry, northern Sulawesi, Indonesia. onset – facts, theories, controversies. Earth-Science Earth and Planetary Science Letters, 163, 381–398. Reviews, 160, 264–299. ELBURG, M.A., VAN LEEUWEN, T., FODEN,J.&MUHARDJO, HUANG, W., DUPONT-NIVET,G.ET AL. 2015. What was M. 2002. Origin of geochemical variability by arc–con- the Paleogene latitude of the Lhasa terrane? A reassess- tinent collision in the Biru area, southern Sulawesi ment of the geochronology and paleomagnetism of (Indonesia). Journal of Petrology, 43, 581–606. Linzizong volcanic rocks (Linzhou basin, Tibet). Tec- FORSYTH,D.&UYEDA, S. 1975. On the relative importance tonics, 34, 594–622, https://doi.org/10.1002/(ISSN) of the driving forces of plate motions. Geophysical 1944-9194 Journal International, 43, 163–200, https://doi.org/ INGALLS, M., ROWLEY, D.B., CURRIE,B.&COLMAN, A.S. 10.1111/j.1365-246X.1981.tb02758.x 2016. Large-scale subduction of continental crust GARZANTI, E., BAUD,A.&MASCLE, G. 1987. Sedimentary implied by India–Asia mass-balance calculation. record of the northward ﬂight of India and its collision Nature Geoscience, 9, 848–853. with Eurasia (Ladakh Himalaya, India). Geodinamica JI, W.Q., WU, F.Y., CHUNG, S.L., LI, J.X. & LIU, C.Z. Acta, 1, 297–312. 2009a. Zircon U–Pb geochronology and Hf isotopic GARZANTI, E., RADEFF,G.&MALUSÀ, M.G. 2018. Slab constraints on petrogenesis of the Gangdese Batholith, breakoff: a critical appraisal of a geological theory as southern Tibet. Chemical Geology, 262, 229–245. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

JI, W.Q., WU, F.Y., LIU, C.Z. & CHUNG, S.L. 2009b. LIU, H.F. 1993. Division of Linzizong volcanic rock system Geochronology and petrogenesis of granitic rocks in and belong to time in Lhasa area. Tibet Geology, 2, Gangdese batholith, southern Tibet. Science in China 59–69 [in Chinese with English abstract]. Series D: Earth Sciences, 52, 1240–1261, https://doi. LUSTRINO, M. 2005. How the delamination and detachment org/10.1007/s11430-009-0131-y of lower crust can influence basaltic magmatism. Earth- JI, W.Q., WU, F.Y. ET AL. 2016. Eocene Neo-Tethyan Science Reviews, 72,21–38. slab breakoff constrained by 45 Ma oceanic island MA, L., WANG,Q.ET AL. 2013a. Early Late Cretaceous basalt-type magmatism in southern Tibet. Geology, (c. 93 Ma) norites and hornblendites in the Milin area, 44, 283–286. eastern Gangdese: lithosphere–asthenosphere interac- KAPP, P., DECELLES, P.G. ET AL. 2007. The Gangdese retroarc tion during slab roll-back and an insight into early thrust belt revealed. GSA Today, 17,4–9, https://doi. Late Cretaceous (c. 100–80 Ma) magmatic ‘flare-up’ org/10.1130/GSAT01707A.1 in southern Lhasa (Tibet). Lithos, 172–173,17–30. KOHN,M.&PARKINSON, C.D. 2002. Petrologic case for MA, L., WANG,Q.ET AL. 2013b. Late Cretaceous (100– Eocene slab breakoff during the Indo-Asian collision. 89 Ma) magnesian charnockites with adakitic affinities Geology, 30, 591–594. in the Milin area, eastern Gangdese: partial melting of LEE, H.Y., CHUNG, S.L., LO, C.H., JI, J., LEE, T.Y., QIAN,Q. subducted oceanic crust and implications for crustal &ZHANG, Q. 2009. Eocene Neotethyan slab breakoff in growth in southern Tibet. Lithos, 175–176, 315–332. southern Tibet inferred from the Linzizong volcanic MA, L., WANG, B.D. ET AL. 2014. Petrogenesis of the Early record. Tectonophysics, 477,20–35. Eocene adakitic rocks in the Napuri area, southern LEE, H.Y., CHUNG, S.L. ET AL. 2012. Geochemical and Sr– Lhasa: partial melting of thickened lower crust during Nd isotopic constraints on the genesis of the Cenozoic slab break-off and implications for crustal thickening Linzizong volcanic successions, southern Tibet. Jour- in southern Tibet. Lithos, 196–197, 321–338. nal of Asian Earth Sciences, 53,96–114. MA, Y., YANG, T., BIAN, W., JIN, J., ZHANG, S., WU,H.&LI, LEE, T.Y. & LAWVER, L.A. 1995. Cenozoic plate reconstruc- H. 2016. Early Cretaceous paleomagnetic and geochro- tion of Southeast Asia. Tectonophysics, 251,85–138. nologic results from the Tethyan Himalaya: insights LEECH, M.L., SINGH, S., JAIN, A.K., KLEMPERER, S.L. & into the Neotethyan paleogeography and the India– MANICHAVASGAM, R.M. 2005. The onset of India–Asia Asia collision. Scientific Reports, 6, 21605, https:// continental collision: early, steep subduction required doi.org/10.1038/srep21605 by the timing of UHP metamorphism in the western MAFFIONE, M., VAN HINSBERGEN, D.J.J. ET AL. 2015. Forearc Himalaya. Earth and Planetary Science Letters, 234, hyperextension dismembered the south Tibetan ophio- 83–97, https://doi.org/10.1016/j.epsl.2005.02.038 lites. Geology, 43, 475–478, https://doi.org/10.1130/ LI, S.M., WANG, Q., ZHU, D.C., STERN, R.J., CAWOOD, P.A., g36472.1 SUI, Q.L. & ZHAO, Z.D. 2018. One or two Early Creta- MAGNI, V., VAN HUNEN, J., FUNICIELLO,F.&FACCENNA,C. ceous arc systems in the Lhasa Terrane, southern Tibet. 2012. Numerical models of slab migration in continen- Journal of Geophysical Research: Solid Earth, 123, tal collision zones. Solid Earth, 3, 293–306. 3391–3413, https://doi.org/10.1002/2018JB015582 MAGNI, V., FACCENNA, C., VAN HUNEN,J.&FUNICIELLO,F. LI, Z.Y., DING, L., LIPPERT, P.C., SONG, P.P., YUE, Y.H. & 2013. Delamination v. break-off: the fate of continen- VAN HINSBERGEN, D.J.J. 2016. Paleomagnetic con- tal collision. Geophysical Research Letters, 40, straints on the Mesozoic drift of the Lhasa terrane 285–289. (Tibet) from Gondwana to Eurasia. Geology, 44, MAHÉO, G., GUILLOT, S., BLICHERT-TOFT, J., ROLLAND,Y.& 727–730, https://doi.org/10.1130/G38030.1 PÊCHER, A. 2002. A slab breakoff model for the Neo- LIANG, X.F., CHEN,Y.ET AL. 2016. 3D imaging of subduct- gene thermal evolution of South Karakorum and ing and fragmenting Indian continental lithosphere South Tibet. Earth and Planetary Science Letters, beneath southern and central Tibet using body-wave 195,45–58. finite-frequency tomography. Earth and Planetary Sci- MARTINOD, J., HUSSON, L., ROPERCH, P., GUILLAUME,B.& ence Letters, 443, 162–175. ESPURT, N. 2010. Horizontal subduction zones, conver- LIN, T.H., CHUNG, S.L., KUMAR, A., WU, F.Y., CHIU, H.Y. & gence velocity and the building of the Andes. Earth and LIN, I.J. 2013. Linking a prolonged Neo-Tethyan mag- Planetary Science Letters, 299, 299–309. matic arc in South Asia: zircon U-Pb and Hf isotopic MCKENZIE,D.&PRIESTLEY, K. 2008. The influence of lith- constraints from the Lohit Batholith, NE India. Terra ospheric thickness variations on continental evolution. Nova, 25, 453–458. Lithos, 102,1–11. LIPPERT, P.C., VAN HINSBERGEN, D.J.J. & DUPONT-NIVET,G. MO, X.X., ZHAO, Z.D. ET AL. 2003. Response of volcanism 2014. Early Cretaceous to present latitude of the central to the India–Asia collision. Earth Science Frontiers, 10, proto-Tibetan Plateau: A paleomagnetic synthesis with 135–148 [in Chinese with English abstract]. implications for Cenozoic tectonics, paleogeography, MO, X.X., HOU, Z.Q., NIU, Y.L., DONG, G.C., QU, X.M., and climate of Asia. In:NIE, J., HORTON, B.K. & ZHAO, Z.D. & YANG, Z.M. 2007. Mantle contributions HOKE, G.D. (eds) Toward an Improved Understanding to crustal thickening during continental collision: evi- of Uplift Mechanisms and the Elevation History of the dence from Cenozoic igneous rocks in southern Tibet. Tibetan Plateau. Geological Society of America, Spe- Lithos, 96, 225–242. cial Papers, 507,1–21. MO, X.X., NIU, Y.L., DONG, G.C., ZHAO, Z.D., HOU, Z.Q., LIU, A.L., WANG,Q.ET AL. 2018. Origin of the c. 50 Ma ZHOU,S.&KE, S. 2008. Contribution of syncollisional Linzizong shoshonitic volcanic rocks in the eastern felsic magmatism to continental crust growth: a case Gangdese arc, southern Tibet. Lithos, 304–307, study of the Paleocene Linzizong Volcanic Succession 374–387. in southern Tibet. Chemical Geology, 250,49–67. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

MOLNAR,P.&STOCK, J.M. 2009. Slowing of India’s con- REPLUMAZ, A., KARASON, H., VAN DER HILST, R.D., BESSE,J. vergence with Eurasia since 20 Ma and its implications &TAPPONNIER, P. 2004. 4-D evolution of SE Asia’s for Tibetan mantle dynamics. Tectonics, 28, TC3001, mantle from geological reconstructions and seismic https://doi.org/10.1029/2008TC002271 tomography. Earth and Planetary Science Letters, MOLNAR,P.&TAPPONNIER, P. 1975. Cenozoic tectonics of 221, 103–115. Asia: effects of a continental collision. Science, 189, REPLUMAZ, A., NEGREDO, A.M., VILLASEÑOR,A.&GUILLOT,S. 419–426. 2010. Indian continental subduction and slab break-off MURPHY, M.A. & YIN, A. 2003. Structural evolution and during Tertiary collision. Terra Nova, 22, 290–296. sequence of thrusting in the Tethyan fold-thrust belt ROWLEY, D.B. 1996. Age of initiation of collision between and Indus-Yalu suture zone, southwest Tibet. Geologi- India and Asia: a review of stratigraphic data. Earth and cal Society of America Bulletin, 115,21–34. Planetary Science Letters, 145,1–13. NÁBĚLEK, J., HETENYI,G.ET AL. 2009. Underplating in the SCHÄRER, U., HAMET,J.&ALLÈGRE, C.J. 1984. The Transhi- Himalaya–Tibet collision zone revealed by the malaya Gangdese plutonism in the Ladakh region: a Hi-CLIMB experiment. Science, 325, 1371–1374. U–Pb and Rb–Sr study. Earth and Planetary Science NAIR,R.&CHACKO, T. 2008. Role of oceanic plateaus in the Letters, 67, 327–339. initiation of subduction and origin of continental crust. SCHELLART, W.P. 2004. Quantifying the net slab pull force Geology, 36, 583–586, https://doi.org/10.1130/ as a driving mechanism for plate tectonics. Geophysical G24773A.1 Research Letters, 31, L07611, https://doi.org/10. NEGREDO, A.M., REPLUMAZ, A., VILLASENOR,A.&GUILLOT, 1029/2004GL019528 S. 2007. Modeling the evolution of continental subduc- SEARLE, M.P. 2018. Timing of subduction initiation, arc for- tion processes in the Pamir–Hindu Kush region. Earth mation, ophiolite obduction and India–Asia collision in and Planetary Science Letters, 259, 212–225. the Himalaya. In:TRELOAR, P.J. & SEARLE, M.P. (eds) NIU, Y.L. 2014. Geological understanding of plate tec- Himalayan Tectonics: A Modern Synthesis. Geological tonics: basic concepts, illustrations, examples and new Society, London, Special Publications, 483, https:// perspectives. Global Tectonics and Metallogeny, 10, doi.org/10.1144/SP483.8 23–46. SEARLE, M.P., ELLIOTT, J.R., PHILLIPS, R.J. & CHUNG, S.L. NIU, Y.L. 2017. Slab breakoff: a causal mechanism or pure 2011. Crustal lithosphere structure and continental convenience? Science Bulletin, 62, 456–461. extrusion of Tibet. Journal of the Geological Society, NIU, Y.L., O’HARA, M.J. & PEARCE, J.A. 2003. Initiation London, 168, 633–672, https://doi.org/10.1144/ of subduction zones as a consequence of lateral com- 0016-76492010-139 positional buoyancy contrast within the lithosphere: SENGÖR, A.M.C. 1979. Mid-Mesozoic closure of Permo- a petrologic perspective. Journal of Petrology, 44, Triassic Tethys and its implications. Nature, 279, 851–866. 590–593. ORME, D.A., CARRAPA,B.&KAPP, P. 2015. Sedimentology, SETON, M., MÜLLER, R.D. ET AL. 2012. Global continental provenance and geochronology of the upper Creta- and ocean basin reconstructions since 200 Ma. Earth- ceous–lower Eocene western Xigaze forearc basin, Science Reviews, 113, 212–270, https://doi.org/10. southern Tibet. Basin Research, 27, 387–411. 1016/j.earscirev.2012.03.002 PAN, G.T., DING, J., YAO, D.S. & WANG, L.Q. 2004. STERN, R.J. 2002. Subduction zones. Reviews of Geophys- Guidebook of 1:1 500 000 Geologic Map of the ics, 40, 3-1–3-38, https://doi.org/10.1029/2001R Qinghai–Xizang (Tibet) Plateau and Adjacent Areas. G000108 Cartographic Publishing House, Chengdu, China. ST-ONGE, M.R., RAYNER,N.&SEARLE, M.P. 2010. Zircon PATRIAT,P.&ACHACHE, J. 1984. India–Eurasia collision age determinations for the Ladakh batholith at Chuma- chronology has implications for crustal shortening thang (northwest India): implications for the age of the and driving mechanism of plates. Nature, 311, India–Asia collision in the Ladakh Himalaya. Tectono- 615–621, https://doi.org/10.1038/311615a0 physics, 495, 171–183. PEARCE, J.A. 1982. Trace element characteristics of THORKELSON, D.J. 1996. Subduction of diverging plates and lavas from destructive plate boundaries. In:THORPE, the principles of slab window formation. Tectonophy- R.S. (ed.) Andesites, Orogenic Andesites and Related sics, 255,47–63. Rocks. John Wiley, New York, 525–548. TRUMBULL, R.B., RILLER, U., ONCKEN, O., SCHEUBER, E., PEARCE, J.A. & MEI, H.J. 1988. Volcanic rocks of the 1985 MUNIER,K.&HONGN, F. 2006. The time–space distribu- Tibet Geotraverse: Lhasa to Golmud. Philosophical tion of Cenozoic volcanism in the south-central Andes: Transactions of the Royal Society of London A: Mathe- a new data compilation and some tectonic implications. matical, Physical and Engineering Sciences, 327, In:ONCKEN, P., CHONG,G.ET AL. (eds) The Andes: Active 169–201. Subduction Orogeny. Springer, Berlin, 29–43, https:// PUTIRKA, K., MIKAELIAN, H., RYERSON,F.&SHAW, H. 2003. doi.org/10.1007/978-3-540-48684-8_2 New clinopyroxene–liquid thermobarometers for VAN DER VOO, R., SPAKMAN,W.&BIJWAARD, H. 1999. maﬁc, evolved, and volatile-bearing lava compositions, Tethyan subducted slabs under India. Earth and Plane- with applications to lavas from Tibet and the Snake tary Science Letters, 171,7–20. River Plain, Idaho. American Mineralogist, 88, VAN DE ZEDDE, D.M.A. & WORTEL, M.J.R. 2001. Shallow 1542–1554. slab detachment as a transient source of heat at midli- QUIDELLEUR, X., GROVE,M.,LOVERA, O.M., HARRISON,T.M. thospheric depths. Tectonics, 20, 868–882. &YIN, A. 1997. Thermal evolution and slip history of VAN HINSBERGEN, D.J.J., STEINBERGER, B., DOUBROVINE,P.& the Renbu Zedong Thrust, southeastern Tibet. Journal GASSMÖLLER, R. 2011. Acceleration–deceleration of Geophysical Research: Solid Earth, 102,2659–2679. cycles of India–Asia convergence: roles of mantle Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

GANGDESE MAGMATISM IN SOUTHERN TIBET

plumes and continental collision. Journal of Geophysi- of India and Eurasia? Australian Journal Earth Sci- cal Research: Solid Earth, 116, B06101, https://doi. ences, 59, 321–340. org/10.1029/2010JB008051 WU, F.Y., JI, W.Q., LIU, C.Z. & CHUNG, S.L. 2010. VAN HINSBERGEN, D.J.J., LIPPERT, P.C., DUPONT-NIVET, G., Detrital zircon U–Pb and Hf isotopic data from the MCQUARRIE, N., DOUBROVINE, P.V., SPAKMAN,W.& Xigaze fore-arc basin: constraints on Transhimalayan TORSVIK, T.H. 2012. Greater India Basin hypothesis magmatic evolution in southern Tibet. Chemical Geol- and a two-stage Cenozoic collision between India and ogy, 271,13–25, https://doi.org/10.1016/j.chemgeo. Asia. Proceedings of the National Academy of Sciences 2009.12.007 of the United States of America, 109, 7659–7664, XIONG, Q., GRIFFIN, W.L., ZHENG, J.P., O’REILLY, S.Y., PEAR- https://doi.org/10.1073/pnas.1117262109 SON, N.J., XU,B.&BELOUSOVA, E.A. 2016. Southward VAN HINSBERGEN,D.J.J., LIPPERT,P.C.,LI, S.H., HUANG, trench migration at c. 130–120 Ma caused accretion of W.T., ADVOKAAT, E.L. & SPAKMAN, W. 2018. Recon- the Neo-Tethyan forearc lithosphere in Tibetan ophio- structing Greater India: paleogeographic, kinematic, lites. Earth and Planetary Science Letters, 438, and geodynamic perspectives. Tectonophysics, https:// 57–65, https://doi.org/10.1016/j.epsl.2016.01.014 doi.org/10.1016/j.tecto.2018.04.006, in press. XU, J., ZHENG, Y.Y., SUN,X.&SHEN, Y.H. 2016. Alteration VAN HUNEN,J.&ALLEN, M. 2011. Continental collision and and mineralization at the Zhibula Cu skarn deposit, slab break-off: a comparison of 3-D numerical models Gangdese belt, Tibet. Ore Geology Reviews, 75, with observations. Earth and Planetary Science Letters, 304–326. 302,27–37. XU, R.H., SCHÄRER,U.&ALLÈGRE, C.J. 1985. Magmatism VAN HUNEN,J.&MILLER, M.S. 2015. Collisional processes and metamorphism in the Lhasa Block (Tibet): a geo- and links to episodic changes in subduction zones. Ele- chronological study. The Journal of Geology, 93, ments, 11, 119–124. 41–57, https://doi.org/10.1086/628918 VON BLANCKENBURG,F.&DAVIES, J.H. 1995. Slab breakoff: XU, W.C., ZHANG, H.F., LUO, B.J., GUO,L.&YANG,H. a model for syncollisional magmatism and tectonics in 2015. Adakite-like geochemical signature produced the Alps. Tectonics, 14, 120–131, https://doi.org/10. by amphibole-dominated fractionation of arc magmas: 1029/94TC02051 an example from the Late Cretaceous magmatism in WANG, C., DING, L., ZHANG, L.Y., KAPP, P., PULLEN,A.& Gangdese belt, south Tibet. Lithos, 232, 197–210. YUE, Y.H. 2016. Petrogenesis of Middle–Late Triassic YANG, Z.M., HOU, Z.Q. ET AL. 2016. Cospatial Eocene and volcanic rocks from the Gangdese belt, southern Lhasa Miocene granitoids from the Jiru Cu deposit in Tibet: terrane: implications for early subduction of Neo- petrogenesis and implications for the formation of col- Tethyan oceanic lithosphere. Lithos, 262, 320–333, lisional and postcollisional porphyry Cu systems in https://doi.org/10.1016/j.lithos.2016.07.021 continental collision zones. Lithos, 245, 243–257. WANG, L., ZENG, L.S., GAO,L.&CHEN, Z.Y. 2013. Early YIN,A.&HARRISON, T.M. 2000. Geologic evolution of the Cretaceous high Mg# and high Sr/Y clinopyroxene- Himalayan–Tibetan orogen. Annual Review of Earth bearing diorite in the southeast Gangdese batholith, and Planetary Sciences, 28, 211–280, https://doi. Southern Tibet. Acta Petrologica Sinica, 29, org/10.1146/annurev.earth.28.1.211 1977–1994 [in Chinese with English abstract]. YIN, A., HARRISON, T.M., RYERSON, F.J., WENJI, C., KIDD, WANG, L.Y., GAO, S.B., LI, W.L., MAO, R.W. & HUANG, W.S.F. & COPELAND, P. 1994. Tertiary structural L.L. 2016. The discovery of the Early Cretaceous Zen- evolution of the Gangdese Thrust System, southeast- ong Group volcanic rocks and geological significance ern Tibet. Journal of Geophysical Research: Solid in Jiwa area in south of the Central Lhasa subterrane. Earth, 99, 18 175–18 201, https://doi.org/10.1029/ Acta Petrologica Sinica, 32, 1543–1555 [in Chinese 94JB00504 with English abstract]. YIN, A., HARRISON, T.M. ET AL. 1999. Tertiary deformation WANG, Q., ZHU, D.C. ET AL. 2015. Eocene magmatic history in southeastern and southwestern Tibet during processes and crustal thickening in southern Tibet: the Indo-Asian collision. Geological Society of America insights from strongly fractionated c. 43 Ma granites Bulletin, 111, 1644–1664. 40 39 in the western Gangdese Batholith. Lithos, 239, YUE, Y.H. & DING, L. 2006. Ar/ Ar geochronology, 128–141. geochemical characteristics and genesis of the Linzhou WATSON, E.B. & HARRISON, T.M. 1983. Zircon saturation basin dikes, Tibet. Acta Petrologica Sinica, 22, revisited: temperature and composition effects in a vari- 855–866 [in Chinese with English abstract]. ety of crustal magma types. Earth and Planetary Sci- ZENG, L.S., GAO, L.E., XIE, K.J. & ZENG, J.L. 2011. Mid- ence Letters, 64, 295–304, https://doi.org/10.1016/ Eocene high Sr/Y granites in the Northern Himalayan 0012-821X(83)90211-X Gneiss Domes: melting thickened lower continental WEN, D.R., CHUNG, S.L. ET AL. 2008. Late Cretaceous crust. Earth and Planetary Science Letters, 303, Gangdese intrusions of adakitic geochemical character- 251–266. istics, SE Tibet: petrogenesis and tectonic implications. ZHANG, Z.M., ZHAO, G.C., SANTOSH, M., WANG, J.L., DONG, Lithos, 105,1–11. X. & SHEN, K. 2010. Late Cretaceous charnockite with WHITE, L.T., AHMAD, T., IRELAND, T.R., LISTER, G.S. & FOR- adakitic affinities from the Gangdese Batholith, south- STER, M.A. 2011. Deconvolving episodic age spectra eastern Tibet: evidence for Neo-Tethyan mid-ocean from zircons of the Ladakh Batholith, northwest Indian ridge subduction? Gondwana Research, 17, 615–631. Himalaya. Chemical Geology, 289, 179–196. ZHANG, Z.M., SHEN, K., SANTOSH,M.&DONG, X. 2011. WHITE, L.T., AHMAD, T., LISTER, G.S., IRELAND, T.R. & FOR- High density carbonic fluids in a slab window: evidence STER, M.A. 2012. Is the switch from I- to S-type magma- from the Gangdese charnockite, Lhasa terrane, southern tism in the Himalayan Orogen indicative of the collision Tibet. Journal of Asian Earth Sciences, 42, 515–524. Downloaded from http://sp.lyellcollection.org/ by guest on October 24, 2018

D.-C. ZHU ET AL.

ZHAO, L., PAUL,A.ET AL. 2016. Continuity of the Alpine Belt, southern Tibet: products of slab melting and sub- slab unraveled by high-resolution P-wave tomography. sequent melt–peridotite interaction? Journal of Asian Journal of Geophysical Research: Solid Earth, 121, Earth Sciences, 34, 298–309, https://doi.org/10. 8720–8737. 1016/j.jseaes.2008.05.003 ZHAO, L., XU, X.B. & MALUSÀ, M.G. 2017. Seismic probing ZHU, D.C., ZHAO, Z.D. ET AL. 2011. The Lhasa Terrane: of continental subduction zones. Journal of Asian Earth record of a microcontinent and its histories of drift Sciences, 145,37–45. and growth. Earth and Planetary Science Letters, ZHENG, H., LI, T., GAO, R., ZHAO,D.&HE, R. 2007. Tele- 301, 241–255, https://doi.org/10.1016/j.epsl.2010. seismic P-wave tomography evidence for the Indian 11.005 lithospheric mantle subducting northward beneath the ZHU, D.C., ZHAO, Z.D., NIU, Y.L., DILEK, Y., HOU, Z.Q. & Qiangtang terrane. Chinese Journal of Geophysics, MO, X.X. 2013. The origin and pre-Cenozoic evolution 50, 1418–1426. of the Tibetan Plateau. Gondwana Research, 23, ZHENG, Y.F., CHEN, Y.X., DAI, L.Q. & ZHAO, Z.F. 2015. 1429–1454, https://doi.org/10.1016/j.gr.2012.02.002 Developing plate tectonics theory from oceanic subduc- ZHU, D.C., WANG,Q.ET AL. 2015. Magmatic record of tion zones to collisional orogens. Science in China India–Asia collision. Scientiﬁc Reports, 5, 14289 Series D: Earth Sciences, 58, 1045–1069. https://doi.org/10.1038/srep14289 ZHOU, S., MO, X.X., DONG, G.C., ZHAO, Z.D., QIU, R.Z., ZHU, D.C., LI, S.M., CAWOOD, P.A., WANG, Q., ZHAO, Z.D., 40 39 WANG, L.L. & GUO, T.Y. 2004. Ar– Ar geochro- LIU, S.A. & WANG, L.Q. 2016a. Assembly of the Lhasa nology of Cenozoic Linzizong volcanic rocks from and Qiangtang terranes in central Tibet by divergent Linzhou Basin, Tibet, China and their geological impli- double subduction. Lithos, 245,7–17. cations. Chinese Science Bulletin, 49, 1970–1979 ZHU, D.C., CHUNG, S.L. & NIU, Y.L. 2016b. Recent [in Chinese]. advances on the tectonic and magmatic evolution of ZHU, D.C., CHUNG, S.L., MO, X.X., ZHAO, Z.D., NIU, Y.L., the Greater Tibetan Plateau: a special issue in honor SONG,B.&YANG, Y.H. 2009a. The 132 Ma Comei– of Prof. Guitang Pan. Lithos, 245,1–6. Bunbury large igneous province: Remnants identiﬁed ZHU, D.C., WANG, Q., CAWOOD, P.A., ZHAO, Z.D. & MO, in present-day SE Tibet and SW Australia. Geology, X.X. 2017a. Raising the Gangdese Mountains in 37, 583–586, https://doi.org/10.1130/G30001A.1 southern Tibet. Journal of Geophysical Research: ZHU, D.C., MO, X.X., NIU, Y.L., ZHAO, Z.D., WANG, L.Q., Solid Earth, 122, 214–223, https://doi.org/10.1002/ LIU, Y.S. & WU, F.Y. 2009b. Geochemical investiga- 2016JB013508 tion of Early Cretaceous igneous rocks along an east– ZHU, D.C., WANG,Q.&ZHAO,Z.D.2017b. Constraining west traverse throughout the central Lhasa Terrane, quantitatively the timing and process of continent–conti- Tibet. Chemical Geology, 268, 298–312. nent collision using magmatic record: method and exam- ZHU, D.C., ZHAO, Z.D. ET AL. 2009c. Early Cretaceous ples. Science China Earth Sciences, 60, 1040–1056, subduction-related adakite-like rocks of the Gangdese https://doi.org/10.1007/s11430-016-9041-x