Eocene–Oligocene granitoids in southern Tibet Constraints ...

Earth and Planetary Science Letters 349-350 (2012) 38–52

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Letters

Eocene–Oligocene granitoids in southern Tibet: Constraints on crustalanatexis and tectonic evolution of the Himalayan orogen

Zeng-Qian Hou n, Yuan-Chuan Zheng, Ling-Sen Zeng, Li-E Gao, Ke-Xian Huang, Wei Li,Qiu-Yun Li, Qiang Fu, Wei Liang, Qing-Zhong Sun

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China

a r t i c l e i n f o

Article history:

Received 5 November 2011

Received in revised form

12 June 2012

Accepted 13 June 2012

Keywords:

U–Pb zircon dating

Ar–Ar dating

Eocene–Oligocene granitoids

exhumation

Himalayan orogen

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esponding author. Tel.: þ86 1088364366.

ail address: [email protected] (Z.-Q. Hou

a b s t r a c t

Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline

complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-

skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the

Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of

these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms

involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting

event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–

Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD).

U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from �46 Ma (in

Lhunze) to �30 Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46 Ma granitoids

constrains the timing of shearing to �39–41 Ma.

The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan

Himalaya, with minor �30 Ma granodiorites along the IYS and �35 Ma granites in the Yelaxiangbo

dome, where Indian mid-crustal rocks are exposed. The �35 Ma granites are characterized by variable

Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb

(11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya.

Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites

exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the

amphibolites at �880 1C and �10 kbar. The �30 Ma granodiorites and �42–46 Ma two-mica granites

are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a

much wider range in eNd(t) of –14.9 to –2.5 and (87Sr/86Sr)i of 0.7062–0.7188, and have a Nd isotopic

model age of 1486–1978 Ma, indicating that these magmas were derived from a thickened Indian lower

crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by

the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the

Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in

which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural,

metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis

and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as

slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the

subducted Indian crust during Indo-Asia collision at 55–40 Ma.

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1. Introduction

The Himalaya mountain system, which is the largest activecollisional orogen on Earth, was initiated by India–Asia collisionat �55–50 Ma or earlier (Yin, 2006 and references herein);however, growth mechanisms within the orogen remain uncertain.

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).

The Himalaya collisional orogen has been subjected to intenseCenozoic deformation, high-grade metamorphism, and widespreadsyncollisional anatexis (cf. Le Fort, 1996; Yin, 2006), which has ledto many uncertainties about the tectonic evolution of the orogen.More recent debate has focused on mass exchange between theHimalayan (Indian plate) and Tibetan crust (Asian plate) duringcollision (Yin et al., 2010a, b), and on the origin of the GreaterHimalayan crystalline complex (GHC), a high-grade metamorphicunit that forms the core of the Himalaya (Fig. 1A). Four potentialmodels for the origin of the GHC and the formation of the Himalaya

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Fig. 1. Simplified geological map of southern Tibet, showing the location of the

main tectonic units, gneiss domes, and Eocene–Oligocene granitoids (A);

(B) shows the tectonic framework of the central Himalayan orogen, and

(C) shows an idealized cross-section through southern Tibet. GTS¼Gangdese

thrust system, GCT¼Gangdese central thrust, THTB¼Tethyan Himalayan thrust-

ford belt, STD¼south Tibetan detachment, MCT¼main central thrust, and

MHT¼main Himalayan thrust. Stars and numbers indicate sampling locations

and U–Pb ages of granitoids determined in the present study, from south to north,

1–Yangxiong, 2–Quedang, 3–Dala, 4–Yelaxiangbo, 5–Yelaxiangbo granite, and

6–Zedong, respectively. Biotites 40Ar/39Ar plateau ages of the Eocene granitoids

in the THTB (Fig. 1A) are from Pan et al. (2004).

Z.-Q. Hou et al. / Earth and Planetary Science Letters 349-350 (2012) 38–52 39

have been proposed: (1) Indian basement-involved thick-skinnedthrusting (Le Fort, 1975; Yin, 2006; Yin et al., 2010a), (2) an exoticterrane-involved thin-skinned thrusting (DeCelles et al., 2000, 2001,2002; Robinson et al., 2006), (3) large-scale horizontal channel flowof the Tibetan middle crust (Nelson et al., 1996; Beaumont et al.,2001, 2004; Searle et al., 2003; Godin et al., 2006), and (4) wedgeextrusion and exhumation of a slice of subducted Indian crustduring India–Asia collision (Chemenda et al., 1995, 1996, 2000).

Crust-derived granitoids can be generated by the distinctivedynamic processes that each of these above models predicts, butthe timing, spatial relationships, source region, and dynamicmechanisms involved in crustal anatexis differ among the models(Harris and Massey, 1994; Thompson and Connolly, 1995;Harrison et al., 1997; Searle and Szluc, 2005; Harrison, 2006;King et al., 2007). Thus, constraining the onset and duration ofregional melting under southern Tibet, and determining thespatial distribution and source region of anatectic magmas duringcollision, is a crucial test of the validity of these models. All ofthese variables can be constrained by studying the granitoidsproduced during India–Asia collision and that intruded varioustectonic units in southern Tibet (Fig. 1).

Miocene granites within the parallel high Himalayan leuco-granite (HHL) and the North Himalayan granite (NHG) belts havecontrasting ages, petrogenetic histories, and emplacement styles(Harrison et al., 1997 and references therein). In addition, recentstudies have reported an Eocene (�42–44 Ma) regional meltingevent that formed two-mica granites in southern Tibet (Aikman,2007; Aikman et al., 2008, 2012a, b; Qi et al., 2008; Gao, 2009;Zeng et al., 2011). These studies promote an understanding ofcrustal anatexis related to continental collision, but a genetic linkbetween crustal anatexis and the growth mechanisms involved inHimalayan orogenesis has not been well established.

In order to explore the relationship between crustal anatexisand Himalayan orogenic processes, and to critically compare thepredictions of each of the models mentioned above with direct

observations, a NW–SE-striking, 200-km-long tectonic traversethrough the Indus–Yarlung suture (IYS), the north Himalayanantiform (NHA), the Tethyan Himalayan thrust–fold belt, and thesouth Tibetan detachment system terranes (STD; Fig. 1), wasundertaken during this study, combined with an analysis ofexisting data (e.g., Zeng et al., 2011). Along this traverse, repre-sentative samples of granitoids, exposed within the IYS, the NHA,and the THS (Fig. 1), were taken for age dating and geochemicalanalysis to further constrain the temporal and spatial distribution,geochemical variation, and possible origin of these granitoids.This paper presents new ion microprobe U–Pb zircon and biotite40Ar/39Ar dates, major and trace element compositions, and theSr–Nd isotope systematics of granitoids from this traverse. Theseresults constrain the onset and duration of crust-derived mag-matism and the origin of Eocene–Oligocene granitoids of southernTibet, and provide new insights into the exhumation of high-pressure metamorphic rocks and the development of the Hima-layan orogen.

2. Tectonic setting and structural geology

2.1. Major tectono-stratigraphic units

The Himalayan orogen can be loosely divided into threetectono-stratigraphic units, all juxtaposed by orogen-scale faultsthat are roughly parallel with the trend of the orogen (Fig. 1A;Gansser, 1964; Le Fort, 1975, 1996; Yin and Harrison, 2000;DiPietro and Pogue, 2004). From north to south, in decreasingstratigraphic height, these are: the Tethyan Himalayan sedimen-tary sequences (THS), a deformed package of predominantlyPaleoproterozoic to Eocene metasediments bounded by the IYSand STD; the Greater Himalayan crystalline complex (GHC), ahigh-grade metasedimentary sequence that was exhumedbetween the STD and the main central thrust (MCT); and thelesser Himalayan series (LHS), a sequence of dominantly silici-clastic rocks bounded by the MCT and main boundary thrust(MBT) (Fig. 1A; Le Fort, 1975, 1996; Yin, 2006).

The THS extends for 41500 km along the Himalayan range,and consists of Late Precambrian to Early Paleozoic metasedi-mentary rocks and thick Permian to Upper Cretaceous passiveIndian continental margin sequences (Burchfiel et al., 1992;Garzanti, 1999). From north to south (Fig. 1), the THS consistsof a deformed, broadly southward-younging, Triassic to Cretac-eous sequence of predominantly low-grade metasediments(Aikman et al., 2008), which varies southward from thick Triassicclastic-dominated sediments to Cretaceous marine clastics andcarbonate platform deposits. The GHC consists of Paleoprotero-zoic to Ordovician high-grade metamorphic rocks, and forms analmost continuous belt along the Himalayan range, barringisolated patches (i.e., gneiss domes) along the NHA (Yin, 2006).The GHC in southernmost Tibet has a broadly similar structural,metamorphic, and intrusive history to that the Yelaxiangbo gneissdome that outcrops in the NHA (Fig. 1B; cf. GSY, 2004), suggestingthat the THS is underlain by the GHC in southern Tibet.

The IYS consists of Mesozoic ophiolite and melange relics ofthe Neo-Tethyan Ocean. It was modified around 30 Ma by thenorth-dipping Gangdese thrust system (GTS) along the southernmargin of the Lhasa terrane (Yin et al., 1994; Harrison et al.,2000), and before �14 Ma by the south-dipping overthrustingGangdese central thrust system (GCT) and the Renbu–Zedongthrust (RZT; Yin et al., 1994; Fig. 1B). The IYS juxtaposed the Lhasaterrane (Asian plate) to the north with the southern THS (Indianplate; Fig. 1B).

The south Tibetan detachment (STD) is a north-dipping, low-angle normal-fault system (Burg and Chen, 1984; Burchfiel et al.,

Z.-Q. Hou et al. / Earth and Planetary Science Letters 349-350 (2012) 38–5240

1992) that was active at 23–12 Ma (Scharer et al., 1986; Noble andSearle, 1995; Hodges et al., 1996, Hodges and Hurtado, 1998; Searleet al., 1997; Murphy and Harrison, 1999). Movement along the STDjuxtaposed weakly metamorphosed THS against the GHC (Fig. 1B;Burg and Chen, 1984; Burchfiel et al., 1992; Edwards et al., 1996;Hodges et al., 1996). Well-developed shear-sense indicators inmylonites beneath the STD indicate northeastward or northwest-ward displacement of the hangingwall (e.g., Burchfiel et al., 1992)with a minimum displacement of 35–40 km (Hodges, 2000). Boththe MCT and MBT appear to sole into a common north-dippingdetachment, the main Himalayan thrust (MHT; Zhao et al., 1993),indicating that the main part of the Indian continent was thrustbeneath the Himalayas along this structure (Fig. 1C).

2.2. The Tethyan Himalayan fold–thrust belt

The Tethyan Himalayan fold–thrust belt of south Tibet isbounded by the IYS and the STD. Contractional structures in thebelt are dominated by tight folding and brittle thrusting of Jurassicand Cretaceous strata (cf. Pan et al., 2004). At least two phases ofregional deformation events are recognized in the belt (cf. Aikman,2007; Aikman et al., 2008, 2012a, b). The first comprises S–Nshortening with initial deformation at �50 Ma (Ratschbacheret al., 1994), leading to the formation of a series of W–E-trendingupright or south-vergent folds. The second phase of deformation ischaracterized by S–N shortening, but is associated with displace-ment along numerous S–N-striking thrust faults (e.g., the LhenzeThrust; Fig. 1B) that accommodated 2 km of vertical displacement(Quigley et al., 2006). The total amount of crustal shortening duringthese two phases of deformation is estimated to be more than 130–140 km (Ratschbacher et al., 1994).

2.3. North Himalayan antiform (NHA) and the Yelaxiangbo gneiss

dome

The NHA is a prominent east-trending structural high withassociated gneiss domes that is bounded to the north by the RZTand to the south by the STD (Hauck et al., 1998). The antiformformed in the mid Miocene (cf. Lee et al., 2000) with NHA-relateddeformation superimposed on the Tethyan Himalayan fold–thrustbelt (Fig. 1A; Yin, 2006). At least eight gneiss domes (e.g., theMalashan, Sakya-Kuday, Kangmar, and Yelaxiangbo domes) havebeen recognized in the NHA (Fig. 1A). These gneiss domes are in faultcontact with the overlying THS (Burg and Chen, 1984; Debon et al.,1986; Scharer et al., 1986; Steck et al., 1998). The Indian continentalrocks and high-grade metamorphic rocks, underwent ductile exten-sion during normal slip along the STD (Chen et al., 1990; Aoya et al.,2005; Lee et al., 2006; Lee and Whitehouse, 2007) were exposed inthese domes. Most of these gneiss domes were intruded by Cenozoictwo-mica granites, leucogranites, and a leucocratic dike swarm (Burget al., 1984; Debon et al., 1986) during 28–12 Ma (Scharer et al.,1986; Hodges et al., 1996; Edwards and Harrison, 1997; Harrisonet al., 1997; Searle et al., 1999; Zhang et al., 2004).

The Yelaxiangbo dome in the easternmost NHA of southern Tibetis exposed in a N–S-trending 60�25 km elliptical area surroundedby the overlying THS (Fig. 1B). This dome is capped by normal faultsto both north and south or by outward detachment faults aroundthe core of the dome (Fig. 1B; Zeng et al., 2011). The core of thedome is formed of migmatitic augen biotite orthogneisses andgarnet amphibolites with subordinate pyroxenites, mantled byhigh-grade garnet-bearing metapelites, garnet-graphite schists withU–Pb zircon ages 4500 Ma (Gao, 2009), and THS sediments.At least two types of amphibolite pods have been recognized:(1) layered garnet amphibolite (or retrograde eclogite) in the centerof the dome, with layers subparallel to metapelite and orthogneissfoliations; and (2) lens-like amphibolite within intensively deformed

garnet–graphite schist (Zeng et al., 2011). The former consists ofvariable amounts of garnet, hornblende, plagioclase, and biotite, andthe latter has a similar mineralogy but without garnet. Peakmetamorphic conditions for the garnet amphibolites are 872–892 1C and 9.7–11.3 kbar (Gao, 2009; Zeng et al., 2011), withSHRIMP U–Pb dating of metamorphic zircons indicating a meta-morphic age of �45 Ma (Zeng et al., 2011). The orthogneisses andgarnet-schists underwent multiphase metamorphism, as indicatedby U–Pb dates from metamorphic zircons of 47–37 Ma (Gao, 2009).As mentioned above, the metamorphic rocks in the dome are,barring amphibolites in the core of the dome, similar to those ofthe GHC. Unlike other domes, the Yelaxiangbo dome was intrudedby �35 Ma granite dike swarms along detachment faults (Gao,2009) and by �42–44 Ma two-mica granite plutons which intrudedinto orthogneisses and metasedimentary rocks (Fig. 1B; Aikman,2007; Aikman et al., 2008, 2012a, b; Zeng et al., 2011).

3. Eocene–Oligocene magmatism in southern Tibet

Unlike the Miocene granites that form the east–west-trendingHHL and NHG, recently reported Eocene granitoids (Aikman,2007; Aikman et al., 2008, 2012a; Qi et al., 2008; Zeng et al.,2011) are restricted to the Yelaxiangbo dome and surroundingareas of south Tibet. Our field investigations indicate that thisregional melting event forms a nearly N–S-trending Eocene–Oligocene granitoid belt, approximately orthogonal to both theHHL and the NHG, which extends �150 km from the IYS to theSTD along the Zedong–Lhunze traverse (Fig. 1B). The granitoids inthe belt are exposed around three different tectonic units, i.e., theGTS activing at 30 Ma (Harrison et al., 2000), the Yelaxiangbodome (Zeng et al., 2011), and the Tethyan Himalayan fold–thrustbelt formed since 50 Ma (Ratschbacher et al., 1994) (Fig. 1B).

The northernmost granitoids within the belt occur as stocks orsheets along the south-directed GTS (Fig. 1B), and separate theAndean Gangdese granitoid batholiths of the Lhasa terrane fromrocks of Indian affinity. They intrude either the isoclinally foldedTHS or a remnant Jurassic–Cretaceous arc (Aitchison et al., 2000)juxtaposed over a melange in Zedong (Fig. 1B). Harrison et al. (2000)previously reported a 30-Ma Yajia granodiorite in Zedong, which isgeochemically distinct from the 65–43 Ma Paleocene–Eocene syn-collisional granitoids of the Gangdese range (Chung et al., 2009).

To the south of the GTS and IYS, at least two two-mica graniteplutons and a granite dike swarm are exposed in the Yelaxiangbodome (Fig. 1B). The intensely deformed �42 Ma (U–Pb dating ofzircon) Yelaxiangbo pluton intrudes the amphibolite-facesorthogneiss core of the dome (Zeng et al., 2011), whereas theweakly deformed �44 Ma (U–Pb dating of zircon) Dala pluton(Aikman, 2007; Aikman et al., 2008, 2012a) intrudes a greens-chist-facies mantle that surrounds garnet–graphite and garnet–quartz schists, and THS sediments (Qi et al., 2008; Zeng et al.,2011). The emplacement depth of these plutons is constrained tobetween 12 and 30 km by metamorphic pressure estimates (cf.Aikman et al., 2008). The 35 Ma (U–Pb dating of zircon; Gao,2009) undeformed granite sheets and dikes usually intrudeorthogneisses and metapelites, cross-cut the two-mica granitepluton, and occur along normal or detachment faults around theYelaxiangbo dome (Fig. 1B).

To the south, in the Tethyan Himalayan fold–thrust belt, the42.8 Ma (U–Pb dating of zircon; Zeng et al., 2011) Quedang two-mica granite sheet intrudes shales and sandstones of the THS inthe hangingwall of the Lhunze thrust fault (Fig. 1B). Thesegranitoids occasionally contain granitic enclaves and garnet-gneiss xenoliths. The southernmost group (i.e., the Yangxiongintrusion) consists of numerous intensely deformed two-micagranite stocks and associated dike swarms that were intruded


along the Lhunze thrust fault and into the strongly folded THS,20–40 km northeast of the STD (Fig. 1B).

4. Age dating of the crystallization and deformation of granitoids

To further constrain the timing of the onset and duration ofcrystallization of the Eocene–Oligocene granitoid belt of southTibet, zircon samples were separated from granitoid intrusions atChongmuda (near Yajia), Dala, Quedang, and Yangxiong (nearLhunze) for U–Pb dating (Fig. 1; Suppl. Table 1). Age of shearingdeformation is constrained by 40Ar/39Ar dating of biotites fromthe Eocene granitoids with typical evidence suggestive of defor-mation (Figs. 2 and 4; Suppl. Table 2).

4.1. Zircon U–Pb dating

A �30 Ma age for the Yajia granodiorite in Zedong wasobtained by Harrison et al. (2000) and Chung et al. (2009). Here,we present new ages of the Chongmuda granodiorite (e.g., SampleCMD-34) as dated by laser ablation-inductively coupled plasma-mass spectrometry (Appendix A). The granodiorite is lithologically

Fig. 2. Photomicrographs showing textures and mineral assemblages in Eocene–Oligoce

(B) elongate biotite and muscovite grains within mica ribbons and polygonal elongate re

pluton; (C) two-mica granite from the Quedang pluton with mica ribbon texture; (D) en

mica granite from the coarse-grained Yangxiong pluton with a mica ribbon texture

fine-grained Yangxiong pluton. Bi¼biotite, Ho¼hornblende, Kf¼K-feldspar, Mus¼mus

similar to the Yajia granodiorite, and contains euhedral to subhe-dral biotite and quartz with uniform extinction patterns (Fig. 2A),suggesting that the granodiorite is undeformed. Twenty-five ana-lyses of zircons with relatively high Th/U ratios (0.27–0.91) andwell-developed oscillatory growth zoning yield ages from28.870.7 to 31.270.9 Ma, with a mean age of 29.8270.27 Ma(95% confidence level, MSWD¼1.4; Fig. 3A).

We dated granitoids within the southern part of the granitoidbelt by SHRIMP zircon U–Pb analysis. Zircons from the Dalagranite have consistent core–rim textures. Their rims have typicaloscillatory growth zoning, whereas cores are often irregular orhave oscillatory zoning. Well-developed oscillatory growth zon-ing in zircons is indicative of a magmatic origin, although zirconsfrom the Dala granite have relatively low Th/U ratios (0.03–0.25).Areas of zircon with magmatic zoning yield a mean age of44.5970.37 Ma (MSWD¼1.2; n¼10), identical to the previouslyreported age (Aikman et al., 2008; Qi et al., 2008). However, newdating of magmatic zircons from the host rock (Sample cb-77-1)and an enclave (Sample cb-77-3) of the Quedang granitesheet give an age of �46 Ma (Fig. 3C–D; Suppl.Table 1), which is older than a previously reported U–Pb age(�43 Ma; Zeng et al., 2011). Biotite and quartz grains within the

ne granitoids of southern Tibet. (A) Granite from the Chongmuda pluton (Zedong);

crystallized quartz grains within quartz ribbons in two-mica granite from the Dala

clave of granite from the Quedang pluton with typical porphyritic texture; (E) two-

; (F) preferentially aligned biotite and muscovite in two-mica granite from the

covite, Pl¼plagioclase, Qz¼quartz, Tit¼titanite.

0.0046

0.0050

0.0048

0.0042

0.0044

0.0052

0.020 0.024 0.028 0.032 0.036 0.040 0.044 0.02 0.03 0.04 0.05 0.06 0.070.0060

0.0070

0.0080

0.0090

0.0360.0065

0.0520.0480.044

0.0079

0.040

0.0077

0.0075

0.0073

0.0071

0.0069

0.0067

0.01 0.090.070.050.030.0040

0.0140

0.0120

0.0100

0.0080

0.0060

0.032 0.036 0.040 0.044 0.048 0.052 0.056 0.0600.0062

0.0066

0.0070

0.0074

0.0078

0.0082

0.0086 0.0076

0.0072

0.0068

0.0064

0.0060

0.0056

0.00520.00 0.02 0.04 0.06

Fig. 3. U–Pb concordia diagrams for Eocene–Oligocene granitoids of southern Tibet. U–Pb ages are shown for (A) the Chongmuda granite (Zedong), (B) the Dala granite,

(C) the Quedang granite, (D) an enclave from the Quedang granite, (E) the coarse-grained Yangxiong granite, and (F) the fine-grained Yanxiong granite. Details of the

analytical techniques using during zircon U–Pb dating are given in Appendix A. Cathodoluminescence (CL) images showing the texture, analysis spots, and respective ages

of SHRIMP zircon U–Pb dating are given in Suppl. Fig. 1.


host granitoids occur as ribbon-like structures (Fig. 2C), indicatingintense deformation at sub-magmatic temperatures (cf. Aikmanet al., 2008), whereas the enclaves have a porphyritic structure(Fig. 2D) with little evidence of deformation. Zircons from bothtypes of samples are euhedral to subhedral and have similar core–rim textures. Zircon cores are commonly irregular or haveoscillatory zoning, whereas rims have typical oscillatory growthzoning with low Th/U ratios (0.09–0.24 for host rocks and 0.11–0.24 for enclaves). Fifteen analyses of zircons in the host-rockSample cb-77-1 and 14 analyses of zircons in the enclaveSample cb-77-2 give 206Pb/238U ages between 45.070.7 and

48.171.0 Ma with a mean age of 46.2870.50 Ma (MSWD¼1.4),and between 43.271.7 and 47.070.8 Ma with a mean age of45.4370.41 Ma (MSWD¼1.1), respectively (Fig. 3C–D). Althoughboth host rocks and enclaves underwent variable deformation,their U–Pb ages are within error. As it is unlikely that deformationprocesses intensely change crystallization ages, crystallization ofthe Quedang granitoids and enclaves took place at �46 Ma. Twofactors indicate that these results provide an accurate estimate ofthe timing of crystallization of the Quedang granitoid, relative toprevious results. Firstly, 29 analyses were made of rims of zirconfrom the Quedang granitoid, almost three times more analyses

Fig. 4. a40Ar/39Ar plateau age of biotites from deformed Eocene two-mica granites

of southern Tibet. (A) Dala granite, (B) Quedang granite, (C) enclave from the

Quedang granite, (D) coarse-grained Yangxiong granite, (E) fine-grained Yang-

xiong granite. Details of the analytical techniques used during 40Ar/39Ar dating are

given in Appendix A.


than undertaken by Zeng et al. (2011). Secondly, biotite 40Ar/39Arplateau ages for the Quedang granitoid are also older than thepreviously reported U–Pb age, as discussed below.

Samples of fine-grained (Sample yx-6) and coarse-grained two-mica granites (Sample yx-9) from Yangxiong in the Lhunze areawere collected for SHRIMP zircon U–Pb dating. The occurrence ofquartz with undulatory extinction and biotite in ribbon structures(Fig. 2E) indicate that both rocks underwent deformation. Zirconsfrom Sample yx-9 have core–rim textures and well-developedoscillatory growth zoning, suggesting a magmatic origin, althoughthey have relatively low Th/U ratios (0.06–0.82). These zircons yieldages from 44.470.7 Ma to 50.470.9 Ma, with a mean age of46.4970.71 Ma (MSWD¼2.5; Fig. 3E). We regard this as the ageof crystallization of the coarse-grained granite at Yangxiong. Incontrast, zircons from the fine-grained granite (Sample yx-6) aredominantly sector zoned with very weak oscillatory zoning, andgenerally lack any complex core–rim textures (Fig. 2). The relativelylow Th/U ratios (0.01–0.54), sector zoning, and the absence ofinherited cores within these zircon grains suggest they are ofmetamorphic origin. Ages for these zircons are generally youngerthan those from the coarse-grained granite (Fig. 3E). Apart from oneanalysis (yx-6–8.1), the ages are clustered, with 206Pb/238U agesfrom 37.870.9 Ma to 44.870.8 Ma, and a mean age of39.7071.10 Ma (MSWD¼2.3; Fig. 3F), identical to the �39 Ma40Ar/39Ar plateau age of biotite in the same sample (see below). Thisresult, combined with the deformation texture of the granite, leadsus to interpret this date as the age of deformation and metamorph-ism of the fine-grained Yangxiong granite.

4.2. Ar–Ar dating of biotite

The Eocene granitoids of southern Tibet have undergone variabledegrees of deformation. Quartz in these rocks occurs as ribbons(Fig. 2B) around plagioclase porphyroblasts. The average size ofquartz within the ribbons is less than 300 mm, smaller than thatoutside of the ribbons. The presence of high-angle grain boundariesand strain-free grains suggests that the quartz within these ribbonsis almost completely recrystallized (Simpson, 1985). Biotite andmuscovite grains also have a distinctive ribbon structure, subparallelto the margins of the quartz ribbons. These textures indicate intenseshearing and recrystallization.

Biotite from the Dala granite yields an 40Ar/39Ar plateau age of39.9270.28 Ma (Fig. 4A) and an isochron age of 39.7270.43 Ma,with an intercept on the 36Ar/40Ar axis corresponding to atmo-spheric values (i.e. 40Ar/36Ar¼307.276.1 Ma). The 40Ar/39Ar dat-ing of biotite from the Quedang granitoid yields a plateau age of44.5370.32 Ma, and an isochron age of 42.6570.48 Ma (Fig. 4B).Biotite from enclaves yields an identical plateau but at a slightlyyounger age of 42.6270.30 Ma, and gives an isochron age of44.3970.51 Ma (Fig. 4C).

Of all the rocks in southern Tibet, the granitoids at Yangxiongare the most deformed. Quartz in coarse-grained granite (Sampleyx-9) shows undulatory extinction, while biotite occurs in well-developed ribbons (Fig. 2E). Minerals in the fine-grained granite(Sample yx-6) have a shape-preferred orientation (Fig. 2F). Theplateau age of Sample yx-9 is 40.7270.32 Ma (Fig. 4D), and givesan isochron age of 40.9070.51 Ma, with an 40Ar/36Ar intercept of286.977.5 younger than the 206Pb/238U age determined forzircons from the same sample (46.4970.71 Ma). The plateauage of Sample yx-6 is 39.1970.42 Ma (Fig. 4E), withan isochron age of 39.4170.78 Ma, and an 40Ar/36Ar interceptof 286.4711, identical to the SHRIMP U–Pb age (39.7071.10 Ma) obtained for metamorphic zircon (Fig. 3F).

In summary, all U–Pb zircon dates for these three groups ofgranitoids (Suppl. Tables 1 and 2) indicate magmatic activity at30–46 Ma, with a clear trend in granitoid ages from �30 Ma in


the north to �46 Ma in the south (Suppl. Tables 1 and 2; Fig. 3).40Ar/39Ar ages of biotite with typical ribbon textures are usuallyyounger than the 206Pb/238U ages of magmatic zircons from thesame sample, implying that crystallization of granitic magmaswas followed by deformation at sub-magmatic temperaturesduring 39–41 Ma, barring the youngest �30 Ma granodiorites.

5. Petrology and geochemistry of granitoids

Granitoids in southern Tibet range in composition (Suppl.Table 4) from �30 Ma granodiorite (SiO2¼64.0–71.7 wt%) and�35 Ma granite (SiO2¼72.0–76.1 wt%) to Eocene two-micagranites (SiO2¼68.3–73.0 wt%) that contain plagioclase, quartz,biotite, K-feldspar, and rare garnet phenocrysts. The granodioritesat Yajia and Chongmuda, and the two-mica granites at Yelaxiangbo,Dala, Quedang, and Yangxiong, are relatively Na-rich, withNa2O/K2O ratios of 1.16–2.12, CaO rich (1.98–5.33 wt.%), high-alumina (A/CNK¼0.8–1.7), have relatively low concentrations ofNi (3.1–28.0 ppm) and Cr (6.6–55.3 ppm), and low Mg# values(0.49–0.72). In contrast, granites at Yelaxiangbo contain relativelyhigh but variable Na2O concentrations (3.77–5.39 wt%) withNa2O/K2O varying between 1.07 and 4.44, and have much lowerMgO (0.05–0.3 wt%) and CaO (0.81–1.55 wt%) concentrations(Suppl. Table 4). On a Harker diagram (Fig. 5), the majority ofthese granitoids plot along one of two distinct but relativelyindependent variation trends. Most of the Yelaxiangbo granitesplot along Trend I of the Harker diagram, with decreasing Na2Oand Al2O3 and slightly increasing MgO and CaO, whereas mostof the Eocene two-mica granites plot along Trend II, withdecreasing Na2O, Al2O3, MgO, and CaO (Fig. 5). Some granitoidsat Yelaxiangbo and Yangxiong vary between the two trends(Fig. 5), implying either mixing of two magmatic end-membersor intense assimilation of crustal materials (see below). Twodistinct trends are also present in plots of SiO2 against traceelements (Th, Rb, Sr; Fig. 5). Almost all Eocene two-mica granites,except for the fine-grained granites at Yangxiong, plot along a flattrend (trend II), where Th, Rb, and Sr concentrations are invariantwith increasing SiO2 content (Fig. 5). The Oligocene granites havesteep trends (trend I) that intersect trend II (Fig. 5), implyingdifferent origins for these two suites of granitoids.

Compared with Miocene leucogranites derived from anatexisof the GHC (Zhang et al., 2004), the Eocene–Oligocene granitoidsare significantly enriched in Ba, Ti, Sr, Zr, Hf, and light rare earthelements (LREEs), and are depleted in Cs, Rb, Nb, Ta, and Ti (Fig. 6;Suppl. Table 4). The LREE, Hf, Zr, Ti, Y, and Yb concentrations inthe �35 Ma granites are similar to those of the Miocene leuco-granites (Fig. 6; Le Fort et al., 1987; Zhang et al., 2004).

The Eocene two-mica granites have low heavy rare earthelement (HREE) and Y concentrations (5.8–13.0 ppm); this result,coupled with high values of Sr/Y (18–66) and (La/Yb)N (20–46,where N denotes normalization to Chondrite), indicates a geochem-ical affinity with adakitic rocks found in island arcs (Defant andDrummond, 1990; Peacock et al., 1994; Beate et al., 2001) andcollision zones (Atherton and Petford, 1993; Chung et al., 2003; Houet al., 2004) (Fig. 7). The �30 Ma granodiorites are also adakitic, butmore mafic, and have lower Y (3.4–12.9 ppm) concentrations, YbN

(1.4–4.5) values, and (La/Yb)N ratios (22–88), implying a garnet-stability source with relatively higher horizon of meting zone andlarger degree of melting (Defant and Drummond, 1990) (Fig. 7). Incontrast, the �35 Ma granites are more felsic, have relatively low Yconcentrations (1.96–8.51 ppm), YbN (0.5–2.7) values, and Sr/Y (14–99) and (La/Yb)N ratios (7.5–28.9), falling outside of the adakite fieldin Fig. 7.

The Oligocene granodiorites have a limited range of eNd(t) (–3.36to –2.5) and (87Sr/86Sr)i (0.7062–0.7065; Suppl. Table 5) values, close

to those of mid-Miocene porphyritic granite stocks within the Lhasaterrane (Hou et al., 2004) and a Miocene dacitic dike emplaced atSakya dome in the NHA, south of the IYS (King et al., 2007; Fig. 8).The Eocene two-mica granites have low eNd(t) (–14.9 to –8.9) andhigh (87Sr/86Sr)i (0.7137–0.7188) ratios (Fig. 8), whereas Oligocenegranites have Sr–Nd isotopic signatures similar to those of the two-mica granites, but with relatively high (87Sr/86Sr)i ratios (0.7133–0.7193), much lower than those of Miocene granites of the NHA(Zhang et al., 2004).

6. Discussion

6.1. Origin of �35 Ma granites

Several models have been proposed for the origin of Miocenegranites in the NHG and HHL: (1) fluid-saturated anatexis (Le Fortet al., 1987), (2) decompression melting under fluid-absent condi-tions (Harris and Massey, 1994; Davidson et al., 1997), and (3)melting triggered by shear heating or high heat production duringradioactive decay (Harrison et al., 1997, 1998). However, themajority of these models consider that the leucogranites with highRb/Sr (41.0) and high (87Sr/86Sr)i (40.7300) ratios were derivedfrom melting of mid- to upper-crustal metapelites during meta-morphism (Vidal et al., 1982; Debon et al., 1986; Deniel et al., 1987;Harris and Inger, 1992; Searle et al., 1997; Zhang et al., 2004).However, differences between the Oligocene granites and Mioceneleucogranites of southern Tibet, in terms of age, major and traceelement compositions, and Sr–Nd isotopic signatures, suggest differ-ing origins for these granitoids. Experimental studies demonstratethat melting of metapelites can form Na- and K-rich peraluminousmelts (Patino and Harris, 1998), similar to the Yelaxiangbo granite,with Na/K ratios between 1.07 and 4.44. However, metapelites inthe Yelaxiangbo dome have much high (87Sr/86Sr)i ratios (40.8500)and relatively low eNd(t) (–11.5 to –16.5) values (Zeng et al., 2011).It is unlikely that melting of these metapelites could have produced�35 Ma granites with relatively low (87Sr/86Sr)i (o0.7120) andRb/Sr ratios (0.24–3.35; see Fig. 8), unless other mid-crustal rockswere involved during either melting of the metapelites or emplace-ment of the granitoid magmas. If the rocks involved have relativelylow (87Sr/86Sr)i ratios, the resulting melts should have (87Sr/86Sr)i

ratios similar to those of the Yelaxiangbo granites. However, no mid-crustal rocks in southern Tibet have yet been discovered with(87Sr/86Sr)i ratios lower than those of the Yelaxiangbo granites(Gao, 2009; Zeng et al., 2011). In southern Tibet, the closestapproximation to these mid-crustal rocks with low (87Sr/86Sr)i ratiosis augen gneisses at Yelaxiangbo. However, the gneisses have a widerange of extremely high 87Sr/86Sr ratios (0.8250–1.2000; Zeng et al.,2011), which means that their involvement, even if only in smallabundances, in the petrogenesis of granitoid magmas would havegreatly increased the (87Sr/86Sr)i ratios of the �35 Ma granitesproduced.

Zeng et al. (2011) proposed that melting of amphibolite producedthe Yelaxiangbo two-mica granites, based on the presence oftextures suggestive of partial melting within the Yelaxiangboamphibolites. We consider that the �35 Ma granites were derivedfrom melting of amphibolite with minor metapelite, as supported bytwo observations. Firstly, Sr–Nd isotopic compositions of �35 Magranites are close to those of the Yelaxiangbo amphibolites (Fig. 8),which have a wide range of eNd(t) (–15.6 to 1.9) values with limitedvariation in (87Sr/86Sr)i ratios (0.7109–0.7334; Zeng et al., 2011).Secondly, compared with Miocene leucogranites, the relativelyhigh Sr concentrations (4100 ppm) and high Sr/Y ratios (14–99)of the amphibolites suggest that dehydration melting of hornblendecould release a large amount of Sr into melts but leave a garnetþplagioclaseþhornblendeþpyroxene assemblage in the source region

Fig. 5. Harker diagrams showing variations in major and trace elements versus SiO2 in granitoids of southern Tibet. Data for the Yelaxiangbo two-mica granites (42.6 Ma)

are from Zeng et al. (2011), for the Yajia granodiorite are from Harrison et al. (2000), and for the two-mica granites at Dala and Quedang are from Zeng et al. (2011) and this

study. Details of the analytical techniques used during major and trace element analysis are given in Appendix A.


(Wolf and Wyllie, 1994). These residual minerals are petrographi-cally similar to the garnetþhornblendeþplagioclaseþepidote7biotite melanosomes in partially molten Yelaxiangbo amphibolite(Zeng et al., 2011) that underwent peak metamorphism at �880 1Cand �11 kbar (Gao, 2009). Dehydration melting of hornblende

within an amphibolite source is unlikely to generate the K-rich,peraluminous granites of the Yelaxiangbo dome. The generation ofthe magmas that formed these granites requires dehydration melt-ing of mica (cf. Inger and Harris, 1993; Patino and Beard, 1995;Patino and Harris, 1998; Zhang et al., 2004). In the Yelaxiangbo

0.02

0.1

1

10

1000.02

0.1

1

10

100

1000

5000

Cs Rb Ba Th U K Ta Nb La Ce Sr Nd Hf Zr Sm Ti Y Yb

Sam

ple/

N-M

OR

BS

ampl

e/H

imal

aya

LG

Quedang enclaves

Yangxiong ganitoid (coarse-grained) Yangxiong ganitoid (fine-grained)

Yelaxiangbo (42.6 Ma)Zedong (30 Ma)

Quedang

Dala

Yelaxiangbo (35 Ma)

Fig. 6. Spidergrams for south Tibetan granitoids normalized to N-MORB (A) and

average Miocene Himalayan leucogranite (B). N-MORB values are from Sun and

McDonough (1989).

0 10 20 30 40 500

50

100

150

200

Y

Sr/Y

Fig. 7. Y vs. Sr/Y (A) and YbN vs. (La/Yb)N plots (B) showing variations in Eocene–Olig

granites are adakitic, whereas Oligocene granite from Yelaxiangbo have lower Sr/Y and

from Zeng et al. (2011).


dome, mica is widespread as biotite in amphibolite or as muscovitein metapelite, which implies that the metapelite was involvedduring the partial melting of amphibolite. However, as discussedabove, the extremely high (87Sr/86Sr)i ratios of the metapelite rule itout as the dominant source material for the K-rich peraluminousgranites at Yelaxiangbo.

There are three potential mechanisms for triggering melting of theYelaxiangbo amphibolite and minor metapelite beneath southernTibet: (1) decompression with exhumation and doming of mid-crustal rocks, (2) shear heating during thrusting, and (3) juvenileheating from the lower crust. As mentioned above, peraluminousgranites were generated by partial melting of amphibolite at elevatedpressures (up to 11 kbar). This means that decompressional meltingwith exhumation and doming of mid-crustal rocks is not a reasonablemechanism for granite formation. According to Whittington et al.(2009), in thickened Himalayan crust with a shear zone at 35 kmdepth, strain heating by thrusting for 40 Ma can reach the schistsolidus, thus triggering mid- to upper-crustal anatexis. This meansthat granites related to either thick-skinned (cf. Yin et al., 2010a, b) orthin-skinned thrusting (cf. Schelling and Arita, 1991) after initiationof India–Asia collision at �50–55 Ma would be generated at15–10 Ma, roughly consistent with the onset and duration ofMiocene leucogranite magmatism in the NHG and HHL (Harrisonet al., 1997), but much younger than Oligocene melting in southernTibet. Because the rate of thermal diffusion through hot lithosphericmantle and lower crust is low (Whittington et al., 2009), the amphi-bolite solidus could not be reached by such strain heating alone. Thisimplies that melting of mid-crustal rocks needs additional heat, suchas juvenile heat from the lower crust. This proposal is supported bythe fact that (1) Eocene two-mica granites derived from lower crust(see below) intruded the Yelaxiangbo gneiss dome, potentiallyproviding enough heat for melting of mid-crustal rocks (e.g., amphi-boliteþmetapelite), and (2) the Yelaxiangbo amphibolite yieldeda U–Pb metamorphic zircon age of �45 Ma (Zeng et al., 2011),identical to the age of the Eocene two-mica granites (�42–46 Ma),implying that time-integrated heating from Eocene magmas at�35 Ma triggered mid-crustal anatexis under southern Tibet.

6.2. Origin of 30–46 Ma adakitic granitoids

The 30–46 Ma granitoids of southern Tibet are characterizedby their adakitic geochemical affinity (Fig. 7). Several petrogenetic

(La/

Yb)

Yb

0

50

100

150

0 5 10 15 20 25

ocene granitoids of southern Tibet. Oligocene granodiorites and Eocene two-mica

La/Yb ratios, falling outside of the adakite field. Data for Miocene leucogranite are

8

4

0

-4

-8

-12

-16

-20

-240.70 0.71 0.72 0.73 0.74

ε Nd

(87Sr/86Sr)I

Fig. 8. eNd vs. (87Sr/86Sr)i diagram for south Tibetan granitoids indicating sources

and origin. Data sources: mid-Miocene adakitic stocks from Gangdese (solid

circles) are from Hou et al. (2004) and felsic dykes in southern Tibet (stars) are

from King et al. (2007); amphibolite (diamonds) are from Spencer et al. (1995),

Zeng et al. (2011); end-member Yarlung MORB from the Indus–Tsangpo suture,

and the Amdo orthogneiss (upper crust) and lower crust compositions are from

Miller et al. (1999), Hou et al. (2004); Greater Himalayan leucogranite (GHL) from

King et al. (2010), Zeng et al. (2011). Grey zone indicates isotopically hetero-

geneous high-grade metamorphic rocks within the Indian plate. The modeling

shown here suggests that Oligocene granodiorites that plot along trend I were

formed by mixing of old lower crust and depleted mantle. This indicates

derivation from a thickened lower crustal source containing mantle components,

and Eocene two-mica granites formed by mixing of old lower and middle crustal

material, implying that adakitic melts derived from a thickened lower crustal

source were contaminated by mid-crustal materials or mixed with granite melts

derived from mid-crustal rocks. Details of the analytical techniques used for Sr–Nd

isotopic analysis are given in Appendix A.


models for adakitic magmas have been proposed, involving themelting of: (1) mantle peridotite under hydrous conditions(Williams et al., 2004), (2) subducted oceanic slab (Defant andDrummond, 1990), (3) foundering lower crust (Stern and Hanson,1991; Gao et al., 2004), and (4) thickened mafic lower crust(Atherton and Petford, 1993; Chung et al., 2003; Hou et al., 2004).The geochemical characteristics of the 30–46 Ma adakitic grani-toids may be used to test the validity of the above hypotheses.The absence of parental mafic magmas in southern Tibet and thelack of evidence of involvement of a large volume of mantlematerial (Fig. 8) rule out the possibility of the first model forgeneration of the 30–46 Ma adakitic rocks. In the other models,depletion in HREEs and Y requires melting of a mafic source atdepths within the garnet stability field (e.g., Z50 km; Defant andDrummond, 1990; Wolf and Wyllie, 1991; Atherton and Petford,1993; Rapp and Watson, 1995; Hou et al., 2004; Xiong et al.,2006; Chung et al., 2009). The presence of kyanite inclusions inplagioclase phenocrysts (Zeng et al., 2011), high CaO concentra-tions (0.77–4.17 wt%), and high (CaOþNa2O)/K2O ratios (1.57–17.5) in adakitic rocks also require melting of mafic rocks at highpressures (Patino and Harris, 1998; Zhang et al., 2004; King et al.,2010). Although partial melting of the Yelaxiangbo amphibolite atmid-crustal levels (around 10–11 kbar) under southern Tibet(Gao, 2009; Zeng et al., 2011) might generate the �35 Magranites, it is unlikely to have formed the 30–46 Ma adakiticgranitoids of southern Tibet.

Melting of subducted oceanic slab, for example Neo-Tethyanoceanic crust that was subducted under the Asian plate (i.e., Lhasaterrane) at 120–70 Ma, also cannot produce the 30–46 Ma adakiticgranitoids. Slab-derived melts older than 70 Ma would occur alongthe E–W-trending Gangdese batholith in the Lhasa terrane (Chuet al., 2006) rather than in southern Tibet, and would also haveMORB-like Sr–Nd isotopic signatures (Atherton and Petford, 1993;Hou et al., 2004) rather than the elevated (87Sr/86Sr)i and negativeeNd(t) values observed above. The reaction of a delaminated lower-crust-derived melts with the overlying lithospheric mantle duringascent could form adakites and high-Mg andesites with high bulk-rock Cr, Ni, and Mg# values (Stern and Hanson, 1991; Rapp et al.,1999; Matin et al., 2005; Wang et al., 2006). However, this isinconsistent with the composition of the Eocene–Oligocene grani-toids, and also is not supported by available geophysical data(Schulte-Pelkum et al., 2005; Nabelek et al., 2009).

The last potential source for the formation of adakitic grani-toids in southern Tibet is thickened Indian mafic lower crust. Thisold mafic lower crust underwent collision-related crustal thick-ening from �55 Ma (Aikman et al., 2008; Qi et al., 2008; Zenget al., 2011) with subsequent eclogitization, as documented bygeophysical data (Schulte-Pelkum et al., 2005; Nabelek et al.,2009). Assuming the Sr–Nd isotopic composition of this materialis similar to that of the global lower crust (cf. Miller et al., 1999),the Sr–Nd isotopic signatures of granodiorites at Chongmuda anddacitic dikes at Sayan dome (King et al., 2007) could have beengenerated by melting of thickened mafic lower crustal materialwith the addition of variable amounts of juvenile mantle. ZirconHf isotope data for granodiorites at Chongmuda and Yajia have apositive range of eHf values (2.7–8.4; Chung et al., 2009), alsosuggesting the involvement of juvenile mantle components dur-ing melting. In contrast, the Sr–Nd isotopic signatures of theEocene two-mica granites can be modeled by mixing of old maficlower crustal material with mid-crustal rocks, represented by theYelaxiangbo amphibolites and the GHC (Fig. 8), suggesting thatadakitic melts derived from the old lower crust were contami-nated by mid-crustal materials (e.g., the GHC) or were mixed withgranite magmas generated from the melting of mid-crustal rocks(e.g., amphibolites). Trace element data for the Eocene two-micagranites further support this suggestion. The plots of SiO2 vs. Th,Ba, and Rb for these adakitic granitoids yielded similar flat trends(Fig. 5), which were unlikely produced by fractional crystalliza-tion. The flat trends intersects the steep trends defined by theYelaxiangbo Oligocene granites at SiO2¼73–75% (Fig. 5A–C). AZr/Ba vs. Sr/Ba plot of both granitoid suites produces a roughlinear array (Fig. 5D), indicating a binary mixing pattern (Clynne,1999) that suggests mixing of adakitic and granite melts tookplace at variable ratios.

Derivation of the Eocene two-mica granites by melting of an oldIndian lower crustal source is also supported by Nd–Hf isotope dataand zircon age populations in these rocks. The Eocene two-micagranites have Nd isotopic model ages (TDM) of 1486–1978 Ma(Suppl. Table 4), and inherited zircons within these granites showtwo distinct age ranges at 526–766 and 1370–1943 Ma (Suppl.Table 1), corresponding to igneous/metamorphic events at �500and 1750 Ma in northeastern India (Yin et al., 2010b), respectively.Of the two groups of zircon ages, the age of the younger (526–766 Ma) group is identical to the age of zircons within theYelaxiangbo amphibolites (475–842 Ma; Zeng et al., 2011). Ourunpublished Hf isotopic data for the two-mica granites at Dala(eHfo–6) suggest an old crustal source, which therefore rules outthe involvement of juvenile mantle components during the genera-tion of these granitoid magmas in southern Tibet.

Given this, we infer that the adakitic melts derived from theIndian lower crust probably ascended into the middle crust and thenunderwent a MASH process (crustal melting, melt assimilation,


magma storage, and homogenization; cf. Hildreth and Moorbath,1988), finally forming the Eocene granitoids of southern Tibet.

6.3. A interpretation of the apparent northward younging trend of

the Eocene–Oligocene granitoids

An important characteristic of the Eocene–Oligocene regionalmagmatism in southern Tibet is the apparent northward young-ing trend of the granitoids from the STD to the IYS. Any tectono-thermal explanation of melting of the lower or middle crust togenerate these Eocene–Oligocene granitoids must incorporatethis regional melting event and the resulting magmatic age trend.

It is likely that melting of the lower crust to produce thesouthern Tibet adakitic melts took place at the leading edge of thesubducted Indian continent (see Fig. 9), where break-off of theattached Neo-Tethyan oceanic slab occurred at �50–40 Ma (Yueand Ding, 2006; Gao et al., 2008). Upwelling of asthenosphericmaterial through a slab window could have provided sufficientheat to cause flux-melting of the leading edge of the subductedmaterial. This thermal flux-melting model implies two significantevents. Firstly, melting of the Asian lithospheric mantle overlyingthe northward-subducted Indian continental lithosphere wouldhave been triggered by hot asthenosphere. This has been con-firmed by the occurrence of 50 Ma discrete gabbro bodies (Moet al., 2005) and subsequent 40–38 Ma basaltic lavas (Gao et al.,2008) along the southern edge of the Gangdese batholiths of theLhasa terrane. Secondly, the depth of melting within the Indianmafic lower crust becomes gradually shallower due to time-integrated heating, but the degree of involvement of mantlecomponents increases with time from 46 to 30 Ma. This isconsistent with the fact that the �30 Ma granodiorites are moremafic and contain lower concentrations of Y and Yb than the�42–46 Ma two-mica granites in southern Tibet (see Fig. 7).

This model predicts that the upwelling asthenosphere, actingas a heat source, would have resulted in continuous melting of theleading edge of the Indian continental slab during slow and gentlenorthward subduction at 55–30 Ma (ref. Chung et al., 2005, 2009).A direct consequence of this, if the middle and lower crust werecoupled without large-scale shearing in south Tibet, would havebeen the formation of a south–north-trending granitoid belt thatyoungs southwards. However, this predicted result is the opposite

Fig. 9. North–south sketch cross-section through southern Tibet and the Himalaya durin

�50 Ma resulted in rapid exhumation of slices of the subducted Indian continental sla

middle crust under southern Tibet. Melting of the thickened mafic Indian lower-crust

through a slab window, formed adakitic melts at 46–30 Ma. These melts moved upward

Southward extrusion and exhumation of crustal materials and granitoid magmas at 46

propagation of anatectic mid to lower crust at 30–12 Ma resulted in development

GHC¼Greater Himalayan crystalline complex, THS¼Tethyan Himalayan sequence.

to the situation in south Tibet, where Eocene–Oligocene magma-tism along the Zedong–Lhunze traverse gives ages from 30 to46 Ma, becoming older to the south. Although the emplacementof these magmas was controlled by Cenozoic thrust faults (e.g.,the Lhunze thrust), no compelling evidence demonstrates thatthis thrusting directly resulted in the formation of these grani-toids. Field observations and petrographic studies indicate thatthe Eocene–Oligocene granitoids either intruded into (e.g., Yelax-iangbo pluton) or were rooted in (e.g., Yangxiong stocks) themiddle crust, and in part underwent a MASH process in themiddle crust under southern Tibet (see Fig. 9). Such a MASH orAFC process likely resulted in a slightly younger age for the hybridmagmas, but these processes were unlikely to have caused ageneral change in the age trend for the Eocene–Oligocene gran-itoids. One plausible interpretation is that the middle crustalmaterial under southern Tibet rapidly moved southwards after�55 Ma during northward subduction of the Indian continent; asa consequence, the adakitic magmas derived from the Indianlower crust continuously ascended, and were subsequently storedin the middle crust that was moving southward under southernTibet. Intense deformation of the Eocene granitoids also requiresshearing of the south Tibetan middle crust, where granitoidmagmas were emplaced and stored.

6.4. Tectonic implications

At least four models have been proposed for the formation ofthe GHC and the Himalayan orogen (Yin et al., 2010a, b andreferences therein): thick-skinned thrusting involving Indianbasement (Le Fort, 1975; Yin, 2006), thin-skinned thrustinginvolving an exotic terrane (Schelling and Arita, 1991; DeCelleset al., 2001, 2002; Robinson et al., 2006), mid to lower crustalchannel flow (Nelson et al., 1996; Beaumont et al., 2001, 2004;Searle et al., 2003; Godin et al., 2006), and wedge extrusion andexhumation of deeply subducted Indian continental crust(Chemenda et al., 1995, 2000). Each of these models emphasizesdifferent controlling factors and predicts different protoliths forthe GHC, thus implying distinct dynamic controls during forma-tion of the GHC and the Himalaya. Thrusting models, either thick-or thin-skinned, predict the generation of mid-Miocene granitesin the HHL and NHG during shearing-related heating along a

g late Eocene–early Oligocene times. Break-off of the attached Neo-Tethyan slab at

b and southward transportation along the MHT (main Himalayan thrust) into the

at the leading edge of the slab, triggered by upwelling of asthenospheric material

s into the middle crust and intruded exhumed slices of the Indian continental slab.

–30 Ma formed a granitoid belt, becoming older to the south. Further southward

of the STD and GHC within the Himalaya. LHS¼Lesser Himalayan sequence,


continuously active decollement that cut through the Indianmetamorphic basement. Numerical simulations and thermalmodeling indicate that Himalayan thrusting at 24 Ma (Harrisonet al., 1997) or 40 Ma (Whittington et al., 2009) could havetriggered discontinuous crustal melting to form20–24 Ma granites along the STD and 12–18 Ma granites in theNHA. However, this thrusting- and shearing-related heatingcannot have formed the 30–46 Ma granitoids emplaced in thesouth Tibetan middle crust between the IYS and STD. The channelflow model indicates southward horizontal flow of high-grademetamorphic rocks from the Asian plate in a low-viscosity middlecrust channel, driven by the overburden of a thickened Asianlithosphere during India–Asia collision (cf. Beaumont et al., 2001,2004). Such a low-viscosity channel requires a zone of partialmelting in the middle crust under southern Tibet. Although azone of partial melting at depths of 15–20 km beneath north–south rift zones across the Tibetan–Himalayan plateau has beenidentified by geophysical data (Nelson et al., 1996), and a regionalmelting event beneath southern Tibet is evidenced by the pre-sence of �30–46 Ma granitoids outcropping between the IYS andthe STD, these granitoids are derived mainly from Indian mid- tolower crustal materials rather than the Asian crust. This impliesthat horizontal flow of the Asian middle crust, even if it did occurin the mid-Miocene (cf. King et al., 2007), was unlikely to haveoperated during the India–Asia collision at 50–35 Ma.

In contrast, the temporal–spatial distribution and meltingmechanisms of the �30–46 Ma granitoids in southern Tibet areconsistent with results obtained by thermal–mechanical physicalmodeling of continental subduction (Chemenda et al., 1995, 1996,2000). This modeling confirms that the continental crust can bedeeply subducted into the mantle at a depth of up to 200 km, withsubsequent failure at the front of the subduction zone, forming amajor thrust similar to the MCT. This failure is followed bybuoyancy-driven uplift of a slice of the subducted crust to depthsof 20–30 km, causing a normal displacement (e.g., the STD) alongthe upper surface of the crustal slice. The subsequent erosionalunloading and continuous underthrusting along the first majorthrust plane results in rapid exhumation of crustal slices such asthe GHC (Chemenda et al., 1995, 1996, 2000). In the westernHimalaya, numerous eclogite blocks outcrop as exhumed crustalslices along the northern margin of the Indian plate, and in partare exposed in the GHC (Spencer et al., 1995; Parrish et al., 2006and references therein). They formed at 46.4 Ma and 27.5 kbarduring northward subduction of the Indian continent, and wererapidly exhumed to the middle crust (�35 km depth) at �44 Ma(cf. Parrish et al., 2006). In the central Himalaya, although noeclogite blocks have been observed in southern Tibet, some amphi-bolites with peak metamorphism (10–11 kbar at 45 Ma; Gao, 2009;Zeng et al., 2011) outcrop in the Yelaxiangbo dome, suggesting thatthey were rapidly exhumed to middle crustal depths as crustal slices(�32–34 km depth) at ca. 45 Ma. This finding indicates that thesouthward wedge extrusion and exhumation of slices of subductedIndian continental crust did indeed operate in the Himalaya duringthe early stages of continental collision.

Our study of the Eocene–Oligocene granitoids in southern Tibetprovides important constraints on the mechanisms that operatedduring exhumation of the subducted Indian continental crust andduring orogenesis in the Himalaya. Thermal–mechanical physicalmodeling indicates that rapid uplift of subducted continental crustalslices was a result of failure and the buoyancy of low-densitysubducted upper-crustal materials (Chemenda et al., 1995, 1996,2000). However, such buoyancy-driven uplift is difficult to reconcilewith the presence of high-density eclogites, formed by metamorph-ism of deeply subducted continental crust at depths 4100 km(Parrish et al., 2006 and references therein). Another possibility forthe rapid uplift of these ultrahigh-pressure rock slices is break-off of

the subducted Indian continental lithosphere with the attachedoceanic lithosphere (Yue and Ding, 2006; Chung et al., 2009; Leeet al., 2006), as suggested by the presence of Eocene–Oligocenegranitoids in southern Tibet. Once the leading edge of the subductedIndian continental slab broke free of the attached oceanic litho-sphere, the eclogitic ultrahigh-pressure rock slices that formed atdepths of �100 km would have been rapidly exhumed to themiddle crust (Parrish et al., 2006).

The volume of material that was exhumed into the middlecrust is still an open question. However, the widespread occur-rence of coesite and diamond in 700-km-long, 50-km-wideeclogite and gneiss belts (cf. Liu et al., 2009) in the southeast ofthe Chinese Sulu orogen—the largest ultrahigh-pressure meta-morphic belt in the world—suggests that a huge volume ofsubducted continental material was diachronously exhumed tothe middle crust as ‘‘exotic’’ tectonic slices under eastern China(Liu et al., 2009). This implies that rapid uplift and southwardwedge extrusion of subducted slices of Indian continental mate-rial could supply enough crustal material to enable the formationof the GHC.

Uplift and exhumation of the Indian continental crust wasprobably diachronous along the Himalayan range and multi-phase in southern Tibet. In the western Himalaya, rapid upliftand exhumation of eclogite slices occurred at �44 Ma (cf. Parrishet al., 2006), whereas the uplift of subducted slices of Indian crustprobably occurred before regional melting at �46 Ma in southernTibet, given that the exhumed crustal slice that outcrops atYelaxiangbo is intruded by 42–46 Ma granitoids. Shear deforma-tion of the south Tibetan Eocene granitoids at 39.2–44.5 Ma andat sub-magmatic temperatures suggests that uplift and south-wards movement of the Indian crustal slices continued until�40 Ma. The emplacement of �35 granite dykes along the top-to-outward detachment faults around the core of the Yelaxiangbodome (Fig. 1B) indicates that local extension at �35 Ma causedanother period of uplift and rapid exhumation of the crustal slices(e.g., amphibolites and metapelite) within the middle crust undersouthern Tibet. The formation of �35 Ma granites during meltingof amphibolite and minor metapelite at pressures of 10–11 kbar(Gao, 2009; Zeng et al., 2011) suggests that amphibolite slices orblocks in the middle crust (�35 km depth) were rapid exhumedand domed within the shallow crust.

Here, based on our data and previously published information,we propose a tectonic model for crustal anatexis under southernTibet and orogenesis in the Himalaya (Fig. 9). This modelemphasizes that continental subduction, slab break-off, buoy-ancy-driven uplift, lateral movement, and exhumation of sub-ducted crustal slices were fundamentally important processesduring melting of the thickened Indian lower crust and thetectonic evolution of the Himalayan orogen. This model predictsthat comparable Eocene–Oligocene granitoids with similar appar-ent northward younging trends should exist in other regionsbetween the IYS and STD. Evidence of the occurrence of coevalgranitoids in the northwestern Himalaya is increasing, given thatthree individual intrusions at Dingri have been dated (usingbiotite 40Ar/39Ar) at 43.4, 45.3, and 45.5 Ma, respectively (Panet al., 2004). Moreover, our model consistently fits the majority ofobservations, especially the coherent Eocene–Oligocene eventswithin the Himalaya, such as exhumation of eclogite and amphi-bolite blocks along the northern margin of the Tethyan Himalayaat 45–37 Ma (Parrish et al., 2006; Gao, 2009; Zeng et al., 2011),the occurrence of detachment faults and normal faults around�35 Ma gneiss domes in the NHA, the onset of peak metamorph-ism of high-grade metamorphic rocks in the NHA (45–35 Ma;Lee and Whitehouse, 2007; Zeng et al., 2011) and in the GHC(35–32 Ma; Yin, 2006 and references therein), thrusting withinthe Tethyan Himalayan fold–thrust belt (�50 Ma; Ratschbacher


et al., 1994), and subsequent regional uplift in southern Tibet (Yin,2006; Fig. 9). This model provides an internally self-consistentexplanation for all of these observations within the Himalayanorogen.

Acknowledgments

This study is supported by grants from the Ministry of Scienceand Technology of China (2011CB4031006), IGCP/SIDA-600, NSFC(40730419; 40425014), the Program of the China GeologicalSurvey (1212011121255), and the Funds for Creative ResearchGroups of China (40921001) We are most grateful to the twoanonymous reviewers for critical and constructive reviews of thismanuscript.

Appendix A. Descriptions of analytical methods

Major and trace elements

Major and trace elements analyses were done at the NationalResearch Center for Geoanalysis, Chinese Academy of GeologicalSciences. Major elements were analyzed by wet chemistry andXRF, trace elements and REE were analyzed by ICP-MS. Theroutine analytical precision and accuracy for most elementsmeasured are estimated to be o5%. Mg#¼Mg2þ/(Mg2þ

þFe2þ).

Zircon U–Pb dating

In this study, zircons were separated from granitic samplesusing conventional heavy-liquid and magnetic separation techni-ques. Zircon grains, together with a zircon U–Pb standard(TEMORA; cf. Black et al., 2004), were cast in an epoxy mount,which was then polished to section the grains in half for analysis.Zircon were documented with transmitted and reflected lightmicrographs as well as cathodoluminescence images to revealtheir internal structures, and the mount was vacuum-coated withan �500 nm layer of high-purity gold. Zircon U–Pb isotopicanalyses were performed using the sensitive high-resolution ionmicroprobe (SHRIMP) in Beijing.

Ages of granitoids were obtained using the SHRIMP II equippedat the Beijing SHRIMP center, Institute of Geology, ChineseAcademy of Geological Sciences. The uncertainties of individualanalyses are reported at the 1s level. The mean dates for206Pb/238U analyses are used to indicate crystallization age ofgranitoids, with 95% confidence interval (2s). Operating and dataprocessing procedures follow those established in RSES, Austra-lian National University. Standard material for measurements ofU–Th abundance and U–Pb–Th isotopic ratios of analyzed sam-ples is TEM standard zircon with 206Pb/238U¼0.0668 at 417 Ma.The mass resolution used for determining Pb/U and Pb/Pb isotopicratios is about 5000. Common 206Pb was corrected using non-radiogenic 204Pb.

Samples from the Chongmuda granitoid were analyzed by theLA-ICP-MS at the Geological Lab Center of China University ofGeosciences. The laser ablation was performed with an argoncarrier gas. During the experiments, about 1 min was spent formeasuring gas blank. Calibration was performed using the TEMzircon standard. The Harvard reference zircon 91,500 was used assecondary standards for data quality control. All U–Th–Pb isotoperatios were calculated using the GLITTER 4.0 (GEMOC) software,and common lead was corrected using the common lead correc-tion function proposed by Anderson (2002). The weighted meanU–Pb ages and concordia plots were carried out using Isoplotv. 3.0.

40Ar/39Ar dating

Fresh biotite grains were separated for 40Ar/39Ar dating usingconventional magnetic and gravimetric methods and hand-pick-ing under a binocular microscope. The biotite grains were washedwith methanol and rinsed many times with deionized water in anultrasonic bath, and then irradiated for 48 h at the Beijing NuclearResearch Institute Reactor. Also irradiated was the Fangshanbiotite (ZBH-25) whose age is 132.771.2 Ma and potassiumcontent is 7.6% was used to calculate the irradiation factor J.K2SO4 and CaF2 were used to determine correction factors for theinterfering neutron reactions. The sample was step-heated using aradiofrequency furnace. Its Ar isotopes were measured with aMM-1200B Mass Spectrometer at the Laboratory of IsotopeGeochronology at the Institute of Geology, Chinese Academy ofGeosciences. The procedure for of the analyses and age calcula-tions is the same as that described by Chen et al. (2002).Measured isotopic ratios were corrected for mass discrimination,atmospheric Ar component, blanks and irradiation induced massinterference. The correction factors for the interfering isotopesproduced during the irradiation were determined by analysis ofthe irradiated pure K2SO4 and CaF4 salts. The final results are:(36Ar/37Ar)Ca¼0.000240; (40Ar/39Ar)K¼0.004782; (39Ar/37Ar)Ca¼

0.000806. The blanks of m/e¼40, 39, 37, and 36 are less than6�10�15, 4�10�16, 8�10�17, and 2�10�17 mol, respectively.The decay constant is taken as l¼5.543�10�10 a�1 (Steiger andJager, 1977). 37Ar were corrected for radiogenic decay (half-life35.1 days).

Sr–Nd isotope

Sr–Nd isotopic analysis was done at the Isotope Geology Lab,Chinese Academy of Geological Science (Beijing). Sr and Ndisotopic measurements were done by MC-ICP-MS (MAT-262).NBS987 ratio of 87Sr/86Sr¼0.7102572 (2s), measurement accu-racy of Rb/Sr ratio is better than 0.1%, mass fractionation ofSr isotopes was corrected using 88Sr/86Sr¼8.37521; J&M ratioof 143Nd/144Nd¼0.51112578 (2s), measurement accuracy ofSm/Nd ratio is better than 0.1%, mass fractionation of Nd isotopeswas corrected using 146Nd/144Nd¼0.7219. The initial eNd valuesand 87Sr/86Sr ratios were calculated at t¼46 Ma for the eclogitesand amphibolites.

Appendix B. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2012.06.030.

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