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Constraining the mid-crustal channel flow beneath theTibetan Plateau: data from the Nielaxiongbo gneissdome, SE TibetDan-Ping Yan a b , Mei-Fu Zhou b , Paul T. Robinson b c , Djordje Grujic c , John Malpas b ,Allen Kennedy d & Peter H. Reynolds ca The State Key Laboratory of Geological Processes and Mineral Resources and School ofEarth Sciences and Resources, China University of Geosciences, Beijing, 100083, PR Chinab Department of Earth Sciences, University of Hong Kong, Hong Kong, PR Chinac Department of Earth Sciences, Dalhousie University, Halifax, NS, Canadad SHRIMP Facility, John de Laeter CEMS, Curtin University, Bentley, 6103, Australia

Available online: 24 Jun 2011

To cite this article: Dan-Ping Yan, Mei-Fu Zhou, Paul T. Robinson, Djordje Grujic, John Malpas, Allen Kennedy & Peter H.Reynolds (2012): Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneissdome, SE Tibet, International Geology Review, 54:6, 615-632

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International Geology ReviewVol. 54, No. 6, 20 April 2012, 615–632

Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from theNielaxiongbo gneiss dome, SE Tibet

Dan-Ping Yana,b∗, Mei-Fu Zhoub, Paul T. Robinsonb,c, Djordje Grujicc, John Malpasb,Allen Kennedyd and Peter H. Reynoldsc

aThe State Key Laboratory of Geological Processes and Mineral Resources and School of Earth Sciences and Resources, ChinaUniversity of Geosciences, Beijing 100083, PR China; bDepartment of Earth Sciences, University of Hong Kong, Hong Kong, PR

China; cDepartment of Earth Sciences, Dalhousie University, Halifax, NS, Canada; dSHRIMP Facility, John de Laeter CEMS, CurtinUniversity, Bentley 6103, Australia

(Accepted 8 November 2010)

Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous withshortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, andmigmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weaklymetamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from boththe basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachmentfault occurring within the ductilely deformed layer. Leucogranites crosscut the basement complex, the lower detachment,and the middle crustal layer, but do not intrude the upper detachment or the cover sequence. Three deformational fabricsare recognized: a N–S compressional fabric (D1) in the cover sequence, a north- and south-directed extensional fabric (D2)in the upper detachment and lower tectonic units, and a deformation (D3) related to the leucogranite intrusion. SHRIMPzircon U–Pb dating yielded a metamorphic age of ∼514 million years for the amphibolite and a crystallization age of ∼20million years for the leucogranite. Hornblende from the amphibolite has an 40Ar/39Ar age of 18 ± 0.3 million years, whereasmuscovites from the schist and leucogranite yielded 40Ar/39Ar ages between 13.5 ± 0.2 and 13.0 ± 0.2 million years. Theseresults suggest that the basement was consolidated at ∼510 Ma and then exhumed during extension and silicic plutonism at∼20 Ma. Continuing exhumation led to cooling through the 500◦C Ar closure temperature in hornblende at ∼18 Ma to the350◦C Ar closure temperature in muscovite at ∼13 Ma. The middle crustal ductilely deformed layer within gneiss domesof southern Tibet defines a southward-extruding ductile channel, marked by leucogranites emplaced into migmatites andamphibolites. We propose a model involving thinned upper crust for the initial extension of the Tibetan Plateau in the earlyMiocene.

Keywords: Nielaxiongbo; SE Tibet; gneiss dome; mid-crustal channel flow; South Tibetan Detachment System; middlecrustal layer; leucogranite

1. Introduction

Gneiss domes occurring within the South Tibetan sedi-mentary sequence have similar structural geometries anddeformational successions (Burg et al. 1984; Chen et al.1990; Lee et al. 2000, 2004, 2006; Aoya et al. 2005, 2006;Quigley et al. 2006, 2008), whereas numerous models havebeen proposed to explain their genesis. For example, Burget al. (1984) proposed a thrusting model to explain the for-mation of these domes, whereas Yin (2006) favoured a latewedge extrusion model in which the domes were formedby passive–active roof faulting. Nevertheless, all modelsaccount for the Miocene–Oligocene leucogranites (Zhanget al. 2004; Watts and Harris 2005; Aoya et al. 2006), wellknown in the Greater Himalayan Sequence (GHS) (Le Fort1981; France-Lanord and Le Fort 1988; Inger and Harris

∗Corresponding author. Email: yandp@cugb.edu.cn

1993; Harrison et al. 1997; Grujic et al. 2002). On theother hand, identification of the South Tibetan DetachmentSystem (STDS; Burg et al. 1984; Burchfiel et al. 1992;Edwards et al. 1996; Hodges 2000; Lee et al. 2000, 2004;Searle and Godin 2003; Aoya et al. 2005; Quigley etal. 2006; Lee and Whitehouse 2007; Zhang et al. 2007)(Figure 1) has led some workers to interpret the domesas metamorphic core complexes (e.g. Chen et al. 1990; Liet al. 2003).

A change in shear direction from top-to-north in thenorthern side to top-to-south in southern side in therocks of South Tibet has been related to intrusion of theleucogranites (Aoya et al. 2005, 2006). A similar kine-matic inversion, which has also been observed elsewherein the Himalaya along the footwall of the STDS, suggests

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2010.548153http://www.tandfonline.com

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90°E

86°E

29

°N

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88°E 90°E 92°E

100 km

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Figure 1. A schematic tectonic map showing the distribution of North Himalayan gneiss domes in South Tibet and the South TibetanDetachment System (after Burchfiel et al. 1992; Hodges 2000). The South Tibetan Detachment System is marked by the heavy ticked linesin the hanging wall. YZS, Yarlung Zangbo Suture; STDS, South Tibetan Detachment. MCT, Main Central thrust; MBT, Main BoundaryThrust; A–B, cross section in Figure 9.

southward extrusion of the GHS (e.g. Grujic et al. 1996).Protracted melting of the middle crust during convergenceat ∼26–10 Ma (Harris et al. 2004; Zhang et al. 2004) sup-ports the existence of a mid-crustal ductile channel flow(Godin et al. 2006; Bai et al. 2010) related to the for-mation of the GHS and the STDS (Grujic et al. 1996,2002; Searle and Szulc 2005; King et al. 2007). Thus,the proposed mid-crustal ductile channel flow model mayprovide a link between the metamorphism, ductile defor-mation, and concomitant leucogranite intrusion in the GHSand the STDS (Grujic et al. 1996, 2002; Beaumont et al.2001, 2004; Jamieson et al. 2002, 2004, 2006; Searle andSzulc 2005; Hollister and Grujic 2006; King et al. 2007).However, the different processes of such a channel flow andgeochronological constraints need further determining.

The Nielaxiongbo gneiss dome, the easternmost domein South Tibet, lies about 250 km east of the Kangmardome (Chen et al. 1990; Lee et al. 2000; Aoya et al.2006). It contains leucogranites similar to those elsewherein South Tibet and the Greater Himalayas (BGMRXAR1993; Zhang et al. 2007). Little is known about the timingand origin of the leucogranite in this complex and its rela-tionship to the middle crustal rocks due to a lack of detailedstructural, petrographic, and geochronological data.

To determine the character of the Nielaxiongbo gneissdome and its relationship to the STDS and possibleexistence of mid-crustal channel flow beneath southern

Tibet, we carried out a reconnaissance field investigationand collected samples from all the relevant zones of thedome (Figure 2). This article describes the geology of thegneiss dome and presents new SHRIMP zircon U–Pb and40Ar/39Ar dating results for this body. The principal objec-tives of this article are to provide better constraints onthe deformational/magmatic history of the Nielaxiongbogneiss dome and to elucidate the role of ductile deforma-tion and silicic magmatism in its formation. We furthercombine our data with those from other North Himalayangneiss domes to explain the origin of these enigmatic fea-tures. Our data support a link between uplift of the NorthHimalayan gneiss domes and mid-crustal ductile channelflow in South Tibet.

2. Geological background

Southern Tibet consists of two major tectonic units: theIndian subcontinent to the south and the Lhasa Block tothe north, separated by the Yarlung Zangbo suture zone(Girardeau et al. 1984; Hirn et al. 1984; Zhou et al. 1996;Aitchison et al. 2000; Yin 2000) (Figure 1). The nar-row, 2–10 km-wide Yarlung Zangbo suture zone is markedby various lenses of Cretaceous ophiolites and mélange,Cretaceous forearc sedimentary sequences, and Tertiarymolasse deposits, and consists of highly deformed green-schist, mica-quartz schist (Rowley 1996; Aitchison et al.

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91°54″E

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L2 (n = 46)

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10 m 2 m

2 m

4 km

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(A) (B)

(C)

S2

S0

(D) (E) (F)

92°12″E

Figure 2. Geological map of the Nielaxiongbo gneiss dome (modified from TGSA 2002 and Zhang et al. 2007). Pz1, lower Palaeozoic;Pz2, upper Palaeozoic; T3, Upper Triassic; K1, Lower Cretaceous. Insets A and B, a lower hemisphere Wulff net showing the D2 min-eral lineations (the solid dots are from TGSA 2002, and open squares are our measurements). C-D indicates a section crosscutting theNielaxiongbo dome.

2000, 2003; Malpas et al. 2003; Geng et al. 2006). TheLhasa Block is composed mainly of Precambrian basementrocks, Palaeozoic and Mesozoic cover strata intruded byabundant Mesozoic and Cenozoic granitic plutons (Zhuet al. 2009) (Figure 1). The basement rocks are com-posed mainly of gneiss, amphibolite, and schist, along withsporadic marble and granulite lenses (Dong et al. 2009;Wang et al. 2009; Zhang et al. 2010). These metamor-phic rocks show evidence of extensive migmatization andpolyphase deformation with P–T conditions of ∼1.0 GPaand 750−850◦C (Wang et al. 2009). The sedimentary coverof the Lhasa Block consists of Devonian to Jurassic sedi-mentary rocks together with volcanic intercalations (Genget al. 2006).

The Indian subcontinent includes the Tethys-Himalayasand the GHS. Within the subcontinent, the STDSjuxtaposes the Cambrian–Eocene Tibetan sedimentary

sequence over the GHS, which are composed mainly ofamphibolite- to granulite-facies Proterozoic metasedimen-tary rocks, intruded locally by early Palaeozoic granites(Liu and Zhong 1997; Lee et al. 2000; Ding et al. 2001;Yin and Harrison, 2001; Geng et al. 2006; Goscombeet al. 2006; Zhang et al. 2010). Farther south, the GHSlies above the Main Central Thrust (MCT). The MainBoundary Thrust separates the Lesser Himalayas from theSub-Himalayas (Figure 1). A series of gneiss domes farthernorth form an E–W-trending belt within the Tibetan sedi-mentary sequence (Figure 1). Leucogranites are commonin these domes.

3. Nielaxiongbo gneiss dome

The Nielaxiongbo gneiss dome is a NW-striking, dome-like anticline, 22 km long and 10 km wide. It consists

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of a metamorphic complex (BGMRXAR 1993; TGSA2002; Zhang et al. 2007; Zhang and Guo 2007), a ductilelydeformed middle crustal layer, and the Tibetan sedimen-tary sequence that in this area comprises Upper Triassicand Lower Cretaceous sedimentary strata. The basementcomplex is composed of amphibolite-grade gneissesand locally migmatite, kyanite + staurolite + garnetgneiss, biotite + muscovite + plagioclase gneiss, andbiotite + plagioclase gneiss with well-developed folia-tion. The metamorphic grade of these rocks decreasesstructurally upward around the dome. A similar decreasein metamorphic grade outward from the dome is alsoobserved in middle crustal Palaeozoic rocks overlying thebasement sequence. These rocks pass outward from garnet-mica schist, locally containing staurolite, to mica-bearingquartzite to phyllite. The cover sedimentary sequenceof Upper Triassic and Lower Cretaceous age is a majorcomponent of the Tibetan sedimentary sequence (Figure 1)and is composed of terrigenous flysch (TGSA 2002). Alower detachment separates the basement complex fromthe middle crustal layer, and a major upper detachmentseparates the middle crustal layer from the cover sequence(Figure 2). In addition, minor normal faults within themiddle crustal layer thinned or selectively removed partsof the strata. Several leucogranite plutons and stocksintrude the basement complex, the lower detachment,and the middle crustal layer, but do not intrude the upperdetachment (Figure 2). These relationships are similar

to those described for the Kangmar, Kampa, and otherdomes in South Tibet (Chen et al. 1990; Lee et al. 2000;Watts and Harris 2005; Quigley et al. 2006). At least threedeformational stages (D1–D3) have been identified withinthe Nielaxiongbo gneiss dome.

3.1. N–S compressional top-to-south phase (D1)

The earliest deformational phase D1 is well developed inthe Tibetan sedimentary sequence and locally preserved inthe upper detachment fault and the middle crustal layer. Itis represented by numerous sub-horizontal brittle thrusts,which typically cut the Tibetan sedimentary sequence.Within the Tibetan sedimentary sequence, the deformedplane is the original bedding surface (S0). The folds in thepelitic and psammitic interbeds are sub-harmonic in pro-file with type 1B folds in competent sandstone beds andtype 1C or 2 folds in incompetent shale or siltstone beds(cf. Ramsay and Huber 1987) (Figures 2 and 3A). Thesandstone layers display a fan-shaped spaced cleavage (S1

foliation), whereas the slates have inverse fan-shaped axialplane slaty cleavages (S1 foliation) (inset C in Figure 2).In some cases, S1 is represented by crenulation cleavage(inset F in Figure 2) and spaced cleavage, which has S-Cfabrics indicating southward thrusting and displays a strongflattening strain (Figure 3C). S1 is sub-perpendicular to thebedding S0 on a regional scale. The axial planes of the foldsstrike E–W and dip steeply to north in the northern limb

Figure 3. Field and thin section photos of brittle-ductile deformation and metamorphic mineral assemblages in the upper detachmentfault. (A) Type 1C fold in Triassic interlayered sandstone and argillite, which indicate S0 and S1. Distance along the axial trace is ∼300 m;(B) Triassic sandstone with penetrative cleavage S1; and (C) spaced cleavage with S-C fabric in the foliated sandstone.

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and to south in the southern limb of the dome (Figures 2and 3A–3C), reflecting regional N–S compression.

In general, structural styles in the middle crustal layerare similar to those in the Tibetan sedimentary sequence(inset in Figure 2). The metamorphic mineral assemblage,which defines the foliation (S1), consists of muscovite/sericite + plagioclase + chlorite + quartz (Figure 3B),reflecting deformation under low greenschist-faciesconditions.

3.2. Extensional deformational phase (D2)

Deformational phase D2 includes brittle/brittle-ductiledeformation in the upper detachment fault and ductiledeformation in the middle crustal layer, the basementdetachment, and the basement complex.

The upper detachment fault zone contains breccia andphyllite with phyllitic foliation S2, which is sub-parallel tothe fault (Figure 2). The upper detachment fault crosscutsD1 folds and S1 foliation and thus belongs to the D2 defor-mation (Figure 4A). The metamorphic mineral assemblagealong S2 within this fault consists of muscovite/sericite + chlorite + plagioclase + quartz (Figure 4A),which indicates low to middle greenschist-faciesconditions related to this deformation.

The ductile deformation of the middle crustal layer andbasement detachment (D2) is manifested by the presenceof (1) mylonite (Figures 4 and 5); (2) a penetrative S2

foliation (Figure 4B–4D) defined by the axial planesof intrafolial folds; and (3) N–S-trending ductile shearbands of centimetre size with a consistent top-to-the-northsense of shear, S-C fabric, kinking, and fan-type splittingalong (001) of S1 biotite porphyroblasts, garnet ‘b’-typepressure shadows, and well-developed mineral lineations,which have a N–S trend (Figures 2A, 2B, and 2F, 4Cand 4D). Within the sequence, the upper Palaeozoicrocks are separated from those of the lower Palaeozoicby a minor detachment fault (TGSA 2002). The meta-morphic mineral assemblages consist of muscovite +plagioclase + biotite + garnet + quartz + hornblende ±staurolite (Figure 6A and 6B), indicating loweramphibolite-facies conditions that produced a garnet–staurolite belt along the lower detachment (TGSA2001). The assemblage of muscovite + plagioclase+ biotite + garnet + quartz (garnet belt) in lowerPalaeozoic sequence indicates upper greenschist-faciesmetamorphism and somewhat higher grade assemblages,consisting of hornblende + biotite + plagioclase +garnet + staurolite + quartz (staurolite belt), indicateupper greenschist- to lower amphibolite-facies meta-morphism (Figure 2; TGSA 2002). An assemblage ofmuscovite + plagioclase + biotite + quartz (Figure 4D) inthe upper Palaeozoic strata indicates middle greenschist-facies metamorphism. The ductile deformation of D2

indicates north-directed extensional tectonics at themiddle–upper crustal level.

Ductile deformation at the top of the basement com-plex, which produced a mylonitic foliation (S2) and ashallowly N- or S-dipping mineral lineation (L2) (Figure5A and 5B), is equivalent to the D2 deformation in themiddle crustal layer; however, S-C fabrics and garnet ‘b’-type pressure shadows indicate a top-to-the-south sense ofshear (Figure 5B). The metamorphic mineral assemblagealong the S2 foliation consists of kyanite + sillimanite +K-feldspar + muscovite + hornblende + staurolite + pla-gioclase ± biotite + quartz (Figure 5A and 5B), indicatingamphibolite-facies metamorphism. The local occurrence ofmigmatite indicates partial melting within the complex.

3.3. Deformational phase (D3) related to the intrusionof leucogranites

The leucogranites form two nested bodies of adamellitewith many peripheral dikes and sills. The Quzen body(Figure 5C) intrudes the basement complex, the lowerdetachment fault, and the middle crustal layer, that is, itcuts through the D1 and D2 fabrics but not the upperdetachment fault or cover sequence (Figure 2; Zhanget al. 2007). S3 crenulation and L3 mineral lineation,which define the D3 deformation, dip steeply outwardaround the dome (Figures 2, 4A, and 5B). Thus, D3 wasformed during final emplacement of the leucogranites. Theleucogranites are medium grained and consist essentiallyof plagioclase (An24–26), K-feldspar, muscovite, and quartz(Figure 5D). The Quzen body is intruded by the Renbuobody of fine-grained adamellite with an identical mineral-ogy, that is, plagioclase (An22–24), K-feldspar, and quartzwith minor muscovite, biotite, zircon, apatite, and rutile.The leucogranites crosscut the well-developed D2 folia-tion and L2 lineation in the country rocks. A few samplesof deformed leucogranite (M22 and M1) from the Quzenbody contain remnants of sillimanite (D2) along shearbands (S3) (Figure 2) with steeply outward-dipping minerallineation, indicating that the leucogranite was emplacedinto sillimanite-grade country rocks already at high temper-ature and that the ductile deformation pre-dated the mainemplacement of the plutons.

4. Analytical methods

4.1. SHRIMP dating of zircon

Using conventional heavy liquid and magnetic techniques,zircon grains were separated from two samples, an amphi-bolite (M13) from the basement complex and a leucogran-ite (M22) from the Quzen body that intrudes the basementcomplex (Figure 2). After separation, about 100 grainsof zircon were mounted for U–Pb isotope analysis atthe SHRIMP II lab at Curtin University of Technology,Australia. Analytical and data-reduction procedures aresimilar to those described by Compston et al. (1984). Pb/Uages are based on a value of 564 million years determined

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Figure 4. Field and thin section photos of ductile deformation and metamorphic mineral assemblages in the middle crustal layer.(A) S2 crenulations with axial plane foliation S3. Note that quartz-rich and mica-rich shear bands define the S1 foliation; (B)penetrative S2 foliation within garnet-bearing mylonite of the middle crustal layer; (C) S2 defined by the oriented growth of mus-covite + plagioclase + biotite + quartz (garnet belt) in the middle crustal layer; (D) in the middle crustal layer, the penetrative S2 foliationis defined by alignment of muscovite + plagioclase + biotite + quartz, and σ -type garnet porphyroblasts with pressure shadows; and (E)S1 biotite porphyroblast shows kinking and fan-type splitting along (001) within the penetrative S2 foliation indicating top-to-the-northshearing in the lower detachment shear zone.

by thermal ionization mass spectrometry U–Pb analysis ofthe standard zircon CZ3. Because the 207Pb/206Pb ages aresensitive to the common Pb correction, the 206Pb/238U ageis normally adopted for Phanerozoic samples (Compstonet al. 1984). Individual analyses (Table 1) are presentedas 1σ error boxes on concordia plots and uncertainties inmean ages are quoted at the 95% confidence level (2σ ).

4.2. 40Ar/39Ar dating

For 40Ar/39Ar dating, muscovite was separated from apegmatitic adamellite (M12B) of the Renbuo body, aleucogranite of the Quzen body (M1), and a garnet-mica

schist (M7). Hornblende was separated from an amphi-bolite (M4) of the middle crustal layer. Fresh portions ofeach sample were cut, crushed, and sieved to obtain min-eral grains of 0.2–2 mm diameter. These were washed indistilled water in an ultrasonic bath for 30 min and air dried.Muscovite and hornblende separates, greater than 99%purity, were handpicked under a binocular microscope.Samples were loaded into irradiation disks along with thehornblende MMhb-1 standard (520 ± 2 Ma; Samson andAlexander 1987). The disks were wrapped in aluminiumfoil and vacuum sealed in silica glass tubes. The packageswere then shielded with cadmium and irradiated for 54hours at the McMaster University nuclear reactor. Isotopicanalyses were carried out in the Argon Isotope Laboratory

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Figure 5. Field and thin section photos of the deformation and mineral assemblages in the GHS and leucogranite. (A) Migmatiteof the GHS; (B) gneiss from the core complex showing mylonite foliation (S2) and a metamorphic mineral assemblage of (silli-manite) + hornblende + plagioclase + muscovite + biotite + quartz + K-feldspar; (C) leucogranite intruded into the amphibolite; and (D)leucogranite of the Quzen body.

(A)

M13-747±1 Ma

M22-3 (G1)482±9 Ma

M22-3 (G1)20±0 Ma

M13-651347±7 Ma

(B)

Figure 6. Cathodoluminescence morphologies of zircon grains from (A) amphibolite (M13) and (B) leucogranite (M22) of theNielaxiongbo, showing metamorphic recrystallization and growth in the leucogranite magma.

at Dalhousie University, Canada, using a VG 3600 massspectrometer equipped with an internal tantalum resistancefurnace of the double-vacuum type. Details of the experi-mental procedures are as described in Muecke et al. (1988).All dates are reported using 5.543 × 10−10 a−1 as the totaldecay constant for 40K (Steiger and Jager 1977).

5. Analytical results

5.1. SHRIMP zircon U–Pb age

The amphibolite, M13, from the basement complex con-tains zircon grains with a variety of textures and morpholo-gies. The grains display highly complex internal structuresas shown in the cathodoluminescence images (Figure 6A),

which likely resulted from variable degrees of recrystalliza-tion. Some grains have thin overgrowths with intermediateto high U contents and low and variable Th/U ratios(<1–0.01) (Figure 7A). Many of the recrystallized grainscontain inclusions.

The SHRIMP analytical results for zircon from sampleM13 are given in Table 1 and are plotted on a concordia dia-gram (Figure 7B and 7C); 14 of 18 analyses cluster around500 million years on the concordia diagram, whereas theother 4 have much younger ages (Figure 7B). Two grainswith ages close to 500 million years (M13-5 and M13-6)have very low U contents and one (M13-16) yielded a dis-cordant analysis (Figure 7C). When these three anomalousanalyses are excluded, the remaining 11 form a coherent

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Tabl

e1.

SH

RIM

PU

–Pb

isot

opic

anal

yses

for

zirc

ons

from

the

leuc

ogra

nite

and

base

men

tcom

plex

ofN

iela

xion

gbo

dom

e.

Rat

ios

corr

ecte

dco

mm

onP

bA

ges

(Ma)

Spo

t

206P

b c(%

)

U

(ppm

)

Th

(ppm

)

Pb

(ppm

)T

h/U

%co

ncor

danc

e20

6P

b/23

8U

±%20

7P

b/23

5U

±%20

7P

b/20

6P

b±%

208P

b/23

2T

h±%

206

Pb/

238

U±

207P

b/23

5U

±20

7P

b/20

6P

b±

M22

(Quz

enad

amel

lite

ofle

ucog

rani

te)

M22

-10.

0038

8580

3916

524

0.02

660.

0032

766.

50E

−05

0.02

0961

0.00

1036

0.04

641

0.00

1991

0.00

0835

0.00

0749

210

211

3287

M22

-20.

0085

6786

1813

228

0.02

00.

0035

147.

80E

−05

0.02

2197

0.00

1311

0.04

5812

0.00

2384

0.00

1114

0.00

1134

231

221

063

M22

-30.

0006

2494

816

770

0.18

100

0.07

7685

0.00

1457

0.60

742

0.01

3352

0.05

6708

0.00

0531

0.02

3009

0.00

0643

482

948

28

480

21

M22

-50.

0060

2923

1421

60.

010

0.00

2794

5.80

E−0

50.

0167

270.

0023

410.

0434

20.

0058

97−0

.000

726

−0.0

0436

618

017

20

59

M22

-60.

0101

9335

4348

100.

010

0.00

2907

5.90

E−0

50.

0175

790.

0020

480.

0438

540.

0049

230.

0006

170.

0025

8319

018

20

47

M22

-70.

0033

2773

3514

720

0.02

00.

0030

345.

90E

−05

0.01

8983

0.00

0872

0.04

5385

0.00

1777

0.00

0386

0.00

0620

019

10

37

M22

-70.

1797

941

2956

190.

010

0.00

2849

8.80

E−0

50.

0116

740.

0079

440.

0297

230.

0200

9−0

.006

607

−0.0

1031

418

112

80

41

M22

-80

5339

9014

0.02

120.

0028

635.

50E

−05

0.01

9443

0.00

0508

0.04

9255

0.00

077

0.00

208

0.00

0125

180

201

160

36

M22

-90.

0067

5444

3262

110.

010

0.00

2761

5.60

E−0

50.

0166

240.

0020

540.

0436

750.

0052

16−0

.000

799

−0.0

0251

318

017

20

40

M22

-40.

0043

7868

8912

420

0.02

00.

0032

096.

20E

−05

0.02

012

0.00

1082

0.04

5475

0.00

2176

0.00

0669

0.00

0914

210

201

033

M13

(am

phib

olit

ein

base

men

tcom

plex

)

M13

-187

359

877

0.68

111

0.08

1301

0.00

1149

0.62

8769

0.01

076

0.05

6091

0.00

0451

504

749

57

456

18

M13

-311

0545

593

0.41

980.

0823

250.

0011

820.

6562

710.

0119

690.

0578

160.

0005

5451

07

512

752

321

M13

-472

937

463

0.51

111

0.08

2263

0.00

1163

0.63

6879

0.01

3763

0.05

615

0.00

0824

510

750

09

458

33

M13

-535

924

531

0.68

910.

0796

430.

0011

780.

6409

760.

0189

910.

0583

70.

0013

9449

47

503

1254

452

M13

-634

587

280.

2510

40.

0828

540.

0012

490.

6517

730.

0192

040.

0570

530.

0013

4151

37

510

1249

452

M13

-784

912

267

0.14

102

0.08

3083

0.00

1171

0.65

6325

0.01

2566

0.05

7294

0.00

0648

514

751

28

503

25

M13

-838

812

821

0.33

870.

0510

680.

0007

730.

3801

160.

0089

710.

0539

840.

0008

8132

15

327

737

037

M13

-975

650

169

0.66

118

0.08

3136

0.00

1179

0.63

7348

0.01

328

0.05

5601

0.00

0756

515

750

18

436

30

M13

-10

1889

149

148

0.08

101

0.08

412

0.00

119

0.66

8804

0.01

1436

0.05

7663

0.00

0462

521

752

07

517

18

M13

-11

2653

9020

80.

0398

0.08

5148

0.00

1185

0.68

411

0.01

0726

0.05

827

0.00

0331

527

752

96

540

12

M13

-12

9121

3163

0.00

324

0.00

7692

0.00

0106

0.05

3357

0.00

0916

0.05

0309

0.00

0435

491

531

209

20

M13

-13

942

615

870.

6510

40.

0850

80.

0011

810.

6734

960.

0121

70.

0574

130.

0005

7252

67

523

750

722

M13

-14

1176

851

109

0.72

106

0.08

3446

0.00

1163

0.65

4373

0.01

1898

0.05

6875

0.00

0572

517

751

17

487

22

M13

-15

1292

1630

0.01

320.

0249

260.

0003

620.

1957

10.

0056

110.

0569

450.

0013

0815

92

181

548

951

M13

-16

946

673

900.

7190

0.08

6004

0.00

1399

0.70

7874

0.01

7461

0.05

9695

0.00

0993

532

854

310

593

36

M13

-17

2378

716

0.00

331

0.00

7289

0.00

011

0.04

9306

0.00

2515

0.04

9061

0.00

2298

471

492

151

106

M13

-18

1594

1113

142

0.70

101

0.08

112

0.00

1117

0.63

9228

0.01

0734

0.05

7152

0.00

046

503

750

27

497

18

M13

-19

186

8115

0.44

970.

0796

570.

0013

410.

6309

220.

0380

140.

0574

450.

0032

0549

48

497

2450

912

3

Not

es:20

6P

b c(%

)in

dica

tes

perc

enta

geof

tota

l206P

bth

atis

non-

radi

ogen

ic.C

omm

onP

bco

rrec

ted

byas

sum

ing

206P

b/23

8U

-208P

b/23

2T

hag

e-co

ncor

danc

e.

Dow

nloa

ded

by [

Gua

ngzh

ou I

nstit

ute

of G

eoch

emis

try]

at 0

1:38

15

Mar

ch 2

012

International Geology Review 623

10000

0.12

0.10

0.08

100

100

200

300

400

500

200

300

Upper intercepts at206

Pb/238

U = 470 ± 92 Ma

50 Ma

400

500

0.06

0.04

0.02

0.00

0.10

0.08

0.06

0.04

0.02

0.0000.0 0.2 0.4 0.6 0.8

0.0 0.1 0.2 0.3 0.4

207Pb/

235U

206P

b/2

38U

206P

b/2

38

U

207Pb/

235U

0.5 0.6 0.7 0.8

(A)

(B)

(D)

1000M22

Amphibolite, M13

Leucogranite, M22

M13

100

10

11 10

TH/U=1

TH/U=0.1

TH/U=0.0

1

100

U (ppm)

Th (

ppm

)

1000 10000

0.10

0.09

0.08

0.070.5 0.6 0.7 0.8

206P

b/2

38U

207Pb/

235U

(C)Amphibolite, M13

550

450

514 ± 6 Ma

206Pb/

238U = 514 ± 6 Ma

207Pb/

235U = 511 ± 6 Ma

207Pb/

206U = 503 ± 1 3 Ma

N = 11

0.0045

0.0035

0.0025

0.00150.008 0.012 0.016 0.020 0.024 0.028

Lower intercepts at206

Pb/238

U = 20.3 ± 1.9 Ma

206P

b/2

38U

207Pb/

235U

(E)

26

30

22

18

14

10

Leucogranite, M22

N = 9

Figure 7. (A) Th versus U diagram for the analysed zircons from the Nielaxiongbo gneiss dome. (B and C) Concordia diagrams forthe zircon from the amphibolite (M13). (D and E) Concordia diagrams of zircon from the leucogranite (M22). For sample locations, seeFigure 2.

group with a mean 206Pb/238U age of 514 ± 6 millionyears, a mean 207Pb/235U age of 511 ± 6 million years,and a mean 207Pb/206Pb age of 503 ± 13 million years(Figure 7C). The chi-squared values for these ages are 1.2,2.0, and 2.2, respectively, indicating a small but significantscatter in the ages. The scatter suggests some geologicaldisturbance, consistent with the recrystallization of zirconsshown in the cathodoluminescence images (Figure 6A).The four grains with much younger ages (M13-8, -12, -15, -17) have very low Pb contents coupled with high Ucontents, suggesting some post-crystallization disturbanceof the U–Pb systematics.

Zircon grains from the leucogranite sample, M22(Table 1), show oscillatory zoning and are clearly magmaticin origin (Figure 6B). They have relatively low Th contentswith Th/U ratios of 0.18–0.01 (Figure 7A). When plottedon a concordia diagram (Figure 7D), one sample yielded adiscordant age with an upper intercept 206Pb/238U age of

470 ± 92 million years (Figure 7D). The other nine form asingle cluster on the diagram (Figure 7E), yielding a mean206Pb/238U age of 20.3 ± 1.9 million years (Figure 7E),which is considered to be the best estimate of the crystal-lization age for the leucogranite. The older grain is closein age to zircons from the basement amphibolite and isprobably a xenocryst captured during emplacement of theleucogranite.

5.2. 40Ar/39Ar analytical results

The 40Ar/39Ar data of muscovite and hornblende, cor-rected for interfering isotopes and mass discrimination, aresummarized in Table 2, and the apparent age spectra foreach sample are illustrated in Figure 8. All age errors arequoted at the 95% confidence level (2σ ). They includeerrors in the irradiation correction factors and the errorin the neutron fluence parameter but do not include the

Dow

nloa

ded

by [

Gua

ngzh

ou I

nstit

ute

of G

eoch

emis

try]

at 0

1:38

15

Mar

ch 2

012

624 D.-P. Yan et al.

Tabl

e2.

Res

ults

of40

Ar/

39A

rda

ting

ofth

ese

para

ted

mus

covi

tean

dam

phib

ole.

Sam

ple/

sepa

rate

T(◦

C)

39A

r(m

V)

39A

r(%

)A

ge(M

a)±

1σA

tm.c

ont.

(%)

37A

r/39

Ar

36A

r/40

Ar

39A

r/40

Ar

IIC

(%)

M1/

mus

covi

tea

600

14.8

0.4

8±

590

.70.

030.

0030

820.

0478

930.

1265

029

.80.

810

±3

87.7

00.

0029

770.

0485

380.

0270

043

.81.

27

±3

93.8

00.

0031

780.

0321

590.

0375

097

.22.

711

±1

74.9

00.

0025

40.

0873

490

775

169.

94.

712

±0

620

0.00

210.

1268

390

800

434.

212

.113

±0

340

0.00

1151

0.20

630

825

548.

615

.213

±0

14.9

00.

0005

070.

2650

730

850

427.

111

.913

±0

9.7

00.

0003

30.

2855

320

875

366.

610

.213

±0

11.2

00.

0003

80.

2814

210

900

258.

67.

212

±0

15.6

00.

0005

30.

2686

920

925

177.

34.

912

±0

15.7

00.

0005

350.

2709

030

950

151.

14.

212

±0

150

0.00

051

0.27

2986

097

514

2.6

3.9

12±

014

.20

0.00

0485

0.27

3949

010

0014

7.2

4.1

12±

013

.10

0.00

0446

0.27

7235

010

5027

1.6

7.5

13±

014

.10

0.00

0478

0.27

0901

011

0020

05.

513

±0

7.8

00.

0002

650.

2926

960

1250

60.2

1.6

12±

166

.60

0.00

2259

0.11

5017

014

5045

.51.

211

±2

84.4

00.

0028

630.

0583

950.

02M

7/m

usco

vite

b60

019

.60.

79

±3

75.5

0.08

0.00

2573

0.10

7679

0.29

650

311.

112

±2

61.7

0.06

0.00

2099

0.12

7233

0.17

700

39.4

1.4

10±

272

.20.

050.

0024

540.

1072

880.

1675

091

.23.

412

±0

54.4

0.02

0.00

1845

0.15

0456

0.06

775

246.

49.

212

±0

54.1

00.

0018

350.

1523

740.

0180

079

2.5

29.7

12±

035

00.

0011

870.

2084

660

825

386.

314

.512

±0

20.3

00.

0006

890.

2517

620

850

322.

112

.112

±0

20.1

00.

0006

810.

2534

220

875

222.

58.

312

±0

21.4

00.

0007

280.

2478

810

900

126.

64.

712

±0

26.8

00.

0009

110.

2365

70

925

74.9

2.8

12±

142

.90

0.00

1457

0.19

0072

0.01

950

52.8

1.9

12±

129

.20

0.00

0995

0.23

0051

0.02

975

47.6

1.7

12±

130

.80.

010.

0010

510.

2276

360.

0410

0048

.91.

812

±1

25.2

0.01

0.00

0859

0.23

7156

0.04

1050

68.7

2.5

12±

021

.90.

020.

0007

470.

2468

530.

0411

0051

.71.

913

±1

17.7

0.02

0.00

0606

0.25

7389

0.06

1250

29.4

1.1

13±

258

.20.

170.

0019

80.

1274

690.

414

508.

30.

30

±22

100.

40.

320.

0034

040.

0080

493.

89

Dow

nloa

ded

by [

Gua

ngzh

ou I

nstit

ute

of G

eoch

emis

try]

at 0

1:38

15

Mar

ch 2

012

International Geology Review 625

M12

B/m

usco

vite

c60

05.

70.

217

±32

96.8

0.16

0.00

3281

0.00

7244

0.29

650

8.7

0.3

5±

3599

.40.

030.

0033

680.

0039

430.

2270

021

.30.

90

±23

103.

20.

010.

0034

950.

0039

170

750

151.

56.

410

±1

84.8

00.

0028

720.

0584

880

775

411.

517

.512

±0

33.4

00.

0011

340.

2097

560

800

332.

414

.113

±0

8.9

00.

0003

030.

2854

570

825

332.

514

.113

±0

5.9

00.

0002

010.

2943

390

850

284.

812

.113

±0

50

0.00

017

0.29

6349

087

523

4.4

1013

±0

4.5

00.

0001

540.

2982

250

900

145.

46.

213

±0

9.6

00.

0003

260.

2824

620

925

83.5

3.5

13±

09.

20

0.00

0313

0.28

5742

095

056

2.3

13±

010

.50

0.00

036

0.28

1104

097

548

.62

12±

111

.10

0.00

038

0.28

245

010

0046

.51.

912

±1

11.5

00.

0003

940.

2827

960

1050

803.

412

±0

18.2

00.

0006

210.

2611

850

1100

76.1

3.2

13±

014

.20

0.00

0484

0.26

6207

012

5021

.10.

911

±3

78.6

00.

0026

720.

0796

850.

0114

503.

20.

10

±90

100.

60.

120.

0034

090.

0040

340.

59M

4/m

usco

vite

d65

04.

40.

843

±40

91.1

5.95

0.00

3091

0.00

8238

4.52

750

5.5

10

±24

102.

511

.02

0.00

348

0.01

3081

41.6

985

08.

81.

77

±13

96.7

15.1

50.

0032

830.

0179

4663

.990

010

.62

5±

895

16.1

60.

0032

360.

0383

4495

.88

950

7.5

1.4

7±

1192

.818

.81

0.00

3168

0.03

6481

73.9

897

517

.23.

311

±4

79.8

17.0

80.

0027

360.

0708

4646

.83

1000

44.6

8.7

15±

160

.117

.11

0.00

2073

0.10

3935

.13

1025

39.6

7.7

15±

152

.317

.93

0.00

1844

0.12

4987

37.5

210

507.

41.

412

±6

65.8

18.6

90.

0023

780.

1058

7547

.17

1075

12.8

2.4

13±

464

.718

.54

0.00

229

0.10

2652

43.3

611

0040

.97.

915

±2

54.8

17.9

70.

0019

170.

1173

4937

.13

1125

102

19.8

15±

053

.117

.78

0.00

183

0.12

5074

37.2

711

5087

.517

16±

144

.117

.52

0.00

1549

0.13

9233

34.6

311

7575

.214

.616

±1

38.6

16.7

90.

0013

860.

1496

4232

.71

1250

42.5

8.3

16±

149

.515

.95

0.00

1741

0.12

2174

30.7

413

502.

70.

558

±52

86.2

14.4

70.

0029

310.

0095

78.

4514

503

0.5

0±

103

100.

97.

250.

0034

220.

0032

5817

.42

Not

es:37

Ar/

39A

r,36

Ar/

40A

r,an

d39

Ar/

40A

rar

eco

rrec

ted

for

mas

ssp

ectr

omet

er.

IIC

(%),

inte

rfer

ing

isot

opes

corr

ecti

on.

a Tot

alga

sag

e=

12.8

±0.

2m

illi

onye

ars;

mea

nag

e(7

75–1

250◦

C)=

13±

0.1

mil

lion

year

s(2

σun

cert

aint

y);J

=0.

0023

±0.

0000

23.

bTo

talg

asag

e=

12.7

±0.

2m

illi

onye

ars;

mea

nag

e(7

50–1

250◦

C)=

13±

0.2

mil

lion

year

s(2

σun

cert

aint

y);J

=0.

0022

79±

2.27

9E–0

5.c T

otal

gas

age=

12.8

±0.

4m

illi

onye

ars;

mea

nag

e(7

75–1

100◦

C)=

13.5

±0.

2m

illi

onye

ars

(2σ

unce

rtai

nty)

;J=

0.00

2279

±2.

279E

–05.

dTo

talg

asag

e=

15.4

±1.

5m

illi

onye

ars;

mea

nag

e(1

000–

1250

◦ C)=

18±

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Figure 8. Age and 37Ar/39Ar spectra for muscovite and amphibole from the Nielaxiongbo gneiss dome. (A) M1 muscovite; (B) M7 mus-covite; (C) M12B muscovite; and (D) M4 amphibole. Half-heights of open rectangles indicate the 1σ relative (between-step) uncertainties.Plateau ages (with 2σ uncertainties) are indicated. For sample locations, see Figure 2.

Figure 9. Geological section A–B through the Bhutan Himalayas and Nielaxiongbo gneiss domes. The geology is based on the geologicalmap of Bhutan, INDEPTH geophysical data, Lee et al. (2000), BGMRXAR (1993), and our observations. The topography is from SRTM90 DEM data. The arrows indicate the flow direction of the middle crust ductilely deformed channel. For section locations, see Figure 1.Imposed on both sections are the contours, which indicate resistivity contours, after Unsworth et al. (2005, Figure 3). We suggest that thehigher contours may correspond to the active channel. The concentric dashed lines on the right-hand side indicate isotherms. The contoursin the eastern region are dashed because of the distance to the INDEPTH line (it has been shown that the resistivity varies along theorogen). MCT, Main Central thrust; YZS, Yarlung Zangbo Suture; MBT, Main Boundary Thrust; MFT, Main Frontal Thrust; MHT, MainHimalayan Thrust; KT, Kakhtang Thrust; STDS, South Tibetan Detachment; LHS, Lesser Himalayan Sequence; GHS, Greater HimalayanSequence; SK, Sakteng Klippe (remnants of the Tibetan sedimentary sequence soled by the STD); GBt, Gangdese Batholith; ILC, IndiaLithospheric Continent; IML, India Mantle Lithosphere.

uncertainty in the potassium decay constants. In this arti-cle, a plateau is defined as a sequence of three or moreconsecutive steps that are mutually indistinguishable at 1σ

and encompass at least 50% of the total 39Ar released.All three muscovite samples (M1 from leucogranite,

M12B from pegmatitic adamellite, and M7 from garnet-mica schist) (Table 2) yielded well-defined plateau agesat 13.0 ± 0.1, 13.5 ± 0.2, and 13.0 ± 0.2 million years,respectively (Figure 8). The hornblende sample from themiddle crustal layer (M4) also yielded a well-definedplateau age of 18 ± 0.3 million years.

6. Discussion

There are several major domes in South Tibet, includ-ing the Kangmar, Kampa, Mabja, and Malashan domes(Chen et al. 1990; Lee et al. 2004; Aoya et al. 2006;

Quigley et al. 2006), with the Nielaxiongbo dome beingthe easternmost one. These domes all have a similar struc-ture and are composed of a basement complex, a middlecrustal layer, and a sedimentary cover sequence, each unitbeing separated from one another by detachment faults.The Nielaxiongbo dome has ductilely deformed layer andabundant leucogranites, both of which may shed light on amiddle crustal ductile flow underneath the Tibetan Plateau.In particular, knowledge on the mechanism and timing ofuplift of the basement may provide important constraintson the evolution of such ductile flow.

6.1. Nature of the basement complex

The basement complex in the Nielaxiongbo gneiss domeconsists of amphibolites and granitic gneisses. Zirconsfrom an amphibolite (sample M13) yielded an average

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SHRIMP age of ∼514 million years. However, four grainsfrom this sample have much younger zircon ages of 47, 49,159, and 327 million years (Table 1), suggesting successivethermal disturbance. The youngest ages (47–49 millionyears) coincide approximately with the early stages of col-lision between the Indian and Eurasian plates (Himalayanorogeny) (Yin and Harrison, 2001; Thanh et al. 2010).

The early Palaeozoic age of the basement complex inthe Nielaxiongbo gneiss dome is similar to the ages of thegneiss from the Kangmar dome, which has a U–Pb zir-con age of 560 ± 4 million years (Scharer et al. 1986)and an Rb–Sr isochron age of 485 ± 6 million years(BGMRXAR 1993). Lee et al. (2000) reported similar U–Pb zircon ages of 508 million years for two orthogneissesin the Kangmar dome. Liu et al. (2006) also obtainedSHRIMP zircon U–Pb ages of 513 ± 10 and 502 ± 9million years for the Tanggaximu and Tongbashi plutons,respectively, both intruding basement rocks in the Yadongarea (Figure 1). The similar ages obtained from widely dis-tributed rocks indicate the existence of a common basementbeneath South Tibet.

We interpret these dates as marking the age of meta-morphism and magmatism in the basement rocks, sug-gesting consolidation of the basement during the earlyPalaeozoic. Thus, metamorphism and magmatism in theSouth Tibet basement rocks are thought to be related tothe Pan-African orogeny (Valdiya 1997; Liu et al. 2006),indicating an affinity with the Indian plate.

6.2. Exhumation of the basement complex associatedwith Neogene extension

In the Nielaxiongbo dome, E–W-striking deformationalfeatures, such as brittle thrust planes, the strike of type 1Cor 2 fold axial planes, and fan-shaped spaced crenulationcleavages fans (S1 foliation) (Figures 2, 3, and 4A), docu-ment early crustal shortening and thickening (D1). Previousstudies have dated the onset of crustal thickening at ∼50Ma; understanding in this process, however, remains lim-ited (Rowley 1998; Yin and Harrison 2001; Green et al.2008).

Ductile deformation (D2) produced foliation S2 andmineral lineation L2, S-C fabrics, press shadows withintrafolial folds, shear bands, and splitting of biotite por-phyroblasts (Figures 2, 4, and 5B), indicating top-to-the-north shear sense in the middle ductile crustal layer andshallower tectonic levels. In contrast, the basement com-plex, similar to that in the GHS, has features showingtop-to-the-south sense of shear.

Our 40Ar/39Ar ages obtained from the basement com-plex at Nielaxiongbo range from 18 ± 0.3 million yearsfor amphibole to 13.0 ± 0.2 million years for mus-covite (Figure 8). These ages suggest a cooling rate of30 ± 10◦C/million years through approximately 500◦C

to 350◦C (argon closure temperatures in amphibole andmuscovite, respectively), associated with exhumation of thebasement rocks. Based on K–Ar and 40Ar/39Ar apparentages, exhumation of the basement complex in Kangmaroccurred between 12.5 and 19 Ma (Debon et al. 1985),∼13 Ma (Chen et al. 1990), or 22 Ma (Liu 1984). Amore detailed 40Ar/39Ar thermochronological study of theKangmar dome yielded muscovite cooling ages of 15.24 ±0.05–12.23 ± 0.03 million years and biotite cooling agesof 17.04 ± 0.04–10.94 ± 0.03 million years (Lee et al.2000). The basement ages in Kangmar increase with depthand young northwards within a single structural level. NewU/Pb zircon ages from deformed migmatites and unde-formed granite in the core of the Mabja dome suggest thatextension, synchronous with peak metamorphism, beganat 35.0 ± 0.8 Ma, was ongoing at 23.1 ± 0.8 Ma andceased at 16.2 0.4 Ma (Lee et al. 2004, 2006; Lee andWhitehouse 2007). The young age corresponds with thetime of exhumation of the Himalayan metamorphic belt,which is dated at 16.8 Ma (Liu 1984). Thus, our new struc-tural and geochronological data from Nielaxiongbo andother works from Kangmar dome suggest that the ductiledeformation in the region began at or before ∼35 Ma in adeep tectonic level, resulting in southward ductile flow atthe mid-crustal tectonic level that continued from ∼23 Mato 13 Ma.

In the Nielaxiongbo gneiss dome, later brittle-ductiledeformation (D3) is characterized by spaced cleavage S3

and mineral lineation L3 (Figures 4A and 5B). These fab-rics are concentric to the leucogranite pluton, indicatingthat they were produced by intrusion of the leucogranite.The S3 foliation cuts the lower, but not the upper detach-ment, indicating that the lower detachment pre-dated theleucogranite intrusion.

These deformational features and their ages suggestthat development of D3 and intrusion of the leucogran-ites occurred immediately after, or partly synchronouswith, the north-directed ductile and brittle-ductile deforma-tion of the Tibetan sedimentary sequence and the south-ward ductile flow of the basement complex (GHS), whichformed D2. The metamorphic rocks of the dome, whichdecrease in metamorphic grade upward from migmatiteto kyanite + sillimanite + K-feldspar, to staurolite and gar-net + biotite assemblages, are similar to those at the top ofthe GHS (Waters et al. 2006).

6.3. Implications of silicic magmatism for themid-crustal ductile channel flow model

Leucogranites within the Nielaxiongbo gneiss dome havea zircon U–Pb age of ∼20 million years (M22 in Quzenadamellite), and muscovite, which develops along D2 fab-rics, has 40Ar/39Ar ages of 13 and 13.5 million years. Thisindicates that the leucogranites were emplaced at ∼20 Ma

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and cooled to ∼350◦C at 13.4–13 Ma. Thus, unroof-ing and cooling of the gneiss dome, which was triggeredby intrusion of the leucogranite, would have occurredslightly later than, or partly overlap with, the develop-ment of D2. The 40Ar/39Ar dates of ∼13 Ma constrainthe timing of top-to-the-north movement along the upperdetachment.

A deformed leucocratic dike swarm in the Mabja domeyielded a zircon U–Pb age of 23.1 + 0.8 million years,whereas a post-tectonic two-mica granite in the same bodyyielded a zircon U–Pb age of 14.2 + 0.2 million years and amonazite U–Pb age of 14.5 + 0.1 million years. The old ageis interpreted as the ongoing age for middle crustal ductileextension in the North Himalayan gneiss domes, whereasthe younger ages indicate that ductile deformation ceasedin the middle Miocene (Lee et al. 2006). The younger agesare similar to an age of 14.4 + 0.1 million years reportedby Zhang et al. (2004). It is concluded that emplacement ofthe leucogranite post-dated initiation of the D2 mid-crustalductile flow, indicating that the ductile flow triggered theintrusion of the leucogranite.

It is widely accepted that exhumation of the basementcomplexes in South Tibet was associated with extensionaltectonics and emplacement of granitic magmas (Maluski etal. 1988; Guillot et al. 1995; Harrison et al. 1997; Searleet al. 1997; Li et al. 2003). It is also generally acceptedthat these magmas formed by melting of crustal materi-als (Harris et al. 2004; Zhang et al. 2004). Leucogranites,which are widely distributed in the GHS and gneiss domes,have similar ages between ∼23 and ∼13 million yearsthroughout the region (Searle and Godin 2003; Godin etal. 2006; Quigley et al. 2006). Our SHRIMP U–Pb zir-con age of ∼20 million years for the leucogranite inNielaxiongbo is consistent within analytical errors with our40Ar/39Ar amphibole age for the local amphibolite, sug-gesting that exhumation of the Nielaxiongbo gneiss domewas associated with silicic plutonic activity.

The ductile deformation of the basement complex andthe middle crustal layer within the Nielaxiongbo, Kangmar,Kampa, Mabja, and Malashan domes, as well as similarbodies elsewhere in South Tibet and the Greater Himalayas,took place at mid-crustal levels between 24 and 12 mil-lion years and is probably partly a result of the ductilechannel flow in South Tibet (Chen et al. 1990; Edwardset al. 1996; Grujic et al. 1996, 2002; Hodges 2000, 2006;Beaumont et al. 2001, 2004, 2006; Li et al. 2003; Searleand Godin 2003; Godin et al. 2006; Yin 2006; Zhang et al.2007; Bai et al. 2010; Langille et al. 2010). The leucogran-ites could have been produced by melting of high-grademetamorphic basement rocks within the thickened crust(Le Fort et al. 1987; Inger and Harris 1993; Guillot andLe Fort 1995; Harris et al. 2004). The presence of par-tial melts (migmatites) or granitic magmas would havelubricated the boundaries of the ductile channel within themiddle crust, which may be represented by the extruded

palaeochannel of the Greater Himalayas (Grujic et al. 1996,2002; Beaumont et al. 2001, 2004, 2006; Godin et al. 2006;Hodges 2006). The metamorphic grade decreases upwardin both the gneiss domes and the GHS (Waters et al. 2006);a kinematic transition exists in South Tibet (Aoya et al.2005, 2006); and a kinematic inversion occurs elsewherein the GHS (Grujic et al. 1996). All of these features arecompatible with southward and upward extrusion of themid-crustal ductile channel flow.

6.4. An integrated model for the mid-crustal ductilechannel flow in South Tibet

Evidence for important normal faulting exists on the north-ern side of the Greater Himalayas (Burg and Chen 1984;Burchfiel et al. 1992; Edwards et al. 1996; Edwards andHarrison 1997; Wu et al. 1998), and the Kangmar dome hasbeen related to the same extensional event that producedthe STDS. The STDS and the gneiss domes in southernTibet suggest that shortening and extension were contem-poraneous at different tectonic levels within the Himalayanand South Tibetan crust (Chen et al. 1990; Burchfiel etal. 1992; Hodges et al. 1992, 1993). However, the extru-sion and channel flow hypothesis implies that the normalfaulting of the STDS resulted from southward flow andextrusion of the Greater Himalayas beneath the Tibetansedimentary sequence. The finite-element thermal mechan-ical model of Beaumont et al. (2001, 2004, 2006) andJamieson et al. (2002, 2004, 2006) provided test for thishypothesis, concluding that the formation of the NorthHimalayan gneiss domes was a response to flow dynam-ics in the weak mid-crustal channel rather than regionaltectonic extension or gravitational collapse of the TibetanPlateau. Relative N–S extension may have occurred inthe upper crust but is caused by shear stresses along theupper boundary of the southward-flowing middle crustalchannel (Beaumont et al. 2001, 2004). In addition, geo-dynamic models suggest that squeezing of the channelabove a crustal ramp can destabilize the upper crustaloverburden, facilitating the formation of a dome. Haucket al. (1998) and Yin (2006) have suggested that theNorth Himalayan domes are actually culminations alongthe North Himalayan antiform, which could be above aramp in the Main Himalayan Thrust.

The deformation and metamorphic gradients in theNielaxiongbo dome are most likely related to southwardand upward extrusion of the GHS. Although both themiddle crustal layer and the cover sequence have a north-ward kinematic sense, the contrast of deformational styleand metamorphic grade indicates a southward extrusionof the ductilely deformed middle crustal layer (the GHS)related to N–S extension of the cover sequence (Figure5B). Therefore, both the metamorphic gradient and theductile deformation are consistent with the tectonic modelinvolving southward extrusion between the MCT and the

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STDS (Grujic et al. 1996, 2002, 2006; Beaumont et al.2001, 2004, 2006; Godin et al. 2006).

Thus, we propose that the southward and locallyupward extrusion of the ductile channel flow in the middlecrust in South Tibet was responsible for the formation andexhumation of the gneiss domes in South Tibet (Figure 9).

The ductile channel flow in South Tibet occurs in themiddle crust where it includes at least amphibolite-faciesmetamorphic rocks plus migmatite and magmas producedby partial melting of the metamorphic rocks (Figure 9).Southward flow (D2) in this channel might have startedbetween the STDS and the MCT at ∼20 Ma or even ear-lier at 24–26 Ma (e.g. Godin et al. 2006; Hodges 2006; Leeand Whitehouse 2007), resulting in north-directed shear-ing along the STDS at shallow levels. The consequentemplacement of leucogranitic magmas (D3), which lubri-cated the boundary of the ductile channel flow, formedthe domes (Figure 9). Exhumation of the complex led tocooling through 500◦C and 300◦C between ca. 18 Ma and13 Ma and formed ductile deformation fabrics similar tothose in the Greater Himalayas (Hodges 2000). We suggestthat the upward movement of the ductile channel mighthave been due to continuous N–S shortening in the mid-dle and lower crusts of southern Tibet (Figures 1 and 9).Therefore, the extensional thinning of the upper crust ofthe Tibetan Plateau is marked by the doming and uplift ofductile channel in the STDS.

7. Conclusions

The Nielaxiongbo gneiss dome is composed of a GreaterHimalayan basement, a middle ductilely deformed crustallayer, and a sedimentary cover sequence, all separatedby detachment faults. Both the basement complex andthe middle crustal layer were intruded by leucogranites.The south-directed ductile deformation and associatedhigh-grade metamorphism, partial melting, and top-to-the-north shearing along the two strands of the STDSdefine mid-crust ductile channel flow. The basement rockswere exhumed during the Miocene involving temperaturesbetween approximately 500◦C and 350◦C at 18–13 Ma.Doming of the weak mid-crustal channel flow was probablyinitiated by the silicic plutonism at ∼20 Ma and associatedthinning of the upper crust of southern Tibet, which wascaused by the southward mid-crustal ductile channel flow.

AcknowledgementsThis study was supported by research grants from the NationalBasic Research Programme of China (2009CB421001), NationalScience Foundation of China (40872140, 40921062), the 111Project (B07011), and the National Science and EngineeringResearch Council of Canada. We are grateful to the geologicalteam of the Tibetan Geological Survey for assistance in the field,and we thank Keith Taylor for technical assistance in the ArgonIsotope Laboratory at Dalhousie University.

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