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

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1 This article was downloaded by: [Guangzhou Institute of Geochemistry] On: 15 March 2012, At: 01:38 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneiss dome, SE Tibet Dan-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 c a The State Key Laboratory of Geological Processes and Mineral Resources and School of Earth Sciences and Resources, China University of Geosciences, Beijing, , PR China b Department of Earth Sciences, University of Hong Kong, Hong Kong, PR China c Department of Earth Sciences, Dalhousie University, Halifax, NS, Canada d 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 gneiss dome, SE Tibet, International Geology Review, 54:6, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 International Geology Review Vol. 54, No. 6, 20 April 2012, Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneiss dome, SE Tibet Dan-Ping Yan a,b, Mei-Fu Zhou b, Paul T. Robinson b,c, Djordje Grujic c, John Malpas b, Allen Kennedy d and Peter H. Reynolds c a The State Key Laboratory of Geological Processes and Mineral Resources and School of Earth Sciences and Resources, China University of Geosciences, Beijing , PR China; b Department of Earth Sciences, University of Hong Kong, Hong Kong, PR China; c Department of Earth Sciences, Dalhousie University, Halifax, NS, Canada; d SHRIMP Facility, John de Laeter CEMS, Curtin University, Bentley 6103, Australia (Accepted 8 November 2010) Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous with shortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, and migmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weakly metamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from both the basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachment fault 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 fabrics are recognized: a N S compressional fabric (D 1 ) in the cover sequence, a north- and south-directed extensional fabric (D 2 ) in the upper detachment and lower tectonic units, and a deformation (D 3 ) related to the leucogranite intrusion. SHRIMP zircon U Pb dating yielded a metamorphic age of 514 million years for the amphibolite and a crystallization age of 20 million years for the leucogranite. Hornblende from the amphibolite has an 40 Ar/ 39 Ar age of 18 ± 0.3 million years, whereas muscovites from the schist and leucogranite yielded 40 Ar/ 39 Ar ages between 13.5 ± 0.2 and 13.0 ± 0.2 million years. These results 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 the 350 C Ar closure temperature in muscovite at 13 Ma. The middle crustal ductilely deformed layer within gneiss domes of southern Tibet defines a southward-extruding ductile channel, marked by leucogranites emplaced into migmatites and amphibolites. We propose a model involving thinned upper crust for the initial extension of the Tibetan Plateau in the early Miocene. Keywords: Nielaxiongbo; SE Tibet; gneiss dome; mid-crustal channel flow; South Tibetan Detachment System; middle crustal layer; leucogranite 1. Introduction Gneiss domes occurring within the South Tibetan sedimentary sequence have similar structural geometries and deformational 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 have been proposed to explain their genesis. For example, Burg et al. (1984) proposed a thrusting model to explain the formation of these domes, whereas Yin (2006) favoured a late wedge extrusion model in which the domes were formed by passive active roof faulting. Nevertheless, all models account for the Miocene Oligocene leucogranites (Zhang et al. 2004; Watts and Harris 2005; Aoya et al. 2006), well known in the Greater Himalayan Sequence (GHS) (Le Fort 1981; France-Lanord and Le Fort 1988; Inger and Harris 1993; Harrison et al. 1997; Grujic et al. 2002). On the other hand, identification of the South Tibetan Detachment System (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 et al. 2006; Lee and Whitehouse 2007; Zhang et al. 2007) (Figure 1) has led some workers to interpret the domes as metamorphic core complexes (e.g. Chen et al. 1990; Li et al. 2003). A change in shear direction from top-to-north in the northern side to top-to-south in southern side in the rocks of South Tibet has been related to intrusion of the leucogranites (Aoya et al. 2005, 2006). A similar kinematic inversion, which has also been observed elsewhere in the Himalaya along the footwall of the STDS, suggests Corresponding author. yandp@cugb.edu.cn ISSN print/issn online 2012 Taylor & Francis

3 616 D.-P. Yan et al. 90 E 29 N 29 N 86 E 88 E 90 E 92 E 100 km Figure 1. A schematic tectonic map showing the distribution of North Himalayan gneiss domes in South Tibet and the South Tibetan Detachment System (after Burchfiel et al. 1992; Hodges 2000). The South Tibetan Detachment System is marked by the heavy ticked lines in the hanging wall. YZS, Yarlung Zangbo Suture; STDS, South Tibetan Detachment. MCT, Main Central thrust; MBT, Main Boundary Thrust; 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 convergence at Ma (Harris et al. 2004; Zhang et al. 2004) supports the existence of a mid-crustal ductile channel flow (Godin et al. 2006; Bai et al. 2010) related to the formation 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 may provide a link between the metamorphism, ductile deformation, and concomitant leucogranite intrusion in the GHS and the STDS (Grujic et al. 1996, 2002; Beaumont et al. 2001, 2004; Jamieson et al. 2002, 2004, 2006; Searle and Szulc 2005; Hollister and Grujic 2006; King et al. 2007). However, the different processes of such a channel flow and geochronological constraints need further determining. The Nielaxiongbo gneiss dome, the easternmost dome in South Tibet, lies about 250 km east of the Kangmar dome (Chen et al. 1990; Lee et al. 2000; Aoya et al. 2006). It contains leucogranites similar to those elsewhere in South Tibet and the Greater Himalayas (BGMRXAR 1993; Zhang et al. 2007). Little is known about the timing and origin of the leucogranite in this complex and its relationship to the middle crustal rocks due to a lack of detailed structural, petrographic, and geochronological data. To determine the character of the Nielaxiongbo gneiss dome and its relationship to the STDS and possible existence of mid-crustal channel flow beneath southern 94 E Tibet, we carried out a reconnaissance field investigation and collected samples from all the relevant zones of the dome (Figure 2). This article describes the geology of the gneiss dome and presents new SHRIMP zircon U Pb and 40 Ar/ 39 Ar dating results for this body. The principal objectives of this article are to provide better constraints on the deformational/magmatic history of the Nielaxiongbo gneiss dome and to elucidate the role of ductile deformation and silicic magmatism in its formation. We further combine our data with those from other North Himalayan gneiss domes to explain the origin of these enigmatic features. Our data support a link between uplift of the North Himalayan gneiss domes and mid-crustal ductile channel flow in South Tibet. 2. Geological background Southern Tibet consists of two major tectonic units: the Indian subcontinent to the south and the Lhasa Block to the 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 narrow, 2 10 km-wide Yarlung Zangbo suture zone is marked by various lenses of Cretaceous ophiolites and mélange, Cretaceous forearc sedimentary sequences, and Tertiary molasse deposits, and consists of highly deformed greenschist, mica-quartz schist (Rowley 1996; Aitchison et al.

4 International Geology Review E E E E (B) (A) N N 617 L2 (n = 46) L2 (n = 9) m N N N N E E E E (C) (E) (D) 2m 10 m 2m (F) 2m Basement complex + leucogranites 4 km S2 S0 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 mineral lineations (the solid dots are from TGSA 2002, and open squares are our measurements). C-D indicates a section crosscutting the Nielaxiongbo dome. 2000, 2003; Malpas et al. 2003; Geng et al. 2006). The Lhasa Block is composed mainly of Precambrian basement rocks, Palaeozoic and Mesozoic cover strata intruded by abundant Mesozoic and Cenozoic granitic plutons (Zhu et al. 2009) (Figure 1). The basement rocks are composed mainly of gneiss, amphibolite, and schist, along with sporadic marble and granulite lenses (Dong et al. 2009; Wang et al. 2009; Zhang et al. 2010). These metamorphic rocks show evidence of extensive migmatization and polyphase deformation with P T conditions of 1.0 GPa and C (Wang et al. 2009). The sedimentary cover of the Lhasa Block consists of Devonian to Jurassic sedimentary rocks together with volcanic intercalations (Geng et al. 2006). The Indian subcontinent includes the Tethys-Himalayas and the GHS. Within the subcontinent, the STDS juxtaposes the Cambrian Eocene Tibetan sedimentary sequence over the GHS, which are composed mainly of amphibolite- to granulite-facies Proterozoic metasedimentary 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; Goscombe et al. 2006; Zhang et al. 2010). Farther south, the GHS lies above the Main Central Thrust (MCT). The Main Boundary Thrust separates the Lesser Himalayas from the Sub-Himalayas (Figure 1). A series of gneiss domes farther north form an E W-trending belt within the Tibetan sedimentary sequence (Figure 1). Leucogranites are common in these domes. 3. Nielaxiongbo gneiss dome The Nielaxiongbo gneiss dome is a NW-striking, domelike anticline, 22 km long and 10 km wide. It consists

5 618 D.-P. Yan et al. of a metamorphic complex (BGMRXAR 1993; TGSA 2002; Zhang et al. 2007; Zhang and Guo 2007), a ductilely deformed middle crustal layer, and the Tibetan sedimentary sequence that in this area comprises Upper Triassic and Lower Cretaceous sedimentary strata. The basement complex is composed of amphibolite-grade gneisses and locally migmatite, kyanite + staurolite+ garnet gneiss, biotite + muscovite + plagioclase gneiss, and biotite + plagioclase gneiss with well-developed foliation. The metamorphic grade of these rocks decreases structurally upward around the dome. A similar decrease in metamorphic grade outward from the dome is also observed in middle crustal Palaeozoic rocks overlying the basement sequence. These rocks pass outward from garnetmica schist, locally containing staurolite, to mica-bearing quartzite to phyllite. The cover sedimentary sequence of Upper Triassic and Lower Cretaceous age is a major component of the Tibetan sedimentary sequence (Figure 1) and is composed of terrigenous flysch (TGSA 2002). A lower detachment separates the basement complex from the middle crustal layer, and a major upper detachment separates the middle crustal layer from the cover sequence (Figure 2). In addition, minor normal faults within the middle crustal layer thinned or selectively removed parts of the strata. Several leucogranite plutons and stocks intrude the basement complex, the lower detachment, and the middle crustal layer, but do not intrude the upper detachment (Figure 2). These relationships are similar to those described for the Kangmar, Kampa, and other domes in South Tibet (Chen et al. 1990; Lee et al. 2000; Watts and Harris 2005; Quigley et al. 2006). At least three deformational stages (D 1 D 3 ) have been identified within the Nielaxiongbo gneiss dome N S compressional top-to-south phase (D 1 ) The earliest deformational phase D 1 is well developed in the Tibetan sedimentary sequence and locally preserved in the upper detachment fault and the middle crustal layer. It is represented by numerous sub-horizontal brittle thrusts, which typically cut the Tibetan sedimentary sequence. Within the Tibetan sedimentary sequence, the deformed plane is the original bedding surface (S 0 ). The folds in the pelitic and psammitic interbeds are sub-harmonic in profile with type 1B folds in competent sandstone beds and type 1C or 2 folds in incompetent shale or siltstone beds (cf. Ramsay and Huber 1987) (Figures 2 and 3A). The sandstone layers display a fan-shaped spaced cleavage (S 1 foliation), whereas the slates have inverse fan-shaped axial plane slaty cleavages (S 1 foliation) (inset C in Figure 2). In some cases, S 1 is represented by crenulation cleavage (inset F in Figure 2) and spaced cleavage, which has S-C fabrics indicating southward thrusting and displays a strong flattening strain (Figure 3C). S 1 is sub-perpendicular to the bedding S 0 on a regional scale. The axial planes of the folds strike 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 detachment fault. (A) Type 1C fold in Triassic interlayered sandstone and argillite, which indicate S 0 and S 1. Distance along the axial trace is 300 m; (B) Triassic sandstone with penetrative cleavage S 1 ; and (C) spaced cleavage with S-C fabric in the foliated sandstone.

6 International Geology Review 619 and to south in the southern limb of the dome (Figures 2 and 3A 3C), reflecting regional N S compression. In general, structural styles in the middle crustal layer are similar to those in the Tibetan sedimentary sequence (inset in Figure 2). The metamorphic mineral assemblage, which defines the foliation (S 1 ), consists of muscovite/ sericite+ plagioclase + chlorite + quartz (Figure 3B), reflecting deformation under low greenschist-facies conditions Extensional deformational phase (D 2 ) Deformational phase D 2 includes brittle/brittle-ductile deformation in the upper detachment fault and ductile deformation in the middle crustal layer, the basement detachment, and the basement complex. The upper detachment fault zone contains breccia and phyllite with phyllitic foliation S 2, which is sub-parallel to the fault (Figure 2). The upper detachment fault crosscuts D 1 folds and S 1 foliation and thus belongs to the D 2 deformation (Figure 4A). The metamorphic mineral assemblage along S 2 within this fault consists of muscovite/ sericite+ chlorite + plagioclase + quartz (Figure 4A), which indicates low to middle greenschist-facies conditions related to this deformation. The ductile deformation of the middle crustal layer and basement detachment (D 2 ) is manifested by the presence of (1) mylonite (Figures 4 and 5); (2) a penetrative S 2 foliation (Figure 4B 4D) defined by the axial planes of intrafolial folds; and (3) N S-trending ductile shear bands of centimetre size with a consistent top-to-the-north sense of shear, S-C fabric, kinking, and fan-type splitting along (001) of S 1 biotite porphyroblasts, garnet b -type pressure shadows, and well-developed mineral lineations, which have a N S trend (Figures 2A, 2B, and 2F, 4C and 4D). Within the sequence, the upper Palaeozoic rocks are separated from those of the lower Palaeozoic by a minor detachment fault (TGSA 2002). The metamorphic mineral assemblages consist of muscovite + plagioclase + biotite + garnet + quartz + hornblende ± staurolite (Figure 6A and 6B), indicating lower amphibolite-facies conditions that produced a garnet staurolite belt along the lower detachment (TGSA 2001). The assemblage of muscovite + plagioclase + biotite + garnet + quartz (garnet belt) in lower Palaeozoic sequence indicates upper greenschist-facies metamorphism and somewhat higher grade assemblages, consisting of hornblende + biotite + plagioclase + garnet + staurolite + quartz (staurolite belt), indicate upper greenschist- to lower amphibolite-facies metamorphism (Figure 2; TGSA 2002). An assemblage of muscovite + plagioclase + biotite + quartz (Figure 4D) in the upper Palaeozoic strata indicates middle greenschistfacies metamorphism. The ductile deformation of D 2 indicates north-directed extensional tectonics at the middle upper crustal level. Ductile deformation at the top of the basement complex, which produced a mylonitic foliation (S 2 ) and a shallowly N- or S-dipping mineral lineation (L 2 ) (Figure 5A and 5B), is equivalent to the D 2 deformation in the middle crustal layer; however, S-C fabrics and garnet b - type pressure shadows indicate a top-to-the-south sense of shear (Figure 5B). The metamorphic mineral assemblage along the S 2 foliation consists of kyanite + sillimanite + K-feldspar + muscovite + hornblende + staurolite + plagioclase ± biotite + quartz (Figure 5A and 5B), indicating amphibolite-facies metamorphism. The local occurrence of migmatite indicates partial melting within the complex Deformational phase (D 3 ) related to the intrusion of leucogranites The leucogranites form two nested bodies of adamellite with many peripheral dikes and sills. The Quzen body (Figure 5C) intrudes the basement complex, the lower detachment fault, and the middle crustal layer, that is, it cuts through the D 1 and D 2 fabrics but not the upper detachment fault or cover sequence (Figure 2; Zhang et al. 2007). S 3 crenulation and L 3 mineral lineation, which define the D 3 deformation, dip steeply outward around the dome (Figures 2, 4A, and 5B). Thus, D 3 was formed during final emplacement of the leucogranites. The leucogranites are medium grained and consist essentially of plagioclase (An ), K-feldspar, muscovite, and quartz (Figure 5D). The Quzen body is intruded by the Renbuo body of fine-grained adamellite with an identical mineralogy, that is, plagioclase (An ), K-feldspar, and quartz with minor muscovite, biotite, zircon, apatite, and rutile. The leucogranites crosscut the well-developed D 2 foliation and L 2 lineation in the country rocks. A few samples of deformed leucogranite (M22 and M1) from the Quzen body contain remnants of sillimanite (D 2 ) along shear bands (S 3 ) (Figure 2) with steeply outward-dipping mineral lineation, indicating that the leucogranite was emplaced into sillimanite-grade country rocks already at high temperature and that the ductile deformation pre-dated the main emplacement 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 amphibolite (M13) from the basement complex and a leucogranite (M22) from the Quzen body that intrudes the basement complex (Figure 2). After separation, about 100 grains of zircon were mounted for U Pb isotope analysis at the SHRIMP II lab at Curtin University of Technology, Australia. Analytical and data-reduction procedures are similar to those described by Compston et al. (1984). Pb/U ages are based on a value of 564 million years determined

7 620 D.-P. Yan et al. Figure 4. Field and thin section photos of ductile deformation and metamorphic mineral assemblages in the middle crustal layer. (A) S 2 crenulations with axial plane foliation S 3. Note that quartz-rich and mica-rich shear bands define the S 1 foliation; (B) penetrative S 2 foliation within garnet-bearing mylonite of the middle crustal layer; (C) S 2 defined by the oriented growth of muscovite + plagioclase + biotite + quartz (garnet belt) in the middle crustal layer; (D) in the middle crustal layer, the penetrative S 2 foliation is defined by alignment of muscovite + plagioclase + biotite + quartz, and σ -type garnet porphyroblasts with pressure shadows; and (E) S 1 biotite porphyroblast shows kinking and fan-type splitting along (001) within the penetrative S 2 foliation indicating top-to-the-north shearing in the lower detachment shear zone. by thermal ionization mass spectrometry U Pb analysis of the standard zircon CZ3. Because the 207 Pb/ 206 Pb ages are sensitive to the common Pb correction, the 206 Pb/ 238 U age is normally adopted for Phanerozoic samples (Compston et al. 1984). Individual analyses (Table 1) are presented as 1σ error boxes on concordia plots and uncertainties in mean ages are quoted at the 95% confidence level (2σ ) Ar/ 39 Ar dating For 40 Ar/ 39 Ar dating, muscovite was separated from a pegmatitic adamellite (M12B) of the Renbuo body, a leucogranite of the Quzen body (M1), and a garnet-mica schist (M7). Hornblende was separated from an amphibolite (M4) of the middle crustal layer. Fresh portions of each sample were cut, crushed, and sieved to obtain mineral grains of mm diameter. These were washed in distilled 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 the hornblende MMhb-1 standard (520 ± 2 Ma; Samson and Alexander 1987). The disks were wrapped in aluminium foil and vacuum sealed in silica glass tubes. The packages were then shielded with cadmium and irradiated for 54 hours at the McMaster University nuclear reactor. Isotopic analyses were carried out in the Argon Isotope Laboratory

8 International Geology Review 621 Figure 5. Field and thin section photos of the deformation and mineral assemblages in the GHS and leucogranite. (A) Migmatite of the GHS; (B) gneiss from the core complex showing mylonite foliation (S 2 ) and a metamorphic mineral assemblage of (sillimanite) + hornblende + plagioclase + muscovite + biotite + quartz + K-feldspar; (C) leucogranite intruded into the amphibolite; and (D) leucogranite of the Quzen body. M ±1 Ma M ±7 Ma (A) M22-3 (G1) 482±9 Ma M22-3 (G1) 20±0 Ma Figure 6. Cathodoluminescence morphologies of zircon grains from (A) amphibolite (M13) and (B) leucogranite (M22) of the Nielaxiongbo, showing metamorphic recrystallization and growth in the leucogranite magma. at Dalhousie University, Canada, using a VG 3600 mass spectrometer equipped with an internal tantalum resistance furnace of the double-vacuum type. Details of the experimental procedures are as described in Muecke et al. (1988). All dates are reported using a 1 as the total decay constant for 40 K (Steiger and Jager 1977). 5. Analytical results 5.1. SHRIMP zircon U Pb age The amphibolite, M13, from the basement complex contains zircon grains with a variety of textures and morphologies. The grains display highly complex internal structures as shown in the cathodoluminescence images (Figure 6A), which likely resulted from variable degrees of recrystallization. Some grains have thin overgrowths with intermediate to high U contents and low and variable Th/U ratios (<1 0.01) (Figure 7A). Many of the recrystallized grains contain inclusions. The SHRIMP analytical results for zircon from sample M13 are given in Table 1 and are plotted on a concordia diagram (Figure 7B and 7C); 14 of 18 analyses cluster around 500 million years on the concordia diagram, whereas the other 4 have much younger ages (Figure 7B). Two grains with ages close to 500 million years (M13-5 and M13-6) have very low U contents and one (M13-16) yielded a discordant analysis (Figure 7C). When these three anomalous analyses are excluded, the remaining 11 form a coherent (B)

9 622 D.-P. Yan et al. Table 1. SHRIMP U Pb isotopic analyses for zircons from the leucogranite and basement complex of Nielaxiongbo dome. Spot 206Pbc (%) U (ppm) Th (ppm) Ratios corrected common Pb Ages (Ma) Pb (ppm) Th/U % concordance 206 Pb/ 238 U ±% 207 Pb/ 235 U ±% 207 Pb/ 206 Pb ±% 208 Pb /232 Th ±% 206 Pb/ 238 U ± 207 Pb/ 235 U ± 207 Pb/ 206 Pb ± M22 (Quzen adamellite of leucogranite) M E M E M M E M E M E M E M E M E M E M13 (amphibolite in basement complex) M M M M M M M M M M M M M M M M M M Notes: 206 Pbc (%) indicates percentage of total 206 Pb that is non-radiogenic. Common Pb corrected by assuming 206 Pb/ 238 U- 208 Pb/ 232 Th age-concordance.

10 International Geology Review M22 M13 (A) Th (ppm) TH/U = 1 TH/U = 0.1 TH/U = U (ppm) Amphibolite, M13 (B) 0.10 Amphibolite, M13 (C) 206 Pb/ 238 U 206 Pb/ 238 U Ma Pb/ 235 U Leucogranite, M Upper intercepts at 206 Pb/ 238 U=470±92Ma Pb/ 235 U (D) 206 Pb/ 238 U 206 Pb/ 238 U ±6Ma Pb/ 235 U Leucogranite, M Pb/ 238 U=514±6Ma 207 Pb/ 235 U=511±6Ma 207 Pb/ 206 U=503±13Ma N =11 (E) Lower intercepts at 206 Pb/ 238 U=20.3±1.9Ma N = Pb/ 235 U Figure 7. (A) Th versus U diagram for the analysed zircons from the Nielaxiongbo gneiss dome. (B and C) Concordia diagrams for the zircon from the amphibolite (M13). (D and E) Concordia diagrams of zircon from the leucogranite (M22). For sample locations, see Figure 2. group with a mean 206 Pb/ 238 U age of 514 ± 6 million years, a mean 207 Pb/ 235 U age of 511 ± 6 million years, and a mean 207 Pb/ 206 Pb 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 significant scatter in the ages. The scatter suggests some geological disturbance, consistent with the recrystallization of zircons shown 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 U contents, suggesting some post-crystallization disturbance of the U Pb systematics. Zircon grains from the leucogranite sample, M22 (Table 1), show oscillatory zoning and are clearly magmatic in origin (Figure 6B). They have relatively low Th contents with Th/U ratios of (Figure 7A). When plotted on a concordia diagram (Figure 7D), one sample yielded a discordant age with an upper intercept 206 Pb/ 238 U age of ± 92 million years (Figure 7D). The other nine form a single cluster on the diagram (Figure 7E), yielding a mean 206 Pb/ 238 U age of 20.3 ± 1.9 million years (Figure 7E), which is considered to be the best estimate of the crystallization age for the leucogranite. The older grain is close in age to zircons from the basement amphibolite and is probably a xenocryst captured during emplacement of the leucogranite Ar/ 39 Ar analytical results The 40 Ar/ 39 Ar data of muscovite and hornblende, corrected for interfering isotopes and mass discrimination, are summarized in Table 2, and the apparent age spectra for each sample are illustrated in Figure 8. All age errors are quoted at the 95% confidence level (2σ ). They include errors in the irradiation correction factors and the error in the neutron fluence parameter but do not include the 26 30

11 624 D.-P. Yan et al. Table 2. Results of 40 Ar/ 39 Ar dating of the separated muscovite and amphibole. Sample/separate T ( C) 39 Ar (mv) 39 Ar (%) Age (Ma) ± 1σ Atm. cont. (%) 37 Ar/ 39 Ar 36 Ar/ 40 Ar 39 Ar/ 40 Ar IIC (%) M1/muscovite a ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± M7/muscovite b ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

12 International Geology Review 625 M12B/muscovite c ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± M4/muscovite d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Notes: 37 Ar/ 39 Ar, 36 Ar/ 40 Ar, and 39 Ar/ 40 Ar are corrected for mass spectrometer. IIC (% ), interfering isotopes correction. a Total gas age = 12.8 ± 0.2 million years; mean age ( C) = 13 ± 0.1 million years (2σ uncertainty); J = ± b Total gas age = 12.7 ± 0.2 million years; mean age ( C) = 13 ± 0.2 million years (2σ uncertainty); J = ± 2.279E 05. c Total gas age = 12.8 ± 0.4 million years; mean age ( C) = 13.5 ± 0.2 million years (2σ uncertainty); J = ± 2.279E 05. d Total gas age = 15.4 ± 1.5 million years; mean age ( C) = 18 ± 0.3 million years (2σ uncertainty); J = ± 2.279E 05.

13 626 D.-P. Yan et al. (A) Age (Ma) (C) Age (Ma) Quzen leucogranite 13.0 ± 0.1 Ma % 39 Ar Released Renbuo pegmatitic granite ± 0.2 Ma ± 0.3 Ma % 39 Ar Released % 39 Ar Released (B) Age (Ma) (D) Age (Ma) % 39 Ar Released Garnet-mica schist 13.0 ± 0.2 Ma Amphibolite Figure 8. Age and 37 Ar/ 39 Ar spectra for muscovite and amphibole from the Nielaxiongbo gneiss dome. (A) M1 muscovite; (B) M7 muscovite; (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 geological map of Bhutan, INDEPTH geophysical data, Lee et al. (2000), BGMRXAR (1993), and our observations. The topography is from SRTM 90 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 the higher contours may correspond to the active channel. The concentric dashed lines on the right-hand side indicate isotherms. The contours in the eastern region are dashed because of the distance to the INDEPTH line (it has been shown that the resistivity varies along the orogen). MCT, Main Central thrust; YZS, Yarlung Zangbo Suture; MBT, Main Boundary Thrust; MFT, Main Frontal Thrust; MHT, Main Himalayan Thrust; KT, Kakhtang Thrust; STDS, South Tibetan Detachment; LHS, Lesser Himalayan Sequence; GHS, Greater Himalayan Sequence; SK, Sakteng Klippe (remnants of the Tibetan sedimentary sequence soled by the STD); GBt, Gangdese Batholith; ILC, India Lithospheric Continent; IML, India Mantle Lithosphere. uncertainty in the potassium decay constants. In this article, a plateau is defined as a sequence of three or more consecutive steps that are mutually indistinguishable at 1σ and encompass at least 50% of the total 39 Ar released. All three muscovite samples (M1 from leucogranite, M12B from pegmatitic adamellite, and M7 from garnetmica schist) (Table 2) yielded well-defined plateau ages at 13.0 ± 0.1, 13.5 ± 0.2, and 13.0 ± 0.2 million years, respectively (Figure 8). The hornblende sample from the middle crustal layer (M 4 ) also yielded a well-defined plateau age of 18 ± 0.3 million years. Quigley et al. 2006), with the Nielaxiongbo dome being the easternmost one. These domes all have a similar structure and are composed of a basement complex, a middle crustal layer, and a sedimentary cover sequence, each unit being separated from one another by detachment faults. The Nielaxiongbo dome has ductilely deformed layer and abundant leucogranites, both of which may shed light on a middle crustal ductile flow underneath the Tibetan Plateau. In particular, knowledge on the mechanism and timing of uplift of the basement may provide important constraints on the evolution of such ductile flow. 6. Discussion There are several major domes in South Tibet, including the Kangmar, Kampa, Mabja, and Malashan domes (Chen et al. 1990; Lee et al. 2004; Aoya et al. 2006; 6.1. Nature of the basement complex The basement complex in the Nielaxiongbo gneiss dome consists of amphibolites and granitic gneisses. Zircons from an amphibolite (sample M13) yielded an average

14 International Geology Review 627 SHRIMP age of 514 million years. However, four grains from this sample have much younger zircon ages of 47, 49, 159, and 327 million years (Table 1), suggesting successive thermal disturbance. The youngest ages (47 49 million years) coincide approximately with the early stages of collision between the Indian and Eurasian plates (Himalayan orogeny) (Yin and Harrison, 2001; Thanh et al. 2010). The early Palaeozoic age of the basement complex in the Nielaxiongbo gneiss dome is similar to the ages of the gneiss from the Kangmar dome, which has a U Pb zircon 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 orthogneisses in the Kangmar dome. Liu et al. (2006) also obtained SHRIMP zircon U Pb ages of 513 ± 10 and 502 ± 9 million years for the Tanggaximu and Tongbashi plutons, respectively, both intruding basement rocks in the Yadong area (Figure 1). The similar ages obtained from widely distributed rocks indicate the existence of a common basement beneath South Tibet. We interpret these dates as marking the age of metamorphism and magmatism in the basement rocks, suggesting consolidation of the basement during the early Palaeozoic. Thus, metamorphism and magmatism in the South Tibet basement rocks are thought to be related to the Pan-African orogeny (Valdiya 1997; Liu et al. 2006), indicating an affinity with the Indian plate Exhumation of the basement complex associated with Neogene extension In the Nielaxiongbo dome, E W-striking deformational features, such as brittle thrust planes, the strike of type 1C or 2 fold axial planes, and fan-shaped spaced crenulation cleavages fans (S 1 foliation) (Figures 2, 3, and 4A), document early crustal shortening and thickening (D 1 ). Previous studies have dated the onset of crustal thickening at 50 Ma; understanding in this process, however, remains limited (Rowley 1998; Yin and Harrison 2001; Green et al. 2008). Ductile deformation (D 2 ) produced foliation S 2 and mineral lineation L 2, S-C fabrics, press shadows with intrafolial folds, shear bands, and splitting of biotite porphyroblasts (Figures 2, 4, and 5B), indicating top-to-thenorth shear sense in the middle ductile crustal layer and shallower tectonic levels. In contrast, the basement complex, similar to that in the GHS, has features showing top-to-the-south sense of shear. Our 40 Ar/ 39 Ar ages obtained from the basement complex at Nielaxiongbo range from 18 ± 0.3 million years for amphibole to 13.0 ± 0.2 million years for muscovite (Figure 8). These ages suggest a cooling rate of 30 ± 10 C/million years through approximately 500 C to 350 C (argon closure temperatures in amphibole and muscovite, respectively), associated with exhumation of the basement rocks. Based on K Ar and 40 Ar/ 39 Ar apparent ages, exhumation of the basement complex in Kangmar occurred between 12.5 and 19 Ma (Debon et al. 1985), 13 Ma (Chen et al. 1990), or 22 Ma (Liu 1984). A more detailed 40 Ar/ 39 Ar thermochronological study of the Kangmar dome yielded muscovite cooling ages of ± ± 0.03 million years and biotite cooling ages of ± ± 0.03 million years (Lee et al. 2000). The basement ages in Kangmar increase with depth and young northwards within a single structural level. New U/Pb zircon ages from deformed migmatites and undeformed granite in the core of the Mabja dome suggest that extension, synchronous with peak metamorphism, began at 35.0 ± 0.8 Ma, was ongoing at 23.1 ± 0.8 Ma and ceased at Ma (Lee et al. 2004, 2006; Lee and Whitehouse 2007). The young age corresponds with the time of exhumation of the Himalayan metamorphic belt, which is dated at 16.8 Ma (Liu 1984). Thus, our new structural and geochronological data from Nielaxiongbo and other works from Kangmar dome suggest that the ductile deformation in the region began at or before 35 Ma in a deep tectonic level, resulting in southward ductile flow at the mid-crustal tectonic level that continued from 23 Ma to 13 Ma. In the Nielaxiongbo gneiss dome, later brittle-ductile deformation (D 3 ) is characterized by spaced cleavage S 3 and mineral lineation L 3 (Figures 4A and 5B). These fabrics are concentric to the leucogranite pluton, indicating that they were produced by intrusion of the leucogranite. The S 3 foliation cuts the lower, but not the upper detachment, indicating that the lower detachment pre-dated the leucogranite intrusion. These deformational features and their ages suggest that development of D 3 and intrusion of the leucogranites occurred immediately after, or partly synchronous with, the north-directed ductile and brittle-ductile deformation of the Tibetan sedimentary sequence and the southward ductile flow of the basement complex (GHS), which formed D 2. The metamorphic rocks of the dome, which decrease in metamorphic grade upward from migmatite to kyanite + sillimanite + K-feldspar, to staurolite and garnet + biotite assemblages, are similar to those at the top of the GHS (Waters et al. 2006) Implications of silicic magmatism for the mid-crustal ductile channel flow model Leucogranites within the Nielaxiongbo gneiss dome have a zircon U Pb age of 20 million years (M22 in Quzen adamellite), and muscovite, which develops along D 2 fabrics, has 40 Ar/ 39 Ar ages of 13 and 13.5 million years. This indicates that the leucogranites were emplaced at 20 Ma

15 628 D.-P. Yan et al. and cooled to 350 C at Ma. Thus, unroofing and cooling of the gneiss dome, which was triggered by intrusion of the leucogranite, would have occurred slightly later than, or partly overlap with, the development of D 2.The 40 Ar/ 39 Ar dates of 13 Ma constrain the timing of top-to-the-north movement along the upper detachment. A deformed leucocratic dike swarm in the Mabja dome yielded a zircon U Pb age of million years, whereas a post-tectonic two-mica granite in the same body yielded a zircon U Pb age of million years and a monazite U Pb age of million years. The old age is interpreted as the ongoing age for middle crustal ductile extension in the North Himalayan gneiss domes, whereas the younger ages indicate that ductile deformation ceased in the middle Miocene (Lee et al. 2006). The younger ages are similar to an age of million years reported by Zhang et al. (2004). It is concluded that emplacement of the leucogranite post-dated initiation of the D 2 mid-crustal ductile flow, indicating that the ductile flow triggered the intrusion of the leucogranite. It is widely accepted that exhumation of the basement complexes in South Tibet was associated with extensional tectonics and emplacement of granitic magmas (Maluski et al. 1988; Guillot et al. 1995; Harrison et al. 1997; Searle et al. 1997; Li et al. 2003). It is also generally accepted that these magmas formed by melting of crustal materials (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 years throughout the region (Searle and Godin 2003; Godin et al. 2006; Quigley et al. 2006). Our SHRIMP U Pb zircon age of 20 million years for the leucogranite in Nielaxiongbo is consistent within analytical errors with our 40 Ar/ 39 Ar amphibole age for the local amphibolite, suggesting that exhumation of the Nielaxiongbo gneiss dome was associated with silicic plutonic activity. The ductile deformation of the basement complex and the middle crustal layer within the Nielaxiongbo, Kangmar, Kampa, Mabja, and Malashan domes, as well as similar bodies elsewhere in South Tibet and the Greater Himalayas, took place at mid-crustal levels between 24 and 12 million years and is probably partly a result of the ductile channel flow in South Tibet (Chen et al. 1990; Edwards et al. 1996; Grujic et al. 1996, 2002; Hodges 2000, 2006; Beaumont et al. 2001, 2004, 2006; Li et al. 2003; Searle and Godin 2003; Godin et al. 2006; Yin 2006; Zhang et al. 2007; Bai et al. 2010; Langille et al. 2010). The leucogranites could have been produced by melting of high-grade metamorphic basement rocks within the thickened crust (Le Fort et al. 1987; Inger and Harris 1993; Guillot and Le Fort 1995; Harris et al. 2004). The presence of partial melts (migmatites) or granitic magmas would have lubricated the boundaries of the ductile channel within the middle 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 upward in 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 elsewhere in the GHS (Grujic et al. 1996). All of these features are compatible with southward and upward extrusion of the mid-crustal ductile channel flow An integrated model for the mid-crustal ductile channel flow in South Tibet Evidence for important normal faulting exists on the northern side of the Greater Himalayas (Burg and Chen 1984; Burchfiel et al. 1992; Edwards et al. 1996; Edwards and Harrison 1997; Wu et al. 1998), and the Kangmar dome has been related to the same extensional event that produced the STDS. The STDS and the gneiss domes in southern Tibet suggest that shortening and extension were contemporaneous at different tectonic levels within the Himalayan and South Tibetan crust (Chen et al. 1990; Burchfiel et al. 1992; Hodges et al. 1992, 1993). However, the extrusion and channel flow hypothesis implies that the normal faulting of the STDS resulted from southward flow and extrusion of the Greater Himalayas beneath the Tibetan sedimentary sequence. The finite-element thermal mechanical model of Beaumont et al. (2001, 2004, 2006) and Jamieson et al. (2002, 2004, 2006) provided test for this hypothesis, concluding that the formation of the North Himalayan gneiss domes was a response to flow dynamics in the weak mid-crustal channel rather than regional tectonic extension or gravitational collapse of the Tibetan Plateau. Relative N S extension may have occurred in the upper crust but is caused by shear stresses along the upper boundary of the southward-flowing middle crustal channel (Beaumont et al. 2001, 2004). In addition, geodynamic models suggest that squeezing of the channel above a crustal ramp can destabilize the upper crustal overburden, facilitating the formation of a dome. Hauck et al. (1998) and Yin (2006) have suggested that the North Himalayan domes are actually culminations along the North Himalayan antiform, which could be above a ramp in the Main Himalayan Thrust. The deformation and metamorphic gradients in the Nielaxiongbo dome are most likely related to southward and upward extrusion of the GHS. Although both the middle crustal layer and the cover sequence have a northward kinematic sense, the contrast of deformational style and metamorphic grade indicates a southward extrusion of the ductilely deformed middle crustal layer (the GHS) related to N S extension of the cover sequence (Figure 5B). Therefore, both the metamorphic gradient and the ductile deformation are consistent with the tectonic model involving southward extrusion between the MCT and the