The Malashan Gneiss Dome in South Tibet: Comparative Study with the Kangmar Dome with Special Reference to Kinematics of Deformation and Origin of Associated Granites

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The Malashan gneiss dome in south Tibet: comparative study with the Kangmar dome with special reference to kinematics of deformation and origin of associated granites

M. AOYA1, S. R. WALLIS2, T. KAWAKAMI3, J. LEE4,Y.WANG5 & H. MAEDA6 1Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Tsukuba 305-8567, Japan (e-mail: [email protected]) 2Department of Earth & Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan 3Department of Earth Sciences, Faculty of Education, Okayama University, Okayama, 700-8530, Japan (Present address: Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Tsukuba 305-8567, Japan) 4Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA 5Department of Geology, China University of Geosciences, Beijing 100083, China 6Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Abstract: Despite the importance of Tethys Himalayan or North Himalayan gneiss domes for dis- cussing extrusive flow of the underlying Greater Himalayan sequence, these metamorphic domes in general remain poorly documented. The main exception is the Kangmar dome. The Malashan metamorphic complex, a newly documented North Himalayan gneiss dome, is shown to have strong similarities with the Kangmar dome, suggesting that the North Himalayan gneiss domes have the following features in common: (i) Barrovian-type metamorphism with grade increasing towards a centrally located two-mica granite; (ii) the presence of two dominant ductile deformation stages, D1 and D2, with D2 showing an increasing strength towards the granite contacts; and (iii) the development of a strong D2 foliation (gneissosity) in the outermost part of the granite cores. In addition, field and bulk-chemical studies show: (i) D2 is associated with a dominant top-to-the-north sense of shear (in disagreement with the most recent kinematic studies in Kangmar dome); (ii) the deposition age of associated metasediments is upper Jurassic suggesting that the Malashan dome is located not at the base, but within the middle section of the Tethys Himalaya; and (iii) in contrast to the Kangmar granitic gneiss that is interpreted as Indian basement, three granitic bodies in Malashan all formed as young intrusive bodies during the Himalayan orogeny. These results suggest that the formation mechanism of the North Himalayan gneiss domes needs to be re-evaluated to test the rigidity of the hanging wall assumed in channel flow models.

Contemporaneous activity of thrust faults, with top- the length of the main Himalayan range (Fig. 1). to-the-south movement, and normal faults, with Southward extrusion of the Greater Himalaya, a top-to-the-north movement, is one of the most strik- high-grade metamorphic sequence sandwiched ing characteristics of the Himalayan orogeny (e.g. between the MCT and the STD (Fig. 1), is com- Burg et al. 1984a; Burchﬁel et al. 1992; Hodges monly presented as one of the best documented et al. 1992; Hodges 2000). The south Himalayan examples of large-scale channel ﬂow and extrusion thrust systems such as the Main Central Thrust of middle to lower continental crust (e.g. Beaumont (MCT) and Main Boundary Thrust (MBT) are over- et al. 2001, 2004; Jamieson et al. 2004). In this tec- lain by a normal fault system, the South Tibetan tonic framework, the Greater Himalaya forms the Detachment (STD), which stretches roughly along footwall of the north-dipping STD with the Tethys

From:LAW, R. D., SEARLE,M.P.&GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 471–495. 0305-8719/06/$15.00 # The Geological Society of London 2006. 472 M. AOYA ET AL.

Fig. 1. Tectonic map of south Tibet (modiﬁed from Burchﬁel et al. 1992) showing location of the Kangmar and Malashan areas. ITSZ, Indus–Tsangpo Suture Zone; MBT, Main Boundary Thrust; MCT, Main Central Thrust; STD, South Tibetan Detachment.

Himalaya in the hanging wall (Fig. 1). The Tethys with a formation age of 32–12 Ma (e.g. Scha¨rer Himalaya is dominated by nearly unmetamor- 1984; Hodges et al. 1992, 1996; Edwards & Harrison phosed Ordovician–Eocene sedimentary rocks, 1997; Searle et al. 1997; Harrison et al. 1999; which were deposited on the northern continental Simpson et al. 2000). These studies raise the possi- margin of India (e.g. Hodges 2000 and references bility that the granite cores of the North Himalayan therein). Exposed locally within this domain are gneiss domes might also have been generated by isolated gneiss domes, the North Himalayan syncollisional Himalayan magmatism. If so, the gneiss domes, most of which are associated with North Himalayan gneiss domes may provide a link cores of two-mica granitic gneiss (e.g. Burg et al. between high-grade metamorphism, crustal anatexis 1984b; Lee et al. 2004; Fig. 1). High-grade metase- and regional deformation in the structural section dimentary rocks that mantle the granitic cores show overlying the Greater Himalaya. development of a strong ductile deformational In spite of this potential significance, most of the fabric. Characterizing the kinematics of this defor- North Himalayan gneiss domes have not yet been mation phase is important for documenting the well documented. The main exceptions are the spatial and kinematic relationships between these Kangmar (Burg et al. 1984b, 1987; Chen et al. areas and the normal-slip (top-down-to-the-north) 1990; Lee et al. 2000; Figs 1 & 2) and Mabja motion of the STD. Furthermore, characterizing the domes (Lee et al. 2004, 2006; Zhang et al. 2004). origin and role of the granitic core in the North Hima- In this contribution, we describe another example layan gneiss domes is important for documenting how of the North Himalayan gneiss domes, the these domes formed. Geophysical observations, Malashan metamorphic complex (Figs 1 & 3), including short-wavelength gravity anomalies (Jin which has remained undocumented since its first indi- et al. 1994), and the coincidence of high electrical cation on simplified tectonic maps (Burg & Chen, conductivity, middle crustal low velocities and reflec- 1984; Burg et al.,1984b). In addition to documenting tion bright spots (Chen et al. 1996; Makovsky et al. the Malashan complex, we also use a comparison 1996; Nelson et al. 1996; Alsdorf & Nelson 1999), between the metamorphic, structural and geochem- suggest the presence of partial melt within the ical features of the Malashan dome with the present-day middle crust of Tibet. In addition, field Kangmar dome to summarize: (i) general geological and geochronological studies of leucogranites in the characteristics of the North Himalayan gneiss Greater Himalaya (Fig. 1) indicate that associated domes; and (ii) unresolved problems where disagree- migmatitic sequences can be regarded as a fossil ment exists between researchers and/or between partial-melting zone during the Himalayan orogeny different regions. The first part of this contribution MALASHAN GNEISS DOME, SOUTH TIBET 473

mineral assemblages can be used to define metamorphic zones and to document a Barrovian-type metamorphic facies series with chloritoid-in, garnet-in, staurolite-in and kyanite-in isograds (Burg et al. 1984b; Chen et al. 1990; Lee et al. 2000; Fig. 2). The spatial distribution of the metamorphic zones is nearly concentric around the central Kangmar granite, with metamorphic grade increasing towards this granite (Fig. 2). The Malashan area is located on the NW shore of Paiku Lake about 300 km to the west of the Kangmar dome (Figs 1 & 3). There are three distinct granite bodies exposed in the Malashan area, referred to here as the Paiku, Cuobu and Malashan granites (Fig. 3). The lithologies surrounding the granites are dominantly calc-schist (Fig. 3) making it difficult to recognize the distribution of metamorphic zones in the same detail as in the Kangmar area. In the southern part of Malashan, however, pelitic schist is present on mappable scales (Fig. 3), and thin pelitic schist layers up to several metres thick are also locally present within the calc-schist unit of Malashan. Figure 3 shows the distribution of index minerals in these pelitic schists, and illustrates that the series of mineral assemblages is similar to that in the Fig. 2. Geological map of the Kangmar area (modified Kangmar dome (Burg et al. 1984b). The garnet– from Lee et al. 2000) showing metamorphic zonation staurolite–biotite assemblage found close to the with chloritoid-in, garnet-in and staurolite-in isograds, granite bodies (Fig. 3) is comparable with that and dominance of S1 or S2 foliation in individual outcrops. Kyanite-in isograd located closest to the observed in Kangmar (Fig. 2). However, kyanite Kangmar granite is not shown. is not found in Malashan. In the southern part of Malashan chloritoid is found associated with quartz, plagioclase, muscovite and chlorite deals with the metasedimentary schists with special (Fig. 3); the same assemblage is observed in the focus on the associated deformation, and the second chloritoid-in zone of Kangmar (Fig. 2). These part with the origin of granitic bodies enclosed in observations indicate that the Malashan area was the gneiss domes. We note that the whole Kangmar affected by a Barrovian-type metamorphism similar granitic body is referred to as ‘the Kangmar orthog- to that seen around the Kangmar dome. In addition, neiss’ by most workers, and indeed the greater part Figure 3 shows that the highest-grade assemblage, of it shows development of a gneissose foliation. In garnet þ staurolite þ biotite, is found only in the this study, however, we refer to it as the Kangmar area adjacent to the granite bodies. This distribution granite (Fig. 2) because the strength of the gneissose suggests that metamorphic grade decreases away foliation shows considerable variation within the from the granite bodies, and is comparable to that body. Similarly we also refer to one of the granitic of the Kangmar dome. In the Malashan area andalu- bodies in the Malashan area as the Malashan granite site locally occurs as distinct porphyroblasts in (Fig. 3), despite the fact that parts of it are locally areas adjacent to the Cuobu and Paiku granites strongly deformed and can correctly be described as (Fig. 3). Local presence of andalusite, which over- orthogneiss. grows kyanite, is also reported from the Kangmar area (Burg et al. 1987).

Metasedimentary schists: metamorphic and deformational features Deformation stages and distribution of their associated structures Metamorphic zonation In Kangmar a dominantly ﬂat-lying foliation is The Kangmar dome area is distinguished by its developed in the metasedimentary schists surround- easy access and good exposure. It also shows ing the Kangmar granite (Burg et al. 1984b; Chen widely developed pelitic rocks (Fig. 2), whose et al. 1990; Lee et al. 2000), and the associated 474 M. AOYA ET AL.

Fig. 3. Geological map of the Malashan area based on ﬁeld observations and image processing of ASTER satellite imagery. Presence or absence of four index minerals (chloritoid, biotite, garnet and staurolite) in pelitic schists in addition to quartz, plagioclase, muscovite and chlorite are also shown. Note that in the garnet-bearing samples chlorite is a secondary mineral. L.B., loose block; S.B?, possible slide block. Localities of samples that appear in Figure 12f–h are also shown.

deformation stage is referred to as D2 (Chen et al. data (Lee et al. 2000, 2002) and is shown 1990; Lee et al. 2000). An earlier major defor- in Figure 2. It can be seen from Figure 2 that the mation stage, D1, can also be deﬁned. This S2-dominant outcrops are concentrated in the phase is characterized by development of a area close to the Kangmar granite. This indicates steeper foliation that pre-dates D2 (Chen et al. that the strength of the D2 deformation increases 1990; Lee et al. 2000). The foliations developed towards the Kangmar granite, a point also during D1 and D2 are referred to as S1 and S2, discussed by other researchers (Burg et al. 1984b; respectively. The spatial distribution of S1- and Chen et al. 1990). S2-dominant outcrops in the Kangmar area can be In Malashan we also recognized two major defor- derived from previously compiled and published mation stages, D1 and D2, associated with foliations MALASHAN GNEISS DOME, SOUTH TIBET 475

Fig. 4. Representative meso- and microstructures of calc-schists in the Malashan area. Localities are indicated in Figure 5. Cal, calcite; Pl, plagioclase; Qtz, quartz. (a) Outcrop-scale isoclinal D2 folds (loose block); folded foliation represented by white calcite vein is S1; diameter of lens cap is 6 cm. (b) Outcrop-scale open D2 folds with weakly developed S2; folded foliation is S1; hammer length is 40 cm. (c) Photomicrograph of tight D2 folds with well-developed S2 (plane polarized light); folded foliation is S1.(d) Photomicrograph of open D2 folds (plane polarized light) with trace of folded S1 (dashed line); several plagioclase porphyroblasts that contain straight S1 are indicated. (e) Photomicrograph of porphyroclastic plagioclase (plane polarized light) in sample with strongly developed S2; straight internal S1 foliation is at high angle to external S2; strain shadow between the two porphyroclasts is ﬁlled by calcite, which appears brighter than surrounding calcite–graphite intercalation. (f) Outcrop-scale open D4 folds; folded foliation is S2; hammer length is 40 cm.

S1 and S2, respectively (Fig. 4). Two younger low- are low strain with no associated foliation (Fig. 4f). strain deformation stages, D3 and D4, were also In general, therefore, almost any tectonic foliation defined and recognized as post-D2 folds with flat- observed in the field can be classified as either S1 lying and steeply dipping axial planes, respectively. or S2.S1 overprinted by S2 can be observed in These folds are, however, rare and the D4 structures most outcrops and is characterized by folded S1 476 M. AOYA ET AL. with an axial planar S2 (Fig. 4a–d). In addition, a are shown in Figure 6a & b, respectively. In calc- microstructural criterion for differentiating schists of the Malashan area L2 can be recognized between S1 and S2 is provided by porphyroblastic mainly by the development of iron oxide streaks plagioclase (An 10–60 with complex zoning pat- associated with pyrite (Fig. 7a) and locally devel- terns), which is widely developed in calc-schist oped calcite pressure shadows around plagioclase units (Fig. 4d & e). The plagioclase grains com- grains. Comparison of the S2 and L2 data between monly contain a well-preserved straight internal Malashan and Kangmar areas (Fig. 6a & b) shows foliation mainly defined by graphite and muscovite that, in both areas, S2 is dominantly flat-lying inclusions. In samples that contain open D2 micro- and L2 trends approximately north–south, although folds, the trace of the folded S1 defines zigzag in the Malashan area the trend is slightly rotated patterns due to the presence of straight sections of towards the NE–SW. These data show that S1 protected by overgrown plagioclase (Fig. 4d). the maximum stretching direction during D2 is This microstructure indicates that the D2 defor- broadly north–south in both areas. We interpret this mation post-dates the growth of the porphyroblastic to indicate that the D2 deformation represents a plagioclase. In addition, the straight internal foli- regional north–south trending middle-crustal flow ation can be traced continuously into the external on a scale that encompasses at least the two regions. S1 (Fig. 4d) implying static, post-D1 growth of the host plagioclase. In summary, the growth of Kinematics of D2 deformation plagioclase porphyroblasts occurred between D1 and D2. This relationship is useful for determining Two contrasting views of the kinematics of the D2 whether an observed foliation is S1 or S2, especially deformation in Kangmar have been published: (i) when an S2 overprinting an earlier S1 cannot be a dominantly top-to-the-north sense of shear directly identified in the outcrop. In cases where (Chen et al., 1990); and (ii) bulk pure shear the internal foliation within plagioclase is at a reflected by a combination of top-to-the-north and high angle to the external penetrative foliation top-to-the-south senses of shear in the northern (Fig. 4e), it can be deduced that the external foli- and the southern flanks of the dome, respectively ation is S2. Using this microstructural scheme in (Lee et al. 2000). To obtain new insights into this combination with field-structural observations, we issue, and to carry out a comparison with the were able to establish that S2 is developed to a Kangmar examples, we collected information on greater or lesser degree throughout the study area the kinematics of deformation in Malashan. shown in Figure 3. White veins composed dominantly of calcite are In outcrops where S2 is strongly developed, it is widely developed throughout the calc-schists of generally difficult to recognize the older foliation, Malashan and in general have a length of a few S1, and in these cases S1 is commonly only recog- centimetres and a width of a few millimetres nized in the hinges of tight to isoclinal D2 folds (Fig. 7). When observed on S2 planes, the intersec- (Fig. 4a). In field surveys of these types of outcrops tions of these white veins with S2 are dominantly the orientation of the S1 tends to be left unrecorded. subperpendicular to L2 (Fig. 7a & e) suggesting In contrast, the S1 foliation is easy to measure in that their formation was related to the D2 defor- outcrops where overprinting by D2 is relatively mation. Syn-D2 formation of these veins is also weak (Fig. 4b). Using these characteristics of field supported by the observations that they commonly measurements, we classified outcrops in the Mala- cross-cut the S2 (Fig. 7b & c) but are locally slightly shan area into Type (a) outcrops where only S2 folded (Fig. 7c). The white veins can, therefore, was measured and Type (b) outcrops where both be regarded as veins filling voids produced by foli- S2 and S1 were measured. The results are plotted ation boudinage (e.g. Platt 1980) that formed during in Figure 5, which shows a concentration of Type D2. On surfaces oriented approximately parallel to (a) outcrops in the area close to the granite L2 and subnormal to S2, the boudin necks are bodies. Because Type (a) outcrops represent stron- commonly oblique to S2 (Fig. 7b), and this feature ger development of D2 deformation compared to can be used to infer the shear sense associated Type (b) outcrops, we conclude that the strength with D2 strain. For strain histories with a non-zero of D2 deformation in Malashan increases towards rotational component, the finite stretching direction, the central granite bodies. This is directly analogous represented by the orientation of the meso-scale to the Kangmar area. foliation, progressively rotates towards the flow plane and away from the instantaneous stretching direction (Fig. 7d). Syndeformational extensional Orientation of D2 structures cracks, or boudin necks in the present case, develop Stereonet plots of S2 foliation and L2 stretching perpendicular to the instantaneous stretching direc- lineation (developed on S2) in Malashan, and com- tion and, as a result, the foliation and cracks show parable data for Kangmar (Lee et al. 2000) an oblique relationship (Fig. 7d). In the case of MALASHAN GNEISS DOME, SOUTH TIBET 477

Fig. 5. Geological map of the Malashan area showing distribution of D2-dominant outcrops. Localities of photos presented in Figures 4, 7 and 8 are also shown.

Figure 7b, the shear sense can be determined to be porphyroblasts as shear-sense indicators it is top-to-the-north. important to note that other kinematic indicators In addition to the oblique boudin necks, the fol- clearly document non-coaxial deformation in the lowing shear sense indicators for D2 are locally region, and that the calcareous schists hosting developed: (i) vein sets representing shortening the porphyroblasts are homogenous on the milli- and stretching quarters of bulk strain (e.g. Passchier metre scale (with the exception of the porphyroblasts 1990; Wallis 1992; Fig. 8a & b); (ii) asymmetry of themselves) and lack development of microlithons lenses or clasts (Fig. 8c & d); (iii) rigid-body that could complicate kinematic interpretation (e.g. rotation of porphyroclastic plagioclase recognized Passchier & Trouw 1996, p. 178). All the examples by continuation of the internal S1 into external S2 shown in Figure 8 indicate top-to-the-north to NE (Fig. 8e); (iv) grain-shape preferred orientation of senses of shear. calcite grains (e.g. Means 1981; Fig. 8f); and (v) The distribution of the determined D2 shear outcrop-scale shear bands (S-C0 fabric). In using senses in Malashan is summarized in Figure 9. As 478 M. AOYA ET AL.

(a) Malashan by agreement between more than two indicators (This study) document top-to-the-north to NE senses of shear (Fig. 9). These data suggest that the D2 deformation in Malashan can be correlated with a top-to-the-north deformation that is observed on the STD (e.g. Burg et al. 1984a; Burchﬁel et al. 1992). In comparison with the two contrasting kinematic views proposed for D2 deformation in Kangmar, the results of our studies support Chen et al.’s (1990) interpretation that the shear sense of D2 is dominantly top-to-the-north and, as indicated above, that D2 deformation may be correlated to the activity of the STD.

Sedimentary age: structural level of Malashan Field studies show that a semi-continuous Ordovician to Eocene sedimentary succession is (b) Kangmar S2 (pole) preserved in the Tethys Himalaya in south Tibet (Lee et al., 2000) L2 (Burg & Chen 1984; Burchfiel et al. 1992; Liu & Einsele 1994). The depositional age of the Mala- shan metasedimentary rocks, therefore, provide’s a constraint on the structural level of the Tethys Himalaya affected by the D2 top-to-the-north deformation. In our field studies, fossil ammonites of Tithonian (Upper Jurassic) age were collected from ellipsoidal calcareous concretions within low-grade calcareous shales of the Malashan area (Fig. 10a–d). The whorl is slightly crushed and dis- torted by compaction and the sutures are not visible. However, the following diagnostic features can be observed: (i) evolute, widely umbilicate rounded whorls; and (ii) sharp, highly sigmoidal major ribs numbering 26 per half-whorl, which are bipartite on the outer flanks (Fig. 10a–c). Reconstruction of the original shape suggests the outer whorl was originally highly inflated (Fig. 10b). The major ribs tend to be tripartite in the outermost whorl (Fig. 10c). These features indicate that the specimen Fig. 6. Stereographic plot (equal-area lower- is assignable to the genus Aulacosphinctoides hemisphere projection) of S2 and L2 data from: A. infundibu- (a) metasedimentary schists of the Malashan area (Spath 1923), and can be identified as lum (Uhlig 1903–1910, pp. 371–372, pl. XLVI, (n ¼ 66 for S2 and n ¼ 55 for L2); a single representative datum is selected for each individual site; Figs 1a–c), which occurs in the Middle Spiti and (b) the Kangmar area (slightly modified from Lee Shales in the Himalayan area. Aulacosphinctoides et al. 2000) including the Kangmar orthogneiss. occurs characteristically from the Lower to Middle Tithonian, corresponding to the upper part of the Menkatum formation in south Tibet (Wester- is clear from this figure, shear sense is dominantly mann & Wang 1988). The metasediments of the top-to-the-north to NE, with only three exceptions Malashan area are dominantly calcareous (Fig. 3) out of 23 determinations. There is no systematic and lithologically similar regardless of meta- trend showing a concentration of top-to-the-south morphic grade. We therefore conclude that the shear-sense indicators in a particular region, medium-grade metasedimentary rocks of the Mala- suggesting that the presence of top-to-the-south shan area that occur in proximity to granite bodies shear senses is only of local significance. Moreover, and are strongly affected by D2 deformation, also all of the most reliable data that were determined have a Jurassic sedimentary age. MALASHAN GNEISS DOME, SOUTH TIBET 479

Fig. 7. Boudin necks developed in calc-schists of the Malashan area and used as a D2 shear-sense indicator. Localities of (a)–(c) are indicated in Figure 5. (a) Boudin necks on S2; white boudin necks dominantly composed of calcite are developed subperpendicular to L2 deﬁned by dark streaks associated with pyrite grains; length of pencil is 15 cm. (b) Side view of (a); series of white veins are commonly oblique to S2 indicating a top-to-the-north sense of shear. (c) Another side view of (a); boudin neck in centre is slightly folded. (d) Schematic illustration showing interpretation of oblique boudin necks as a shear-sense indicator (example of simple shear). (e) Stereographic plots showing subperpendicular relationship between L2 and boudin–S2 intersections around the Malashan granite. 480 M. AOYA ET AL.

Fig. 8. D2 shear-sense indicators locally observed in calc-schist of the Malashan area; dominant foliation in all figures is S2; field localities for all figures are shown in Figure 5. (a, b) Vein sets; folded small veins in (a) and boudinaged vein in (b) represent shortening and stretching quarters, respectively, and indicate top-to-the-NE sense of shear. (c) Asymmetry of the shape of quartz-rich lens indicating top-to-the-north sense of shear. (d) Asymmetry of deflection pattern around pyrite clast showing top-to-the-NE sense of shear. (e) Rigid-body rotation of porphyroclastic plagioclase recognized by continuation of the internal S1 foliation into external S2 foliation (represented by an elongate calcite grain as indicated by the black arrow; plane polarized light); top to the NE sense of shear is indicated. (f) Grain shape preferred orientation of recrystallized calcite grains oblique to S2 foliation indicating top-to-the-north sense of shear (crossed nicols).

A stratigraphic section of the Gyirong area metasediments is separated from the Greater Hima- (Burchﬁel et al. 1992; Fig. 10e), extending from laya by a thick sedimentary sequence, which starts the STD to the southern part of the Malashan area at the STD with Ordovician-aged units and con- (see Fig. 1 for sectional line), shows clearly that tinues up to the Triassic. This suggests that the the Jurassic unit corresponding to the Malashan top-to-the-north deformation (D2) observed in the MALASHAN GNEISS DOME, SOUTH TIBET 481

Fig. 9. Distribution of L2 and associated D2 shear senses in the Malashan area. Shear senses are indicated by movement direction of the overlying unit and are grouped into two types: those determined by one indicator and those determined by agreement of two or more indicators. Methods used to determine shear senses are also indicated for each locality.

Malashan area affected a structurally high level of that their contact represents an extensional detach- the Tethys Himalaya which is separate from the ment fault. In contrast, Lee et al. (2000) concluded STD (Fig. 10e). that the contact was originally an unconformity. In both cases the Kangmar granite is regarded as a part of the Indian basement. This assumption is Origin of granite bodies: structural based mainly on the results of conventional and geochemical constraints U–Pb zircon chronology for the Kangmar granite that yielded ages of 566–507 Ma (Scha¨rer 1986; Q1 The original pre-penetrative-deformation relation- Lee et al. 2000). To further investigate the origin ship between the Kangmar granite (orthogneiss) of granitic bodies in the Tethys Himalaya, we and its surrounding metasedimentary rocks is dis- next present the results of our structural and bulk- puted. Chen et al. (1990) suggested that the two chemical studies on the three granite bodies in the units were originally unrelated to each other and Malashan area (Fig. 3). 482 M. AOYA ET AL.

Fig. 10. Sedimentary age and stratigraphic level of the Malashan metasedimentary unit in the Tethys Himalaya. (a–d) Ammonite specimen (Aulacosphinctoides infundibulum) from southwestern part of the Malashan area: (a) left-side view; (b) ventral view; (c) right-side view; (d) sample locality. (e) Stratigraphic section of rocks exposed in the Gyirong area (simpliﬁed from Burchﬁel et al. 1992); geographic position of sectional line is indicated in Figure 1; only relative thicknesses are shown. Jurassic unit corresponding to the Malashan metasedimentary rocks is indicated.

The Malashan granite: striking similarity fabrics suggests the outer margin of the Kangmar with the Kangmar granite granite is affected by a D2 shear zone that displays increasing strain towards the contact. Among the Field observations of previous workers (Burg et al. three granite bodies in Malashan (Fig. 3), only the 1984b; Chen et al. 1990; Lee et al. 2000) provide Malashan granite (Fig. 11) exhibits a strongly clear evidence that the Kangmar granite (orthog- mylonitized outermost part that can be described neiss) and its surrounding metasedimentary schists as orthogneiss (Fig. 11c). The orientations of the share the same D2 fabric. The effect of D2 is particu- mylonitic foliation and its associated stretching larly strong in the outermost part of the Kangmar lineation defined by the arrangement of mica granite where it is recognized by development of grains and quartz rodding are subparallel to S2 a strong mylonitic foliation subparallel to S2 of and L2 in the adjacent metasedimentary schists the adjacent metasedimentary schists. In contrast, (Fig. 11a & b). This suggests that the granite the central part of the Kangmar granite is less is affected by the same D2 deformational fabric as strongly foliated (our own observations). This the surrounding units. A significant difference is spatial change in the strength of D2 deformational that the mylonitic foliation of the Malashan MALASHAN GNEISS DOME, SOUTH TIBET 483 granite is commonly associated with two sets of Brodie & Rutter 1985). In summary, we suggest shear bands (S-C or S-C’ fabric) indicating top- that the top-N and the top-S shear bands represent to-the-north and top-to-the-south senses of shear two different deformation stages. No convincing (Fig. 11c). We will refer to these shear bands as off-set of one set of shear bands by the other set top-N and top-S shear bands hereafter. To investi- has thus far been observed, and this might indicate gate how these shear bands are related to the defor- that the two sets developed simultaneously (i.e. they mation history in the Malashan area, we carried out should form conjugate sets) as demonstrated in the microstructural observations and measured the footwall to the STD in the Everest Massif (Law orientation distribution of biotite grains. et al. 2004). However, in this interpretation, asym- Our microstructural observations reveal the pre- metric K-feldspar indicating a top-to-the-north sence of top-to-the-north asymmetric mica tails sense of shear should also be present. In the Mala- developed on K-feldspar clasts in domains where shan area, however, asymmetric K-feldspars in the top-N shear bands dominate (Fig. 11d). In con- three samples, MSA9, MSA12 and MSW14, from trast, in domains dominated by top-S shear bands, three different localities (Fig. 11a) all indicate the asymmetry of K-feldspar clasts is in agreement top-to-the-south sense of shear. These observations with a top-to-the-south sense of shear. In this support the interpretation that the two sets of shear second case the sense of shear is indicated by the bands are due to two distinct deformation phases shape of the K-feldspar grain itself (Fig. 11d). rather than representing conjugate sets. The same This type of K-feldspar grain will be referred to type of overprinting structure with two distinct as an ‘asymmetric K-feldspar’. Because these sets of shear bands with opposed senses of shear grains exhibit simultaneous extinction under has recently been reported in the French Massif cross-polarized light, with no evidence for lattice Central (Duguet & Faure 2004). distortion by plastic deformation (Fig. 11d), it is To examine which shear band is dominant on the highly unlikely that the grain-shape asymmetry sample scale and to characterize the relative was formed by a deformation phase that post- strength of the deformation in each outcrop, we dates the growth of K-feldspar. We conclude, there- carried out measurements of the orientation distri- fore, that top-to-the-south deformation occurred bution of biotite grains for eight granite samples synchronously with growth of the K-feldspar. including five from the Malashan granite. The Further observation reveals that plagioclase shapes of the biotite grains were approximated by inclusions overgrown by asymmetric K-feldspars straight lines drawn on photos of polished slabs commonly show prominent wavy extinction, and cut perpendicular to the foliation and parallel to locally exhibit euhedral shapes (Fig. 11e). The the stretching lineation (Fig. 11c & f), and the wavy extinction locally cross-cuts concentric com- angle and length of the lines were measured. The positional zoning of plagioclase, indicating that the lines approximating to the orientation and length observed wavy extinction can be attributed to of the biotite grains were drawn using the following plastic deformation of plagioclase grains. Plastic criteria: (i) short round grains were not drawn; (ii) deformation of plagioclase occurs at temperatures small grains (,c. 0.2 mm in length) were not .5008C (e.g. Brodie & Rutter 1985; Fitz Gerald drawn; (iii) long grains with gentle curvatures & Stu¨nitz 1993) implying that the top-to-the-south were divided into two or more shorter straight-line deformation occurred under relatively high- segments; and (iv) all measurable grains within temperature conditions. The euhedral plagioclase the frame determined at the start of measurement inclusions (Fig. 11e) further suggest that they (20–70 cm2) were traced. For the angular measure- grew in free space, possibly in the melt, as plagio- ments the direction of the foliation is taken to be clase is only rarely euhedral in metamorphic rocks zero degrees and, in the case of the Malashan mylo- (e.g. Spry 1969; Hiroi et al. 1995). In contrast, nitic samples (Fig. 12a–d), the following sign con- domains with strong development of top-N shear vention was used: negative angles for the top-N bands include not only K-feldspar but also strain- (NE) shear bands and positive angles for the top-S free plagioclase clasts (Fig. 11d) indicating that (SW) shear bands (Fig. 11c). top-to-the-north shearing occurred at lower temp- The complete set of our angle measurement is eratures where plastic deformation of plagioclase shown in Figure 12. The first important point is was more limited. In top-N domains quartz com- that in all the mylonitic samples from the marginal monly shows wavy extinction and development of part of the Malashan granite, the negative orien- subgrains. These observations indicate that during tations are generally more abundant than positive. the top-to-the-north stage, plastic deformation of This asymmetry is highlighted by the solid quartz, rather than feldspar, was significant and shading (Fig. 12a–d). Combining these data with that rapid recovery was not possible. These micro- our observation that both top-N and top-S shear structural features suggest a lower deformation bands are clearly developed in these samples temperature of approximately 300–5008C (e.g. (Fig. 11c), we interpret the angle data to indicate 484 M. AOYA ET AL.

WebColor MALASHAN GNEISS DOME, SOUTH TIBET 485 that top-N shear bands are dominant. That is, the major- and trace-element XRF analyses were mylonitic foliation of the Malashan granite rep- carried out. The results are shown in Table 1, and resents a top-to-the-north deformation and, there- are plotted in Figure 13 as SiO2-variation diagrams, fore, can be regarded as S2 (Fig. 11a). In contrast together with published data from the Tethys Hima- the top-S shear bands are only locally observed layan and Greater Himalayan granites (see caption and can be regarded as a remnant feature of an of Fig. 13 for data sources). These plots (Fig. 13) older deformation, which can be correlated with indicate that for most elements the Malashan and D1. The widespread dominance of S2 in Malashan Cuobu granites show strong similarities with other (Fig. 5) makes it difficult to determine the shear Tethys Himalayan granites including the Kangmar sense of D1 in the metasedimentary schists. granite. In contrast, the chemistry of the Paiku However, we could determine the shear sense of granite is similar to the Greater Himalayan leuco- D1 at one outcrop where local pelitic schists pre- granites, as represented by relatively low contents serve relatively planar S1 (MSW28 in Fig. 11a). of TiO2 and MgO, and by relatively high contents The shear sense shown by a d-type porphyroclast of K2O and Rb (Fig. 13). Locally low CaO, high of garnet is top-to-the-south (Fig. 11g), consistent MnO and the wide range of Na2O contents for with the sense of shear shown by the earlier defor- samples of the Paiku granite are also similar to mation in the mylonitic samples of the Malashan the Greater Himalayan leucogranites (Fig. 13). granite. A change of shear sense from top-to-the- These data show that some of the Tethys Himalayan south during D1 to top-to-the-north during D2 is granites are chemically closely comparable to the also described in pelitic schists of the Kangmar Greater Himalayan leucogranites, which are dome (Chen et al. 1990). widely accepted to have formed during the Himala- Another important result of the biotite orientation yan orogeny (32–12 Ma: Searle et al. 2003 and measurements is the distinctly weaker development references therein). An additional important point of foliation in the central part of the Malashan is that the Malashan and Cuobu granites show granite (Figs 11f & 12e) relative to the outermost very limited chemical variation relative to the com- parts (Figs 11c & 12a–d). This contrast means positional variation shown in Figure 13, and the that in the Malashan granite the D2 deformation is Malashan and Cuobu granites, therefore, have strik- concentrated in its outermost part, and that a D2 ingly similar bulk-chemical compositions. Even for shear zone is developed in the marginal part of a single pluton in the Tethys Himalaya, internal the Malashan granite, a direct analogy with the compositional variation greater than that seen for Kangmar granite. It is difficult to be confident both the Malashan and Cuobu granites is commonly about the nature of the weak foliation developed observed (Debon et al. 1986; Zhang et al. 2004). It in the central part of the Malashan granite is highly improbable that a similarity on this level is (Fig. 11f). It may be S2,S1 or some foliation repre- simply coincidence, and we therefore conclude that senting magmatic flow. Regardless of this interpret- the Malashan and Cuobu granites were derived ation, the central part of the Malashan granite is from the same type of original magma. clearly less strongly affected by D2 than the margin.

Bulk chemistry of granites in the The Cuobu and Paiku granites: intrusive Malashan area origin and relative timing of intrusion To investigate the bulk-chemical characteristics of To constrain the origin of the Malashan granite, the three granite bodies in Malashan (Fig. 3), which has many similarities to the Kangmar

Fig. 11. Summary of structural features of the Malashan granite. Samples cut perpendicular to foliation and parallel to lineation. K, K-feldspar; P, plagioclase; Q, quartz. (a) Structural map of the region around the Malashan granite; symbols used for stretching lineation are the same as those in Figure 9; localities of samples that appear in Figure 11c–g and 11a–e are indicated. (b) Stereographic plot of S2 and L2 from the outermost part of the Malashan granite; a single representative data point is selected for each outcrop. (c) Mylonitic granite sample (MSA12); sets of shear bands indicating top-to-the-north and south senses of shear are developed. (d) Photomicrograph of mylonitic granite sample (MSA9; crossed nicols); the upper part of the micrograph is dominated by top-to-the-north shear bands while the lower part is associated with local top-to-the-south shear bands. (e) Close-up photo of the central part of K-feldspar clast in the lower part of (d); plagioclase inclusions commonly show wavy extinction and locally exhibit euhedral shapes. (f) Slab photo of weakly foliated granite sample (MSA36); scale is same as in (c). (g) Photomicrograph of pelitic schist sample that preserves relatively planar S1 (plane polarized light); top-to-the-south sense of shear is indicated for D1 by asymmetric tails around garnet porphyroclast in centre. 486 M. AOYA ET AL.

Fig. 12. Histograms showing distribution of angles between biotite grains and foliation in granite samples from the Malashan area. Data were taken in planes cut perpendicular to the foliation and parallel to the stretching lineation. The horizontal axes represent 20 angle bins each of 98 width; orientation of foliation is taken as zero degrees. For mylonitic samples of the Malashan granite (a–d), the signs of the angles are deﬁned so that the top-N (NE) shear bands are recorded as minus and the top-S (SW) as plus (see Fig. 11c). For other less deformed samples without development of shear bands (e–h) orientation was not marked at the time of sampling. The vertical axis represents the length- normalized frequency (ratio of measured total biotite length in each bin against that in the whole measurement). For samples (a–d) excesses of bin height against those of oppositely signed bins are shown by black shading to highlight the asymmetry of the angular-data distribution. (a–d) Mylonitic samples from the outermost part of the Malashan granite; see Figure 11a for localities. (e) Weakly foliated sample from central part of the Malashan granite; see Figure 11a for locality. (f, g) Weakly foliated sample from the marginal part of the Cuobu granite; see Figure 3 for localities. (h) Weakly foliated sample from the marginal part of the Paiku granite; see Figure 3 for locality. Table 1. Major- and trace-element bulk-chemical compositions of granite from the Malashan area

Body Malashan Malashan Malashan Malashan Malashan Malashan Malashan Malashan Malashan Cuobu Cuobu Cuobu Cuobu Paiku Paiku Paiku Paiku Paiku vein Sample MSA28 MSA36 MSK7 MSK17 MSK40 MSL11 MSL16 MSW14 MSW18 MSA4 MSK4a MSL4a MSW3 MSL21 MSW21 MSW22 MSW23 MSW22V Foliation moderate weak strong strong weak strong moderate strong moderate weak weak weak weak moderate weak moderate moderate weak (wt.%) SiO2 72.00 72.98 72.22 71.88 72.75 72.56 72.30 72.30 72.13 73.30 72.65 73.10 72.41 74.18 72.66 72.80 74.99 74.13 TiO2 0.23 0.20 0.23 0.23 0.21 0.23 0.24 0.24 0.23 0.21 0.22 0.21 0.23 0.06 0.02 0.13 0.01 0.01 Al2O3 15.71 15.55 15.66 16.10 15.52 15.43 15.62 15.66 15.66 15.41 15.74 15.56 15.62 15.37 15.81 15.81 15.03 15.25 Fe2O3 1.53 1.37 1.52 1.39 1.44 1.58 1.54 1.57 1.56 1.33 1.48 1.39 1.49 0.73 0.49 1.24 tr 0.28 MnO 0.03 0.03 0.03 0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.03 0.02 0.02 0.02 tr 0.12 MgO 0.55 0.44 0.53 0.50 0.50 0.55 0.55 0.55 0.55 0.49 0.52 0.47 0.51 0.11 0.10 0.30 tr 0.01 CaO 1.72 1.42 1.51 1.64 1.39 1.47 1.53 1.48 1.56 1.32 1.25 1.26 1.21 0.93 0.76 1.01 1.26 0.34 Na2O 3.62 3.58 3.76 3.76 3.49 3.62 3.53 3.49 3.66 3.47 3.60 3.70 3.47 3.94 3.88 3.73 4.72 2.79 K2O 3.90 3.86 3.77 3.66 3.99 3.81 3.92 3.84 3.86 3.76 3.76 3.65 3.94 4.17 5.69 4.32 4.08 6.73 P2O5 0.09 0.09 0.11 0.10 0.10 0.11 0.09 0.10 0.10 0.12 0.12 0.12 0.13 0.08 0.05 0.10 0.07 0.13 Total 99.38 99.52 99.33 99.29 99.40 99.40 99.35 99.26 99.34 99.42 99.38 99.48 99.04 99.60 99.50 99.40 100.10 99.80

(ppm) Cr115131312192216161387143610tr3 Ga 18 18 19 19 17 18 17 18 19 20 20 19 20 21 29 22 15 21 Zr 85 74 82 81 80 84 91 90 84 88 86 82 88 36 52 43 39 23 Th 8 8 12 13 10 10 9 11 14 14 10 8 11 4 6 6 4 4 Co 12 12 10 9 13 11 12 11 12 11 10 11 11 11 10 12 11 11 Rb 206 212 209 189 213 224 220 206 226 207 213 208 211 326 466 354 272 489 Nb 8 7 7 4 7 8 8 8 9 3 7 6 5 11 21 10 7 10 Ni 8 9 9 8 9 10 12 10 11 9 9 9 9 7 8 8 6 6 Sr 187 166 190 229 177 176 173 180 172 203 189 194 204 58 81 118 64 tr Cu tr 13 1 tr 2 1 2 tr 38 tr tr tr tr tr tr tr tr tr Y 20181512171622181914161313172514209 Ba 558 523 532 633 547 517 529 498 546 549 523 498 600 142 151 395 64 tr Zn 50 46 51 57 58 51 49 51 49 58 59 55 57 26 39 22 1 8 Pb 51 49 51 54 53 52 48 49 49 46 46 47 47 48 42 47 47 41

Major elements were analysed at Kyoto University, and trace elements at Nagoya University (tr ¼ trace). Sample MSW22V is a synkinematic vein taken from the Paiku granite and plotted as a data point for the Paiku granite in Figure 13 (see caption of Fig. 14f for details). 488 M. AOYA ET AL.

Fig. 13. Plots of bulk-chemical data for granites from the Malashan area on SiO2 variation diagrams for 12 elements. Data for the Tethys Himalayan granites from Lagoi Kangri (Gyaco La), Mabja, Kangmar and Kari La regions (Debon et al. 1986; Zhang et al. 2004), and those of the Greater Himalayan leucogranites around Manaslu, Shishapangma and Kula Kangri regions (Dietrich & Gansser 1981; Le Fort 1981; Debon et al. 1986; Guillot & Le Fort 1995; Inger & Harris 1993; Searle et al. 1997; Zhang et al. 2004) are also plotted. MALASHAN GNEISS DOME, SOUTH TIBET 489 granite, we will focus on the Cuobu and Paiku that andalusite grew signiﬁcantly later than the granites in this section. main phase of Barrovian-type metamorphism. The ﬁrst indicator of the nature of the Cuobu Thirdly, formation of skarn is observed in one granite is given by the observation that it contains area adjacent to the Cuobu granite (Fig. 14c & d). dykes originating from the main body (Fig. 14a). The andalusite and skarn are best explained as the A second important observation is that the pelitic products of contact metamorphism. The above schists adjacent to the Cuobu granite contain anda- three features, therefore, indicate that the Cuobu lusite (Fig. 3). The andalusite generally replaces granite is an intrusive body. An important question biotite (Fig. 14b) and locally staurolite, indicating for our study is the timing of its emplacement

Fig. 14. Geological features of the Cuobu and Paiku granite. And, andalusite; Bt, biotite; Cpx, clinopyroxene; Grt, garnet; Pl, plagioclase; Qtz, quartz. (a) Field photo showing dykes (arrowed) originating from the Cuobu granite. (b) Photomicrograph showing occurrence of andalusite in a pelitic schist adjacent to the Cuobu granite (crossed nicols); andalusite replaces biotite developed along S2.(c, d) Outcrop photo (C) and slab photo (d) of skarn found in the region adjacent to the Cuobu granite. (e) Photomicrograph showing occurrence of slightly boudinaged andalusite in pelitic schist adjacent to the Paiku granite (crossed nicols). (f) Slab photo of sample from marginal part of the Paiku granite (MSW22); this is the only Paiku sample that can be used for angle measurement of biotite grains (Fig. 12h) because of the general low biotite content. The white vein in the left side cross-cuts S2 but is pulled apart in the direction of S2, as suggested by the muscovite grains on both edges of the vein. We consider the syn-D2 nature of the vein to justify plotting this vein sample (MSW22V; Table 1) as part of the Paiku granite (Fig. 13). 490 M. AOYA ET AL. relative to deformation. The observations that the (thick lines in Fig. 12), is reﬂected in two indicators: dykes cross-cut S2 (Fig. 14a) and that andalusite (i) the concentration of data in the central bin overgrows S2 (Fig. 14b) indicate that emplacement (0 + 4.58); and (ii) the standard deviation (s) for post-dated at least a part of D2 deformation. In a particular set of data. As the deformation addition, garnet-rich layers in the skarn are slightly becomes stronger the concentration in the central boudinaged in a north–south direction (Fig. 14d) bin becomes greater and the standard deviation indicating that skarn formation and granite empla- becomes smaller. This systematic trend is rep- cement pre-dated at least a part of D2. These two resented in Figure 15a with the Malashan granite observations lead to the conclusion that emplace- showing stronger deformation than the Paiku and ment of the Cuobu granite occurred during D2 Cuobu granites. The Malashan and Cuobu granites, deformation. Although large-scale dykes and and even the measured Paiku sample (MSW22), all skarn were not observed around the Paiku granite, have similar bulk-chemical compositions (Table 1; it is also associated with formation of andalusite Fig. 13). In addition, all three granite bodies are that post-dates the garnet–staurolite–biotite assem- commonly surrounded by calc-schist (Fig. 3). blage in the adjacent pelitic schists (Figs 3 & 14e). Therefore, the strain contrast shown in Figure 15a This suggests that the Paiku granite also intrudes the cannot be explained by differences in rheological surrounding schists. In this case the andalusite is properties at the pluton contacts. The most likely locally slightly pulled apart within S2 (Fig. 14e) explanation is that there were differences in the suggesting that intrusion of the Paiku granite also timing of emplacement of the granite bodies with took place during the D2 phase. respect to the D2 deformation. The Cuobu granite To compare the strength of the D2 deformation was intruded during the later stage of D2 and experi- experienced by the three granite bodies, the orien- enced only a short period of D2 deformation tation distribution of biotite was also measured for (Fig. 15b). In contrast, formation of the Malashan samples from the Cuobu and Paiku granites granite was earlier, and this body was more strongly (Fig. 12f–h). The three samples are taken from affected by D2 (Fig. 15b). The intermediate strain the marginal parts of the two granites (see Fig. 3 obtained from the Paiku sample suggests that intru- for localities) to enable comparison with the marginal sion of the Paiku granite took place during D2 but samples of the Malashan granite (Fig. 12a–d). For at a time earlier than intrusion of the Cuobu granite the Paiku granite, only one sample (MSW22; (Fig. 15b). Fig. 14f) was measurable because of the generally low biotite content. Comparison of Figure 12a–d Summary: origin of granites with Figure 12f–h shows that the Paiku and in the Malashan area Cuobu granites are signiﬁcantly less deformed than the Malashan granite. The strength of defor- As already mentioned, the Paiku granite is chemi- mation, or deviation from random distribution cally equivalent to the Greater Himalayan

Fig. 15. (a) Summary of the angle measurements for biotite grains shown in Figure 12. (b) Interpretation of the strain contrast between the Malashan, Cuobu and Paiku granites as shown in (a). The strain contrast is interpreted to reflect their different timings of emplacement with reference to D2 deformation. MALASHAN GNEISS DOME, SOUTH TIBET 491 leucogranites (Fig. 13), which have crystallization deformational core to the surrounding metasedimen- ages of 32–12 Ma. Assuming that the Paiku tary units. In other words, the Malashan complex is a granite is also Himalayan, syn-D2 intrusion of the North Himalayan gneiss dome cored by the Paiku granite (Fig. 15b) leads to the interpretation Malashan granite. We propose that the association that D2 is a Himalayan deformation. The Cuobu of features (i)–(iii) can be regarded as a definition granite can also be regarded as Himalayan granite for the North Himalayan gneiss domes. We do not because its intrusion post-dates that of the Paiku interpret the Cuobu and Paiku granites as cores to granite (Fig. 15b). In addition, the striking bulk- the Malashan dome, because: (i) their emplacement chemical similarity between the Cuobu and is documented to be significantly later than the main Malashan granites (Fig. 13) suggests the Malashan Barrovian-type metamorphism (Fig. 14b) whose granite is also a Himalayan intrusion. This consider- distribution defines the granite core; and (ii) they ation leads us in turn to the conclusion that the D1 show only minor effects of the D2 deformation event is also a Himalayan deformation, because (Fig. 12f–h), forming a significant strain gap at D1 affects the Malashan granite, which we conclude their contacts. That is, they form neither a meta- to be a Himalayan intrusion (Fig. 11c & d). This morphic nor a deformational core to the surrounding conclusion is supported by SHRIMP spot dating metasedimentary schists. In the case of the two of zircons from the Malashan and Cuobu granites examples discussed in this study, and also the (Aoya et al. 2005). We also suggest that intrusion recently reported example of the Mabja dome of the Malashan granite occurred during the D1 (Lee et al. 2004; Fig. 1), the flow direction of D2 phase (Fig. 15b). This is based on the syn-D1 was similarly orientated north–south (Fig. 6). growth of asymmetric K-feldspar (Fig. 11d), associ- There is, however, considerable local variation in ated high-temperature deformation (.5008C) and the orientation of this lineation and it may differ magmatic growth of plagioclase inclusions from one region to another. It remains to be seen to (Fig. 11e), which are suggestive of a partially what extent this is a general feature of the North molten stage: the Malashan granite experienced Himalayan gneiss domes. the D1 deformation when it was crystal mush. Our main conclusion is that the Malashan granite, Contradictory interpretations of the which has many similarities with the Kangmar North Himalayan gneiss domes granite, can be interpreted as an intrusive body formed during the Himalayan orogeny. The combination of structural and geochemical studies presented in this contribution leads to the conclusion that D2 deformation in the Malashan Discussion dome took place during the Himalayan orogeny. This is also the case for the Kangmar and Mabja General features of the North Himalayan domes where geochronologic and thermochronolo- gneiss dome gic studies indicate a late Oligocene to Miocene age for D2 deformation (Lee et al. 2000, 2006). With the main exception of the association of two However, as mentioned above, there is disagree- weakly deformed granites (Cuobu and Paiku gran- ment concerning the kinematics of the D2 ites), the Malashan metamorphic complex shows a deformation in Kangmar; i.e. dominantly top-to- number of striking similarities with the Kangmar the-north shear (Chen et al. 1990) or bulk pure dome: (i) presence of Barrovian-type metamorphism shear with a combination of top-to-the-north and whose grade increases towards the granite bodies top-to-the-south on the north and south flanks of located in the central part of the metamorphic the dome respectively (Lee et al. 2000). Our regions (referred to as granite cores hereafter; kinematic study in Malashan indicates a top-to- Figs 2 & 3); (ii) presence of two major deformation the-north sense of shear, directly comparable with stages, D1 and D2, in metasedimentary schists and the results of Chen et al. (1990) from Kangmar. increasing strength of the D2 deformation towards In our study we have determined the map-scale dis- the granite cores (Figs 2 & 5); (iii) development of tribution of D2 shear senses and then assessed the strong D2 foliation (gneissosity) in the outermost reliability of each shear-sense determination by parts of the granite cores (Fig. 12a–e); and (iv) a documenting whether it was made by two or more roughly north–south flow direction associated with independent methods (Fig. 9). In the case of the D2 deformation, as suggested by distribution of L2 Kangmar dome it is at present difficult to conclude (Fig. 6). In the case of the Malashan area, we con- which of the present views (Chen et al. 1990; Lee sider the Malashan granite to be a granite core et al. 2000) is correct because the map-surface dis- similar to the Kangmar granite, because it contains tribution of the determined shear senses has yet to D2 shear zones in its outermost part (Fig. 11a–c), be published. It is therefore necessary to examine forming not only a metamorphic but also a the kinematic nature of the D2 deformation in the 492 M. AOYA ET AL.

Kangmar dome, and in other still enigmatic gneiss 2001, 2004; Searle et al. 2003; Zhang et al. domes to assess whether the D2 deformation can 2004), and the strongest basis for this idea is the kinematically be correlated with activity on the above-mentioned Cambrian to Ordovician U–Pb STD at the scale of the whole Tethys Himalaya. ages for the Kangmar granite (Scha¨rer 1986; Lee Q3 Another contradiction revealed in the present et al. 2000). In this interpretation the ductile D2 study concerns the origin of the granite cores. We shear zones developed around the marginal part of interpret the Malashan granite, the core of the the granite cores themselves are regarded as a Malashan dome, as an intrusive body formed direct continuation of the STD. The stratigraphic during the Himalayan orogeny. Our conclusion is position of the Malashan area in the Tethys based on the presence of three granite bodies that Himalaya (Fig. 10e), however, suggests that the show variations in their bulk composition and in Malashan granite, the core of the Malashan dome, development of D2 deformational fabrics, and is is unlikely to be a part of the Greater Himalaya. supported by radiometric dating (Aoya et al. The Greater Himalayan sequence has ages up to 2005). It is the variation in granitic bodies of the Ordovician (c. 480 Ma; DeCelles et al. 2000), Malashan area that allows us to draw this conclusion. while the Tethys Himalaya preserves a sedimentary In this sense, for some types of study, the Malashan succession from Ordovician to Eocene (Liu & area has clear advantages in its geological setting Einsele 1994). To propose that the Malashan over the Kangmar area. We suggest, therefore, that granite is part of the Greater Himalayan sequence the Kangmar granite may also be a Himalayan would therefore imply that an unreasonably large intrusive based on its striking similarities with the amount of the succession, everything between Malashan granite shown in this study. Even if we the Jurassic and Ordovician, had been locally tecto- exclude from our discussion the inference that the nically excised from within the structural pile. Paiku granite is Himalayan, the evidence demon- In this study we suggest an alternative interpretation strating that the Cuobu and Paiku granites are where the Malashan granite is an intrusive body that syn-D2 intrusions (Fig. 15b) remains unchanged. formed during the Himalayan orogeny. This idea is Because D2 is well constrained to be a Himalayan compatible with the stratigraphic position of the deformation in the Kangmar and Mabja areas, and Malashan area (Fig. 10e) and is supported by the Malashan granite is chemically equivalent to SHRIMP spot dating of zircons from the Malashan the Cuobu granite (Fig. 13), it is difficult to escape and Cuobu granites (Aoya et al. 2005). Himalayan the conclusion that the Malashan granite is a Hima- formation ages of 28–8 Ma have also been obtained layan intrusion. This leaves a major geochronologi- for several nearly undeformed granites in the cal problem: how to reconcile our proposal with the Tethys Himalaya (Scha¨rer 1986; Harrison et al. Q4 results of conventional U–Pb zircon dating of the 1997; Zhang et al. 2004), and the age range partly Kangmar granite, which yields ages of 562 + 4 overlaps that of the Greater Himalayan granites Q2 Ma (Scha¨rer 1986) and 508 + 1 Ma (Lee et al. (32–12 Ma). Zhang et al. (2004) further show 2000). The major difference of c. 50 Ma between that largely undeformed granites from the Mabja the two studies greatly exceeds the estimated and Lhagoi Kangri regions and even the gneissose errors. One possibility is that the U–Pb ages Kangmar granite have Nd–Sr isotopic compo- obtained using mineral separates of zircon from the sitions similar to the Greater Himalayan leucogra- Kangmar granite do not represent a single magmatic nites and metasediments. These studies suggest event but complexly mixed results reflecting most that the variously deformed Tethys Himalayan common ages of xenocrysts. This explanation can granites were derived from melting of the Greater account for the large range in estimated ages for Himalayan sequence. The original magma of the the Kangmar granite and is compatible with our Malashan granite, therefore, probably originated present study. If the North Himalayan gneiss in the Greater Himalaya, migrated upward domes are confirmed to be dominantly cored by through the lower Tethys Himalaya, and was then intrusions formed during the Himalayan orogeny, emplaced into the Jurassic sediments located in at least a part of the concentrically distributed meta- the middle- to upper-stratigraphic section of the morphic isograds surrounding the granite cores may Tethys Himalaya (Fig. 10e). This implies that be explained as the result of deep-seated contact the D2 shear zone observed around the Malashan metamorphism. granite is not directly comparable to the STD, but represents a deformation that occurred within the Implications for channel flow models Tethys Himalaya, at structurally higher levels than the STD. The North Himalayan gneiss domes have recently The two main conclusions of this study, (i) the been viewed as windows into the Greater Himalayan origin of the Malashan granite as Himalayan intru- sequence exposed on hinges of a large-scale anti- sive and (ii) correlation of the D2 deformation with form by several researchers (e.g. Beaumont et al. activity of the STD, are strongly supported by MALASHAN GNEISS DOME, SOUTH TIBET 493 radiometric age determinations reported by Aoya We would like to thank T. Argles, J.-P. Burg and R. Law, et al. (2005). These age determinations for for- whose comments helped improve this study. R. Law is also acknowledged for his editorial assistance. This mation of granite bodies and for the D2 deformation have important implications for channel flow research was supported by a JSPS grant-in-aid awarded to models of the region (e.g. Beaumont et al. 2001, S. Wallis and a grant from Central Washington University awarded to J. Lee. 2004; Jamieson et al. 2004), which are strongly dependent on the strength of the upper crust. As already discussed, the D2 deformation in Malashan References occurred within the Tethys Himalaya (Fig. 10), ALSDORF,D.&NELSON, D. 1999. Tibetan satellite which in the channel flow models behaves as the magnetic low; evidence for widespread melt in rigid hanging wall to a channel of middle- to the Tibetan crust? Geology, 27, 943–946. lower-crustal flow. If, as we suggest, the D2 defor- AOYA, M., WALLIS,S.R.,TERADA,K.,LEE,J., mation of the Malashan area occurred simul- KAWAKAMI,T.,WANG,Y.&HEIZLER,M.2005. taneously with the activity of the STD, it indicates North–south extension in the Tibetan crust that the top-to-the-north deformation represented triggered by granite emplacement. Geology, 33(11). Q5 by the STD also affected the hanging wall and, BEAUMONT, C., JAMIESON, R. A., NGUYEN,M.H.& therefore, that the hanging wall was at least locally LEE, B. 2001. 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