TECTONICS, VOL. 29, TC5011, doi: /2009tc002533, 2010

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1 TECTONICS, VOL. 29,, doi: /2009tc002533, 2010 Metamorphism, melting, and channel flow in the Greater Himalayan Sequence and Makalu leucogranite: Constraints from thermobarometry, metamorphic modeling, and U Pb geochronology Michael J. Streule, 1 Michael P. Searle, 1 David J. Waters, 1 and Matthew S. A. Horstwood 2 Received 12 May 2009; revised 24 March 2010; accepted 11 May 2010; published 28 September Department of Earth Sciences, University of Oxford, Oxford, UK. 2 NERC Isotope Geoscience Laboratory, Keyworth, Nottingham, UK. Copyright 2010 by the American Geophysical Union /10/2009TC [1] The Makalu leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation parallel sheets along the top of the Greater Himalayan Sequence. It is part of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms ± Bt leucogranites but, unlike most other Himalayan granites, also locally contains coarsegrained cordierite. The cordierite bearing leucogranite intrudes through and overlies lower sheets of normal tourmaline granites and represents the most recent phase of magmatism. Cross cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting ( 700 C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800 C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in leucogranites record the last low pressure decompression phase of melting. P T determinations detail peak sillimanite grade metamorphism at 713 C/5.9 kbar, with a secondary cordierite overprint at618 C/2.1kbar;this P T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different leucogranites. The main phase of leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite leucogranite and the migmatitic Barun gneisses at 15.6 ± 0.2 and 16.0 ± 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700 C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene. Citation: Streule, M. J., M. P. Searle, D. J. Waters, and M. S. A. Horstwood (2010), Metamorphism, melting, and channel flow in the Greater Himalayan Sequence and Makalu leucogranite: Constraints from thermobarometry, metamorphic modeling, and U Pb geochronology, Tectonics, 29,, doi: /2009tc Introduction [2] The channel flow model developed in the Himalaya suggests that, during the Early to Middle Miocene, a partially molten low viscosity layer of middle crust was extruded southward by ductile flow processes above a major thrust sense ductile shear zone (Main Central Thrust Zone) and below a major normal sense ductile shear zone (South Tibetan Detachment Zone). These shear zones are bounded at the surface today by brittle faults: the Main Central Thrust (MCT) and South Tibetan Detachment (STD). Deep seismic profiling combined with broadband seismic and magnetotelluric data have imaged a high conductivity layer beneath southern Tibet at km depth [Nelson et al., 1996; Wei et al., 2001; Schulte Pelkum et al., 2005]. This layer is bounded by strong reflector horizons that can be traced to the surface outcrops of the STD and MCT [Hauck et al., 1998; Searle and Szulc 2005]. The bright spots imaged underneath Tibet are thought to represent leucogranite melts forming today, at similar P T conditions and depths to the Miocene leucogranites seen in outcrop in the Himalaya [Searle et al., 2003, 2006; Gaillard et al., 2004; Scaillet and Searle, 2006]. [3] Structural mapping, fabric analysis and P T profiles across the Greater Himalayan Sequence (GHS) and U (Th) Pb geochronology formed the foundations for the channel flow model [Searle and Rex, 1989; Burchfiel et al., 1992; Hodges et al., 1992, 1996; Grujic et al., 1996, 2002; Searle et al., 1999, 2003, 2006, 2008; Jessup et al., 2008]. Together with deep seismic profiling across southern Tibet, these data provided the structural geometry of the bounding shear zones, P T conditions across the GHS and timing constraints on metamorphism and melting that provided the input parameters for the model. Thermal mechanical and numerical models showed that channel flow is a viable process if it was 1of28

2 linked to focused surface erosion at the Himalayan front [Beaumont et al., 2001, 2004, 2006; Jamieson et al., 2004, 2006]. [4] Channel flow and exhumation of the GHS was coeval with movement on a lower, south vergent, thrust sense ductile shear zone, the Main Central Thrust Zone (MCTZ) [Heim and Gansser 1939; Gansser 1964; Stephenson et al., 2000, 2001; Searle et al., 2008] and an upper low angle, top down to the north, normal sense ductile shear zone, the South Tibetan Detachment Zone (STDZ) [Caby et al., 1983; Burg et al., 1984; Burchfiel et al., 1992; Carosi et al., 1998, 1999; Grujic et al., 2002; Searle et al., 2003, 2006; Law et al., 2004; Cottle et al., 2007]. Following the definitions of Stephenson et al. [2000, 2001] and Searle et al. [2008], we map the MCT at the surface as the brittle thrust fault that underlies all Tertiary inverted metamorphic rocks within the ductile shear zone (MCTZ), and the STD as the brittle normal fault that structurally overlies all the right way up metamorphic isograds within the ductile shear above the partially molten core of the GHS (STDZ) [Searle et al., 1992, 1997b, 2003, 2006; Law et al., 2004; Cottle et al., 2007, 2009b; Jessup et al., 2008]. Despite a few previous studies [e.g., Guillot et al., 1993], Searle and Godin [2003] showed that all leucogranites along the Himalaya, many of which display high temperature ductile fabrics, crop out within the GHS metamorphic rocks in the footwall of the STD, and therefore, their intrusion ages can be used to constrain the transition from high grade ductile deformation to low temperature brittle exhumation of the paleochannel. [5] In the Everest Makalu region, two low angle normal faults have been mapped: the lower Lhotse detachment which separates sillimanite grade gneisses, migmatites, and leucogranites below from the Everest series above [Searle, 1999a, 1999b, 2003; Searle et al., 2003, 2006] and the upper Qomolangma detachment which separates the Everest series below from unmetamorphosed Palaeozoic sedimentary rocks above [Burchfiel et al., 1992; Murphy and Harrison, 1999; Searle 2003; Law et al., 2004]. Jessup et al. [2008] constrained P T conditions of metamorphism of the Everest series staurolite schists as C and 6.2 kbar with a decrease in pressure to 3 kbar recorded by cordierite overgrowths. This implies that the Qomolangma detachment represents the STD in a strict sense and the lower Lhotse detachment is a narrow ductile shear zone within the STDZ. Cottle et al. [2007] described the STD section in Dzakaa Chu, northwest of Makalu as a 1000 m thick, low angle (<35 ) zone of distributed ductile shear that records a ductile to brittle progression without discrete brittle detachment faults. In the core of the ductile channel U (Th) Pb geochronology revealed that GHS metamorphism lasted at least 20 million years and peaked around Ma, ending by 16 Ma when later sets of leucogranite dykes cross cut earlier ductile fabrics [Cottle et al., 2009b]. Heating as a result of crustal thickening, internal radiogenic heating and decompression have all been invoked as causes of anatexis in the Himalaya [Harrison et al., 1999; Harris, 2007]. Multisystem geochronology including 39 Ar/ 40 Ar and fission track dating also show that rapid cooling occurred immediately following melting along the entire GHS as the channel flow extrusion was proceeding [Arita et al., 1997; Blythe et al., 2007; Catlos et al., 2001; Copeland and Harrison, 1990; Crouzet et al., 2007; Godin et al., 2001; Hubbard and Harrison, 1989; Macfarlane, 1993; Vannay and Hodges, 1996; Searle et al., 1992, 1997b]. There is no evidence for present day active motion along either the MCT or the STD in the Himalaya, so we assume that channel flow operated only during the Early Middle Miocene. It is however possible that similar P T conditions and crustal melting are occurring today at midcrustal depths beneath the southern part of the Tibetan plateau. [6] This study focuses on the Greater Himalayan Sequence (GHS) in the Makalu region of northeast Nepal, immediately east of the Everest region, a sequence of Indian plate passive margin sedimentary rocks and orthogneisses that have been subsequently metamorphosed, melted, and exhumed during the Tertiary Himalayan orogeny. We have mapped a profile along the Arun valley from the MCT north to the peak of Makalu (Figure 1). Here we report P T conditions of metamorphism and U (Th) Pb ages of both metamorphic host rocks and leucogranites. The metamorphic P T paths of Miocene leucogranites, their host gneisses and their migmatite source rocks are required in order to understand processes of crustal anatexis and exhumation. 2. The Greater Himalayan Sequence [7] Following India Asia collision and closing of the NeoTethys Ocean at approximately 50 Ma [Searle et al., 1997a; Zhu et al., 2005; Green et al., 2008] subduction of the leading margin of the Indian plate led to high pressure eclogite facies metamorphism, preserved only in two regions of the western Himalaya (Kaghan valley, Pakistan, and Tso Morari, Ladakh). In the Ama Drime massif, northeast of Makalu earlier eclogites have been overprinted by a granulite facies metamorphism that is correlated with sillimanite grade metamorphism and muscovite dehydration melting [Cottle et al., 2009a; Groppo et al., 2007; Corrie et al., 2010]. Following initial deep subduction and rapid exhumation of eclogites, crustal shortening and thickening led to widespread kyanite grade metamorphism during the latest Eocene Oligocene. Subsequent heating and decompression led to widespread sillimanite grade metamorphism and associated crustal anatexis. As a result a normal sequence of Barrovian metamorphic isograds developed through the thickened crust [LeFort, 1986; Searle and Rex, 1989]. An outline of the tectonostratigraphy of the Himalaya is shown in Figure 1a. [8] The GHS is now exposed as a highly strained package of metamorphic rocks, migmatites, and leucogranites exhumed from midcrustal depths, with the majority of the GHS being a core of sillimanite grade gneisses with in situ partial melts. Toward the top of the GHS melts become more widespread and coalesce to form large foliationparallel sheets, which spectacularly form some of the highest peaks of the Himalaya (e.g., Shivling, Manaslu, Makalu, Shisha Pangma, Kangchenjunga, and the base of the Everest Nuptse Lhotse massif). Extreme topography allows the 3 D configuration of these plutons to be appreciated [e.g., Searle, 1999a, 1999b, 2003]. The GHS exhibits an inverted and compressed isograd sequence along the base (the MCTZ 2of28

3 Figure 1. (a) Map of the outline tectonostratigraphy of the Himalaya; redrawn from Searle and Szulc [2005]. (b) Geological map of the GHS in eastern Nepal between the MCT and STD in the Arun Valley. Sample localities are also labeled. The location of the Lhotse and Qomolangma detachments and the geological mapping in the Everest area is from Searle et al. [2003]. All other geology is from this study. above the MCT) and right way up and compressed isograds along the top (the STDZ below the STD) that appear to be recumbently folded around the sillimanite core [Searle and Rex, 1989]. These observations of shear zones are interpreted to result from the ductile south vergent recumbent folding of isograds as a result of ductile channel flow and exhumation [Searle et al., 2003, 2006, 2007; Jessup et al., 2006; Cottle et al., 2009b]. 3. Makalu Himalaya and Geological Mapping [9] Geological mapping and sample collection was undertaken in the upper Barun and upper Hongu valleys of eastern Nepal, around the peaks of Makalu, Baruntse, and Chamlang (Figure 1b). In contrast to the neighboring Everest region [Carosi et al., 1998, 1999; Searle, 1999a, 1999b, 2003; Simpson et al., 2000; Searle et al., 2003, 2006; Law et al., 2004; Jessup et al., 2006, 2008; Cottle et al., 2007] the Makalu region is somewhat poorly known. Previous reconnaissance studies in the Makalu Arun region were conducted by Pognante and Benna [1993], Lombardo et al. [1993], and Goscombe et al. [2006] following the initial pioneering studies of Bordet [1961]. Schärer [1984] published U Pb ages of 21.9 ± 0.2 and 24.0 ± 0.4 Ma of monazite from samples of the Makalu granite but no field relationships were given so the geological context of this age is not well known. [10] The STD cuts the nearby Everest Lhotse massif where in this particular location it is bifurcated into two normal sense shear zones; a structurally lower Lhotse detachment and a higher Qomolangma detachment (Figure 2b). Projecting the level of the Lhotse detachment toward the Makalu Baruntse area confirms that this field area forms the structurally highest levels of the GHS [Searle, 2003]. The Makalu leucogranite is a large intrusion which is also laterally extensive, forming the majority of the Makalu and Baruntse peaks, and is also linked to the base of the Everest Lhotse massif. The STD low angle normal faults crop out along the hills north of the Kangchung glacier in Tibet with the highest peaks along the Nepal Tibet border, Makalu (8475 m) and Chomolonzo (7790 m), composed entirely of leucogranite [Searle, 2003]. [11] Detailed mapping of the upper Barun and Hongu valleys demonstrated the presence of two distinct types of granite; a dominant normal Grt + Tur + Ms ± Bt bearing granite and an additional cordierite bearing granite that forms the structurally highest levels of the Makalu summit rocks. A few cross cutting feeder dykes mapped adjacent to the upper Barun glacier appear to have channelled magma up to the upper sheet. Regional metamorphic foliations dip and mineral stretching lineations plunge to the north and northeast (Figure 3a), but are more distorted close to the main intrusive areas. [12] As a result of exhumation associated with the STDZ, rocks show top to the north, bottom to the south sense of shear, with migmatitic leucosomes exhibiting shear sense indicators illustrating the immediately prekinematic to synkinematic nature of crustal anatexis (Figure 3b). Field 3of28

4 Figure 1. (continued) 4of28

5 relationships show that partial melting persists down section into the migmatised Barun gneiss, the likely source rocks for the Makalu granite where strained leucosomes are hosted in sillimanite grade gneisses (Figure 2d). Leucosomes coalesce to form foliation parallel sills which permeate the rocks and occasional cross cutting dykes which channel magma upwards (Figure 2c). Figure 2 5of28

6 [13] The Barun gneiss is composed of orthogneisses and metapelites with the assemblage: Qtz + Bt + Pl + Grt + Sil ± Kfs ± Crd and uncommon calc silicates composed of the assemblage: Qtz + Bt + Ca + Cpx ± Ol ± Kfs. A prominent band of kyanite bearing metapelite separates the upper Barun gneiss from the lower Num orthogneiss. Goscombe and Hand [2000] mapped the MCT along this band but the fact that strongly deformed sillimanite bearing gneisses also occur structurally beneath this horizon suggests that the Num gneiss clearly belongs to the GHS and the MCT occurs beneath the Num Gneiss below a zone of inverted isograds. The Num gneiss is a mylonitised felsic orthogneiss containing the assemblage: Qtz + Pl + Bt + Ms ± Kfs ± Grt ± Sil and lies immediately above the zone of inverted metamorphic isograds which form the MCTZ. The sillimanitebearing Num gneiss and Barun gneiss together with the Makalu leucogranites in between the bounding shear zones have a structural thickness of 30 km. 4. Petrography [14] Here we consider the petrography of a number of metapelites, leucogranites, and migmatites, which are present in the upper part of the GHS in the Everest Makalu area. Sample localities are shown in Figure 1b. In the field, one can observe a widespread cordierite overprint in the uppermost structural levels of metamorphic rocks, which is also where the majority of leucogranites occur Leucogranites and Migmatites [15] Isotopic studies using Sr, Nd, and Pb from elsewhere in the Himalaya imply a metasedimentary crustal protolith source [e.g., Deniel et al., 1987; Harris et al., 1995; Guillot and LeFort, 1995] for the leucogranites seen in the GHS. However, the leucogranites are often heterogeneous across individual intrusive complexes, suggesting multiple phases of melt input, from variable crustal sources [Harris and Massey, 1994]. Migmatites in the Himalaya contain biotite but not muscovite implying that melting has taken place by dehydration of muscovite indicating a temperature of anatexis of 700 C 800 C [e.g., White et al., 2001] at pressures of 4 8 kbar. Experimental studies further confirm that melting took place in the region of 710 C and 790 C at 8 kbar [Petö, 1976; Patiño Douce and Harris, 1998]. The resultant leucogranites formed typically have the assemblage Ms + Bt + Grt + Tur ± Crd ± Sil. [16] The Makalu leucogranite forms a multiphase intrusion, with two distinct groups of leucogranite, based on their mineralogy. The majority of leucogranites (e.g., sample A1) are Grt + Tur + Ms ± Bt bearing with occasional late stage pegmatitic veins rich in Ms (Figures 4a and 4b). Well formed tourmaline suns were often present (Figure 4c). This suite of granites was considered normal and is very similar to the granites in the rest of the GHS of the Himalaya. However, certain granites contain large cordierite crystals (e.g., samples A19, A6; Figure 4d). These were found in situ in dykes on the south face of Makalu (Figure 2c) that appear to be feeding the uppermost structural levels of the intrusion. In addition to this a copious amount of cordierite bearing leucogranite debris, fed from the summit slopes of Makalu, indicates that most upper levels of the Makalu intrusion are formed of this lithology. Field evidence shows that the cordierite bearing leucogranites are the most recent phase of intrusion relating to decompression and exhumation of the GHS. Mineralogy of all relevant samples is shown in Table 1. [17] Such pristine and prominent cordierite bearing leucogranites have not been recognized elsewhere in the Himalaya except in only a few other specific locations within the GHS leucogranites; e.g., Langtang and Bhutan [Inger and Harris, 1993, Visona and Lombardo 2002; Kellett et al., 2009]. Cordierite bearing leucogranites are also present in the Nanga Parbat syntaxis in northwest Pakistan where they have extremely young U Pb ages of Ma [Zeitler et al., 2001a, 2001b; Crowley et al., 2005, 2009]. These Pleistocene cordierite leucogranites from Nanga Parbat are much smaller melt fractions than the large Makalu intrusion and represent extremely rapid and recent exhumation due to the unique tectonic setting of the syntaxial area. In other orogenic belts, cordierite granite terranes have also provided evidence for rapid, high temperature decompression [e.g., Brown and Dallmeyer, 1996]. Therefore, in the Makalu Baruntse area, the unusually wide range of leucogranites present should provide as long a record as possible of crustal anatexis in the GHS, in particular the most recent phases which may represent granite formation induced by decompression and exhumation of the GHS Migmatites and Sillimanite Cordierite Gneisses [18] The host Barun gneisses to the Makalu leucogranites are migmatitic for around 15 km structurally below the granites and are the most likely source rocks. In outcrop, leucosome veins can often be seen ponding together, coalescing magma, and then feeding into dykes which appear to channel magma both laterally and structurally upwards. Limitations on exposure and terrain preclude regional scale mapping of magma injection systems, but in the uppermost structural levels around Makalu, injection conduits become larger implying the upward migration of melts. [19] A typical assemblage of the migmatitic source rocks is Grt + Fsp + Qtz + Bt ± Sil (sample A81) and occasionally prismatic sillimanite can be seen pseudomorphing after Figure 2. Cross section model of the GHS (redrawn from Searle et al. [2007]). Aspects of the model are displayed as field photographs from this study from the upper Barun valley. (a) The south face of Makalu showing uppermost structural levels of the GHS where leucogranites become widespread, with approximate kilometre scale thick sills. (b) Everest Lhotse massif seen looking west from the Upper Barun glacier. The higher Qomolangma Detachment (QD) forms the STD while the lower Lhotse Detachment (LD) forms part of the STDZ below. (c) Sill and dyke networks on the lower south face of Makalu feeding the upper levels of Makalu. (d) Leucosomes in migmatitic Barun gneiss seen near the location of sample A81 (see Figure 1). 6of28

7 kyanite (A73/A74) giving evidence for the P T evolution of these samples. In the uppermost structural levels of the mapped Makalu area a secondary metamorphic overprint is seen. Cordierite coronas are seen around commonly large garnets (sample A45; Figure 5A). Although sillimanite is dispersed through the rock matrix, it is absent from the coronas, implying that cordierite growth also consumed sillimanite along with the garnet. The following reaction may be occurring [Davidson et al., 1997], Grt þ Sil þ Qtz þ H 2 O! Crd: Figure 3. (a) Stereonet plot of structural data observed as mineral stretching lineations and metamorphic foliations in the Barun Gneiss. (b, c) Field photos showing syn kinematic leucosomes in migmatites demonstrating top to the north sense of shear (b) from the locality of sample A74 and (c) from the locality of A81. At the uppermost structural levels cordierite is prominent and garnet is often absent as seen in the gneisses which host the leucogranites (e.g., sample A32; Table 1). In this sample, cordierite and K feldspar are ubiquitous. Original compositional banding reveals two distinct layers in the rock; one layer demonstrates abundant sillimanite, but biotite is absent, while another layer exhibits biotite but no sillimanite. The biotite appears to be undergoing resorption with frequently embayed margins (sample A32; Figure 5B). Here the following reaction is inferred to be occurring [Davidson et al., 1997], Bt þ Sil þ Qtz! Crd þ Kfs þ H 2 O; with the reaction terminating due to the loss of one of the reactants in each of the layers. Therefore in all of the Figure 4. Petrography of Makalu Leucogranites. (a and b) Normal leucogranites showing at least six phases of intrusion. (c) Tourmaline suns from the upper Hongu. (d) Cordierite in leucogranites from the south face of Makalu. 7of28

8 Table 1. Mineral Assemblages of Samples Used for Metamorphic and Geochronological Analysis Sample P T U Pb Rock Type Grt Bt Ms Kfs Pl Sil Ky Tur Qtz Chl Crd Notes: Petrography A 1 x Granite x x x x x x Tur found included within Grt A 6 Granite x x x x x x x Melt pod: Sil present within Crd A 19 x Granite x x x x x (x) Bt is minor, Crd is altered to pinnite with Chl, opaques and minor Ms A 32 x Metapelite x x x x x x Large Sil patches in Bt Qtz Pl Kfs Crd layered matrix A 42 x Metapelite x x x x x Large Sil patches, Grt has Crd coronas with yellow pleochroic haloes A 45 x x Metapelite x x x x x x (x) x Crd corona around Grt (Sil inclusions), Grt is cracked and breaking down to Chl A 47 x Metapelite x x x x x (x) x Grt breaking down with Crd corona. Bt in micro cracks in Grt A 73 x Metapelite x x x x (x) x Prismatic Sil after Ky A 74 x Metapelite x x x x x (x) x Prismatic Sil. Grt is rare A 81 x x Metapelite x x x x x x Grt is poikiloblastic, well formed elongate prismatic Sil cordierite bearing rocks in the upper structural levels, there is petrographic evidence for a transition from sillimanite grade to a lower pressure environment. [20] The prominence of cordierite in the gneisses may correspond geologically to the cordierite bearing leucogranites in this area; therefore, given the field and petrological observations, this paper will attempt to establish petrological and geochronological links between the metamorphic evolution from higher to lower pressure and the two main episodes of granite generation. 5. Metamorphic Mineral Chemistry [21] Specific samples were selected to illuminate as much of the metamorphic history as possible. Samples A73/A74 Figure 5. Petrography of Sillimanite grade Barun gneisses with cordierite overprint. (a) Outcrop and thin section (cross polarised light) view of large garnets with cordierite coronas. (b) Photomicrograph of gneiss (sample A32) showing original compositional layering. 8of28

9 which contained sillimanite pseudomorphs after kyanite formed as a result of increasing temperature and/or decreasing pressure and records a snapshot of P T evolution in this part of the GHS. A garnet bearing migmatite, A81, records conditions of melting, during the main phase of leucogranite formation, while samples A42/A45/A47 which contain cordierite record details of decompression of the GHS. [22] Minerals were analyzed with a JEOL JSM 840A scanning electron microscope in the Department of Earth Sciences, Oxford, equipped with an Oxford Instruments Isis 300 energy dispersive analytical system. Accelerating voltage was 20 kv, with a beam current of 6 na, and a live counting time of 100 s. It was calibrated with a range of natural and synthetic standards, and a ZAF correction procedure was used. Mineral assemblages of the relevant samples are listed in Table 1, and representative mineral analyses are shown in Table 2. Ilmenite was often present as an accessory mineral Garnet [23] Garnets in the migmatitic sample (A81) were compositionally homogeneous although poikiloblastic with large inclusions of Qtz, Bt, and Pl (Figure 6a). Garnets in the other samples were inclusion free and weakly zoned with normal prograde garnet growth profiles (Figure 6b). Samples that have undergone a cordierite overprint (A42/ A45/A47) contained garnets that were overall slightly more calcic compared to those of the structurally lower samples but still showed evidence of prograde growth zoning (Figure 6c) Biotite [24] All the samples contained biotite but no muscovite, implying that P T conditions exceeded the muscovite dehydration melting curve. Within each sample biotite in the matrix was largely homogeneous, but between samples some Fe Mg variability occurred. Matrix biotite also had a high Ti content; cations per formula unit. Biotite was additionally found in cracks within garnet in most samples. This had virtually no Ti content, but similar X Fe and X Mg ratios compared to their matrix counterparts Feldspar [25] Of the metamorphic samples analyzed only some contained plagioclase feldspar, which was largely homogeneous across the suite of samples in the composition range oligoclase andesine. Additionally or alternatively, some samples contained alkali feldspar in the compositional range (Na ,K )AlSi 3 O Cordierite [26] Cordierite found around garnet in three samples (A42/A45/A47) formed an almost pure Mg Fe solid solution in the range (Mg ,Fe ,Mn 0.02 )Al 4 Si 5 O 18 with additional minor Mn. Cordierite was largely homogeneous within samples. Higher Fe or Mg contents in cordierite corresponded to higher Fe or Mg contents in the directly adjacent garnet and therefore support the notion of cordierite replacement of garnet. 6. P T Determinations [27] P T conditions were calculated using THERMOCALC v3.30 which utilizes the self consistent thermodynamic dataset of Holland and Powell [1998; Powell and Holland, 1988, 1994]. Different groups of mineral analyses from Table 2 were used within their petrological context; if garnets were zoned then rim analyses were used as these are assumed to be in equilibrium with the matrix phases. Sets of analyses of matrix minerals were used where all the phases were in contact with one another and in close proximity to any porphyroblast phases that were present; these criteria were used to assume equilibrium. The presence of ilmenite in these samples facilitated the use of Ti contents of biotite [Henry et al., 2005] as an additional check on the accuracy of temperature estimates of THERMOCALC. Results are shown in Table 3 and Figure High Temperature Stage [28] Samples A73 and A74 are sillimanite rich black gneisses with sparse leucosome material, with the evidence of the former presence of kyanite (pseudomorphed by sillimanite). Selecting garnet and matrix biotite analyses directly adjacent to one another gave a result of 554 ± 73 C and 6.1 ± 1.2 kbar, which lies within the kyanite field, but close to the sillimanite field. Ti is present in the matrix biotite, which is remote from the garnet; this content corresponded to a temperature of 687 C using the method of Henry et al. [2005]. This therefore represents some chemical disequilibrium across a single sample and is interpreted to represent the garnet and immediate matrix recording kyanite grade conditions and the remaining matrix biotite used alone is interpreted to represent the transition to sillimanite grade conditions. In this sample, biotite was also present in cracks in garnet but this contained no Ti. With regard to the petrography and chemistry, we interpret this biotite to represent a late breakdown of garnet, which was unable to equilibrate Ti with any part of the rock matrix and thus of little use in P T determination. [29] Sample A81 is rich in leucosome layers but contains no cordierite. Such migmatites are the likely source rocks of the Makalu leucogranite and thus represent peak anatectic P T conditions. Garnets were homogeneous as was biotite in the matrix and in garnet. Although imprecise, a THERMOCALC result of 713 ± 107 C and 5.9 ± 1.8 kbar are in close agreement with the Ti in biotite thermometer (Table 3) and as such are thought to be accurate. These conditions directly correlate to laboratory experiments where samples of metapelitic rock from the GHS were melted at C and 6 8 kbar to produce leucogranites analogous to those observed in the Himalaya [Patiño Douce and Harris, 1998]. During experimental melting at these conditions, muscovite dehydration was determined to be the primary melting process. 9of28

10 Table 2. Representative SEM EDS Mineral Analysis of Relevant Samples A81 Bt Matrix Grt Matrix Kfs Matrix Pl Matrix A47 Bt Matrix Crd Corona Grt Rim Grt Core Kfs Matrix Pl Matrix A32 Bt Kfs Pl A6 Bt Crd Kfs Pl A1 Bt Matrix Grt SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Total No. of O Si Ti Al Fe Mn Mg Ca Na K Cations Kfs Pl A74 Bt Matrix Bt Near Grt Grt Ave Grt Rim Pl Matrix Kfs Matrix A42 Bt Matrix Bt Grt Cracks Crd Corona Grt Rim Grt Core A45 Bt Matrix Bt Grt Cracks Crd Corona Grt Pl Matrix A73 Bt Grt Rim Pl Matrix SiO TiO Al2O FeO MnO MgO CaO Na2O K 2 O Total No. of O Si Ti Al Fe Mn Mg Ca Na K Cations of 28

11 Figure 6. (a) BSE image of a garnet from migmatite sample A81. (b) BSE image of a garnet ringed by cordierite in Barun gneiss sample A42. Position of linescan is shown. (c) Chemical profile across garnet. Changes in Mn, Mg, and Ca content are compensated by a change in the Fe content (not shown). [30] These analyzed samples belong to the same structural unit in the GHS (the Barun gneiss) and are therefore interpreted to have followed the same P T evolution. Each sample records a P T snapshot during this evolution, and these can be linked to infer a continuous prograde P T path involving burial to 6.1 kbar followed by decompression and heating across the muscovite dehydration melting reaction (Figure 7). This clockwise P T path is characteristic of the modeled P T paths determined for large convergent orogens such as the Himalaya [Jamieson, 1991; Jamieson et al., 2002, 2004]. Table 3. P T Data From Samples Plotted in Figure 7 THERMOCALC v.3.30 Sample avt sd(t) avp sd(p) Corr Ti in Bt Temp (±24) Notes A A A Using matrix Bt A A Using Bt in garnet, no Ti A Using Bt/rim garnet 11 of 28

12 Figure 7. THERMOCALC P T determinations from samples of the upper GHS; data are in Table 3. A decompression P T path is exhibited by some samples, although A74 may represent disequilibrium in the sample. Error ellipses are calculated in THERMOCALC and displayed at 1s level. The muscovite dehydration melting curve is from White et al. [2001], and the Al silicate phase diagram is from Holdaway and Mukhopadhyay [1993] Low Pressure Stage [31] Cordierite coronas around garnet were seen in three of the samples analyzed (A42, A45, and A47). Cordierite in metapelites has been experimentally determined to occur at pressures below kbar at temperatures of 640 C 719 C [Holdaway and Lee, 1977] for an Fe end member system but will occur at higher pressures with Fe Mg solid solution. THERMOCALC average P T determinations using matrix biotite compositions described a P T path of decreasing pressure and temperature (A45, A47) which ended with the P T recorded by A42 which was at the lowest grade conditions. This served to quantify the cordierite decompression stage at 618 C ± 58 C and 2.1 ± 0.9 kbar. These are some of the lowest pressures recorded in GHS rocks to date; comparable work by Kellett et al. [2009] obtained pressures of below 2.8 kbar in Bhutan. Ti contents in matrix biotite were additionally used following the method of Henry et al. [2005] to assess the level of matrix equilibrium. Results are shown in Table 3 and show that temperature estimates of THERMOCALC are in accordance with those using Ti in biotite. [32] These P T determinations can be used to extend the clockwise P T path from high temperature metamorphism to low pressure, which therefore exhibits exhumation of the upper structural levels of the GHS (Figure 7). The transition from high temperature to low pressure metamorphism lies within the melt field of muscovite dehydration melting [White et al., 2001], which is consistent with the absence of muscovite and persistence of biotite and also implies that leucogranites may have been produced for a protracted period [e.g., Zhang et al., 2004] during this transition. 7. Metamorphic Modeling [33] The observation of snapshots of a continuum of metamorphism in the upper GHS led to the motivation to model the metamorphism thermodynamically using pseudosections. This will serve to more tightly constrain the entire P T evolution. In particular, the use of pseudosections provides the most rigorous evaluation of changing P T conditions exhibited in metamorphic rocks [Powell and Holland, 2008]. Additionally, the identification of melt extraction during metamorphism is an additional factor that must be incorporated into pseudosection modeling. Pseudosections were drawn using THERMOCALC v3.30 and the NCKFMASH model system, including melt Bulk Rock Compositions [34] Multiple BSE images of thin sections were analyzed to determine the modal proportions of the phases present in all relevant samples. These were then combined with mineral analyses (Table 2) to determine a bulk rock composition. These results are shown in Table 4. [35] The composition data demonstrate that during crustal anatexis the extracted melts (e.g., A1, A6) are enriched in 12 of 28

13 Table 4. Modal Proportions and Bulk Rock Compositions Used for Pseudosection Analysis Modal % Sample Bt Grt Qtz Pl Kfs Crd Sil Rock Type A Migmatite A Crd Gneiss A Leucogranite A Crd L Granite Composition (Molar Proportions): NCKFMASH system Sample SiO 2 H 2 O Al 2 O 3 CaO MgO FeO K 2 O Na 2 O A A A A SiO 2 in particular relative to their original protolith (A81). Correspondingly, the restitic rocks (e.g., A47) are relatively depleted in melt components. It is therefore within this framework that melt production was modeled using THERMOCALC Pseudosections [36] Initially, a pseudosection for sample A6, a cordierite bearing leucogranite, was constructed. As this is thought to be one of the latest formed melts, the pseudosection is useful to interpret when final crystallization of melts took place. The solidus, marked by a heavy line in Figure 8a, lies at approximately 700 C. The P T value for A47, the restitic, cordierite bearing gneiss, lies close to the solidus and, along with the P T trajectory determined in Figure 7, indicates that final crystallization of the Makalu granites took place at approximately 4 kbar and 700 C. This result was further tested by plotting the Mg isopleths of cordierite. Cordierite in A6 has an X Mg value of 0.5 that also closely intersects with the solidus at 4 kbar and 700 C, further confirming the validity of this result. [37] A pseudosection was also constructed for A81 in order to represent a moderately fertile GHS protolith that is the likely source for much of the leucogranite seen in the GHS. The bulk rock composition was determined from the palaeosome domains, excluding the large domains of leucosome as it is possible that these represent introduced melt. Additionally the H 2 O content was increased to 2% to represent a higher volatile content prior to melting. The resulting bulk composition contains muscovite bearing assemblages in the subsolidus region and will undergo dehydration melting of muscovite at temperatures above 700 C (Figure 8b). [38] The large arrow and dashed line mark the solidus and P T trajectory determined previously for the cordierite granite (A6). Peak pressures determined in this study for the rocks of this terrane do not exceed 6.1 kbar (A74); this corresponds to similar pressures determined for Miocene crustal melting elsewhere in the Himalaya [e.g., Walker et al., 1999; Harris et al. 2004], although higher pressures corresponding to the earlier crustal thickening are often recorded in rocks at this structural level [e.g., Vance and Harris, 1999; Walker et al., 1999]. Figure 8b demonstrates that at a pressure of approximately 6 kbar only a few percentage melt is produced at the intersection with the solidus. We interpret this to represent the production of granites such as sample A1. However, if the P T path of the terrane intersects and crosses the field marked cd bi g liq sill significantly more melt is produced as cordierite enters the assemblage. P T determinations imply this is the case and so these two distinct phases of leucogranite production are accurately modeled using pseudosections, and this provides a sound base from which to investigate the geochronology of these samples. The section of the P T path shown in Figure 8b is analogous to those determined by Groppo et al. [2009] for anatectic metapelites found further south in the Arun Valley. 8. U Pb Geochronology [39] Laser ablation multicollector inductively coupled plasma mass spectrometry (LA MC ICP MS) was performed at the NERC Isotope Geosciences Laboratory, UK. A Nu Plasma MC ICP MS (Nu Instruments, Wrexham, UK) and a UP193SS laser ablation system (New Wave Research, UK) were used to analyze the samples with an ablation spot size of 15 or 20mm. Laser fluence was kept at 2 3Jcm 2. Procedures for U (Th) Pb dating were the same as those reported in the work of Cottle et al. [2009a]. Reference materials 91500, Manangotry, and FC 1 [Horstwood et al., 2003] were used for U Pb normalization of zircon, monazite, and xenotime, respectively. For the leucogranites monazite, xenotime, and zircon were separated from crushed rock samples and analyzed from grain mounts. Prior to analysis grains were imaged by backscatter SEM to identify any different growth zones or crystal defects. [40] Multistage monazite growth has been used to date multiple metamorphic stages in rocks in the Himalaya [Kohn et al., 2005] and Karakoram [Fraser et al., 2001; Foster et al., 2004; Streule et al., 2009] previously and was therefore used here as the primary target for metamorphic geochronology [Parrish, 1990]. A common feature of metamorphic monazite is that it is often chemically zoned [Martin et al., 2007]. Of particular importance is zoning of yttrium content, which is related to the episodic growth of monazite and garnet during protracted metamorphism [e.g., Foster et al., 2000, 2002]. Y systematics in a rock may be complex as the element can be hosted by a number of dif- 13 of 28

14 Figure 8. (a) Pseudosection for A6 using the NCKFMASH model system to determine the final P T conditions of melt crystallisation. The solidus is marked in bold, and the P T trajectory of decompression determined in Figure 7 is shown, along with the P T result of cordierite gneiss A47 (error ellipse at 1s level). (b) Pseudosection for A81 using the NCKFMASH model system. Contours of melt produced are added above the solidus (marked in bold). The P T trajectory and solidus for A6 is added as a dashed line. Significant melt production occurs across the cordierite in line; the narrow field marked cd bi g liq sill. Mineral abbreviations arethoseusedinthermocalc[holland and Powell, 1998]. ferent minerals, principally garnet, monazite, allanite, and xenotime [Spear and Pyle, 2002]. However, in the metamorphic samples analyzed allanite and xenotime were absent, and so Y is principally partitioned between the garnet and monazite and therefore growth (or dissolution) of garnet during metamorphism will intrinsically affect the Y budget in a rock [Pyle and Spear, 1999] and subsequently trigger monazite dissolution (or growth). Therefore element maps derived from Ce La,Y La,Th Ma, and U Ma X ray spectra were composed for selected monazite grains from each sample. For sample A32, a Bt Qtz Sil Crd Fsp gneiss which hosted cordierite bearing leucogranites on the south face of Makalu, abundant monazite was separated. To exploit the textural configuration of monazites found in other metamorphic samples which potentially have a complex melting and metamorphic signature (A45, A81), in situ dating of monazite was done using petrographic thin sections following trace element mapping. Additionally for these samples Y + REE contents of monazites were analyzed. This not only quantified the actual compositions of any different zones but also gave an indication as to their origin [Ayres and Harris, 1997]. Data for all U Pb analyses can be seen in Table 5. All age uncertainties are quoted at the 2 sigma level Leucogranite Results [41] The normal leucogranite (A1) contained monazite and zircon. Zircons exhibited large cores interpreted to represent inheritance from the protolith in agreement with other Himalayan leucogranites [e.g., Noble and Searle, 1995; Searle et al., 1999]. The younger magmatic phase relating to Himalayan anatexis was present as thin rims generally less than the spot size of the ablation (see Figure 9). The data define a regression (MSWD = 0.52; Figure 9 and Table 5) with an upper intercept at 454 ± 42 Ma (inheritance) and a lower intercept at 23.8 ± 0.2 Ma interpreted to represent the timing of anatexis; two concordant analyses directly dating the rim at this age were obtained. The upper intercept was determined from the younger concordant and mixed ages defining a regression line. Monazites were rare and allowed for only two analyses, which were discordant and therefore of little independent use. [42] The cordierite bearing leucogranite (A19) contained accessory zircon, monazite, and xenotime. U Pb age data are plotted in Figure 10 and presented in Table 5. The results illustrate multiple anatectic events represented by this sample. As with the normal leucogranite (A1) zoned zircons have inherited cores and magmatic rims. The data define a regression (MSWD = 1.4) with an upper intercept at 508 ± 10 Ma, a similar inherited age to other leucogranites of the Himalaya [e.g., Noble and Searle, 1995; Searle et al., 1999] and a lower intercept age of 23.2 ± 0.8 Ma (see Figure 10). This magmatic age is within uncertainty of the result for sample A1. [43] Younger events are then recorded by monazite and xenotime in sample A19. In this sample the monazites do not appear zoned and different grains possessed different U Pb ages; they are therefore thought to be mainly of magmatic origin. Data points are plotted on Tera Wasserberg diagrams in Figure 10, and regression lines relative to likely commonpbvaluesof [Stacey and Kramers, 14 of 28

15 Table 5. Results of LA MC ICP Mass Spectrometry a Spot I.D. 206 Pb (mv) 207 Pb (mv) 238 U (mv) U (ppm) 207 Pb/ 206 Pb 1s % 206 Pb/ 238 U 1s % 238 U/ 206 Pb 207 Pb/ 235 U 1s % Rho* A32:Mnz Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E A1:Zrn Zrn E Zrn E Zrn E Zrn E Zrn E A1:Mnz Mnz E Mnz E A19:Zrn Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E Zrn E A19Mnz Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E A19Xnt Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E of 28

16 Table 5. (continued) Spot I.D. 206 Pb (mv) 207 Pb (mv) 238 U (mv) U (ppm) 207 Pb/ 206 Pb 1s % 206 Pb/ 238 U 1s % 238 U/ 206 Pb 207 Pb/ 235 U 1s % Rho* Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E Xnt E A45:Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz AMnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz Mnz A81:Mnz Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E Mnz E a U contents (ppm) are estimated from 238 U signal in mv, scaled relative to standard Manangotry Monazite, FC 1 xenotime, or Zircon, where U concentrations are known. Error is estimated at ±20%. 16 of 28

17 Figure 9. U Pb concordia plot of Zircon (not common Pb corrected) for sample A1. Error ellipses are at 2s. Additionally a BSE image of a typical zircon from A1 is shown, demonstrating the zoning pattern. 1975] were fitted. The oldest result of 29.9 ± 1.0 Ma broadly correlates to an age of 32.2 ± 0.4 Ma recorded from the Everest area [Simpson et al., 2000]. There is no direct evidence as to what magmatic or metamorphic stage this particular age may pertain to. However, given the interpretation that kyanite grade metamorphism was recorded in the nearby Everest area at a similar time and similar structural level it seems likely that this age also corresponds to earlier, possibly kyanite grade metamorphism in this area; the metamorphic monazite from this part of the P T cycle appears to have been incorporated into the granite. Following this event other monazites display ages between 18.3 ± 0.2 and 19.1 ± 0.2 Ma (discordant points regressed as described above) and additionally one grain is dated at 16.1 ± 0.2 Ma (Figure 10). These multiple melt events illustrated by the monazites are also represented by two populations of xenotime grains (19.2 ± 0.2 and 15.6 ± 0.2 Ma) present in the sample. These xenotimes contained significant common lead, and the data have been regressed using a Tera Wasserberg plot in the same way as the monazite data. Uranium and lead contents are also variable between the different age populations of accessory minerals and are best exhibited by the xenotimes (Table 5); the younger grains have between 41,000 and 68,000 ppm uranium, while the older grains range from 68,000 to 85,000 ppm uranium. This indicates a variable and heterogeneous magma source has fed the Makalu leucogranite through time. [44] This cordierite bearing leucogranite therefore appears to record protracted melting from at least 19 Ma to approximately 16 Ma within the GHS, with repeated episodes of melt injection into the uppermost structural levels. We interpret the older monazite age at 30 Ma to be inherited from the earlier metamorphic history of this sample. Episodic leucogranite emplacement has also been modeled on a ka time scale in the Himalaya [Scaillet and Searle, 2006] and therefore episodic emplacement appears to be a viable process for the construction of large leucogranite bodies in the Himalaya Gneiss Results [45] Sample A81 is the migmatitic gneiss. Trace element mapping of monazite from this sample demonstrated zoning (Figure 11). Most of the monazites came from the restitic part of the sample, whereas monazite 20 is much more homogeneous and is present in the leucosome of the rock. Broadly, the zoned monazites were characterized by a low Y core, which were mantled by higher Y intermediate layers. Finally, a very thin rim of high Y content was present, although the leucosome monazite (20) appeared to be entirely composed of high Y material. Y + REE profiles are shown in Figure 12. These generally demonstrate a LREE enrichment and significant Eu depletion, which is a characteristic signature of crustal melting [Ayres and Harris, 1997]. Y contents were below the detection limit for the low and intermediate zones and were wt% for high Y rims. [46] The size of the zoning was often on a scale that was smaller than the spot size of ablation, and thus some spots sampled mixtures of more than one zone. Ablation spots may also have sampled more than one zone as ablation 17 of 28

18 Figure 10. U Pb concordia plots of Zircon (not common Pb corrected), a Tera Wasserburg plot of Monazite and a Tera Wasserburg plot of Xenotime from sample A19. See text for description of analysis used for the plotting of common Pb regression lines on Tera Wasserburg plots. Error ellipses are at 2s. Additionally a BSE image of a typical zircon from A19 is shown, demonstrating the zoning pattern. proceeded down hole. The U Pb results were complex and showed variable amounts of common Pb (see Figure 13). Correlating each ablation spot to the Y zone (or mixture of zones) that was sampled yields the conclusion that three phases of monazite growth have occurred in this sample, each corresponding to the high, medium, and low Y zones. Furthermore, the single leucosome monazite, which is of homogeneous Y concentration, correlates with the thin outer Y rich rims of the zoned monazites. If recorded U Pb ratios changed as ablation proceeded this was noted at the time as a possible mixture of zones in the third dimension. As ablation of spots on monazite 20 proceeded down hole of the order mm, no change in the recorded U Pb ratios occurred, implying that the grain is homogeneous in all directions; if it were the outermost rim of a grain only that is observed it would expected that ablation would very quickly penetrate through the rim and mixed ages would be obtained. Common Pb regression lines were plotted through the data for the three Y domains to determine their age. The regression lines were all anchored at an upper intercept of 4985 ± 11 Ma which corresponds to likely common Pb values as determined by Stacey and Kramers [1975]. This yielded lower intercept ages of 19.3 ± 0.1, 17.9 ± 0.4, and 16.0 ± 0.6 Ma. [47] For sample A45, the cordierite overprinted gneiss, a similar approach to that described above was taken. Of the 18 of 28

19 Figure 11. Ce La,Y La,Th Ma, and U Ma X ray maps of monazites from sample A81. Yttrium maps are annotated with the analysis spot number which corresponds to the data in Table 5. monazites that were zoned only two discrete zones could be clearly identified (Figure 14); a low Y core, mantled by high Y overgrowths; Y + REE profiles are shown in Figure 12. Here overall Y contents were higher than in sample A81 possibly as a result of garnet dissolution. Cores contained from below the detection limit to 1.02 wt% Y, while rims contained wt% Y. Rim analyses in particular contained a pronounced Eu anomaly, characteristic of in situ crustal melting. The intricacy of the zoning, which is also likely to be complex in the third dimension as ablation proceeds down hole, and the size of the grains led to many spots being a mixture of varying proportions of two zones. Correlating the position of the ablation spots to the elemental maps, three sets of data were identified; those where analysis had occurred wholly within the core of the monazite (in all dimensions), those that were wholly within the rims, and those that were mixtures of the two. Common Pb lead regression lines were plotted, in the same way as sample A81, through the constraining populations assuming they represent the end member ages (Figure 15). These yielded ages of 34.8 ± 0.5 and 18.7 ± 0.7 Ma; the relatively poor constraint on the younger age is due to only applying the regression through the few spots that were with certainty not mixed ages. MSWD values close to unity imply that the points selected are indeed of a single age population. Pb loss is not thought to be an aspect of this sample as all the zoning shows sharp boundaries, implying that post peak diffusion of elements has not occurred. Further, for timescales of the order of 20Ma, Pb diffusivity rates are extremely low in monazites at temperatures <1200 C [Cherniak et al., 2004]. The REE profiles, in particular lower Eu anomalies and higher La/Yb ratios, although overlapping and therefore of limited use indicate that the older age of 34.8 Ma may be a metamorphic signature, while the younger age of 18.7 Ma may be related to crustal anatexis. [48] In sample A32 abundant rounded monazite was present, commonly mm across and often with small apatite inclusions. X ray mapping revealed complexly zoned crystals, the detail not visible by BSE imaging (Figure 16), but generally all crystals showed two zones of equivalent composition. Due to the small size of these crystals and the 15 mm spot size limit, individual targeting of zones was not 19 of 28

20 Figure 12. (a) Y + REE profiles for monazites from A81. Coloured lines correspond to rim, intermediate, and core analyses. Y content is below the detection limit for intermediate and core analyses. (b) Y + REE profiles for monazites from and A45. Core analyses show lower Y contents and a less pronounced Eu anomaly. Figure 13. U Pb Tera Wasserburg plot of U Pb data from monazites in A81. See text for description of analysis used for the plotting of common Pb regression lines. Error ellipses are at 2s. 20 of 28

21 Figure 14. CeLa, YLa, ThMa, and UMa X ray maps of monazites from sample A45. Yttrium maps are annotated with the analysis spot number which corresponds to the data in Table of 28

22 Figure 15. U Pb Tera Wasserburg plot of U Pb data from monazites in A45. See text for description of analysis used for the plotting of common Pb regression lines. Error ellipses are at 2s. possible. U Pb data show a spread of ages with variable amounts of common Pb (Figure 16). The presence of common Pb is attributed to unavoidable ablation of sub 2 mm apatite inclusions. As a result the data can be interpreted in multiple ways. [49] If the upper intercept of a regression line is anchored at [Stacey and Kramers, 1975] as has been used for the samples already analyzed then all error ellipses are not intercepted, and the MSWD is large, implying that multiple populations of aged grains are present. If the line is not anchored, then the fit of the regression line is better, but the upper intercept is at a much lower value to that used previously. There is no reason to believe that this sample should have a different common Pb history to the others, and so this interpretation also proves unsatisfactory. For samples A81 and A45, the presence of multiple compositional zones has yielded multiple age domains and from this point of view a single regression line through the data is also inappropriate. Therefore, we interpret the results to represent mixtures, in varying quantities, of two age zones, correlating to the two compositional zones of the monazites. Individually regressing each analysis along a line anchored to yields maximum and minimum intercepts of 22.6 ± 0.5 and 21.1 ± 0.5 Ma, respectively. These ages do not necessarily represent the end members of mixing which are therefore dated at 22.6 and 21.2 Ma and are interpreted to represent metamorphic processes most likely linked to melting, occurring in the GHS during this period. The poor constraints on the data preclude any further, detailed interpretation Summary of Geochronology Results [50] All the U Pb geochronology data from this study in the Makalu area are shown on Figure 17. Our data clearly demonstrates the complex metamorphic and anatectic evolution of the GHS and is related to data collected by other workers from the immediate vicinity in eastern Nepal [Murphy and Harrison, 1999; Simpson et al., 2000; Viskupic and Hodges, 2001; Catlos et al., 2002; Searle et al., 2003; Viskupic et al., 2005]. Crustal thickening along the Himalaya is presumed to have started at the onset of India Asia collision approximately 50 Ma resulting in regional Barrovian facies kyanite and sillimanite grade metamorphism. Metamorphism in eastern Nepal is here dated by growth of new monazite at 34.8 ± 0.5 and 29.9 ± 1.0 Ma. This can be related to the peak of Barrovian metamorphism in the Everest area dated by Simpson et al. [2000] at 32.2 ± 0.4 Ma and by Viskupic and Hodges [2001] at 28.4 ± 0.2 Ma and demonstrates the longevity of high grade metamorphism prior to crustal melting. Whether there is any significant difference in P T conditions between these peaks is unclear and indeed unlikely; monazite growth is likely to be triggered 22 of 28

23 Figure 16. BSE images and Y La maps of monazites from sample A32. U Pb data are shown on a Tera Wasserburg and 206 Pb/ 238 U age plot. See text for details discussion of results. by fluid and trace elements budgets, not necessarily by large changes in pressure of temperature. Anatexis began in earnest from approximately 23.8 Ma onwards leading to peak metamorphism recorded by the monazite growth in migmatites at 19.3 ± 0.1 Ma. Schärer [1984] similarly dated Makalu granites at 21.9 ± 0.2 and 24.0 ± 0.4 Ma using monazite and isotope dilution, thermal ionisation mass spectrometry (ID TIMS), which may further demonstrate the protracted multiphase nature of this intrusion (Figure 4B). However, the identification of zoning in both monazite and zircon seen in this study casts doubt on the precise interpretation of older ages obtained by this whole crystal method. Melting in the Everest region has also been dated from Ma [Simpson et al., 2000] to 16.2 ± 0.8 Ma [Murphy and Harrison, 1999]. [51] Subsequently decompression into the cordierite bearing field at 18.7 ± 0.7 Ma recorded by cordierite bearing gneisses marks the onset of exhumation, which was then followed by further decompression melting up to 15.6 Ma, when the GHS in this region began to cool through the melting curve. In the Ama Drime massif to the north, eclogites are found within the GHS and record a subsequent granulite overprint at conditions of 750 C at pressures as low as 4 kbar at 13 14Ma [Cottle et al., 2009a; Groppo et al., 2007; Corrie et al., 2010]. While no eclogites are found in the Upper Barun area specifically the GHS here and in Ama Drime area has a common P T t path through the decompression stage at 4 kbar and is therefore assumed that the GHS is structurally coherent across these areas. The history of the eclogites in Ama Drime prior to this is not as yet wholly clear [Cottle et al., 2009; Corrie et al. 2010]. [52] In the Everest region to the west the transition to lower pressure conditions (4 kbar) is recorded to have occurred at 22.7 ± 0.2 Ma [Simpson et al., 2000]; significantly earlier than in the Makalu area. However due to the lack of detailed imaging of the monazites dated in the Everest study and 23 of 28