High Himalayan Leucogranites Research Articles

Role and origin of water-fluxed melting in the generation of High Himalayan leucogranites

Knowledge of the role water has played in the evolution of Earth’s continents is crucial for improving our understanding of how its layers (e.g. crust and mantle) interact with each other and co-evolved. Orogenic belts are the preferred pathway for material circulation, heat transfer, and water cycling. The Himalayan orogen preserves a large number of syn- and post-collisional rocks, of which the Himalayan leucogranites are key to studying the role of water in crustal reworking. Although some geochemical proxies support water-fluxed melting in the source of Himalayan leucogranites, the timing of water addition, the scale of water-fluxed melting, and how water affects or controls the chemical and physical properties of the partial melts are not well constrained. Here, we filtered the published data on High Himalayan leucogranites to ensure that they represent the geochemical characteristics of primary melts. Our results show that the primary leucogranites with high Th/U and low Rb/Sr ratios were produced by water-fluxed melting as a result of the dissolution of monazite, melting of plagioclase, and formation of peritectic K-feldspar. Furthermore, thermodynamic modeling demonstrates that water-fluxed melting generates a high water content and a large volume of felsic water-unsaturated melt with chemical compositions similar to those of the High Himalayan leucogranites. We suggest that the external water is released by the breakdown of hydrous crustal minerals during dehydration melting and prograde metamorphism.

Mantle inputs to Himalayan anatexis: Insights from petrogenesis of the Miocene Langkazi leucogranite and its dioritic enclaves

Oligocene and Miocene Himalayan anatexis is generally thought to have been induced by intracrustal heating or processes without the involvement of mantle-derived heat and materials, suggesting that the Himalayan leucogranites are typical examples of purely crustal melts. This study focuses on a Miocene leucogranite at Langkazi within the Himalayan orogen, an intrusion that contains a large number of dioritic enclaves. These enclaves have typical igneous textures, contain acicular apatites, and have back-veining structures, quenched margins, and crystallization ages identical to the hosting two-mica granites, indicating that these enclaves are magmatic. Although the enclaves are evolved, the most primitive samples contain high concentrations of MgO (up to 4.3wt.%), Cr (up to 159ppm), and Ni (up to 102ppm), are strongly enriched in large-ion lithophile elements, are depleted in high-field-strength elements, have negative εNd(t) values (–8.6 to –6.1), and have relatively high 87Sr/86Sr(i) values (0.7085–0.7137), suggesting that they were derived from a relatively enriched region of the lithospheric mantle source. Whole-rock geochemical data indicate that the hosting Langkazi leucogranite formed from magmas generated by the partial melting of metapelite material within the High Himalaya crystalline sequence, and these magmas subsequently mixed with mantle-derived melts that are now represented by the Langkazi dioritic enclaves. This indicates that mantle-derived material played an important role in the generation of the Langkazi intrusions. The whole-rock geochemical compositions of samples from the study area also indicate that the primary melts that formed the Langkazi enclaves were significantly contaminated by the relatively juvenile Himalayan lower crustal material, suggesting in turn that these mantle-derived magmas underwent MASH (crustal melting, melt assimilation, magma storage, and homogenization) processes at the base of crust, introducing heat to the lower crust. This mantle-derived heat may have induced partial melting of the Himalayan lower crust, forming adakite-like magmas. Although the small volumes of these dioritic enclaves suggest that this heat may have assisted the genesis of the north Himalayan granites and the High Himalayan leucogranites, whereas in situ crustal radiogenic heating, shear heating, and decompression melting dominated the melting processes in this region.

The origin of Eo- and Neo-himalayan granitoids, Eastern Tibet

Two distinctive modes of post-collisional magmatism are evident along a transect through the eastern Himalaya at ca. 92°E. The first comprises Eocene igneous complexes emplaced into the central Tethyan Himalaya Series (THS) (Dala granitoids, 44.1±1.2Ma) and the core of the Yala-Xiangbo dome (Yala-Xiangbo granitoids, 42±5Ma). Trace element characteristics and Rb–Sr and Sm–Nd isotopes indicate that the Dala granitoids formed from sub-equal mixtures of Gangdese-type magmas and rocks of the Greater Himalayan Crystallines and/or THS. The Yala-Xiangbo granitoids are geochemically complex but interpreted to be evolved equivalents of the Dala granitoid suite. The undeformed Dala plutons constrain the timing of deformation of THS metasediments to be >44Ma and their compositions indicates that components of the Indian foreland sequences had been accreted to the hanging wall of the main Himalayan decollement by that time. The second magmatic mode comprises Miocene intrusions (Arunachal- and Tsona-types) similar in most respects to the High Himalayan Leucogranites documented throughout the range. Although fluid-absent melting of metapelites appears to be the dominant origin of Miocene Himalaya leucogranites, thermometry and trace element data for the Arunachal-type intrusions suggest that they formed under fluid-present conditions. Sr and Nd-isotopes indicate that Lesser Himalayan metasediments were involved during the partial melting that produced the Arunachal-type leucogranites.

Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya

Four distinct granitoid suites outcrop across a N–S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1±1.2Ma and 42±5Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Dala and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40Ma until ca. 15Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there.

Petrochemical characteristics of leucogranite and a case study of Bengbu leucogranites

Leucogranites have a relatively narrow variation in SiO2 content (70.5%–75.5%). Giving similar SiO2 content, leucogranites have relatively higher Al2O3 (>13.5%) and lower TFeO + MgO (<2.5%) contents than those of normal granites. These petrochemical characteristics suggest that leucogranites are derived from partial melting at relatively low temperature and are not significantly affected by fractional crystallization. In the present study, we propose that the Al2O3 vs SiO2 and TFeO + MgO vs SiO2 diagrams can be used to distinguish leucogranites from normal granites. In addition, we report the major element compositions of the Jurassic granitic intrusions from Jingshan-Tushan-Mayishan in the Bengbu area, east-central China. Using the Al2O3 vs SiO2 and TFeO + MgO vs SiO2 diagrams and the comparison with the High Himalayan leucogranites in mineralogical and petrochemical characteristics, we suggest that the Jingshan-Tushan-Mayishan intrusions belong to a leucogranite belt. Similar to those of the High Himalayan leucogranites, the Bengbu leucogranites have low Mg# values, indicating that they resulted from low temperature dehydration partial melting of the subducted continental crust of the South China Block at the crustal depth.

Geology of the western Arunachal Himalaya in parts of Tawang and west Kameng districts, Arunachal Pradesh

The Western Arunachal Himalaya comprises migmatitic gneisses of the Sela Group intruded by Tertiary leucogranite (tourmaline, muscovite and K-feldspar bearing), metasedimentaries of Lumla and Dirang Formations and granite gneiss of the Bomdila Group. The litho units of Sela Group are completely migmatised with distinct leuco, melano and mesosomes (Neosome). The mineral assemblages indicate a sedimentary protolith (para gneiss). Concordant metabasic bodies are observed (rarely discordant) indicating older dykes/sills. The migmatites of Sela Group are of Significantly higher grade than the tectonically underlying Dirang Formation, characterized by kyanite ± biotite± staurolite ± sillimanite assemblages. The tourmaline bearing pegmatitic High Himalayan leucogranite crops out in the higher altitudes of the High Himalayan Sequence. The primary mineral assemblages recorded are tourmaline + muscovite + biotite + garnet apart from quartz and K-feldspar. This intrudes the migmatite of Sela Group parallel as well as oblique to earlier gneissosity as lens shaped bodies. The Sela's are thrust over the metasedimentary rocks of Dirang Formation. Earlier workers marked this thrust as the Main Central Thrust (MCT). During the present studies the whole Dirang Formation was found to be a high strained zone with many shear planes, almost all the lithounits are sheared with prolific graphite/carbon emplacement along the permeable shear planes. The Dirang Formation comprises quartzite, kyanite ± staurolite + garnet ± biotite schist, crystalline limestone and carbon phyllite. Basic and ultrabasic rocks intrude these lithounits. The Bomdila Gneiss, which forms a part of the Lesser Himalayan Sequence, is a large body of granite orthogneiss. The primary mineral assemblage includes quartz, K-feldspar, biotite and garnet. Though the gneisses have an intrusive relationship with the Dirang metasediments the contact is marked by a thrust. Metamorphic grade increases from the base of Dirang to the top of the Sela Group. The base of the Dirang Formation is characterized by biotite + garnet assemblages followed by kyanite + staurolite at the top the Dirang Formation. The base of the Sela Group is marked by kyanite ± garnet ± biotite followed by sillimanite ± biotite at the top of Sela Group. Two phases of deformations are recorded from Sela and Dirang rocks with type III interference pattern folds.

Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya

Abstract The crustally derived High Himalayan leucogranites (HHL) are characterized by strong isotopic heterogeneity and occurrence of magmatic muscovite. Such attributes indicate that the HHL were non-convecting magma bodies and crystallized at pressure-equivalent depths of more than 8.5 km. We have performed one-dimensional thermal modelling in order to simulate the process of incremental growth of a laccolith whose roof is tectonically removed during intrusion, in a context of crustal exhumation due to channel flow. The objective is to define under what conditions HHL laccoliths emplaced close to active normal faults may be built without convecting while crystallizing muscovite. The results indicate that for a HHL thickness in the range 5–10 km, denudation rates cannot be higher than 4 mm a −1 , and are more likely below 3 mm a −1 . At such denudation rates, the intrusion process needs to start at depths of c. 22 km, except when the final laccolith thickness is 10 km, in which case the depth of first-emplaced magmas cannot exceed 18 km. Thick HHL laccoliths (&gt;7 km) may require a minimum denudation rate, on the order of 1 mm a −1 , to prevent wholesale convection and allow muscovite crystallization. Yet, emplacement of such thick HHL laccoliths during normal faulting implies that the top part of the leucogranite nearly reaches the surface while its base is still fed by active intrusions. Overall, such relatively low denudation rates suggest that, when HHL were intruded, the overlying crustal column was not undergoing vigorous erosion. Within the framework of a crustal channel flow, this suggests that the zone of focused erosion during the Miocene was located to the south of the current exposures of the HHL belt. Our results also show that to explain the steep cooling histories documented in many HHL, denudation must have been active after HHL solidification, especially when they were intruded close to their source region. However, to preserve the HHL from exhumation and erosion until the present time, the average denudation rate after emplacement cannot have exceeded 0.5 mm a −1 .

Mechanisms and timescales of felsic magma segregation, ascent and emplacement in the Himalaya

Abstract We combine field, petrological, geochemical and experimental observations to evaluate the timescales of compaction-driven and shear-assisted melt extraction and ascent in the Himalaya. The results show that melt migration via compaction and channelling is inescapable and operates on timescales of less than 1 million years and possibly as short as 0.1 million years. Field and petrological data show that such a fast and efficient melt transfer results from a combination of favourable factors, including: (1) low but constant melt viscosity (10 4.5 Pa s) during extraction and ascent; (2) grain size coarsening of the source rocks in response to prolonged heating prior to melting; and (3) high source fertility and thus high melt fraction, owing to elevated modal amounts of muscovite in leucogranite sources. All three factors dramatically increase source permeability. Calculations show that shear-assisted melt extraction had a time interval recurrence in the range 10 000–100 000 years (10–100 ka), leading to sill thicknesses of 1–30 m. Yet melts falling at the low end of the viscosity range when coupled to high shear velocities may lead to veins several hundred metres thick. The deepest structural levels (e.g. central Zanskar Range) show that in-situ melts formed where pure shear compaction was greatest and where simple shear was also operative. Magma extracted from migmatite leucosomes was injected along planes of weakness parallel to the ductile shear fabric, probably by some form of hydraulic fracturing crack propagation mechanism. Large High Himalayan leucogranite (HHL) bodies (e.g. c. 5 km thick sills at Manaslu, Makalu and northern Bhutan) may thus represent inflated laccoliths assembled via dykes that tapped a 100–300 m melt layer produced by compaction of the Greater Himalayan Series (GHS). Thermal simulations show that such melt layers may have incubation times of several million years. Although transport time for magmas associated with the HHL is short, the time for assembly may take several million years for the largest HHL, as geochronological data indicate (up to 5 million years for Manaslu, Shisha Pangma). Transport of leucogranite melt from mid-crustal levels towards the surface was concomitant with active low-angle normal faulting along the South Tibetan Detachment (STD) normal fault, a structure that effectively formed the lid to the extrusion of a partially molten layer of mid-crustal rocks (channel flow). Rapid cooling of the granites emplaced at the top of the GHS implies rapid extrusion and lateral flow of GHS rocks beneath the STD during the period c. 20–17 Ma. Weakening of the crust by partial melting is thus likely to be pulsatory in time, and future thermomechanical models should incorporate such aspects to model tectonic evolution of hot orogens.

Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north‐east India

AbstractIn the Sikkim region of north‐east India, the Main Central Thrust (MCT) juxtaposes high‐grade gneisses of the Greater Himalayan Crystallines over lower‐grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large‐scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th–Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14–15 and 12–10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14–15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25 °C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25 °C and 6 ± 1 kbar. The P–T–t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid‐crustal extrusion and the role of out‐of‐sequence thrusts in convergent plate tectonic settings.

Records of the evolution of the Himalayan orogen from in situ Th–Pb ion microprobe dating of monazite: Eastern Nepal and western Garhwal

In situ Th–Pb monazite ages from rocks collected along two transects (the Dudh Kosi-Everest, eastern Nepal and the Bhagirathi River, Garhwal Himalaya, India) perpendicular to the Main Central Thrust (MCT) suggest a striking continuity of tectonic events across the Himalaya. The youngest age reported in this study, 5.9±0.2Ma (MSWD=0.4), from matrix monazite grains collected beneath the MCT in the Garhwal region is consistent with several age data from rocks at similar structural levels in central Nepal, providing support for widespread Late Miocene MCT activity. The lateral parallelism of orogenic events is further manifested by the 20.7±0.1Ma age of a High Himalayan leucogranite from an injection complex that outcrops along the Dudh Kosi-Everest transect, resembling ages of these bodies reported elsewhere. The youngest monazite grain analyzed along the Dudh Kosi-Everest transect is 10.3±0.8Ma. The absence of 7–3Ma monazite ages in eastern Nepal may reflect a different nappe structure, which obscures the reactivated ramp equivalent exposed in the Garhwal and central Nepal. Garnets from the MCT hanging wall (Greater Himalayan Crystallines) and footwall (Lesser Himalaya) display different major element zoning, and the patterns are useful for constraining the location of the thrust system that separates the two lithologies. Pressure–temperature paths for two upper Lesser Himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone. Along the Dudh Kosi-Everest transect, upper Lesser Himalayan monazite grains from three rocks record a clear signal at 14.5±0.1Ma (MSWD=8.4), and the ∼23Ma age that characterizes the hanging wall is notably absent. Monazites collected within a large-scale Greater Himalayan Crystallines fold yield the ∼14Ma age, consistent with the structure forming due to MCT-related compression. Paleo-Mesoproterozoic (1407±35Ma) matrix monazite grains are found within an augen gneiss unit located beneath the MCT, whereas Cambro-Ordovician (436±8;548±17Ma) inclusions are preserved within garnets of the Greater Himalayan Crystallines. The presence of 45.2±2.1Ma grains from lower structural levels of the Greater Himalayan Crystallines indicates the unit realized conditions conducive for monazite growth during the Eocene.

Experimental study on dehydration melting of natural biotite-plagioclase gneiss from High Himalayas and implications for Himalayan crust anatexis

Here we present the results of dehydration melting, melt morphology and fluid migration based on the dehydration melting experiments on natural biotite-plagioclase gneiss performed at the pressure of 1.0–1.4 GPa, and at the temperature of 770–1028°C. Experimental results demonstrate that: (i) most of melt tends to be distributed along mineral boundaries forming “melt filmrd even the amount of melt is less than 5 vol%; melt connectivity is controlled not only by melt topology but also by melt fraction; (ii) dehydration melting involves a series of subprocesses including subsolidus dehydration reaction, fluid migration, vapor-present melting and vapor-absent melting; (iii) experiments produce peraluminous granitic melt whose composition is similar to that of High Himalayan leucogranites (HHLG) and the residual phase assemblage is Pl+Qz+ Gat+Bio+Opx±Cpx+Ilm/Rut±Kfs and can be comparable with granulites observed in Himalayas. The experiments provide the evidence that biotite-plagioclase gneiss is one of source rocks of HHLG and dehydration melting is an important way to form HHLG and the granulites. Additionally, experimental results provide constraints on determining the P-T conditions of Himalayan crustal anatexis.

Fluid-enhanced melting during prograde metamorphism

Anatexis is a commonly recognized feature of high-grade metamorphism, but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A suite of small-volume, deformed, two-mica leucogranites has been recognized within the High Himalayan Crystalline Series of the Garhwal Himalaya. These granites are consistently more siliceous than minimum-melt granite compositions and are characterized by low Rb/Sr ratios, high Ba, low abundances of HFS elements and positive Eu anomalies. Such trace-element characteristics contrast strongly with the geochemistry of the well-studied Early Miocene leucogranites of the High Himalaya, derived from fluid-absent melting. Sm–Nd garnet dating of one deformed granite indicates a crystallization age of 39±3 Ma, c. 15 Ma before the emplacement of the more voluminous High Himalayan leucogranites. Whilst some entrainment of restitic phases cannot be excluded, trace element signatures suggest a low temperature (&lt;650°C) crustal melt formed under conditions of high H 2 O activity. Positive Eu anomalies and unusually low Rb/Sr ratios are indicative of rapid, disequilibrium melting. Fluid-enhanced melting may be a common feature of prograde upper amphibolite-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15 Ma. These melts will only crystallize within this period if they segregate from their protoliths. Subsequent dating of long-lived melts would indicate erroneously young ages for the prograde melting events. However, melts formed in this way may be recognized by their distinctive trace-element chemistry. The persistence of early formed melts within an orogen provides insights into the prograde heating path, and may be critical in controlling the rheology of the middle crust, and hence its deformational history.

A model for the origin of Himalayan anatexis and inverted metamorphism

The origin of the paired granite belts and inverted metamorphic sequences of the Himalaya has generally been ascribed to development of the Main Central Thrust (MCT). Although a variety of models have been proposed that link early Miocene anatexis with inverted metamorphism, recent dating studies indicate that recrystallization of elements of the MCT footwall occurred in the central Himalaya as recently as ∼6 Ma. The recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the High Himalayan leucogranites and North Himalayan granites, which differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active thrust that cuts through Indian supracrustal rocks that had previously experienced low degrees of partial melting. Numerical simulations assuming a shear stress of 30 MPa indicate that continuous slip on the Himalayan decollement beginning at 25 Ma could trigger partial melting reactions leading to formation of the High Himalayan granite chain between 25 and 20 Ma and the North Himalayan belt between 17 and 8 Ma. The ramp‐flat geometry we apply to model the Himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle Miocene. Late Miocene, out‐of‐sequence thrusting within the broad shear zone beneath the MCT provides a mechanism to bring these rocks to the surface in their present location (i.e., well to the north of the present tectonic front) and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the MCT. We envision that formation of the MCT Zone involved successive accretion of tectonic slivers of the Lesser Himalayan Formations to the hanging wall and incorporate these effects into the model. The model predicts continued anatexis up to 400 km north of the Himalayan range, consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south Tibetan rift.

Granite-migmatite genetic link: the example of the Manaslu granite and Tibetan Slab migmatites in central Nepal

In central Nepal, the Tibetan Slab is made up of biotite-gneisses (metapelites and metagreywackes), orthogneisses (metagranites) and migmatites. Melanosomes are generally biotite-(± muscovite)-bearing, but locally they may be tourmaline-rich when associated with boron-rich granitic material. Leucosomes occur as lenses conformable with the foliation, veins, patches, or as fillings in shear zones and extensional structures. Field relationships, and mineralogical and chemical data show that three processes may have contributed to the formation of the Tibetan Slab leucosomes: metamorphic differentiation or disequilibrium partial melting (low-Zr tonalitic leucosomes), in-situ equilibrium partial melting (high-Zr leucosomes and some granitic leucosomes) and injection of externally-derived melts (most granitic and some tonalitic leucosomes). The Manaslu pluton belongs to the High Himalayan leucogranite belt and was emplaced at the top of the Tibetan Slab. It corresponds to a muscovite-biotite leucogranite that has been assumed to derive from melting of the Tibetan Slab gneisses (Formation I). Phase relationships, a more magnesian chemistry of the ferromagnesian minerals from the Tibetan-Slab migmatites as compared to the Manaslu leucogranite, the microtextures of accessory phases, and trace-element compositions (lower U, Li, F and higher Sr, Eu, Y, Yb contents in the migmatite leucosomes) show that the in situ Himalayan migmatites, at the crustal level presently exposed, have not been produced under the same P-T-X H 2O conditions as the Manaslu leucogranite magma. While the Formation I was the probable source for the Manaslu granite, migmatites within the formation are not the remanants of a melting process from which the Manaslu granite was derived. Both the Tibetan Slab migmatites and the Manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels, and of percolation of melts and hydrothermal fluids through the crust.

The Timing of Metamorphism, Magmatism, and Cooling in the Zanskar, Garhwal, and Nepal Himalaya

Following India-Asia collision, which is estimated at ca. 54-50 Ma in the Ladakh-southern Tibet area, crustal thickening and timing of peak metamorphism may have been diachronous both along the Himalaya (pre-40 Ma north Pakistan; pre-31 Ma Zanskar; pre-20 Ma east Kashmir, west Garhwal; 11-4 Ma Nanga Parbat) and cross the strike of the High Himalaya, propagating S (in Zanskar SW) with time. Thrusting along the base of the High Himalayan slab (Main Central Thrust active 21-19 Ma) was synchronous with N-S (in Zanskar NE-SW) extension along the top of the slab (South Tibet Detachment Zone). Kyanite and sillimanite gneisses in the footwall formed at pressure of 8-10 kbars and depths of burial of 28-35 km, 30- 21 Ma ago, whereas anchimetamorphic sediments along the hanging wall have never been buried below ca. 5-6 km. Peak temperatures may have reached 750 on the prograde part of the P-T path. Thermobarometers can be used to constrain depths of burial assuming a continental geothermal gradient of 28-30 °C/km and a lithostatic gradient of around 3.5-3.7 km/kbar (or 0.285 kbars/km). Timing of peak metamorphism cannot yet be constrained accurately. However, we can infer cooling histories derived from thermochronometers using radiogenic isotopic systems, and thereby exhumation rates. This paper reviews all the reliable geochronological data and infers cooling histories for the Himalayan zone in Zanskar, Garhwal, and Nepal. Exhumation rates have been far greater in the High Himalayan Zone (1.4-2.1 mm/year) and southern Karakoram (1.2-1.6 mm/year) than along the zone of collision (Indus suture) or along the north Indian plate margin. The High Himalayan leucogranites span 26-14 Ma in the central Himalaya, and anatexis occurred at 21-19 Ma in Zanskar, approximately 30 Ma after the collision. The cooling histories show that significant crustal thickening, widespread metamorphism, erosion and exhumation (and therefore, possibly significant topographic elevation) occurred during the early Miocene along the central and eastern Himalaya, before the strengthening of the Indian monsoon at ca. 8 Ma, before the major change in climate and vegetation, and before the onset of E-W extension on the Tibetan plateau. Exhumation, therefore, was primarily controlled by active thrusts and normal faults, not by external factors such as climate change.

Rare-earth patterns in zircons from the Manaslu granite and Tibetan Slab migmatites (Himalaya) : insights in the origin and evolution of a crustally-derived granite magma

The Tibetan Slab gneisses are currently considered as the source for the High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucogranite (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucidate the zircon growth conditions and, indirectly, the processes involved in incipient anatexis and evolution of granitic magmas. In the migmatites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145–700 ppm) and chondrite-normalized (Yb/Sm) N ratios (≤81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically indistinguishable from those of the gneisses and melanosomes. Zircons from the high-Zr tonalitic leucosomes and granitic leucosomes are euhedral and show higher Yb contents (409–2820 prim) and (Yb/SM)N ratios (≥145) than those of the gneisses and melanosomes. The euhedral shapes and distinctive REE patterns of zircons from the high-Zr leucosomes and granitic leucosomes are consistent with crystallization from a melt, whereas the morphological and chemical similarities of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical change. In the Manaslu granite, zircons have rounded cores with REE patterns distinct from those of the rims (e.g., 250 ppm ≤Yb≤710 ppm in the core, 965 ppm ≤Yb≤ 2775 ppm in the rim) but comparable to those from the Tibetan Slab gneisses suggesting inheritance. The rim compositions, however, are distinct from those of either zircon types of the Tibetan Slab leucosomes, indicating that the leucosomes cannot be the unsegregated equivalent to the Manaslu granite parental magma. Comparison of the rim compositions with fractional crystallization models suggests that the range in zircon Sm and Yb contents are consistent with zircon crystallization from a monazite-saturated, xenotime-undersaturated melt. The Yb contents in the different zircons studied, and their variation within a single zircon, further suggest boundary-layer effects and magma compositional heterogeneity, in agreement with previous models which considered that the Manaslu granite resulted from the aggregation of magma batches.

Detection of inherited monazite in the Manaslu leucogranite by [formula omitted] ion microprobe dating: Crystallization age and tectonic implications

Although leucogranites are among the least petrologically variable of all igneous rocks, ironically they are among the most difficult to reliably date. The High Himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt. For a variety of reasons linked to the minimum melt composition of these leucogranites, these dating studies have not been entirely successful. We report results of a new ion microprobe dating method based on the decay of 232Th to 208Pb in monazite that has directly revealed the presence of inherited Pb in monazite from the Manalsu granite, casting doubt on its previously accepted age. Monazite ages obtained from this leucogranite yield two distinct populations, a large number of ages with a normal distribution and mean age of 22.4 ± 0.5 Ma (±2 S.E.) that we interpret to be the crystallization age, and a smaller inherited fraction with an age of ca. 600 Ma. Because formation of the granite is thought to be related to slip on the Main Central Thrust, both this date and a second less precise result from a structurally similar pluton near Mt. Everest indicate that the Main Central Thrust was active in the interval 24-22 Ma, but do not constrain its initiation. Together with crosscutting relationships, these data require that movement on the North Himalayan Fault occurred prior to 22 Ma at both locations.

Geochemical constraints on the bimodal origin of High Himalayan leucogranites

Major and trace element and Rb-Sr isotope systematics of the Manaslu leucogranite, Central Nepal, have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis. Biotite and tourmaline are, for the most part, mutually exclusive, with a predominance of two-mica leucogranites over tourmaline leucogranites. The 87Sr 86Sr initial isotopic ratios (Sr i) indicate that leucogranitic melts were derived from two different sources, the two-mica leucogranites having a metagreywacke origin (with Sr i < 0.752 and ϵNd < −15) and the tourmaline leucogranites a metapelitic one (Sr i> 0.752; ϵNd > − 15). Such a bimodal nature of the source zone does not directly influence the magmatic evolution, except that probably the higher initial boron content in the metapelitic rocks may increase the Na 2O K 2O ratio. In contrast, the amount of water present during melting principally controls in part anatectic processes and element behaviour. Water-saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two-mica leucogranites whereas water-absent conditions prevailed during melting of metapelitic layers and favoured biotite, plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite.

Magmagenesis associated with extension in orogenic belts: examples from the Himalaya and Tibet

Structures in mountain belts which accommodate layer extension may be synchronous with either ongoing thickening or reduction in crustal thickness. Either case may be associated with magmagenesis because they contribute to the thermal regime in the lithosphere, and the chemistry of magmas produced can reveal source compositions and thereby conditions of melting. In the High Himalaya, exhumation of upper-amphibolite facies metamorphic rocks in the footwall of a major normal shear zone occurred during crustal thickening. This rapid decompression enhanced fluid-absent melting reactions in micaceous pelites and produced the High Himalayan leucogranite suites which were subsequently intruded into the zone of normal shear. Similar leucogranites are seen in the Cretaceous North Lhasa Terrane (NLT), in association with a calc-alkaline series. These suites are also crustally derived but their chemistry indicates higher melting temperatures than in the Himalaya, suggesting that heat flow into the base of the crust was enhanced. This thermal anomaly was most probably associated with lithosphere attenuation following an episode of thickening. The Tibetan Plateau is currently a topographically elevated region of thick crust that appears to be undergoing orogenic collapse—active structures are stretching and thinning the crust. The plateau is underlain by a thin, hot mantle lithosphere, supporting theoretical models that suggest that a lithospheric root of a thick orogen would become unstable and be removed by convection. Consequent thermal perturbations in the mantle lithosphère cause melting in suitably fertile horizons, leading to widespread volcanism. Comparisons have been drawn between the Tibetan model and the late stages of the Variscan belt in Europe. Magmatic rocks of diverse composition occur throughout the late Variscan, many of them indicating derivation from a previously enriched lithospheric source, suggesting thermal anomalies in the subcontinental mantle akin to those inferred for Tibet. Taken with structural and sedimentary evidence, these indications strongly support the hypothesis that the Variscan orogen underwent orogenic collapse in the Tibetan style.

Magnetic properties of the High Himalayan leucogranites: Structural implications

The magnetic properties of the High Himalayan leucogranites have been investigated on 527 specimens in three plutons, Everest-Makalu (6 sites) and Manaslu (40 sites) in Nepal, and Gangotri (43 sites) in India. Susceptibility varies between 2 and 100 × 10 −6 SI, with an anisotropy ratio up to 1.16. High field and low-temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline. The latter produces an inverse fabric, i.e. with the minimum axis parallel to mineral lineation. The magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement, and its usefulness in semi-quantitatively estimating petrofabric intensity is demonstrated for the biotite-bearing facies. Natural remanent magnetization was measurable at only two sites in Everest-Makalu, where there are well-defined reverse directions carried by titanomagnetite and pyrrhotite. Comparison of these preliminary results with predicted directions for stable India suggests northward tilting of about 10° and a small clockwise rotation of this massif.