Emplacement of Himalayan leucogranites by magma injection along giant sill complexes: examples from the Cho Oyu,Gyachung Kang and Everest leucogranites (Nepal Himalaya)

The upper part of the High Himalayan slab in north central Nepal is comprised of a thick layer-parallel sheet of biotite + muscovite + tourmaline ± garnet ± sillimanite ± cordierite leucogranite up to 3–4 km thick and dipping north at 5–20°. These strongly peraluminous magmas were emplaced into high temperature–low-pressure sillimanite and cordierite bearing gneisses, calc-silicates and rare amphibolites which were metamorphosed at temperatures of 600–650°C some time during the Oligocene–early Miocene. Parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks. In the mountains of Cho Oyu, Gyachung Kang, Pumori, Lingtren and the base of the Everest massif, these leucogranites form part of a single structural horizon bounded at the top by the Lhotse Detachment, the lower of two N-dipping normal faults of the South Tibetan Detachment (STD) system, and below by the Khumbu Thrust (KT), an out-of-sequence fault which was partly responsible for the uplift, erosion and exhumation of the leucogranites. A model for the emplacement of these leucogranites is proposed, where they represent viscous minimum melts, produced by melting of a pelitic protolith, similar to the underlying sillimanite grade gneisses, through muscovite breakdown, either during fluid-absent melting at <750°C, or fluid-saturated melting at <650°C. These leucogranites may have intruded up to ∼40 km horizontally from their source, but were emplaced by hydraulic fracturing along multiple sills into recently metamorphosed high temperature–low pressure rocks of the middle crust. The entire mid-crustal region where the granites were formed and emplaced was later uplifted along the hangingwall of the Khumbu Thrust, and by the structurally lower Main Central Thrust (MCT) to their present position. The location of the leucogranites at the top of the slab, but never intruding across the STD normal faults and the complete lack of leucogranites further down the slab rule out frictional heating along the MCT as a viable heat source and also rule out diapirism as a viable emplacement mechanism. High radioactivity of the crustal source, percolation of fluid from the migmatitic source into sills and dykes during simple shear, heat focussing due to a large thermal conductivity contrast across the STD, and decompression during active low-angle normal faulting above, are all viable processes to explain leucogranite melting and emplacement.     

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya)has been modelled as the accretion of successive sills. Theleucogranite is characterized by isotopic heterogeneities suggestinglimited magma convection, and by a thin (<100 m) upper thermalaureole. These characteristics were used to constrain the maximummagma emplacement rate. Models were tested with sills injectedregularly over the whole duration of emplacement and with twoemplacement sequences separated by a repose period. Additionally,the hypothesis of a tectonic top contact, with unroofing limitingheat transfer during magma emplacement, was evaluated. In thislatter case, the upper limit for the emplacement rate was estimatedat 3·4 mm/year (or 1·5 Myr for 5 km of granite).Geological and thermobarometric data, however, argue againsta major role of fault activity in magma cooling during the leucograniteemplacement. The best model in agreement with available geochronologicaldata suggests an emplacement rate of 1 mm/year for a relativelyshallow level of emplacement (granite top at 10 km), uninterruptedby a long repose period. The thermal aureole temperature andthickness, and the isotopic heterogeneities within the leucogranite,can be explained by the accretion of 20–60 m thick sillsintruded every 20 000–60 000 years over a period of 5Myr. Under such conditions, the thermal effects of granite intrusionon the underlying rocks appear limited and cannot be invokedas a cause for the formation of migmatites. KEY WORDS: granite emplacement; heat transfer modelling; High Himalayan Leucogranite; Manaslu; thermal aureole     

Geology and tectono‐metamorphic evolution of the Himalayan metamorphic core: insights from the Mugu Karnali transect,Western Nepal (Central Himalaya)

New structural and tectono‐metamorphic data are presented from a geological transect along the Mugu Karnali valley, in Western Nepal (Central Himalaya), where an almost continuous cross‐section from the Lesser Himalaya Sequence to the Everest Series through the medium‐high‐grade Greater Himalayan Sequence (GHS) is exposed. Detailed meso‐ and micro‐structural analyses were carried out along the transect. Pressure (P)–temperature (T) conditions and P–T–deformation paths for samples from different structural units were derived by calculating pseudosections in the MnNKCFMASHT system. Systematic increase of P–T conditions, from ~0.75 GPa to 560 °C up to ≥1.0 GPa–750 °C, has been detected starting from the garnet zone up to the K‐feldspar + aluminosilicate zone. Our investigation reveals how these units are characterized by different P–T evolutions and well‐developed tectonic boundaries. Integrating our meso‐ and micro‐structural data with those of metamorphism and geochronology, a diachronism in deformation and metamorphism can be highlighted along the transect, where different crustal slices were underthrust, metamorphosed and exhumed at different times. The GHS is not a single tectonic unit, but it is composed of (at least) three different crustal slices, in agreement with a model of in‐sequence shearing by accretion of material from the Indian plate, where coeval activity of basal thrusting at the bottom with normal shearing at the top of the GHS is not strictly required for its exhumation.     

Rb−Sr age of Gaik granite,Ladakh batholith,northwest Himalaya

The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial⁸⁷Sr/⁸⁶Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.     

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 Na₂O/K₂O 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.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

尼泊尔低喜马拉雅推覆带油源对比及油气成藏

尼泊尔低喜马拉雅推覆带油气苗来源不清极大地影响了该区油气勘探.在地质-地球物理综合调查的基础上,利用油气地球化学、碳同位素及生烃史模拟对尼泊尔代莱克地区油源和成藏过程进行了研究.结果表明:①尼泊尔代莱克地区油苗产于Padukasthan断裂,可分两期,第一期呈含油断层泥产出,氯仿沥青"A"为149～231 μg/g,RR.为0.81％,氯仿沥青"A"的δ13C相对较重(-26.24‰～-27.10‰),族组分具有正碳同位素序列,发黄绿色荧光,为典型的低熟煤成油,第二期呈液态油产出并遭受微生物降解,金刚烷IMD指数为0.33～0.45,R.为1.24％～1.53％,3,4-DMD含量46％～47％,全油δ13C为-29.50‰～-29.45‰,族组分碳同位素趋于一致,发蓝色荧光,为海相成因高熟油;②第一期油来源于Surkhet群的Melpani组和Gondwana群煤系烃源岩,为Ⅲ型有机质低熟阶段的产物,第二期来源于Surkhet群的Swat组浅海陆棚相黑色页岩,为Ⅱ1型有机质生油高峰的产物,两期油与Lakharpata群过成熟黑色泥岩和Siwalik群未熟泥岩没有亲缘关系;③尼泊尔低喜马拉雅推覆带具有"多源多期、推覆增熟、砂体控储、披覆控聚"的油气成藏模式,油气成藏过程可划分为沉积浅埋、构造圈闭形成、深埋油藏形成、气藏形成和晚期改造定型5个演化阶段;④尼泊尔低喜马拉雅推覆有利于Gondwana群、Surkhet群深埋增温、持续快速生烃和晚期成藏,对比邻区巴基斯坦的含油气盆地,尼泊尔低喜马拉雅推覆带及相邻类似地区具备良好的油气成藏条件.     

Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence,Annapurna Himalaya,Central Nepal: Insights from titanite petrochronology

The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite ⁴⁰Ar/³⁹Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.     

Geomorphological and Pedological Investigations on the Glacial History of the Kali Gandaki (Nepal Himalaya)

In semi-arid orographic left tributaries of the Kali Gandaki at the northern and western flank of the Nilgiri Himal, glacio-geomorphological and pedological investigations were carried out on prehistoric moraines. Geomorphological relief analysis was derived from other literature and the own fieldwork of the author. The resulting glacial chronology was used as benchmark to explore the limits of different pedological dating methods regarding the degree of soil development. These methods are based on iron fractionation, total element contents and particle size distribution. In general the different glacial stages are mirrored correctly in the relative graduation of soil development. The ratio of well crystallised pedogenic iron-oxides to the total iron content and the ratio fine clay to total clay are most suitable, because they are almost independent from existing changes in the lithological composition. The total element based weathering indices are less suitable, because they react highly sensitive to the geology dependent shift to higher carbonate contents. Most of the grain size based weathering indices are inapplicable because of the typically high textural variability within till deposits.     

Linked fluid and tectonic evolution in the High Himalaya mountains (Nepal)

Fluid inclusions were studied in a quartz lens from the structurally highest unit of the Himalaya mountains in Nepal from a textural, geometrical, chemical and isotopic point of view. Six types of fluid inclusions were distinguished. One of these types consists of annular inclusions; this shape is attributed to a confining pressure increase in a non-isotropic stress field. Two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity. The bulk composition of the fluid was dominated by CO₂ (>84 mol%) and H₂O. The composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and/or that one single fluid was reworked in situ by decrepitation. Stable isotope of fluids and minerals indicate (1) that fluids were buffered by surrounding rocks for O and C and (2) that at least two types of water (metamorphic and meteoric) were involved. Finally, a P-T-t-- path is proposed for the sample, taking into account the southward thrusting along the Main Central Thrust, the northward tectonic denudation of the Himalaya mountains inducing tectonic burying below the Annapurna Range, and lastly, rapid uplift.     

Geomorphological and pedological investigations on the glacial history of the Kali Gandaki (Nepal Himalaya)

In semi-arid orographic left tributaries of the Kali Gandaki at the northern and western flank of the Nilgiri Himal, glacio-geomorphological and pedological investigations were carried out on prehistoric moraines. Geomorphological relief analysis was derived from other literature and the own fieldwork of the author. The resulting glacial chronology was used as benchmark to explore the limits of different pedological dating methods regarding the degree of soil development. These methods are based on iron fractionation, total element contents and particle size distribution. In general the different glacial stages are mirrored correctly in the relative graduation of soil development. The ratio of well crystallised pedogenic iron-oxides to the total iron content and the ratio fine clay to total clay are most suitable, because they are almost independent from existing changes in the lithological composition. The total element based weathering indices are less suitable, because they react highly sensitive to the geology dependent shift to higher carbonate content. Most of the grain size based weathering indices are inapplicable because of the typically high textural variability within till deposits.     

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet).The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics.In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics.The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults.Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone.A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.     

Preliminary results on the last-high-glacial glaciation of the Rolwaling Himal and the Kangchenjunga Himal (Nepal, E- Himalaya)

On the basis of empirical findings and inductive conclusions, the least extent of glaciation during the last ice age can be shown for two areas on the southern slopes of the Himalayas in Nepal – the Rolwaling Himal and the Kangchenjunga Himal. For the Rolwaling Himal, the snow line during the last ice age is calculated at 4000 m asl., i.e., 1500 m below the recent snow line. The last high-glacial snow line in the Kangchenjunga Himal is thought to be at 4350 m asl., the snow line depression here is 1150 m. This revised version was published online in July 2006 with corrections to the Cover Date.     

喜马拉雅然巴淡色花岗岩中石英微量元素的成分特征及其指示意义

为进一步推进喜马拉雅淡色花岗岩稀有金属成矿问题的研究,需全面厘清喜马拉雅淡色花岗岩的岩石学成因和岩浆分异演化过程,并寻找可以判别和指示其岩浆结晶分异程度及成矿潜力的矿物-岩石-地球化学指标。近年研究发现,石英晶格中的微量元素成分特征可以很好的反映岩浆来源和分异演化过程。为此,本文对喜马拉雅然巴岩体中两种演化程度不同的花岗岩（演化程度较低的二云母花岗岩和典型高分异成因的白云母花岗岩）中的石英进行了微量元素成分分析。结果显示,然巴淡色花岗岩石英中含量较高的元素包括Li、Be、Na、K、Al、Ca、Sc、Ti、Ge,这些元素含量在两类淡色花岗岩中存在较大的重叠,但白云母花岗岩中石英普遍具有更高的Be和更低的Ti含量,同时Li、Na、Al的含量高值均出现在白云母花岗岩中。另外,相对二云母花岗岩,白云母花岗岩中石英具有明显增高的Al/Ti（>10）和Ge/Ti（>0.1）比值。综上,本文认为石英中的Be、Ti元素含量以及Al/Ti、Ge/Ti比值有潜力成为指示喜马拉雅淡色花岗岩分异演化程度和稀有金属矿化的有效指标。根据石英中Ti含量估算然巴二云母花岗岩的结晶温度为666~491℃,白云母花岗岩的结晶温度为559~358℃。该结果证实喜马拉雅淡色花岗岩在成岩过程中经历了显著的降温,其岩浆体系可以演化至低于一般花岗岩体系固相线的温度,这为深入理解喜马拉雅淡色花岗岩的岩浆演化过程提供了重要的物理学约束。

     

Infilling of the Younger Kathmandu–Banepa intermontane lake basin during the Late Quaternary (Lesser Himalaya,Nepal): a sedimentological study

The Kathmandu and Banepa Basins, Central Nepal, are located in a large syncline of the Lesser Himalayas. The Older Kathmandu Lake evolved during the Pliocene and early Pleistocene; the Younger Kathmandu Lake, which is the focus of this study, is infilled with late Quaternary sediments. Three formations, arranged in stratigraphical order, the Kalimati, Gokarna and Thoka Formations formed during the infilling stage of this lacustrine basin. Structural and textural sedimentological analyses, a chemical survey across the basin and mineralogical investigations of fine‐grained sediments form the basis of this palaeogeographical study. The basin under investigation was covered by a perennial freshwater lake before 30 000 yr BP. The lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin. A fairly low gradient was favourable for the formation of diatomaceous earths, carbonaceous mudstones and siltstones, which were laid down in the centre of the lake and in small ponds. Towards the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. Crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones. The proximal parts of the alluvial–fluvial sedimentary wedge contain debris flows that interfinger with fine‐grained floodplain deposits. Three highstands of the water‐level (>30 000 yr BP, 28 000–19 000 yr BP, 11 000–4000 yr BP (?)) have been recognised in the sedimentary record of the younger Kathmandu Lake in the Late Quaternary. Second‐order water‐level fluctuations are assumed to be triggered by local processes (damming by tectonically induced landslides). First‐order water‐level fluctuations are the result of climatic changes. Copyright © 2003 John Wiley & Sons, Ltd.     

Microfacies analysis and paleoenvironmental significance of palustrine carbonates in the Thakkhola-Mustang Graben (Nepal Himalaya)

The Thakkhola-Mustang Graben represents the extensional tectonic phase of the Tibetan Plateau uplift and whole Himalayan orogeny. It is situated at the northern side of the Dhaulagiri and Annapurna Ranges and south of the Yarlang Tsangpo Suture Zone. Stratigraphically, the oldest sedimentary units are the Tetang and Thakkhola Formations (Miocene), while the Sammargaon, Marpha and Kaligandaki Formations lying disconformably above these formations represent Plio-Pleistocene units. In this study, different lacustrine carbonates and calcretes were investigated within different lithological units and depositional environments to interpret the palaeoenvironmental and palaeoclimatological evolution of the area.Geological mapping, construction of columnar sections and carbonate sampling were carried out in the field, and stable oxygen and carbon isotope analyses and thin section analyses were done in the laboratory. Lacustrine facies contained abundant pelletal, charophytic algae, oncolitic algal micritic palustrine limestones with ostracods, and micritic mudstones with root traces. Stable carbon and oxygen isotope analysis from the carbonates show a range of δ¹³C values from −0.6‰ to 11.1‰ (V-PDB) and δ¹⁸O values from −13.5‰ to −25‰ (V-PDB).Discontinuous growth of oncolites and spherical pellets (25–40 μm in diameter) in micritic limestone, algal mats and charophyte algae indicate the presence of both shallow and deep water carbonates. Ostracods in dark micritic carbonates indicate quiet and calm water conditions. Microfabrics of the carbonates suggest that they were deposited in a flat and shallow lacustrine environment. The δ¹⁸O values of the investigated limestones of the Thakkhola-Mustang Graben suggest that it attained the current elevation level prior to the east-west extension of the Himalaya.     

Geochemical Constraints on Leucogranite Magmatism in the Langtang Valley, Nepal Himalaya

A complex of crustally derived leucogranitic sills emplacedinto sillimanite-grade psammites in the upper Langtang Valleyof northern Nepal forms part of the Miocene High Himalayan graniteassociation. A series of post-tectonic, subvertical leucograniticdykes intrude the underlying migmatites, providing possiblefeeders to the main granite sills. The leucogranite is peraluminous and alkali-rich, and can besubdivided into a muscovite–biotite and a tourmaline–muscovitefacies. Phase relations suggest that the tourmaline leucogranitescrystallized from a water-undersaturated magma of minimum-meltcomposition at pressures around 3–4 kbar. Potential metasedimentaryprotoliths include a substantial anatectic migmatite complexand a lower-grade mica schist sequence. Isotopic constraintspreclude the migmatites as a source of the granitic melts, whereastrace-element modelling of LILEs (Rb, Sr, and Ba), togetherwith the Nd and Sr isotopic signatures of potential protoliths,strongly suggest that the tourmaline-bearing leucogranites havebeen generated by fluid-absent partial melting of the muscovite-richschists. However, REE and HFSE distributions cannot be reconciledwith equilibrium melting from such a source. Systematic covariationsbetween Rb, Sr, and Ba can be explained by variations in protolithmineralogy and P–T–a_H2O. Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction-melts(F{small tilde} 12%) efficiently extracted from their protolithsunder conditions of low water activity, whereas the heterogeneoustwo-mica granites may result from melting under somewhat highera_H2O conditions. The segregation of low-degree melts from sourcewas probably by deformation-enhanced intergranular flow andmagma fracturing, with the mechanisms of migration and emplacementcontrolled by variations in the uppercrustal stress regime duringlate–orogenic extensional collapse of the thickened crust.     

Fluid inclusion study of the Higher Himalayan quartzitic pelites, Garhwal Himalaya, India: Implications for recrystallization history of metasediments

Quartzitic pelites forms a part of Higher Himalayan Crystalline of higher geotectonic zone in Garhwal Himalaya. Quartzitic pelites (locally known as Pandukeshwar Quartzite) in Garhwal Himalaya is sandwiched between high grade metamorphic rocks of Central Crystallines and Badrinath Formation. Fluid inclusion studies are carried out on the detrital, and recrystallized quartz grains of quartzitic pelites to know about the fluid phases present during recrystallization processes at the time of maximum depth of burial. The quartzitic pelite (Pandukeshwar Quartzite) essentially consists of recrystallised quartz with accessory minerals like mica and feldspar. Fluid microthermometry study reveals the presence of three types of fluids: (i) high-salinity brine, (ii) CO₂-H₂O and (iii) H₂O-NaCl. These fluids were trapped during the development of grain and recrystallization processes. The high saline brine inclusions and CO₂-H₂O fluid with the density of 0.90 to 0.97 gm/cm³ are remnants of provenance area. CO₂ density in detrital quartz grains characterise the protolith of the sandstone as granite or metamorphic rock. The H₂O-NaCl fluids involved in the recrystallization processes at temperature-pressure of 430-350°C; 4.8 to 0.5 Kbars as constrained by fluid isochores of CO₂-H₂O and H₂O-NaCl inclusions and bulging and subgrain development during recrystallization processes. The re-equilibration of the primary fluid due to elevated internal and confining pressure is evident from features like ‘C’ shaped cavities, stretching of the inclusions, their migration and decrepitation clusters. The observed inclusion morphology revealed that the rocks were exhumed along an isothermal decompression path.