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D. Rameshwar Rao ¹, N. S. Gururajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248 001, IN

Abstract

An inverted metamorphic gradient has been documented within the rocks of the Kullu-Rohtang Pass-Khoksar region, with increase in grade of metamorphism from biotite through garnet, staurolite and kyanite to sillimanite zones. Microtextures, mineral equilibria and geothermobarometry of mineral assemblages indicate deformation at a minimum temperature of 500°C and 4.7 kbar pressure in the Kullu region to a maximum temperature of 635°C and 6.8 kbar pressure in the Rohtang Pass region of Higher Himalaya. The lower arnphibolite facies rocks further north of Rohtang Pass indicate P-T conditions of 4.5 kbar and 450°C respectively. The range of P-T conditions imply tectonic burial of 16 to 23 km and geothermal gradient variation of 31 °C/km to 28°C/km. There is a maximum P-T difference of 2100 bars and 135°C upsection between samples near Kullu to samples along the Rohtang Pass, with dP/dT values of around 16 bars/°C. The inverse metamorphism of the region is due to synto post-metamorphic folding of isograds accompanied by thrusting of normal Barrovian metamorphic sequence, that is subsequently followed by a retrogressive cooling path during exhumation.

Keywords

Metamorphism, Inverted Sequence, Geothermometry, Geobarometry, Kullu, Khoksar, Rohtang Pass, Himachal Pradesh, Himalaya.

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D. Rameshwar Rao ¹, Rajesh Sharma ¹, N. S. Gururajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehradun - 248 001, IN

Abstract

The granulite-gneiss terrain of Schirmacher region, East Antarctica includes mafic to acid granulites pervasively intruded by stringers, bands, lenses and stocks of felsic rocks. The late stage felsic rocks are the leucocratic rocks and are the focus of this paper. They have concordant to discordant relation with the associated felsic and other granulitic rocks of the region.

The fluid inclusions, show three stages of f1uid entrapment. The monophase CO₂ inclusions, with densities in the range of 0.996 to 0.811 g/cm³, show pressures of about 5.1 ± 0.6 kbars at inferred temperatures of 675 ± 25°C and indicate peak metamorphism after crystallization. Early stage deformed aqueous inclusions, however, represent initial crystallization. Entrapment of these inclusions was followed by carbonic aqueous inclusions, whose CO₂ densities range from 0.816 to 0.624 g/cm³ with increasing H₂O fluid entrapment occurring during the retrogressive stages. The increased concentration of aqueous fluids has resulted in biphase aqueous inclusions in trails. Their density and homogenization temperatures indicate a pressure of 2 ± 0.5 kbars.

The P-T trend observed from fluid inclusion studies is ref1ected from mineral equilibria studies. The GMBPQ assemblage of these rocks gave temperatures and pressures of 675 ± 25°C and 5.5 ± 0.3 kbars respectively. Further, the two-feldspar thermometry for these rocks record a reequilibrated temperatures of 444 ± 9°C. These P-T conditions of the leucogneisses suggest around 18 km of crust to have been removed, indicating > 50 km thick crust during the amphibolite facies metamorphism corresponding to geothermal gradients of around 37°C/km. A successive decrease in estimated P-T conditions along with density and compositional information suggests retrogression and compressional cooling path for these rocks.

Keywords

Geothermobarometry, Fluid Inclusion Studies, Granulite, Antarctica.

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N. S. Gururajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Debra Dun, IN

Abstract

Microstructural and geochemical analysis have been carried out on the mylonitic augen gneisses of the Chail thrust zone, in the Satluj valley of Himachal Pradesh. The microstructural features of K-feldspar, plagioclase, quartz and mica indicate that these gneisses have undergone an early deep level ductile deformation followed by a high level brittle deformation. The mylonites and blastomylonites represent ductile deformation products and the cataclasite represents brittle deformation product. The mylonitization is related to thrusting events during the Himalayan Orogeny.

Available geochronological data with high initial strontium ratio suggests that the protoliths of the mylonitic augen gneisses (Svtype granitoid) intruded the Chait formation during Precambrian times. The major element chemistry, with the absense of normative corundum values and low Al-index values indicate a mantle origin.

Keywords

Geochemistry, Himalayan Geology, Satluj Valley, Himachal Pradesh, Deformation Microstructures.

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N. S. Gururajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun-248001., IN

Abstract

The crystalline thrust sheets of the Lesser and Higher Himalayan region of the Satluj valley, Himachal Pradesh, contain porphyroblasts of gamet, chloritoid, staurolite, kyanite and albite. The inclusion trails in the prophroblasts indicate that the porphyroblasts overgrew during various stages of S2 crenulation cleavage development related to D2 deformation. The sites for nucleation and growth of porphyroblasts are provided by deformation partitioning. The dissolution of porphyroblast margins is not common as suggested by other workers. Metamorphism during D1 was low grade.

Keywords

Structural Geology, Metamorphic Rocks, Himachal Pradesh.

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N. S. Gururajan ¹, B. K. Choudhuri ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun -248 001, IN

Abstract

The Trans-Himalayan Lohit Plutonic Complex occurs to the east of the Eastern Syntaxis in eastern Arunachal Pradesh and the investigations were carried out mainly along the Lohit River section. The plutonic complex intrudes the metavolcanics of probable arc affinity in the west and high-grade metamorphic rocks of the Asian continental margin in the east. It can be sub divided into western and eastern belts, separated by Walong Thrust. The complex consists of multiple intrusions and they arc:: (1) an early phase of metaluminous, tholeitic to calc- alkaline gabbro and quartz diorites with minor diorites and hybrid rocks; (2) an intermediate phase of calc-alkaline, metaluminous, leucotonalite of high Al-trondhjemitic or adakitic composition; (3) a last phase of peraluminous leucogranite and associated veins of pegmatite and aplite. The enrichment of large-ion-lithophile elements (LILE) relative to high field strength elements (HFSE) of these rock types suggest that they were emplaced in a subduction related environment and derived from different source materials. The major and trace element variations in the early gabbro-quartz diorite, along with the low initial ⁸⁷Sr/⁸⁶Sr ratio (0.703-0.704) suggest that they were derived from a parental tholeitic magma, from which the intrusives have been differentiated under increasing water pressure conditions. The geochemical characteristics of the trondhjemites, such as the high Na₂O, AI₂O₃, Sr, Sr/Y ratios and low K₂0, Y and depletion of heavy rare earth elements (HREE) are similar to those of itdakite and Archaean high Al- trondhjemite. They might have been derived either by melting of the subducted oceanic: crust or by partial melting of newly under- plated basaltic crust beneath the thickened continental arc leaving residual ;garnet and amphibole. The leucogranites with high K₂0, Rb, K_O/Na₂O, K/Rb, Rb/Sr ratios, and high initial ⁸⁷Sr/⁸⁶ Sr ratio (0.713), along with depletion in HREE and absence of negative Eu anomaly, suggest that they were derived from anatectic melting of a mixture of leucotonalite and rnetasedimentary rocks, induced by thrust related thickening during collision. The collision related convergence has resulted in the development of major intra-continental thrusts, which at present steeply dipping in the Lohit Plutonic Complex. Possibly during Miocene these thrusts have been reactivated into right-lateral strike-slip faults.

Keywords

Eastern Syntaxis, Trans-Himalaya, Plutonic Complex, Geochemistry, Subduction, Tectonic Implications, Arunachal Pradesh.

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D. Rameshwar Rao , N. S. Gururajan , J. Swami Nath ¹

Affiliations
1 15B, Oceania Apts., Thiruvalluvar Nagar, Chennai - 600 041, IN

Abstract

No Abstract.

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