Petrological and Thermal Constraints on the Evolution of Lesser Himalayan Metapelites, West Sikkim, Himalaya

Bediya, Prakash (2013) Petrological and Thermal Constraints on the Evolution of Lesser Himalayan Metapelites, West Sikkim, Himalaya. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Lesser Himalayan sequence in West Sikkim mainly contains Phyllite, Schist and Quartzofeldspathic gneiss. Rocks, here, are subjected to local ductile deformation, since, contain folded schistosity (S2). Analysis of the chemical compositions of mineral reveals that garnets from metapelites of the study area are Alamandine rich.There is no significant change in Almandine but Pyrope shows increasing trend toward higher grade and increase in TiO2 in biotite is also evident. Plagioclase is present in all metamorphic grade with about 20 mol% of Ca. P-T conditions of the metamorphism in the study area are obtained by conventional geothermobarometric calculations by simultaneously solving the expressions for garnet-biotite exchange equilibrium and the garnet aluminosilicate-plagioclase-quartz net transfer reaction. Following the GB –GMBP method the “peak” P-T condition of garnet grade and staurolite grade rock is 3.3 kbar and 457 ºC and 7.9 kbar and 620 ºC respectively whereas “peak” P-T condition of 620 ºC at 7.9 kbar and 660 ºC at 8.7 kbar using the GB – GASP geothermobarometer of Ganguly et al. The peak P-T conditions ranging from 3.3 kbar, 457 °C at garnet grade to 8.7 kbar, 660 °C at sillimanite grade in west Sikkim. And hence the metamorphic field gradient is found to be positive, 40 °C /kbar, in this region. Modelling of phase equilibria in pelitic rocks in LHS, west Sikkim, Himalaya is done by construction of pseudosections with average bulk composition of metapelites through the application of Gibb’s free energy minimization technique. The pseudosection demonstrates that the stability fields of metamorphic index assemblages and thermobarometric results coincide with each other. Finally, from the combined study of petrography, mineral chemistry, thermobarometry and, pseudosection calculations it can be concluded Progressive increase in metamorphic grade, toward structurally higher level, is preserved in LHC in, West Sikkim, Himalaya- therefore an IMS. And this IMS shows folded nature along with main central thrust, which leads us to conclude that MCT and IMS are Pre Teesta dome formation.

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