Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry

Abstract

The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North-Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post-tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K-feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P-T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley & Sons, Ltd.

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