Geothermobarometry and Barrovian Metamorphism of Darjeeling-Mangpu Region, Eastern Himalaya

Abstract

Along the Darjeeling-Mangpu region, polyphase deformation and barrovian metamorphism has been delineated in relation to the Main Central Thrust (MCT). The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, k-feldspar and plagioclase show various texture resulting by the continuous and discontinuous reactions in the different zones. From the Microprobe data of the coexisting minerals show that X Mg varies in order garnet<staurolite<biotite<chlorite. White mica which is the solid solution between muscovite and phengite is the commonest mineral within the lower grade; garnet is almandine rich showing normal zoning in the lower part of the MCT and in the upper parts shows reverse zoning. In the lower grade rocks the oriented chlorite, mica grains and rolled garnet formed syntectonically and after succeeded by cross cutting mica, chlorite and idioblastic garnet. P-T condition estimated by different models of geothermobarometry, the temperature value range from 570 C for garnet zone and 780 C for sillimanite zone and the pressure range from 5 kbar to 7.5 kbar. Different models have been proposed to explain inverted metamorphism but recently the channel flow model which defines a flow of a weak crustal layer in between relatively rigid yet deformable crustal slab is most appropriate. If the channel flow is operating at the same time of focused denudation, this can result in exhumation of channel material within a narrow, nearly symmetric zone.

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

“…Ray (, ) mapped the inverted metamorphic zones in Darjeeling and Eastern Sikkim, respectively. All these preliminary findings in the Darjeeling–Sikkim Himalaya were further studied by different workers (e.g., Ghosh, ; Jangpangi, ; Acharyya, ; Gangopadhyay, ; Mukherjee, ; Bhattacharyya and Das, ; Mohan et al ., ; Dasgupta et al ., ; Lal et al ., ; Neogi, ; Nandini and Thakur, ; Prakash and Tewari, , , ; Tewari and Prakash, ). The purpose of the paper is to describe the following: (a) the textural relations with respect to time relations between the phases of deformation and metamorphic crystallization, to probe into the possibility of single or polyphase metamorphism; (b) mineral chemistry, substitution of cations and their ratios and their relations to prograde metamorphism, continuous and discontinuous reactions; (c) phase relations in the different metamorphic zones on AFM projection and the problem of extra phases not compatible with the topology of the projection; (d) nature of continuous and discontinuous metamorphic reactions on the isograds and different metamorphic zones, and their application to explain the compositional changes with grade and growth or resorption in zoned minerals (e.g.…”