Field and Textural Relationship in Pelitic Schists and Gneisses from the area around Mangpu, Darjeeling District, West Bengal

Abstract

The metasedimentary rocks of the area around Mangpu constitute a portion of the hinge zone of the northern limb of the major synform of Lower Darjeeling Himalaya. The rocks display evidences of multiple deformation and at least three major phases of deformation have been recognized. The time relations between the phases of deformation (D1, D2, 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 syn-tectonic low-grade (greenschist facies) which may be designated (M1, early). This was followed by regional static metamorphism (M1, late) in the post-tectonic phase between D1 and D2 deformations (upper green schist and amphibolite facies). This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations (lower greenschist facies). Within the study area four isograds have been delineated by the first appearance of index minerals in the pelitic schists and gneiss which display Barrovian type of metamorphism.

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

“…78A/8 and 78B/5 with input of isograds locations from author's field study. The different zones based on critical mineral assemblages have also been demarcated (modified after Prakash and Tewari, ). [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…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.…”

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

“…In this part of the region, numbers of workers have provided detailed geological study [2][3][4][5][6]. The aim of this study is to carry out a detail account of petrography, mineral chemistry and geothermobarometry of metapelites.…”

Garnet-sillimanite bearing gneisses from Darjeeling, eastern Himalaya: Textural relationship and P–T conditions

“…The term 'Darjeeling Gneiss' was first used by Mallet (1875) for crystalline gneiss of Darjeeling hills of the Darjeeling Himalaya. Later this term has been used extensively by Auden (1935), Acharya (1978), Lal et al (2005), Prakash and Tewari (2013), etc. This region constitutes a part of the lesser Himalaya and consists of metamorphic rocks belonging to Daling-Darjeeling Group (Sinha Roy 1974) of probably Precambrian to Palaeozoic age.…”

“…If it can be assumed that the analysed bulk rock composition was the effective chemical composition of the equilibrated system at the time of crystallization of garnet, the isochemical phase-diagram drawn for this composition and showing different isopleths for garnet can then be used as thermobarometric tool allowing the determination of P-T condition. In Darjeeling-Sikkim area, number of earlier workers have provided a detailed geological and structural account (e.g., Mukul 2000;Gupta et al 2010;Prakash and Tewari 2013), however, the metamorphic conditions of the rocks are not constrained through conventional geothermobarometers and isochemical phase-diagram approach. In this contribution, textural relationship, mineral chemistry and P-T conditions of garnet-sillimanite bearing gneisses are presented.…”