• Sankar Bose

      Articles written in Journal of Earth System Science

    • Mineral reactions and geothermobarometry in a suite of granulite facies rocks from Paderu, Eastern Ghats granulite belt: A reappraisal of the P-T trajectory

      Supratim Pal Sankar Bose

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      High Mg-Al spinel-sapphirine granulites, orthopyroxene-bearing quartzofeldspathic granulites, two pyroxene-bearing mafic granulites and metapelitic gneisses are exposed around Paderu, Eastern Ghats Belt. Geothermobarometry in orthopyroxene-bearing quartzofeldspathic granulites and mafic granulites indicate near isobaric cooling through 90°C from ca. 720°C to 630°C, at 8.0 kbar. However, signatures of ultrahigh temperature metamorphism are recorded from the mineralogy and reaction textures in the high Mg-Al granulites. Mineral reactions deduced in this work, when combined with others described by Lalet al (1987) from the same area and plotted in an appropriate petrogenetic grid in the system FMASO indicate an ACW path comprising a high dT/dP prograde arm reaching Pmax − Tmax = 9.5 kbar, ∼ 1000°C, followed by near-isobaric cooling down to 9 kbar, 900°C and subsequent decompressive reworking.

    • Petrotectonic framework of granulites from northern part of Chilka Lake area, Eastern Ghats Belt, India: Compressional vis-à-vis transpressional tectonics

      Kaushik Das Sankar Bose Subrata Karmakar Supriya Chakraborty

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      Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M1$–$D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2$–$D2­) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the $M_{4}–D_{4}$ stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed $S_{2}/S_{3}$ fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the $M_{3}–D_{3}$ event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.

    • Evolution of fluid from the ultrahigh temperature lower crust to shallower levels: Constraints from silicate–oxide–sulphide–sulphate assemblages of mafic granulites of the Eastern Ghats Belt, India

      Arnob Kumar Mondal Sankar Bose

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      Mafic granulites from key localities of the Eastern Ghats Province preserve Fe–Ti oxides, Cu–Fe sulphides and traces of sulphate minerals along with silicate phases. Two different varieties of mafic granulite exhibit slightly contrasting mineral assemblages. While the massive type of mafic granulite contains minerals assemblage orthopyroxene $+$ clinopyroxene $+$ plagioclase $+$ magnetite $+$ ilmenite $+$ pyrite $+$ pyrrhotite, the migmatitic variety contains garnet as an additional phase. Both oxide and sulphide minerals show contrasting textural characters. Textural analysis and construed mineral reactions imply that the variation of oxide–silicate, oxide–sulphide and sulphate relations is linked to variation of $f$O$_{2}$ during the pre-peak, peak and post-peak stages of metamorphism. The calculated $f$O$_{2}$ values range up to +4 log units relative to the QFM (quartz-fayalite-magnetite) buffer among the samples, except for one sample which shows lower values ($−$10 log units relative to the FMQ (fayalite-magnetite-quartz) buffer). The consistently high $f$O$_{2}$ condition at the lower crust could result from several factors, but the role of the externally derived fluid appears to be plausible. Hot brine solution with CaCl$_{2}$ species can explain the oxidation as well as local metasomatism of the mafic lower crust even though its presence is not verified from direct characterisation like fluid inclusion analysis.

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