Browsing by Author "B. Vishal"

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New Occurrence of Sapphirine-spinel-bearing Granulite from NW of Chilka Lake, Eastern Ghats Belt, Odisha
(Geological Society of India, 2019) D. Prakash; B. Vishal; A.S. Naik; R. Yadav; S.K. Rai; S. Tewari; M.K. Yadav; S. Tiwari; S. Dash; C. Pattnaik
A new locality of sapphirine-spinel-bearing granulites from the Kaithapalli area which lies NW of Chilka Lake, Odisha is reported. The area tectonically forms a northern part of Eastern Ghats belt. It occurs as small enclaves within the khondalite and pelitic granulite. The mineral assemblage includes spinel-sapphirine-garnet-cordierite-orthopyroxene-sillimanite-biotite-k-feldspar-plagioclase-quartz. Development of reaction textures and symplectites are common in the sapphirine-spinel granulites which have been used to describe reaction history. The relative XMg values among various minerals are as follows: cordierite > biotite > sapphirine > orthopyroxene > garnet > spinel. The P-T evolution of these sapphirine-spinel granulites constrained through the pseudosection modelling in the NCKFMASH model system using Perple_X software indicate conditions of ultra-high temperature (UHT) metamorphism. The P-T estimates computed by isopleths define a retrograde trajectory with decompression of c.2.5 kbar from P-Tmax of c.10.5 kbar at c.950 o_C. The sequence of reactions as deduced from the symplectite assemblages, together with pseudosection modelling, from the Kaithapalli area, offer greater opportunities for providing a better picture of petrological evolution of northern part of the Eastern Ghats Belt (EGB). © 2019, Geological Society of India.
Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: Constraints from textural relationship, pseudosection modelling and geothermobarometry
(Cambridge University Press, 2020) D. Prakash; D.K. Patel; M.K. Yadav; B. Vishal; S. Tewari; R. Yadav; S.K. Rai; C.K. Singh
The study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P-T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late-to post-D2 deformation. © 2019 Cambridge University Press.