• Rajib Kar

      Articles written in Journal of Earth System Science

    • Patchy charnockites from Jenapore, Eastern Ghats granulite belt, India: Structural and petrochemical evidences attesting to their relict nature

      Rajib Kar

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      The charnockite patches that occur within leptynite host, in and around Jenapore, northern sector of the Eastern Ghats granulite belt, are disposed in a linear fashion and generally have sharp lithological contact with the host leptynite. Sometimes the patches and foliations of the host are cofolded. Also, the patches sometimes have the internalS1 foliation, while the host leptynite records onlyS2 foliation. Mineralogically and chemically patchy charnockites and host leptynites are distinct entities, and cannot be related by any prograde and retrograde reactions. Particularly important is the peraluminous granitic composition and high Rb/Sr ratios of the leptynites, presumably resulting from biotite-dehydration melting; as against metaluminous granodioritic to tonalitic composition and low Rb/Sr ratios of the patchy charnockites, presumably resulting from hornblende-dehydration melting. The charnockite patches here can be interpreted as caught up patches or xenolith within granitic melt (leptynite). Mg-rich rims of garnet in the charnockite patch were probably caused by heat from the crystallising melt or decompression during ascent of melt.

    • Alkaline intrusion in a granulite ensemble in the eastern ghats belt, India: Shear zone pathway and a pull-apart structure

      S Bhattacharya Rajib Kar

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      The alkaline complex of Koraput, Orissa, India, is one of several bodies in the high-grade Eastern Ghats belt, but this one is an integral part of the high-grade belt and remote from the western boundary against the Bastar craton. The Koraput complex forms a lozenge-shaped intrusion into the metapelitic granulites and is bounded by shear zones. The combined effect of movement along these shear zones, is a northeasterly elongated sygmoidal cavity with maximum width along the northwesterly trending Reidel shear. Thus the Koraput alkaline complex can be considered to have been emplaced in a pull-apart structure, developed in the granulitic country rocks. Moreover, in view of the fact that the western margin of the high-grade Eastern Ghats belt bears clear evidence of collisional features, rather than that of rifting or break-up, the rift-valley model for the alkaline magmatism in this high-grade belt appears untenable.

    • Petrogenesis of granitoid rocks at the northern margin of the Eastern Ghats Mobile Belt and evidence of syn-collisional magmatism

      S Bhattacharya Rajib Kar S Moitra

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      The northern margin of the Eastern Ghats Mobile belt against the Singhbhum craton exposes granitic rocks with enclaves from both the high-grade and low-grade belts. A shear cleavage developed in the boundary region is also observed in these granitoids. Field features and petrography indicate syn-tectonic emplacement of these granitoids. Petrology-mineralogy and geochemistry indicate that some of the granitoids are derived from the high-grade protoliths by dehydration melting. Others could have been derived from low-grade protoliths. Moreover, microstructural signatures in these granitoids attest to their syn-collisional emplacement.

    • Domainal fabric development, associated microstructures and P-T records attesting to polymetamorphism in a granulite complex of the Eastern Ghats granulite belt, India

      Rajib Kar

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      The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation (D1 and D2) attended with foliation (fabric) development ($S_1$ and $S_2$). Two distinct metamorphic events at P-T conditions of ∼900° C at ∼9$kbar$ and ∼600° C at ∼6 $kbar$ are correlated with D1 and D2 respectively. The reaction textures in S1-microdomains are interpreted to be the product of near isobaric cooling at ∼9 $kbar$ from 950° C to 600°C, whereas those in the $S_2$-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D1-M1 high P-T granulite event is interpreted to be Archean in age (ca. 3Ga) on the basis of the isotopic data obtained from the charnockite suite of the area. The later relatively low P-T granulite facies event, attendant to D2-S2 is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt.

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