• Tapas Bhattacharyya

      Articles written in Journal of Earth System Science

    • Anasagar gneiss: A folded granitoid pluton in the Phanerozoic South Delhi Fold Belt, central Rajasthan

      Dhruba Mukhopadhyay Tapas Bhattacharyya Nandini Chattopadhyay Robert Lopez Othmar T Tobisch

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      The Anasagar gneiss was emplaced as a concordant sheet like body along the contact of quartzite and pelitic/semipelitic schist horizons in the northern part of the South Delhi Fold Belt. It is typically a granite gneiss containing megacrysts of K-feldspar set in a recrystallised foliated matrix. The megacrysts are in general converted to granular aggregates, often retaining their crystal outline. Garnet, sillimanite (fibrolite) and rarely staurolite are the metamorphic minerals in the gneiss; these are also present in the enveloping supracrustal rocks. Both the gneiss and the supracrustal rocks are involved in polyphase deformation. F1 isoclinal folds are present only on minor scale in the supracrustal rocks. F2 major and minor folding have affected both the gneiss and the supracrustal rocks. These are asymmetrical folds with alternate flat and steep, locally overturned, limbs and have consistent easterly vergence. F3 folds are upright and coaxial with F2. F4 puckers and large scale warps have E-W to ESE-WNW subvertical axial planes. The gneiss is exposed in the core of an F3 arch on the flat limb of a major F2 antiform whose axial trace is bent by an F4 fold. The intrusion was pre-F2 and late-tectonic with F1. U-Pb zircon dating suggests a crystallization age of 1849 ± 8 Ma. Hence the Anasagar gneiss is older than the late- to post-tectonic ``Erinpura-type'' granite in the South Delhi Fold Belt.

    • Interaction between felsic granitoids and mafic dykes in Bundelkhand Craton: A field, petrographic and crystal size distribution study

      Tanwita Deb Tapas Bhattacharyya

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      In Bundelkhand Craton of central India, mafic dykes intruded when granitoids was partly crystallized. Cuspate–lobate boundary along the contact of granitoids and mafic magma indicates magma mingling in outcrop scale while textural evidence of mingling is represented by acicular apatite morphologies, titanite–plagioclase ocelli and ophitic–subophitic texture, mafic clots, resorbed plagioclase, and hornblende–zircon associations. Mingling also caused thermal exchange and fluid activity along the boundary between two coeval magmas. Crystal size distribution analyses for hornblende in the mafic rocks yield concave up curves which is also consistent with interaction of felsic and mafic magmas.

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