• SARAJIT SENSARMA

      Articles written in Journal of Earth System Science

    • Nature and composition of interbedded marine basaltic pumice in the ~52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

      Sarajit Sensarma Hukam Singh R S Rana Debajyoti Paul Ashok Sahni

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      The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels –one at ~5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ~52–50Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated alteredbasaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO₂, Al₂ O₂, TiO₂ , MgO, alkalies and REE, but very high in Fe₂O₃, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO₂, Al₂O₃, MgO and variable gain in Fe₂O₃, TiO₂, Ni, V, Zr, Zn and REE. Bubble initiationto growth in the ascending basaltic magma (liquidus ~1200–1250◦C) may have occured in ~3 hr. Shortdistance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.

    • Grain-scale anatomy of the Bundelkhand granite: Implications for the interplay of magmatic to sub-magmatic deformation mechanisms

      GOUTAM SARKAR SHRIZA ACHARYA SAYANDEEP BANERJEE ABDUL MATIN SARAJIT SENSARMA HARI B SRIVASTAVA

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      Grain-scale structures of the granitoid rocks from the north-western part of the Bundelkhand craton, central India are analysed with the aid of an optical microscope and electron probe micro analyser. Although field-based studies and quick microscopic observations suggest an overall porphyritic texture ofthe Bundelkhand granitoid, detailed microstructural observations reveal a significant deviation from the first-order igneous porphyritic texture. Here, we show that the Bundelkhand granitoid has three distinct grain-scale structures: (i) original pristine igneous structures, (ii) ductile deformation-related structures, and (iii) brittle fracturing-related structures. Based on microstructural evidences, we argue that the deformation-induced structures (both brittle and ductile) are not restricted to solid state, rather thesestructures initiated in the sub-magmatic stage and nucleated in partially crystallised magma during the magmatic to sub-magmatic event of the crystallisation history.

    • Origin of silicic breccio-conglomerate within the ${\sim}$2.5 Ga LIP rhyolites, Bastar craton (India) and its volcanological and stratigraphic implications

      SARAJIT SENSARMA RUCHI GAUR

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      Recognition of ancient pyroclastic rocks made up of particles of explosive volcanic origin and deposited by primary volcanic processes and/or rapid sedimentation of freshly erupted, texturally unmodified particles vis-a-vis secondary volcaniclastic deposits derived by significant reworking during transport of primary volcanic particles prior to deposition, but long after volcanism (epiclastic) remains a major challenge. A volcanic conglomerate having both rhyolitic clasts and matrix within the ${\sim}$2.5 Ga Bijli Rhyolite in the Dongargarh large igneous province in the Bastar craton, is earlier interpreted to be of epiclastic origin, primarily because of the presence of large rounded rhyolite clasts imparting conglomeratic appearance to the deposit, and thereby considered representing a significant time break (unconformity) between explosive Bijli volcanism and the deposit. Based on new field and petrography studies, we identified the ${\sim}$125 m thick volcanic conglomerate as rapidly sedimented texturally unmodified rhyolitic brecciaconglomerate linked to coeval incipiently welded pyroclastic flow that occurred during caldera collapse related to mafic-recharge-mediated Bijli volcanism, without significant time break. We ascribed the rounding of rhyolite clasts to surface tension of hot crystallising molten magma in plastic state and partly to mechanical interactions of particles on steep slopes in such volcanic settings. This study may help clarify origin of similar deposits in deformed metamorphosed provinces elsewhere.

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