• Debajyoti Paul

      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.

    • Petrography and geochemistry of carbonatite breccia from Amba Dongar carbonatite complex, Gujarat in the Deccan Large Igneous Province suggest mantle origin


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      This study reports petrography, geochemistry, and ${\delta}^{13}$C and d18O composition of carbonatite clasts in carbonatite breccias from the Amba Dongar carbonatite (ADC) complex in the Deccan Large Igneous Province. Petrography reveals early-emplacement of calcio- and ferro-carbonatite. The mineralogy and elemental abundances of calcio- and ferro-carbonatite clasts in breccias are similar to those of the laterformed ring dyke carbonatites, indicating a common source. In the ${\delta}^{13}$C–${\delta}^{18}$O space, calciocarbonatite clasts and carbonatites of main ring dyke of Amba Dongar plot within the extended primary carbonation field, indicating its mantle origin. Positive ${\delta}^{18}$O (${\ge}$+10%) values of clasts show the role of recycled crust and hydrothermal alteration. We propose a model for the origin of carbonatite breccia and later-formed ring dyke of ADC, in which a parental carbonated silicate melt forms carbonatite melt and silicate melt through liquid immiscibility at crustal depths, and intrusion of these melts forming dykes causes updoming (stages I–III). In stages IV–VI, episodic evacuation of the carbonatite magma chamber initiates caldera subsidence leading to extensive brecciation of early-formed carbonatites. Later, the lateral spread of the magma chamber leads to the formation of the carbonatite ring dyke.


      $\bullet$ The ${\delta}^{13}$C and ${\delta}^{18}$O compositions of Amba Dongar carbonatite breccias and carbonatites of the ring dyke lie within the primary carbonatite field indicating its mantle origin.

      $\bullet$ Mineralogy and geochemistry of carbonatite clasts and carbonatites reveal a common source.

      $\bullet$ Evacuation of melt from magma chamber and regeneration of carbonatite melt initiates caldera subsidence and extensive brecciation of the subsurface carbonatite dykes.

      $\bullet$ Genesis of Amba Dongar complex is similar to that of the Alnö carbonatite complex.

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