• TAPAN PAL

      Articles written in Journal of Earth System Science

    • Felsic tuff from Rutland Island – A pyroclastic flow deposit in Miocene-sediments of Andaman-Java subduction complex

      Tapan Pal Biswajit Ghosh Anindya Bhattacharya S K Bhaduri

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      The bedded felsic tuff exposed in Rutland Island, Andaman, consists of two facies:

      white massive tuff with ill-defined bedding contacts (facies-A) and

      dominantly green tuff exhibiting well-developed turbidite sequence with up-section change from a massive unit to plane laminated units to ripple drift lamination (facies-B).

      The felsic tuff is vitric to crysto-vitric in nature and contains broken crystals of quartz, feldspar, biotite and glass shards of different shapes and sizes. The bulk chemistry indicates trachyte to dacite compositional range, and high values of Zr relative to Nb and Y suggest convergent margin tectonic setting of the tuff. Dominance of cuspate shards rather than blocky shards in both the facies indicates subaerial eruption of the pyroclasts. Recurrence of turbidites as well as good sorting of crystals and glass suggest that subaerially erupted ash was transported in subaqueous condition. The bed pattern supports rapid deposition of facies- A from high concentration turbidity flow whereas facies-B could be produced by decrease in grain size and suspended-load fallout rate of turbidity current. Regional correlation suggests that felsic volcanism in Sumatra was the source for such early Miocene to middle Miocene tuff.

    • Metasomatically controlled sillimanite–corundum deposit: A case study from Sonapahar, Meghalaya, Northeast India

      SUSOBHAN NEOGI TAPAN PAL

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      The Sonapahar sillimanite-corundum deposit of Meghalaya, Northeast India occurs within high-grade metapelites and exhibits an unusual association of sillimanite-quartz, corundum-quartz, sapphirine-phlogopite-corundum-sillimanite rock. Pockets of sillimanite and corundum occur within the host quartz sillimanite schist with contrasting chemical signature, where the SiO$_{2}$ /Al$_{2}$ O$_{3}$ ratio is 2.5–3 for the host quartz sillimanite schist and 0.12–0.7 in sillimanite rocks. Earlier it was explained that the sillimanite-corundum rock was produced from metamorphosed paleosol, but the present study suggests that metasomatism had a major role to modify the host quartz-sillimanite schist into sillimanite-corundum rocks during post-peak metamorphism. Field disposition and structural control of the area show that pockets of sillimanite-corundum rock are restricted near the shear zones. Shear zones were the pathways for movement of hydrothermal fluids, conversion of host bulk chemistry by selective leaching and, finally, enrichment of sillimanite-quartz or corundum-quartz pockets within the quartz sillimanite schist.

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