Compositional variation in magma through early neogene in the Northeast Indian Ocean: A testimony from glass shards |
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| Authors: | J P Shrivastava V Sharma |
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| Institution: | 1. Department of Geology, University of Delhi, Delhi, 110 007, India
2. 225, Aarohi Residency, Bopal, Ahmedabad, 380 058, India
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| Abstract: | We report unusual occurrence of glass shards with diverse morphologies and compositions in the volcanic ash associated with the early Neogene marine stratigraphic succession (early Miocene to early middle Miocene) of Andaman-Nicobar Islands, Northeast Indian Ocean. These small, ash-size (200 to 800 μm) broken pieces of glass shards when viewed under Scanning Electron Microscope (SEM), represent distinctive — platy, sickle, bicuspate, concentric, angular, horn shape and slivers with broken angular bubble wall — morphologies. Glass shards are colourless. But, a few are grey or reddish-brown, indicate high Fe content. Chilled, juvenile, angular and blocky shards show fragments of highly viscous, silicic magma. Spindle and ribbon-shape shards form from a low viscosity basalt and rhyolite. Electron Probe Micro Analyzer (EPMA) was used to measure low concentration variations of major oxides within individual amorphous silicate solid glass shards whose disordered atomic structure is that of a liquid derived from a silicate melt. Major elemental chemistry of early Miocene glass shards from Colebrook island show low silica, alkalis, high FeO(T) MgO and CaO, whereas, early middle Miocene glass shards from Inglis island show high silica, alkalis, low FeO(T), MgO and CaO contents. These data-sets when plotted on ternary TotalAlkali-Silica and Na2O+K2O-MgO-FeO(T) diagrams show that their data plots lie within the basaltic-andesite, tephri-phonolite, rhyolite and trachyte fields. These glass shards which were present in the provenance, formed by explosive eruption of lavas, ranging in composition from basalt to rhyolite with andesite/ basalt-andesite being the most common magma types erupted sub-areally, implying island arc type of tectono-magmatic setting for the formation of these lavas. However, more evolutionary variant rhyolite was most likely formed by crystal fractionation. |
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