Spherulites and thundereggs from pitchstones of the Deccan Traps: geology,petrochemistry, and emplacement environments |
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Authors: | Pooja?V?Kshirsagar Email author" target="_blank">Hetu?C?ShethEmail author Sheila?J?Seaman Badrealam?Shaikh Poonam?Mohite Trupti?Gurav D?Chandrasekharam |
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Institution: | (1) Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai, 400076, India;(2) Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA; |
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Abstract: | Spherulites and thundereggs are rounded, typically spherical, polycrystalline objects found in glassy silicic rocks. Spherulites
are dominantly made up of radiating microscopic fibers of alkali feldspar and a silica mineral (commonly quartz). They form
due to heterogeneous nucleation in highly supercooled rhyolitic melts or by devitrification of glass. Associated features
are lithophysae (“stone bubbles”), which have an exterior (sometimes concentric shells) of fine quartz and feldspar, and internal
cavities left by escaping gas; when filled by secondary silica, these are termed thundereggs. Here, we describe four distinct
occurrences of spherulites and thundereggs, in pitchstones (mostly rhyolitic, some trachytic) of the Deccan Traps, India.
The thundereggs at one locality were previously misidentified as rhyolitic lava bombs and products of pyroclastic extrusive
activity. We have characterized the thundereggs petrographically and geochemically and have determined low contents of magmatic
water (0.21–0.38 wt.%) in them using Fourier transform infrared spectroscopy. We consider that the spherulite-bearing outcrops
at one of the localities are of lava flows, but the other three represent subvolcanic intrusions. Based on the structural
disposition of the Deccan sheet intrusions studied here and considerations of regional geology, we suggest that they are cone
sheets emplaced from a plutonic center now submerged beneath the Arabian Sea. |
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