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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.

The chemical and isotopic compositions of thermal springs located along the Son-Narmada-Tapti (SONATA) mega lineament in central India have been investigated. The issuing temperatures of the thermal waters vary from 31 • to 89 • C for the thermal springs and 24 • to 25 • C for the cold springs. These thermal springs are located on the Archean Chotanagpur Gneissic Complex (CGC) in the eastern part of peninsular India. The thermal springs are mostly alkaline in nature with pH varying from 7.5 to 9.5. Piper diagram suggests that the chemistry of the thermal waters is compatible with the granitic host rocks through which the waters circulate. Mineral saturation index suggests that the thermal waters are saturated with cristobalite and quartz at lower temperatures (less than ∼130 to 150 • C), and calcite and forsterite at higher temperatures (∼160 • to 250 • C). The estimated reservoir temperature based on chemical geothermometers is in the range of 132 •-265 • C, which favours a medium enthalpy geothermal system. Oxygen isotope fractionation of Bakreswar and Tantloi thermal springs highlights a higher reservoir temperature than estimated by chemical geothermometer. Positive gravity anomalies over Bakreswar and Tantloi areas strongly suggest a basement/mantle upliftment or mafic intrusion which could account for the heat source close to the surface. However, the large negative gravity anomaly depression around the Surajkund and Katkamsandi thermal springs indicates presence of deep seated faults. Keywords. Hazaribagh thermal spring; Bakreswar and Tantloi thermal spring; geochemistry of thermal water.

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