Deep boreholes around the Koyna-Warna seismic zone in the southwestern part of the Deccan Volcanic Province (DVP) enable a rare access to the basement beneath the thick (typically 1-2 km) lava pile, the nature of which has been enigmatic for long. Utilizing the drill core from borehole KBH-1 near Rasati village, we present here in situ Laser Ablation – Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and LA-MC (multi collector)-ICPMS zircon U-Pb age and Hf-isotopic composition for two samples along with a brief description of the petrology and geochemistry of the representative lithounits of the basement section. The KBH-1 basement section comprises predominantly grey migmatite gneisses of granodiorite, tonalite and quartz monzodiorite composition apart from minor pink monzogranite. The grey gneisses show geochemical affinity to Neoarchean Tonalite-Trondhjemite-Granodiorite (TTG) suites and modern calc-alkalic granitoids. Zircons from a granodiorite and a monzogranite samples yield consistent U-Pb ages of 2710± 63 Ma and 2700±49 Ma (2σ errors). The initial 176Hf/177Hf values lie in a narrow range (0.281162 to 0.281283) corresponding to εHf(T) values of +3.7 to +8.0 indicating that the magmatic precursors of the KBH-1 gneisses represent juvenile magmatism around 2700 Ma. In terms of the composition and age, a correlation between the gneisses in the KBH-1 borehole and the Neoarchean basement gneisses of the Eastern Dharwar Craton (EDC) is emphasized arguing for the extension of the EDC to the Koyna-Warna region.
During the last quadrennial period, (2012-2016), many aspects of the Precambrian geology of the Indian peninsula were revisited by Indian geoscientists and their collaborators with the advantage of modern analytical techniques for geochronology as well as new perspectives on global geodynamics in space and time. Important contributions were made to the Archean and Proterozoic geological evolution of the Dharwar, Singhbhum, Bundelkhand and Bastar cratons, the granulite gneiss terrains, especially the Southern Granulite Terrain (SGT) and the Eastern Ghats Granulite Terrain (EGGT) and also the Proterozoic sedimentary Basins of peninsular India. Significantly, these studies have provided evidence for Paleo-to Eoarchean crustal remnants apart from placing precise age constraints on geological events. Most authors favoured the interpretation of Archean geodynamics in a plate tectonics conceptual framework for the assembly of Indian terranes in the supercontinent reconstructions, as reviewed on the basis of a short compilation of published research articles.
Abstract. Anaerobic microbial sulfate reduction and oxidative sulfur cycling have been studied in long sediment cores mainly acquired as part of IODP explorations. The most remarkable observation in many of these studies is the existence of an active sulfur cycle in the deep subsurface sediments that have very low organic carbon content and are presumably refractory. Here we investigate the interstitial sulfate concentrations and sulfur isotope ratios in a 290 m long core collected from the eastern Arabian Sea at a water depth of 2663 m. Continuous decrease in pore water-sulfate concentrations with depth (up to 75 mbsf) coupled with enrichment in δ34SSO4 values suggests organoclastic sulfate reduction (OSR) processes attributed to the activity of sulfate-reducing bacteria (SRB) and retention of labile organic substrates amenable to the SRBs. Below a depth of 75 mbsf, the absence of a further reduction in sulfate concentrations indicates insufficient labile substrate to drive SRB. An increase in sulfate concentrations at the deeper subsurface (below 128.5 mbsf) coupled with decreasing δ34SSO4 values may be attributed to a ferric-oxyhydroxide driven oxidation of Fe-sulfide. This study reveals that even under deep aerobic water columns, organic matter may continue to be a source of labile organic substrates at significantly deeper subsurface. Enhanced sulfate concentrations in the deeper depths may be attributed to the oxidation of sulfides via ferric-oxyhydroxides buried deep within the sediment. A microbiological investigation may reveal further details of the sulfur cycle at the deep surface.
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