Evidence for Neoarchean Basement for the Deccan Volcanic Flows around Koyna-Warna Region, Western India: Zircon U-Pb Age and Hf-isotopic Results

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 (2 sigma errors). The initial Hf-176/Hf-177 values lie in a narrow range (0.281162 to 0.281283) corresponding to epsilon(Hf(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.