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 ε_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.

Published online 22 December 2016.The Southern Granulite terrane, southern India, comprises a vast exposure of deep crust and forms a key region in the reconstruction of the Gondwana supercontinent. An E-W–trending crustal-scale shear zone, the Palghat-Cauvery suture zone system, which formed during the late Neoproterozoic–Cambrian (0.75–0.50 Ga), marks a prominent terrane boundary separating terranes with a predominantly late Neoarchean (ca. 2.5 Ga) regional granulite metamorphism to the north from those with an Ediacaran–Cambrian (0.63–0.50 Ga) regional granulite metamorphism to the south. Focusing on the younger granulite metamorphic domains, we present here new zircon U-Pb ages and Hf isotopic compositions for 11 charnockite orthogneisses from the Madurai, Trivandrum, and Nagercoil blocks and contribute to the resolution of the age of their magmatic protoliths. This study shows that the charnockite orthogneisses south of the Palghat-Cauvery suture zone relate to a minimum of four distinct episodes of felsic magmatism centered at: ca. 2.62–2.46 Ga, ca. 2.05–1.84 Ga, ca. 1.0–0.9 Ga, and ca. 0.80–0.76 Ga, pertaining to the Siderian, Orosirian, and Tonian Periods. Hafnium isotope analyses of zircon grains from the charnockite gneisses suggest that the protoliths of the ca. 2.05–1.98 Ga gneisses from the Trivandrum and Nagercoil blocks and the ca. 1.0–0.9 Ga gneisses along the southeastern Madurai block involved a significant juvenile magma component, while the protoliths of charnockite gneisses elsewhere in the Madurai block formed mainly through recycling of older crust up to ca. 3.2 Ga. A regional granulite-facies metamorphic imprint during the Ediacaran–Cambrian marked an advanced stage in the amalgamation of the Madurai, Trivandrum, and Nagercoil blocks into the East African orogen and its collision with the Dharwar craton.T. Vijaya Kumar, Y.J. Bhaskar Rao, Diana Plavsa, Alan S. Collins, J.K. Tomson, B. Vijaya Gopal, and E.V.S.S.K. Bab