Elusive origin of the subsurface tephra in the Deccan volcanic province, India

We present comprehensive petrography, bulk-rock major element oxide, trace and rare earth element geochemistry, Nd isotopic compositions, and downhole-induced-magnetic field measurements of ~10 and 20-m thick basaltic tephra, which was found beneath the surface in the two deep bore-hole well-cores KBH-8 (Paneri) and KBH-10 (Udgiri), respectively, in the Koyna–Warna region of the Deccan volcanic province. The two bore-holes are located 18 km apart in the north–south direction. Despite the spatial difference, the tephra exhibit identical mineralogy and geochemical patterns, and high magnetic susceptibility. It consists of clinopyroxene, plagioclase and opaque oxides. The geochemical attributes confirm tholeiitic nature of the pre-erupted magma. The enriched high field strength element and rare earth element contents are typical of intraplate volcanic sources that produce tholeiitic flood basalts. Further, the bulk-rock initial Nd isotopic compositions ( ε Nd (t=65 Ma) = +0.24 and +2.02, respectively) and incompatible trace element ratios (Nb/Th≥8 and La/Nb~1) preclude contribution from the continental crust, therefore, consistent with an asthenospheric mantle source. The trace and rare earth element geochemistry of the tephra is rather in contrast to the geochemical attributes of the basaltic lava pile in the Deccan Plateau, and the Reunion Island basalts, as well. Furthermore, the average bulk magnetic susceptibility and the corresponding induced-magnetic field measured in the tephra is significantly high and in stark contrast to the underlying Deccan Trap basalt. Therefore, the origin of the tephra appears inexplicable unless the source of eruption is elsewhere from the present site of deposition.