Petrology and geochemistry of the Deccan basalts from the KBH-7 borehole, Koyna Seismic Zone (Western Ghats, India): Implications for nature of crustal contamination and sulfide saturation of magma

Abstract

The Deccan basalt erupted at the K-T boundary and comprises multiple flow units with spatial variations in physiography and geochemistry. The Koyna borehole-7 (KBH-7 of MoES-BGRL) of the Koyna Seismic Zone is situated in the western part of the Deccan continental flood basalt province, where the lava pile is 1251 m thick. Thirty-one samples of four flow units (named FA, FB, FC, FD; 1248.20–1110.78 m) from the lower part and another four flow units (named FV, FW, FX, FY; 202.79–42.85 m) from the upper part of the KBH-7 borehole show significant petrographic, mineral chemical and bulk-rock geochemical variations. Individual flows are characterized by higher abundances of plagioclase and clinopyroxene phenocrysts in their lower than upper parts; conversely, their upper parts are relatively enriched in magnetite and ilmenite. In addition, the flows become amygduloidal near their tops. Seven samples from the lower part of the FB flow unit have relatively higher MgO (≈ 8.94–12.9 wt%) and lower TiO2 (≈ 1.43–1.67 wt%) contents compared to samples from other flow units and are identified as low-Ti high-Mg basalt. Clinopyroxene phenocrysts from low-Ti high-Mg basalts are more magnesian (Mg-number ≈ 62.4 to 82.3) and chromium-rich, and not in equilibrium with plagioclase phenocrysts in the rocks. The magnesian-rich clinopyroxene phenocrysts were probably concentrated in the flow by flow differentiation. The trace element geochemistry indicates that the flows in the KBH-7 borehole are crustally contaminated. Extremely low Nb/Ta (<10) values of a few basaltic samples indicate contamination by the Gondwana sediments (e.g., Gondwana greywacke Nb/Ta ≈ 3.8–12.8) along with the tonalitic crust (Nb/Ta ≈ 5.59–10.2). The major and trace element geochemical character of the KBH-7 basalts can be explained by assimilation and fractional crystallization (AFC) modeling using Archean tonalitic crust and Gondwana sediments as major contaminants. AFC modeling reveals that 6.5–14.5% partial melting of the primitive mantle followed by 64–85% AFC in the crustal magma chamber accounts for the variable geochemical character of the KBH-7 basalts. Despite assimilation by crustal components, the basalts do not show any signs of sulfide supersaturation or segregation of an immiscible sulfide liquid, unlike the Nadezhdinsky formation of the Siberian trap flood basalt that produced the Noril'sk Ni-sulfide deposits. The variable concentration of chalcophile elements (Ni ≈ 11–858 ppm, Cu ≈ 32–342 ppm) or ratios like Cu/Zr (> 1) and Ni/MgO (≈ 1.9–163) of the KBH-7 basalts also indicate the absence of an earlier sulfide liquation of the magma. This may be related to the absence of crustal sulfur from the tonalitic basement rocks or the Gondwana sediments (greywacke) in conjunction with high iron content (FeO(T) ≈ 11.9–15.9 wt%) of the basaltic magma.