Platinum-group elements and Nb–Ta geochemistry of the Deccan basalts from Kumbharli and other Ghat sections and the Koyna Seismic Zone, Maharashtra (India): crustal contamination and implications for sulfide saturation of the magma

The Deccan basalts from the Kumbharli, Nadlapur, Bijaynagar, Hazarmacchi and Surli Ghat sections (surface samples) and the Koyna borehole-7 (KBH-7, subsurface samples) from Maharashtra (western India) demonstrate a two-stage crystallization process. Olivine and chromite were crystallized earlier in a shallow crustal magma chamber while clinopyroxene and plagioclase phenocrysts were crystallized in the subvolcanic conduits forming the crystal mush and carried to the surface by the evolved Fe–Ti-rich ascending basaltic magma. The high modal abundance of plagioclase at the top and clinopyroxene at the bottom part of the Kumbharli Ghat section is responsible for an increase in Sr and a decrease in Sc content towards the upper flows. The surface and subsurface basalt samples contain Pd = 5–31 ppb, Pt = 4–15 ppb, Ir = 1–4 ppb, Ru = 1–3 ppb and Rh = 3–5 ppb. Negative relations of Pd/Ir, Pd/Ru and Pd/Rh with MgO from both sets of samples indicate partitioning of Ir, Ru and Rh to early fractionated cumulus chromite (or olivine). Magnetite is a late crystallizing phase from the evolved Fe–Ti-rich basaltic magma. With an increasing modal abundance of magnetite, there is an increase in FeO total , TiO 2 and V with Pd indicating the fractionation of Pd by magnetite. The geochemical characters show that the surface basalt samples are less contaminated compared with the subsurface KBH-7 basalts. High Nb (6.8–13.6 ppm) and low Ta (0.2–0.37 ppm) content and elevated Nb/Ta ratios of the surface samples from the ghat sections indicate the possible interaction of the basaltic magma with the carbonate metasomatized subcontinental lithospheric mantle. On the other hand, a high degree of partial melting with limited crustal contamination can also elevate the Nb/Ta ratios of the basalts from the ghat sections. Deccan basalts from the study areas do not bear the signature of sulfide supersaturation and subsequent immiscible sulfide segregation. This is evidenced by their high chalcophile element concentrations (Ni = 79–103 ppm, Cu = 121–259 ppm) and ratios like Cu/Zr ( ≥ 1) and Ni/MgO (13.7–21.8) with low Cu/Pd (2140–29 938) compared with other Ni–Cu sulfide mineralized continental flood basalts. The lack of crustal sulfur in the various contaminants is perhaps the main reason for this sulfide-undersaturated character of the Deccan basalts of the current study.