Limited crustal contamination in large igneous province basalts: Sr-Nd-Pb-Os isotope evidence from the Western Ghats, Deccan Traps

The origin of basalts from the Western Ghats lava sequences of the Deccan Large Igneous Province (LIP) was investigated through combined major and trace element and isotopic analysis. Sr-Nd-Pb isotopic compositions are strongly variable and show a time-related shift from enriched, crustal-like values for the late Cretaceous Kalsubai and Lonavala Subgroup basalts to more depleted mantle-like values for the early Paleogene Wai Subgroup basalts. In contrast, Os isotopic data are relatively uniform from base to top of the Western Ghats (initial 187Os/188Osi at 66 Ma: 0.12-0.21) and generally similar to those of the present-day Réunion mantle plume. The basalts from the Wai Subgroup Ambenali Formation (187Os/188Osi 0.120) may show a moderate contribution from the sub-continental lithospheric mantle. In general, the combined isotopic and trace element compositions and 187Os/188Osi in particular show that assimilation of the Proterozoic to Archean Indian crust was generally low (<3 wt.% of starting magma mass) and did not exceed 8 wt.% in any of the studied lavas. Similar results have been previously reported for other Phanerozoic LIPs. Therefore, we suggest that the emplacement of LIPs as short-lived eruptive pulses interrupted by relatively longer hiatus periods possibly hindered effective and prolonged heating of the basement rocks above their solidus temperature. Most likely, this obstacle to crustal contamination was particularly effective for the Wai basalts, which are interlayered with numerous sedimentary or paleosol strata probably formed during periods of volcanic quiescence when the crustal basement may have returned to low sub-solidus temperatures.