Evolution of carbonatite complexes of the Deccan flood basalt province: Stable carbon and oxygen isotopic constraints

Abstract

The stable carbon and oxygen isotopic composition of carbonatites from three carbonatite‐alkaline complexes (Amba Dongar, Mundwara and Sarnu‐Dandali) of Deccan province, India, can be placed into two groups, primary and secondary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely composed of a mantle having δ18O similar to that of mean upper mantle (5 to 8‰) but higher δ13C (>−5.5‰). The higher δ13C, particularly that of a batch of parent magma for Amba Dongar (−3.4‰), suggests the incorporation of recycled inorganic crustal carbon in these carbonatites. As in the plume model for generation of continental flood basalts and associated carbonatite‐alkaline complexes, the fluid‐rich rim of the plume head acts as the source for the carbonatites, so the above incorporation of crustal carbon could have been facilitated through the migration of CO2‐rich, 13C enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correlated variations of δ13C and δ18O in unaltered calcite carbonatites in all three complexes are consistent with the formation of these rocks by fractional crystallization from CO2‐rich carbonate magmas, derived from parent carbonated silicate magmas through liquid immiscibility. The extreme enrichment of δ13C and δ18O in some carbonatites and metasomatic rocks is a result of postcrystallization alteration process caused by CO2‐bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a magmatic‐hydrothermal fluid could have caused the alteration, whereas rocks of Mundwara and Sarnu‐Dandali appear to have been altered by meteoric‐hydrothermal fluids.