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

The stable carbon and oxygen isotopic composition of carbonatites from thre e carbonatite-alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of Deccan province, India, can be placed into two groups, primary and secon dary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely compose d of a mantle having delta(18)O similar to that of mean upper mantle (5 to 8 parts per thousand) but higher delta(13)C ( > -5.5 parts per thousand). T he higher delta(13)C, particularly that of a batch of parent magma for Amba Dongar (-3.4 parts per thousand), suggests the incorporation of recycled i norganic crustal carbon in these carbonatites. As in the plume model for ge neration of continental flood basalts and associated carbonatite-alkaline c omplexes, the fluid-rich rim of the plume head acts as the source for the c arbonatites, so the above incorporation of crustal carbon could have been f acilitated through the migration of CO2-rich, C-13 enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correl ated variations of delta(13)C and delta(18)O in unaltered calcite carbonati tes 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 extrem e enrichment of delta(13)C and delta(18)O in some carbonatites and metasoma tic rocks is a result of postcrystallization alteration process caused by C O2-bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a ma gmatic-hydrothermal fluid could have caused the alteration, whereas rocks o f Mundwara and Sarnu-Dandali appear to have been altered by meteoric-hydrot hermal fluids.