Strontium isotope systematics of Amba Dongar and Sung Valley carbonatite-alkaline complexes, India: evidence for liquid immiscibility, crustal contamination and long-lived Rb/Sr enriched mantle sources

Abstract

The results of a Sr isotopic study of coexisting alkaline silicate rocks and carbonatites of two Cretaceous alkaline complexes of India, Amba Dongar (Deccan Flood Basalt Province) and Sung Valley (Rajmahal–Bengal–Sylhet Flood Basalt Province) are reported. The overlapping nature of initial Sr isotopic ratios of alkaline rocks and carbonatites of both the complexes is consistent with a magmatic differentiation model. Modelling of initial 87Sr/ 86Sr variation in alkaline rocks of Amba Dongar is consistent with a process of crustal assimilation by the parent magma undergoing simultaneous fractional crystallization of silicate rocks and silicate–carbonate melt immiscibility. A maximum of ∼5% crustal contamination has been estimated for the parent magma of Amba Dongar, the effect of which is not seen in the Sr isotope ratio of carbonatites generated by liquid immiscibility. A two point Rb–Sr isochron of the Sung Valley carbonatites, pyoxenite and a phlogopite from a carbonatite yielded an age of 106±11 Ma, which is identical to the 40Ar– 39Ar age of this complex. The same age for the carbonatites and the alkaline silicate rocks, similar initial Sr ratios and the higher Sr concentration in the former than the latter favour the hypothesis of liquid immiscibility for the generation of the Sung Valley. The higher initial 87Sr/ 86Sr ratio for these complexes than that of the Bulk Earth indicates their derivation from long-lived Rb/Sr-enriched sources.