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

The results of a Sr isotopic study of coexisting alkaline silicate rocks an d carbonatites of two Cretaceous alkaline complexes of India, Amba Dongar ( Deccan Flood Basalt Province) and Sung Valley (Rajmahal-Bengal-Sylhet Flood Bascalt Province) are reported. The overlapping nature of initial Sr isoto pic ratios of alkaline rocks and carbonatites of both the complexes is cons istent with a magmatic differentiation model. Modelling of initial Sr-87/Sr -86 variation in alkaline rocks of Amba Dongar is consistent with a process of crustal assimilation by the parent magma undergoing simultaneous fracti onal crystallization of silicate rocks and silicate-carbonate melt immiscib ility. A maximum of similar to 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 phlog opite from a carbonatite yielded an age of 106 +/- 11 Ma, which is identica l to the Ar-40-Ar-39 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 liq uid immiscibility for the generation of the Sung Valley. The higher initial Sr-87/Sr-86 ratio for these complexes than that of the Bulk Earth indicate s their derivation from long-lived Rb/Sr-enriched sources. (C) 2000 Elsevie r Science Ltd. All rights reserved.