Isotope and rare earth element chemistry of carbonatite-alkaline complexesof Deccan volcanic province: implications to magmatic and alteration processes

Results of different isotopic and trace element studies on three carbonatit e-alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Decca n flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0 +/- 0.3 Ma by the Ar-40-Ar-39 met hod. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting the ir derivation from a common parent magma, probably through liquid immiscibi lity. The rare earth element abundance in these rocks also supports the liq uid immiscibility hypothesis. Further investigation revealed that the paren t magma of this complex has been contaminated (similar to 5%) by the lower crustal material, which is clearly reflected in the initial Sr-87/Sr-86 var iation of alkaline rocks but not in the carbonatites. Sr study also suggest s that the mantle source of Amba Dongar like the other two complexes is a R b/Sr enriched source. The temporal and spatial relationships of all the thr ee complexes with the Deccan flood basalts support the hypothesis of reunio n plume origin for these. Fractional crystallization and subsequent hydroth ermal/meteoric alteration are found to have controlled the stable carbon an d oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have i ncorporated recycled crustal carbon. In a plume origin scenario such incorp oration indicates the entrainment of C-13-enriched subcontinental lithosphe ric mantle by the plume. (C) 2000 Elsevier Science Ltd. All rights reserved .