Age and geochemistry of the Kamthai carbonatites, Rajasthan, western India

The Kamthai carbonatites form part of the Sarnu-Dandali alkaline complex (SDAC) of Rajasthan, western India. The SDAC is one of several alkaline intrusive complexes emplaced prior to the Deccan continental flood basalt eruptions. Considered as one of the earliest Deccan-Reunion plume related magmatic activities, the rocks of the complex hold clues to many tectonomagmatic processes such as plume-lithosphere interaction, mantle melting prior to flood basalt volcanism, and carbonatite-plume relationship, apart from the outstanding questions pertaining to the origin of carbonatites themselves, and their association with alkaline silicate rocks. To understand some of these processes vis-à-vis the evolution of the complex, we have carried out a detailed field, petrographic, geochronological (⁴⁰Ar/³⁹Ar), geochemical, and Sr-Nd-Pb-C-O isotopic investigation. Phlogopites from carbonatites yield an age of 68.6 Ma, identical to the ages determined for the three associated phonolite dykes. Interestingly, an earlier study reports the presence of older (89-86 Ma) subvolcanic and volcanic bodies in the complex, thus suggesting recurrent alkaline magmatism. Carbonatites of Kamthai occur as veins, dykes, and small plugs, along with dykes/plugs of ijolite, nephelinite, syenite and phonolite etc. The SDAC intrudes into the basement made up of Malani Rhyolites. The stable C-O isotopic compositions of unaltered carbonatites (δ¹³C_PDB= -6.6 to -4.6 ‰; δ¹⁸O_SMOW=5.5 to 9.5 ‰), which are predominantly calcite carbonatites, not only confirm the magmatic nature of the rocks but also show evidence of fractional crystallization. The chondrite-normalized rare-earth element patterns of the carbonatites and alkaline silicate rocks show LREE enriched patterns, with the former possessing abnormally high contents of LREE. The average (⁸⁷Sr/⁸⁶Sr)_i and ε_Nd(t=68.5 Ma) for carbonatites are 0.7043±0.0001 and 2.4±0.2, respectively, which are indistinguishable from those for the alkaline silicate rocks (⁸⁷Sr/⁸⁶Sr)_i= 0.7045±0.0003; ε_Nd(t)=2.4±0.4), which suggests common parentage. All these data point towards a petrogenetic link between the 68.6 Ma carbonatites and alkaline silicate rocks of the SDAC, either through liquid immiscibility or fractional crystallization of a common parental magma. Overlapping initial Sr-Nd isotopic ratios of these rocks with those of the least contaminated Deccan lava flows and the Reunion island rocks suggest a possible genetic link between the SDAC and the Deccan-Reunion plume.