Abstract
The Newania carbonatite complex of Rajasthan, India is one of the few dolomite carbonatites of the world, and oddly, does not contain alkaline silicate rocks thus providing a unique opportunity to study the origin and evolution of a primary carbonatite magma. In an attempt to characterize the mantle source, the source of carbon, and the magmatic and post-magmatic evolution of Newania carbonatites, we have carried out a detailed stable carbon and oxygen isotopic study of the complex. Our results reveal that, in spite of being located in a metamorphic terrain, these rocks remarkably have preserved their magmatic signatures in stable C and O isotopic compositions. The δ13C and δ18O variations in the complex are found to be results of fractional crystallization and low temperature post-magmatic alteration suggesting that like other carbonatites, dolomite carbonatites too fractionate isotopes of both elements in a similar fashion. The major difference is that the fractional crystallization of dolomite carbonatites fractionates oxygen isotopes to a larger extent. The modes of δ13C and δ18O variations in the complex, −4.5 ± 1‰ and 7 ± 1‰, respectively, clearly indicate its mantle origin. Application of a multi-component Rayleigh isotopic fractionation model to the correlated δ13C versus δ18O variations in unaltered carbonatites suggests that these rocks have crystallized from a CO2 + H2O fluid rich magma, and that the primary magma comes from a mantle source that had isotopic compositions of δ13C ~ −4.6‰ and δ18O ~ 6.3‰. Such a mantle source appears to be a common peridotite mantle (δ13C = −5.0 ± 1‰) whose carbon reservoir has insignificant contribution from recycled crustal carbon. Other Indian carbonatites, except for Amba Dongar and Sung Valley that are genetically linked to Reunion and Kerguelen plumes respectively, also appear to have been derived from similar mantle sources. Through this study we establish that dolomite carbonatites are generated from similar mantle source like other carbonatites, have comparable evolutionary history irrespective of their association with alkaline silicate rocks, and may remain resistant to metamorphism.
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