Petrology, geochemistry and genesis of rift-related carbonatites of Ambadungar, India

Abstract

The Ambadungar (Amba Dongar) alkaline carbonatite complex is emplaced in the Deccan traps igneous province. A wide range of carbonatites and alkaline rocks are exposed around Ambadungar. The alkaline rocks have been classified as tinguaite, phonolite and/or phononephelinite, melanephelinite, and syenite and/or nepheline syenite whereas carbonatites vary from calcio-carbonatites to ferro- and silicocarbonatites. The enrichment in large-ion lithophile elements (LILE), P, and rare-earth elements (REE) in carbonatites is considered to result from fractionation of a mantle derived magmatic liquid, i.e. nephelinitic magma, by liquid immiscibility which also produced melanephelinite and/or phononephelinite with high field strength elements (HFSE) such as Ca, Mg, Fe, and Mn in the alkaline silicate liquid fraction. The La:Lu ratios of the carbonatites are typical of igneous rocks and vary between 590 and 1945, similar to many known magmatic carbonatites. The δ¹³C concentration varies between -2 and -8 whereas δ¹⁸O-values vary between 7.7 and 26.8. The δ¹³C concentration is typical of primary igneous carbonatites but δ¹⁸O enrichment is thought to be the result of post-magmatic processes such as interaction with meteoric water and re-equilibration with hydrous fluids at low temperatures.