A mineralogical–geochemical–petrological study of deformed carbonatites from Padu–Nongryngkoh Jaintia and East Khasi Hills, Meghalaya, northeastern India

This study presents the first analyses of the mineral chemistry, geochemistry and petrogenesis of magnesio-carbonatite dykes from the recently identified Padu–Nongryngkoh region, Meghalaya (India). In the field, carbonatites occur as dykes emplaced in the intensely deformed and layered basement of gneissic rocks. Padu carbonatites consist primarily of calcites and dolomites along with varied proportions of apatite, forsterite, phlogopite, spinel, pyrochlore, ilmenite, magnetite and zircon. Apatites from these carbonatites show magmatic characteristics with marked enrichment of chlorine and heavy rare earth elements (REEs). However, these carbonatites display lower REE concentrations compared to the average REE abundances of global carbonatites, and are marked by extreme depletion of Ni, Ti, Rb, Nb, Ta, Zr and Hf. These carbonatites are associated with an episodic network of dykes and sills in the Padu–Nongryngkoh–Jarain area formed by possible upwelling of the Kerguelen Plume. Low REE concentrations in the Padu carbonatites may be due to either lack of REE replenishment in the magma source, or due to a differentiated alkaline hydrated melt as the source for these rocks.