Indian Kimberlites and Related Rocks: Petrology and Geochemistry

Abstract Potassic, ultrapotassic, ultramafic rocks occur mostly as dykes and few diatremes in the southern and central Indian cratons The ultramafic diatreme in Majhgawan in central India is (he only diamond-producing primary source at the present time Several occurrences in southern India also contain diamond but are not being commercially exploited Available isotopic age data on these rocks suggest a Proterozoic age (Table 1), a feature that is distinctive compared to other ages on similar rocks barring the Premier pipe in southern Africa We present petrographic, mineralogical and geochemical data on careiully selected samples from several localities Mineralogically there are differences in the content of olivine, phlogopite, perovskite and spinel among the localities In common with occurrences elsewhere, the olivines show a range from 83 to 93 mole percent The macrocrystal olivines are more magnesian Among the phlogopites, a two-fold division on the basis of Mg number is discernible Most of the ilmenites are distinctly less magnesian than the southern African kimberlites Geochemically the samples show a wide variation in major element abundances Despite geographical separation and petrographic differences, Zr and HI maintain a perfect positive correlation In common with similar rocks, all the samples studied show an enrichment of both compatible and incompatible elements Compared to established and accepted kimbeilites of southern Africa and lamproites of Western Australia, the Indian samples show general similarity but also divergences in geochemical parameters This is believed to suggest a transitional nature of the Indian potassic, ultralmafic rocks between kimberlites and lamproites Petrogenesis of the occurrences is discussed