Mineralogy and geochemistry of kimberlites NK-2 and KK-6, Narayanpet Kimberlite Field, eastern Dharwar craton, southern India: evidence for a transitional kimberlite signature

Abstract

We have carried out a mineralogical and geochemical study on two previously uncharacterized kimberlites of coherent facies, NK–2 and KK–6, from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton (EDC), southern India. Both these kimberlites have a distinct inequigranular texture resulting from two generations of variously sized olivine macrocrysts and groundmass microphenocrysts. Other groundmass phases include clinopyroxene (diopside), serpentine, phlogopite, calcite, spinel, perovskite and apatite. Electron-microprobe analyses of various groundmass minerals support their kimberlitic affinity. Primitive Mg-numbers, Ni and Cr contents and high enrichment in incompatible trace elements are hallmarks of these kimberlites. From major- and trace-element geochemistry, crustal contamination seems to be minimal. The LILE and HFSE display excellent correlations amongst themselves and also with each other, thereby implying limited effects of post-emplacement hydrothermal alteration. Ratios of highly mobile and immobile incompatible trace elements (e.g., La, Ce, Nb, Ba, Pb and Th) strongly resemble those of transitional kimberlites of southern Africa and are confined to the arrays of mixing lines between Group-I and Group-II kimberlites. The REE abundances of NK–2 and KK–6 kimberlites suggest variable degrees of partial melting. Petrogenetic modeling depicts them to be products of a greater degree of melting. A mixed Group-I and Group-II “southern African” kimberlite source is inferred for the generation of NK–2 and KK–6 magmas, as well as for several other EDC kimberlites. This can explain several overlapping petrological and geochemical characters of Group-I and Group-II kimberlites displayed by the kimberlites of southern India. Lower whole-rock Ti and Fe contents in the NK–2 and KK–6, compared to those in other non-diamondiferous kimberlites from the NKF, suggests that they are promising diamond prospects; we recommend their bulk processing.