Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, Eastern Dharwar Craton, southern India: insights from groundmass mineralogy, bulk-chemistry and perovskite oxybarometry

Abstract

The Mesoproterozoic Narayanpet Kimberlite Field (NKF) is located ~200 km north of the well-known Wajrakarur Kimberlite Field (WKF) in the Eastern Dharwar Craton, southern India. Whereas a majority of the WKF occurrences are diamondiferous and contain mantle xenoliths and xenocrysts, their paucity is a characteristic feature of the NKF. This has been attributed alternately to the derivation of NKF magmas from a shallower depth, or to variability in thickness of the Sub-Continental Lithospheric Mantle (SCLM) beneath the Eastern Dharwar Craton. Recently, exploration by De Beers resulted in the discovery of a number of new kimberlite occurrences from the NKF, with some of their geochemical features and radiogenic isotope systematics subsequently becoming available. In this paper, we present detailed petrography, groundmass mineral composition and new bulk-rock geochemistry data for a number of NKF rocks and attempt to further constrain their origin. We also investigate the influence of redox conditions on diamond prospectivity by estimating oxygen fugacity (fO₂) from Fe–Nb oxybarometry on NKF groundmass perovskites. We identify for the first time in the NKF the presence of volcaniclastic (fragmental textured) facies kimberlite belonging to the diatreme portion of the intrusion. Rarity of olivine macrocrysts and the presence of diopside are hallmarks of the NKF kimberlites, in contrast to archetypal kimberlites of southern Africa. Mineral components of the groundmass display features that are characteristic to both archetypal kimberlites and to orangeites, and it is not straight forward to apply conventional mineral-genetic schemes in the nomenclature of the NKF pipes. Low fO₂ of the NKF magma (ΔNNO (nickel–nickel oxide) = −1.9 to −3.2), indistinguishable from that of diamondiferous kimberlites world-wide, indicates that redox conditions were favourable for diamond prospectivity, and that magmatic emplacement could, instead, have played a major role in their low diamond potential.