Petrology and Sr-Nd isotope systematics of the Ahobil kimberlite (Pipe-16) from the Wajrakarur field, Eastern Dharwar craton, southern India

Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite (Pipe-16) from the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Tirich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of (i) liquidus mineral composition, (ii) bulk-rock chemistry, and (iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The TDM Nd model age (~2.0 Ga) of the Ahobil kimberlite is (i) significantly older than those (1.5-1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC, (ii) indistinguishable from those of the Mesoproterozoic EDC lamproites, and (iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO₂, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.