The geology and geochemistry of the Wadagera kimberlite and the characteristics of the underlying subcontinental lithospheric mantle, Dharwar craton, India

Abstract

The Wadagera Group 1 kimberlite is situated close to the northern bank of the Krishna River in northern Karnataka, India. It is the largest pipe in a cluster discovered by De Beers during an exploration programme in the area in 2002. The pipe has been dated at 1 083.8 +/-5.3 Ma which broadly corresponds to published ages of kimberlites in the adjacent Narayanpet cluster. The pipe was discovered during follow-up of an airborne multispectral scanner survey. Ground magnetic survey and drilling data indicate that the pipe has an area of approximately 6 ha. The kimberlite intrudes exposed Peninsular Gneiss Complex of the Indian Shield and limited petrographic analysis suggests that only magmatic kimberlite has been preserved. The kimberlite surface sample shows extreme calcretisation and does not yield any mantle garnets. Garnet xenocrysts are recovered in drill samples from about 20 m depth. This paper analyses the mantle derived xenocrysts from the Wadagera kimberlite to propose a Subcontinental Lithospheric Mantle (SCLM) architecture sampled by the kimberlite. The major element garnet mineral chemistry represents a Ti lherzolite population. This and other major element systematics of the xenocryst population indicates melt metasomatism of the SCLM sampled. Harzburgitic garnets are almost absent.There is also a very strong chromiteclinopyroxene-garnet-equilibrium (CCGE) trend, which together with the spinel data suggests that the kimberlite has sampled fertile mantle at relatively shallow depth. The major element geochemical data supports the absence of diamond reported in this kimberlite.