Geology, Petrology and Geochemistry of Narayanpet Kimberlites in Andhra Pradesh and Karnataka

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Abstract Apart from the well known occurrence of kimberlites in south India at Wajrakarur in Anantapur district, kimberlites were located by Geological Survey of India (GSI), during 1984-85 around Maddur and Narayanpet areas in Mahabubnagar district of Andhra Pradesh. Landsat imagery and photo-interpretation studies followed by intense geological traverses including stream sediment surveys, launched during 1994-96, led to the discovery of 20 additional kimberlites in the western part of Mahabubnagar district, Andhra Pradesh and in the adjoining Gulbarga district, Karnataka, thus reporting kimberlite for the first time from the state of Karnataka. A total of 30 kimberlites have been reported in this field named as "Narayan pet Kimberlite Field" (NKF). The kimberlites are emplaced along E-W and NW-SE trending major faults and associated NE-SW tear faults in the Peninsular Gneissic Complex carrying enclaves of Dharwar greenstones of Gadwal schist belt. These kimberiites occur both as small bodies of a few metres across and also as dykes with a strike length of about 2 km. The characteristic mineral assemblages and textures indicate that these bodies correspond to hypabyssal facies or root zone kimberlites. The indicator mineral suite present in these rocks are olivine, chrome diopside, magnesian ilmenite, chrome spinel, ai1d rare pyrope garnet Mantle xenoliths are rare, nevertheless spinel Iherzolite; abundantly found in two kimberlites. All the kimberlites of NKF are depleted in SiO2 (~ 35%), potassic (K2O >Na2O) and magnesium rich (MgO >20%). The REE abundance of these kimherlites is consistent with their derivation by partial melting of mantle and subsequent fractional crystallisation of the melt. The similarity of petrological and geochemical characters of these kimberlites indicate that all the kimberlites of NKF are derived from similar source in the upper mantle. Preliminary processing and testing of some of the kimberlite bodies have not yielded any diamonds so far. However, the reported stray diamond finds in the Krishna and Bhima River gravels indicate the presence of diamondiferous kimberlites further south and southwest of the area.