Imaging mantle lithosphere for diamond prospecting in Southeast India

Abstract

An understanding of the tectonic makeup of an area through study of surface geological features, together with depth information on the nature of the underlying lithosphere, forms the key to diamond exploration strategy. Although diamonds have been reported from the Indian craton for many centuries, the search for their carrier rocks dates back only to the beginning of the twentieth century. This study focuses on a wide area in southeast India, parts of which are sources of both diamondiferous and nondiamondiferous kimberlites and lamproites. Using Ps (SV and SH) and Sp receiver functions, we recovered depth images of the lithospheric mantle beneath southeast India encompassing the Eastern Dharwar-Bastar cratons and the adjoining Eastern Ghats mobile belt. These images reveal the presence of two signifi cant velocity anomalies of contrasting nature at different depths beneath the study region. High-velocity features are observed between 160 and 220 km depth (Lehmann discontinuity depth, or L-depth), and a complex low-velocity contrast layer is delineated at ~80- 100 km depth. Analyses of results from several other studies that include regional geology, geophysics, geochemistry, and geochronology allow us to infer that the positive velocity contrasts at L-depth represent preserved oceanic remnants of a ca. 1.6 Ga paleosubduction event in southeast India. Analysis of selected geothermobarometry data in conjunction with other evidence presented in this study indicates that the craton beneath southeast India is underlain by a thick lithospheric root/keel in excess of 200 km, suggesting an environment conducive to diamond stability. Consequently, we interpret the complex low-velocity contrast layer feature as a midlithospheric discontinuity and not a shallow lithosphere-asthenosphere boundary. The diamond formation potential of the area is discussed in light of a working model that incorporates the Mesoproterozoic paleosubduction scenario (ca. 1.6 Ma event) and subsequent kimberlite and/or lamproite intrusions. Wide regions covering the Godavari graben and adjoining areas are identifi ed as potential zones for diamond exploration. © 2013 Geological Society of America.