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. on June 7, 2015 lithosphere.gsapubs.org Downloaded from DAS SHARMA AND RAMESH 332 www.gsapubs.org | Volume 5 | Number 4 | LITHOSPHEREnamely, the Bastar and the Eastern Dharwar cratons, using Ps (P-to-s conversions) and Sp (S-to-p conversions) receiver functions (Vinnik, 1977;Farra and Vinnik, 2000). Based on these seismological results and available geological, geochemical, geochronological, and thermobarometric data, we present a comprehensive geodynamic model to illustrate diamond genesis in the cratonic blocks of southeast India. We also use these results to delineate prospective diamondiferous zones in the study. Our results show that the present-day lithosphere beneath southeast India is indeed thick and extends beyond the well-defi ned diamond stability fi eld.

Continental scale body wave tomography of India: Evidence for attrition and preservation of lithospheric roots

“…The regions of Shillong plateau (H4) and significant portions of the Dharwar craton (H1) are also underlain by high velocity roots. Earlier, the high velocity jumps across the seismic discontinuities between 160 and 220 km observed using receiver functions were interpreted as preserved thick roots in the regions marked by H1 [ Sharma and Ramesh , ]. The high velocity region H3 is the most consistent feature observed in various tomography studies [ Zhou and Murphy , ; Li et al ., ].…”

Mineralogy of the P-12 K-Ti-richterite diopside olivine lamproite from Wajrakarur, Andhra Pradesh, India: implications for subduction-related magmatism in eastern India

“…1.6 Ga). Das Sharma and Ramesh (2013) suggest that a thick lithospheric root underlies southeast India, with the Archean Eastern Dharwar craton and the Proterozoic Eastern Ghats mobile belt being underlain by a relict subducted slab within the upper mantle. The depth (160-220 km) of this feature also coincides with the diamond stability field.…”

“…Das Sharma and Ramesh (2013) have interpreted data obtained primarily from passive seismological studies of southeast India, together with constraints from several other geophysical geological, and geochronological studies, to indicate the preservation in the subcontinental lithospheric mantle of relict subducted oceanic slab material at depths of 160-220 km. The origins of this material are related to suturing of the Eastern Dharwar craton and Eastern Ghats mobile belt possibly during the Mesoproterozoic (ca.…”

“…Diamonds (◊), circles (o) and crosses (x) in the figure refer to the locations of kimberlites, lamproites and deformed alkaline rocks and carbonatites (DARC), respectively. Bu Bunder lamproites, M Majhgawan lamproite field, B Basna kimberlite field, Na Nawapara lamproite field, Mp Mainpur kimberlite field, Tk Tokapal kimberlite field, Ra Ramadugu lamproite field, N Narayanpet kimberlite field, R Raichur kimberlite field, T Tungabhadra kimberlite field, W Wajrakarur kimberlite field, Nl Nallamalai lamproite field, K Krishna lamproite field, and D Damodar valley lamproites (modified after Leelanandum et al 2006;Burke and Khan 2006;Das Sharma and Ramesh 2013) ascent of a mantle plume causing partial melting of the sources. Partial melting is more probably related to decompressional melting resulting from uplift and extensional tectonics, as intially suggested by Chalapathi Rao et al (2004).…”

Role of trace element Pedogeochemistry in Diamond Exploration- A first Report from Lattavaram Kimberlite Cluster, Wajrakarur Field, Eastern Dharwar Craton, southern India

“…In addition, the Cuddapah Basin of Proterozoic age, has recorded more than 47 occurrences of lamproites in its northwestern and northeastern margins, distributed in five lamproite fields: Banganapalle Lamproite Field (BLF), Krishna Lamproite Field (KLF), Nallamalai Lamproite Field (NLF), Ramadugu Lamproite Field (RLF), Vattikodu Lamproite Field (VLF) and Somasila Lamproite Field (SLF) (Naqvi, 2005;Sridhar and Rau, 2005;Alok Kumar et al, 2013;Chalapathi Rao et al, 2014, Ahmed et al, 2016. The WKF, endowed with more than 45 kimberlite occurrences, is the largest among all kimberlite fields in its areal extent spanning over ~9500 km 2 (Das Sharma and Ramesh, 2013;Shaikh et al, 2016).…”