Petrology and petrogenesis of Mesoproterozoic lamproites from the Ramadugu field, NW margin of the Cuddapah basin, Eastern Dharwar craton, southern India

Abstract

Petrography, mineral chemistry, and major and trace element data are presented for the newly discovered Mesoproterozoic (1.33–1.43 Ga) lamproites from the Ramadugu field (RLF), at the NW margin of the Paleo-Mesoproterozoic Cuddapah basin, in the Eastern Dharwar craton (EDC), southern India. RLF lamproites are emplaced as dykes, have a NW–SE trend and their petrography reveal the effects of low-temperature alteration. However, their textural features, mineralogy and geochemistry are closely similar to other well-characterised lamproites worldwide, including examples from the Eastern Dharwar craton, Leucite Hills, West Kimberley, Smoky Butte and Labrador. The RLF magmas have undergone varying degrees of olivine + clinopyroxene fractionation; yet their compatible and incompatible trace element concentrations are sufficiently high to signal a primitive character. Incompatible element ratios suggest limited contamination by continental crust. Geochemical evidence indicates the derivation of RLF magmas from metasomatised harzburgite within the garnet stability field. Rare earth element inversion modelling further highlights substantial involvement of the sub-continental lithospheric mantle in their genesis. The RLF lamproites are geochemically similar to the well-known extension-related ultrapotassic lavas from eastern Virunga and western Anatolia, and exclude an affinity with orogenic lamproites, such as those from the Mediterranean region. Bulk-rock geochemical models, recently developed to infer diamond potential, reveals that RLF lamproites are non-prospective. Lamproites of the RLF, together with those from the Krishna lamproite field and Cuddapah basin are interpreted as an expression of extensional events in the Eastern Dharwar craton possibly related to the break-up of the supercontinent of Columbia between 1.5 and 1.3 Ga.