Petrology and Geochemistry of a Calc-alkaline Lamprophyre-Diorite Dyke from Dancherla Area, in the Vicinity of Wajrakarur Kimberlite Field, Eastern Dharwar Craton

ABSTRACT The Eastern Dharwar Craton hosts several Proterozoic volatilerich mafic-ultramafic alkaline intrusive magmatic rocks. Geological and geochemical studies on such rocks provide significant insights into the nature of the continental lithospheric/sub-lithospheric mantle and Precambrian geodynamic processes. In this study, we present petrology and geochemistry of a newly found calc-alkaline lamprophyre-diorite dyke from Dancherla area in the vicinity of the Wajrakarur Kimberlite Field of the Eastern Dharwar Craton. The lamprophyre consists of a high modal abundance of amphibole (magnesio-hornblende) phenocrysts and microphenocrysts along with clinopyroxene (diopside) microphenocrysts set in a feldspathic groundmass. The inequigranular texture of the lamprophyre gradually transitions into the equigranular (salt and pepper-like) texture of diorite along the trend of the dyke, with amphiboles and feldspar in almost equal proportions in the latter. Changing mineral mode and texture within the dyke points to the derivation of these rocks by crystal fractionation from the same hydrous parental melt. The amphibole and clinopyroxene-dominated crystal fractionation are evident from clustered phenocrysts of these minerals in the lamprophyres, giving rise to the glomeroporphyritic texture. As CaO is preferentially incorporated in clinopyroxene and amphibole, the decrease in CaO contents and CaO/Al2O3 ratios with decreasing MgO, an index of differentiation supports fractional crystallization of clinopyroxene and amphibole. The predominance of calcic amphiboles, elevated Th/Yb and Zr/Y ratios, and significant depletion of high field strength elements suggest a calc-alkaline nature of the Dancherla lamprophyre. High LILE/HFSE in these rocks, like that found in the global calc-alkaline lamprophyres and arc-related magmatic rocks, reveals a subduction-fluid-aided enriched continental lithospheric mantle source of the parental magma. The timing of enrichment of the mantle source is suggested to be Neoarchean, during which the hot oceanic lithospheres accreted to the Western Dharwar Craton leading to the amalgamation of micro-blocks and cratonization of the Dharwar Craton.