Archaean Meta Lamprophyres from Nuggihalli Schist Belt, Hassan District - Karnataka

The Archaean Dharwar greenstone belts of Karnataka predominantly consist of iron enriched tholeiitic Metabasalts, basaltic komatiites, sulphidic chert bands and argillite-graywacke sequence. In Nuggihalli schist belt, in association with mafic-ultramafic rocks, calc-alkaline lamprophyres (Spessartite-Kersantite suite), comparable with Kamblada-Yilgam of Western Australia, have been recorded for the first time. Gold mineralisation is associated with these meta-lamprophyres.

The lamprophyres, in Nuggihalli belt, is a highly banded hornblende-biotite rich quartzo-felspathic rock, forming sill like bodies at the contact of mafic-ultramafic rocks, intensely co-folded along with the enclosing rocks, recording three phases of deformation.

REE values, plotted against MORB values confirm lamprophyric affinity with increased values for Sr, K, Rb, Zr and Y. In Rock/Chondrite values plot, very pronounced increase in values of Ba, Rb, K, P, Zr and Ti is indicated while a pronounced depletion in Nb is brought out, characteristic of Calc-alkaline lamprophyres.

Search for similar rocks in many ultramafic belts in Southern Peninsular Shield will be of great Significance in working out the tectonic evolution ofthese greenstone belts and also serve as favourable target areas for gold exploration.