GEOCHEMISTRY AND PETROGENESIS OF DOLERITE DYKES IN AND AROUND KOLAR SCHIST BELT, SOUTH-INDIA

The Kolar Schist Belt and adjoining areas of Dharwar Craton are consid ered to be accreted terranes. Two distinct sets of dolerite (diabase) dykes, trending E-W and N-S are found in the area. The major minerals in the dykes are plagioclase feldspars (50-65 mol.% anorthite) and cli nopyroxenes (augite, sub-calcic augite and magnesian pigeonite). The K olar dyke samples are essentially tholeiitic except for one sample whi ch resembles a basaltic komatiite. Mineral, as well as, whole rock che mistry of the dyke samples indicate an iron enrichment trend. The mode lling of major and trace elements of Kolar dykes suggest that their pa rental magmas must have been derived from Fe- enriched sources with hi gher Fe/Mg ratios than primitive mantle. Source for parental magma to the basaltic komatiite dyke has been inferred as garnet lherzolite, wh ereas, for all others it could be komatiite with variable Fe/Mg ratios . The Kolar dyke samples are enriched in light rare earth elements (LR EE) and have variable abundances of heavy rare earth elements (HREE) r elative to chondrites. The modelling of major, trace, and rare earth e lement data indicate that the generation of parental magmas took place from sources similar to komatiites, by low percentages (< 30%) of par tial melting, within the lithosphere at pressures less than 3GPa. Magm as representing the dyke samples could have undergone small extents of fractional crystallisation and assimilation of granitoids. It is sugg ested that the partial melting of lithospheric sources is related to i ts stretching and elevation of geothermal gradient, possibly, as a res ult of intrusion of picrite/komatiite melts into the lithosphere.