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 considered 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 c1inopyroxenes (augite, sub-calcic augite and magnesian pigeonite).

The Kolar dyke samples are essentially tholeiitic except for one sample which resembles a basaltic komatiite. Mineral, as well as, whole rock chemistry of the dyke samples indicate an iron enrichment trend. The modelling of major and trace elements of Kolar dykes suggest that their parental magmas must have been derived from Fe- enriched sources with higher Fe/Mg ratios than primitive mantle. Source for parental magma to the basaltic komatiite dyke has been inferred as garnet lherzolite, whereas, for all others it could be komatiite with variable Fe/Mg ratios.

The Kolar dyke samples are enriched in light rare earth elements (LREE) and have variable abundances of heavy rare earth elements (HREE) relative to chondrites. The modelling of major, trace, and rare earth element data indicate that the generation of parental magmas took place from sources similar to komatiites, by low percentages (<30%) of partial melting, within the lithosphere at pressures less than 3GPa. Magmas representing the dyke samples could have undergone small extents of fractional crystallisation and assimilation of granitoids. It is suggested that the partial melting of lithospheric sources is related to its stretching and elevation of geothermal gradient, possibly, as a result of intrusion of picrite/kornatiite melts into the lithosphere.