ORIGIN OF POLYPHASE GNEISSES ASSOCIATED WITH THE KRISHNARAJPET-GREENSTONE-BELT, DHARWAR CRATON

Hornblende, biotite and feldspar rich gneisses (HG, BG and FG respecti vely), in that order of formation are associated with the Krishnarajpe t schist belt in Dharwar craton, a greenstone belt coeval with, and si milar to, the Holenarasipur and Nuggihalli schist belts. These gneisse s show compositional differences with SiO2, K2O and Rb increasing and TiO2, Al2O3, Fe2O3, CaO, MgO, Sr, Zr and Ni decreasing from HG to BG a nd FG. The overall REE is more in BG (SIGMAREE = 332.90), while it is similar in HG (SIGMAREE = 173.36) and FG (SIGMAREE = 159.03) with comp arable LREE/HREE ratios (BG = 12.71; HG = 10.26; FG = 10.95). The over all chemistry of the three phases suggests a single source of origin f or all the phases. While the field characteristics suggest successive origin from HG to BG and FG, the chemical signatures also support this mode of origin from HG to FG through BG. Available geochronological d ata on the gneisses indicate that probably the HG, a partial melt of t he mafic schists of the greenstone belt, was recycled at lower crustal levels in Late Archaean forming BG and FG. The time gap between the p hases appears to be less but no detailed age data exist on the differe nt phases to support this.