Origin of Polyphase Gneisses Associated with the Krishnarajpet Greenstone Belt, Dharwar Craton

Hornblende, biotite and feldspar rich gneisses (HG, BG and FG respectively), in that order of formation are associated with the Krishnarajpet schist belt in Dharwar craton, a greenstone belt coeval with, and similar to, the Holenarasipur and Nuggihalli schist belts. These gneisses show compositional differences with SiO₂, K₂O and Rb increasing and TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Sr, Zr and Ni decreasing from HO to SO and FG. The overall REE is more in BG (ΣREE = 332.90), while it is similar in HG (ΣREE = 173.36) and FG (ΣREE = 159.03) with comparable LREE/HREE ratios (BG = 12.71; HG = 10.26; FG = 10.95). The overall chemistry of the three phases suggests a single source of origin for all the phases. While the field characteristics suggest successive origit from HG to BO and FG, the chemical signatures also support this mode of origin from HG to FG through BG. Available geochronological data on the gneisses indicate that probably the HG, a partial melt of the mafic schists of the greenstone belt, wns recycled at lower crustal levels in Late Archaean foming BG and FG. The time gap between the phases appears to be less but no detailed age data exist on the different phases to support this.