ABSTRACT

The early Proterozoic gneiss-charnockites from Dharmapuri area, southern India are investigated to understand the crustal evolution processes of the area. The granulite facies metamorphism which converted the tonalitic gneisses to enderbites in the area was isochemical, except for the depletion of LIL elements. The enderbites of granulite zone have high K/Rb, Ba/Rb, K/U and K/Th but low U, Th, K/Sr, Rb/Sr and Th/U compared to the tonalitic gneisses of the transition zone. The petrochemical studies show a compositional continuum, without bimodal distribution pattern, from mafic gneisses to enderbites/tonalitic gneisses. Their variation suggests that the rocks were derived from igneous protoliths. The REE pattern of tonalitic rocks show LREE enrichment, HREE depletion and variable positive Eu anomalies. The REE concentrations, however, do not show any signs of fractionation with increasing grade of metamorphism. It is also observed that ages obtained from whole rock Rb-Sr and Sm-Nd systematics remain same, which probably suggests that the high grade metamorphism followed crustal generation processes within a short span. The protolithic orthogneisses were probably derived either by fractional crystallization of basaltic/dioritic magma involving hornblende, plagioclase and iron oxides as the major precipitating phases, or by partial melting of an amphibolite source leaving plagioclase and clinopyroxene as the main residual phases. A fractional crystallization model is more preferred because of the presence of horn-blende cumulate rocks and because there is a continuous compositional range from gabbro to granodiorite through tonalite.