The transition zone between Archean low- and high-grade rocks in southern India represents eroded crustal levels representative of 15-20 km. It is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite, both of which appear to have developed from the tonalitic gneisses. Tonalitic gneisses and charnockites are similar in major and trace elements composition while granitic gneisses are relatively enriched in Rb, K, Th, Ba and light rare earth element (REE) and depleted in Cr and Sc. All three rock types exhibit enriched light REE patterns with variable positive Eu anomalies. Total REE content decreases with increasing Eu/Eu and SiO2 and with decreasing Fe2O3T and MgO in the tonalitic gneisses and charnockites. An internally consistent model for the production of the tonalitic gneisses involves partial melting of an enriched mafic source with variable ratios of hornblende to clinopyroxene. This source, in turn, is derived from an ultramafic mantle relatively enriched in incompatible elements. Granitic gneisses form from tonalitic gneisses by alkali metasomatism from chloride-bearing fluids with high H2O/CO2 ratios purged from the lower crust by CO2, and charnockites are produced from tonalitic gneisses (and granitic gneisses) by ischochemical CO2 metamorphism following the alkali metasomatism.