Abstract. Fluid inclusion studies of rocks from the area suggests progressive decrease of the late Archaean amphibolite-facies to granulitewith increasing grade. Oxygen fugacities calcufacies transition zone of southern India provide lated from orthopyroxene-magnetitequartz are support for the hypothesis that C0,-rich H,Oconsiderably higher than the graphite C0,-0, poor fluids were a major factor in the origin of the buffer, which explains the absence of graphite in high-grade terrain. Charnockites, closely asthe charnockites. sociated leucogranites and quartzo-feldspathicThe present study quantifies the nature of the veins contain vast numbers of large C0,-rich vapours in the southern India granulite metainclusions in planar arrays in quartz and feldspar, morphism. It remains to be determined whether whereas amphibole-bearing gray gneisses of C0 Microthermometry of CO, inclusions shows that miscible CH, and N, must be small, probably less than 10 mol. %combined. Densities of CO, increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO, equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750"C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions.Calculations on the stability of the charnockite assemblage biotite-orthopyroxene-K-feldsparquartz show that an associated fluid phase must have less than 0 . 3 5 H 2 0 activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO, content of biotite stabilizes it to lower H,O activities, and the steady increase of biotite TiO, southward in