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The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO 2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet-orthopyroxene) and pressures (garnet-orthopyroxene-plagioclasequartz) are near 750°C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H 2 O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO 2 -rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5-0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite-magnetite-quartz, ferrosilite-hematite-quartz and magnetite-hematite) in conjunction with garnet-orthopyroxene exchange temperatures, samples with XIlm Hm >0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlm Hm <0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O 2 ) arrays result in self consistent conditions ranging from 660°C and 10−16 bar to 820°C and 10−11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene-orthopyroxene-magnetite-ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlm Hm >0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a hightemperature oxidation event. Moderately oxidized samples (0.1

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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