Fluid inclusions in rocks from the amphibolite‐facies gneiss to charnockite progression in southern Karnataka, India: direct evidence concerning the fluids of granulite metamorphism

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Abstract
Fluid inclusion studies of rocks from the late Archaean amphibolite‐facies to granulite‐facies transition zone of southern India provide support for the hypothesis that CO2,‐rich H2O‐poor fluids were a major factor in the origin of the high‐grade terrain. Charnockites, closely associated leucogranites and quartzo‐feldspathic veins contain vast numbers of large CO2‐rich inclusions in planar arrays in quartz and feldspar, whereas amphibole‐bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed‐facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H2O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H2OMicrothermometry of CO2inclusions shows that miscible CH4and N2must be small, probably less than 10mol.%combined. Densities of CO2increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO2equation 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‐feldspar‐quartz show that an associated fluid phase must have less than 0.35 H2O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO2content of biotite stabilizes it to lower H2O activities, and the steady increase of biotite TiO2southward in the area suggests progressive decrease of aH2O with increasing grade. Oxygen fugacities calculated from orthopyroxene‐magnetite‐quartz are considerably higher than the graphite CO2‐O2buffer, which explains the absence of graphite in the charnockites.The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO2‐flushing of the crust can, by itself, create large terranes of largeion lithophile‐depleted granulites, or whether removal of H2O‐bearing anatectic melts is essential.