Fluids in Granulite Facies Metamorphism: A Comparative Oxygen Isotope Study on the South India and Adirondack High-Grade Terrains

Abstract

Oxygen isotope compositions have been determined for samples from the South India Archean to Pro-terozoic and northwest Adirondack late-Proterozoic high-grade terrains. $\delta^{18}O$ values of charnockites and related orthogneiss protolith from the prograde amphibolite facies to granulite facies transition zone in South India range from 5.9 to 8.4‰. Charnockites, khondalites (sillimanite gneisses), and retrogressed gneisses from the interior of the granulite facies terrain average 2‰ higher than orthogneisses of the transition zone. Individual minerals in incipient charnockitic net veins, as at Kabbaldurga, Karnataka, and Ponmudi, Kerala, have $\delta^{18}O$ identical to minerals of their non-charnockitic host rocks. Quartz $\delta^{18}O$ values of the northwestern Adirondack granites and gneisses average 3-4‰ higher than those from the Indian rocks. The two terrains are thus isotopically distinct, which may reflect differences in premetamorphic history or different degrees of communication with external oxygen reservoirs during metamorphism. The low $\delta^{18}O$ of the South India terrain, including metasedimentary rocks, suggests the pervasive influence of a deep crustal or upper mantle igneous reservoir, whereas the elevated $\delta^{18}O$ of the Adirondack rocks suggests that the isotopic effects of near-surface, low-temperature processes in the protoliths survived subsequent fluid-deficient granulite-facies metamorphism. The near-identity of $\delta^{18}O$ of individual minerals from charnockitic veins and non-charnockitic host gneiss in the South India terrain indicates either that fluids causing charnockitic alteration were nearly in isotopic equilibrium with the country rocks or that oxygen isotopes were set in premetamorphic processes and little altered by incipient charnockite formation. Therefore we do not have direct isotopic evidence of fluid action in the origin of the much-discussed South Indian charnockitic net-veining. Isotopic temperatures are 100°C or more lower than peak metamorphic temperatures in both terrains, as inferred from mineralogic thermometry, indicating extensive postmetamorphic re-equilibration to lower temperatures, even in the fluid-deficient Adirondack terrain. An exception seems to be the quartz-garnet pair, which yields temperatures of 750° to 850°C for South India. Diffusion theory calculations show high closure temperatures in garnet, providing an explanation for the refractory behavior of this pair. It is possible that quartz-garnet will provide a useful oxygen isotope thermometer for granulites when favorable circumstances for postmetamorphic preservation are realized.