Superimposed metamorphic events in high-grade terrains cause multi-stage redistribution of oxygen isotopes and may lead to contradictory conclusions regarding the fluid regime. An attempt is made to separate contributions of the "main" granulite episode from those of the secondary events by analyzing 18O/16O distribution in the fresh granulites and their retrogressed
counterparts both between minerals (local scale) and between rocks (outcrop scale to km-scale). The material represents high-grade Precambrian rocks of the Proterozoic Rayner Complex (RC) of Enderby Land (East Antarctica), and Archean rocks of the Southern Marginal Zone (SMZ) of the Limpopo Belt (South Africa). Petrological data lead to the estimation of peak metamorphic temperatures of about 730-750 °C in the RC (Grew, 1981; Black et al., 1987) and about 850 °C in the SMZ (Stevens and van Reenen, 1992).
In most cases, isotopic reminiscence on peak temperatures among mineral phases is destroyed due to diffusive exchange down to relatively low temperatures. Mineral O-isotope fractionations indicate temperatures of 750-450 °C, regardless of the peak temperatures. Secondary fluid infiltration events further obscure the primary isotopic signatures. However, unaltered rocks are characterized by fractionation patterns consistent with closed-system exchange, while the retrogressed rocks, especially in shear-zones, underwent open-system fluid interaction with concomitant reequilibration.
In contrast, the whole-rock systems generally preserve isotopic signatures acquired during large scale fluid rock interaction at peak conditions. On the outcrop and on the regional scale O-isotope distribution was established among different granulite facies rocks at peak metamorphic temperatures and their 18O/16O ratios (the whole-rock d18O values) can be used therefore to deduce temperature and scale of this fluid-rock interaction as well as the isotopic composition of the interacting fluid (Krylov and Mineev, 1994). Combining both, the mineral and the whole-rock fractionation patterns, it is possible to distinguish between fluid-rock interactions at different stages of metamorphic evolution and to evaluate their specific conditions.
Black, L.P., Harley, S.L., Sun S.S. & McCulloch, M.T., J. Metamorphic Geol. 5, 1-26 (1987).
Grew, E.S., J. Petrol. 22, 297-336 (1981).
Krylov, D.P. & Mineev, S.D., Geochim. Cosmochim. Acta 58, 4465-4473 (1994).
Stevens, G. & Van Reenen, D.D., Precambrian Res. 55, 303-319 (1992).