FORMATION OF WOLLASTONITE-BEARING MARBLES DURING LATE REGIONAL METAMORPHIC CHANNELED FLUID-FLOW IN THE UPPER CALCSILICATE UNIT OF THE REYNOLDS RANGE GROUP, CENTRAL AUSTRALIA

Granulite facies marbles from the Upper Calcsilicate Unit of the Reynolds Range, central Australia, contain metre-scale woIlastonite-bearing layers formed by infiltration of water-rich (X(CO2) = 0.1-0.3) fluids close to the peak of regional metamorphism at c. 700 degrees C. Within the wollastonite marbles, zones that contain <10% wollastonite alternate on a millimetre scale with zones containing up to 66% wollastonite. Adjacent wollastonite-free marbles contain up to 11% quartz that is uniformly distributed. This suggests that, although some wollastonite formed by the reaction calcite + quartz = wollastonite + CO2, the woilastonite-rich zones also underwent silica metasomatism. Time-integrated fluid fluxes required to cause silica metasomatism are one to two orders of magnitude higher than those required to hydrate the rocks, implying that time-integrated fluid fluxes varied markedly on a millimetre scale. Interlayered millimetre- to centimetre-thick marls within the wollastonite marbles contain calcite + quartz without wollastonite. These marls were probably not infiltrated by significant volumes of water-rich fluids, providing further evidence of local fluid channelling. Zones dominated by grandite garnet at the margins of the marl layers and marbles in the wollastonite-bearing rocks probably formed by Fe metasomatism, and may record even higher fluid fluxes. The fluid flow also reset stable isotope ratios. The woIlastonite marbles have average calcite (Cc) delta(18)O values of 15.4 +/- 1.6 parts per thousand that are lower than the average delta(18)O(Cc) value of wollastonite-free marbles (c. 17.2 +/- 1.2 parts per thousand). delta(13)C(Cc) values for the wollastonite marbles vary from 0.4 parts per thousand to as low as -5.3 parts per thousand, and correlations between delta(18)O(Cc) and delta(13)C(Cc) values probably result from the combination of fluid infiltration and devolatilization. Fluids were probably derived from aluminous pegmatites, and the pattern of mineralogical and stable isotope resetting implies that fluid flow was largely parallel to strike.