PALEOHYDROLOGY OF THE CALCSILICATE AUREOLE OF THE BEINN AN DUBHAICH GRANITE, SKYE, SCOTLAND - A STABLE ISOTOPIC STUDY

The contact aureole developed in siliceous carbonates surrounding the Beinn an Dubhaich granite, Skye, shows textural and stable isotope evi dence for infiltration of aqueous fluids during both prograde and retr ograde metamorphism. Strongly depleted isotope compositions of reactio n-product calcite correlate with high silica and fluorine contents, de monstrating a strong link between isotopic alteration and metasomatism by fluids with a significant magmatic component, even at the margins of the aureole. The oxygen and carbon isotope compositions of the carb onates form a linear cluster with a positive slope of about five, cons istent with the depletion of isotope compositions by the infiltration of magmatic and/or meteoric fluids. Rayleigh fractionation during devo latilization played a minor role in determining the final isotope comp osition. Stable isotope compositions of coexisting calcite-dolomite pa irs show varying amounts of isotopic disequilibrium, which correlate w ith the inferred fluid infiltration mechanism. Much of the calcite in dolostones is the product of infiltration-driven reactions along fract ures, and is greatly depleted isotopically relative to the host dolomi te, especially at talc grade. At higher grades the calcite-dolomite fr actionation is smaller, probably due to both increased fluid-rock inte raction and a greater tendency for fluid infiltration to be pervasive on the grain-scale. Limestones generally show near-equilibrium fractio nation of oxygen and carbon owing to the overwhelming compositional in fluence of the host calcite. Veins formed during late-stage hydrotherm al circulation have strongly O-18-depleted compositions relative to th e host rock. No small-scale spatial patterns to the isotopic depletion were observed, but the extent of fluid infiltration was greatest in t he west of the aureole. Fluid infiltration was clearly highly heteroge neous, with no evidence of a consistent flow direction. It is not poss ible to determine fluid fluxes or flow directions from one-dimensional flow models based on continuum flow in the Beinn an Dubhaich aureole.