RETROGRADE FLUIDS IN GRANULITES - STABLE ISOTOPE EVIDENCE OF FLUID MIGRATION

Widespread retrograde alteration assemblages document the migration of mixed H2O-CO2 fluids into granulite facies rocks in the Adirondack Mountains. Fluid migration is manifest by (1) veins and patchy intergrowths of chlorite +/- sericite +/- calcite, (2) small veins of calcite, many only identifiable by cathodoluminescence, and (3) high-density, CO2-rich or mixed H2O-CO2 fluid inclusions. The distinct and varied textural occurrences of the alteration minerals indicate that fluid-rock ratios were low and variable on a local scale. Stable isotope ratios of C, O, and S have been determined in retrograde minerals from samples of the Marcy anorthosite massif and adjacent granitic gneisses (charnockites). Retrograde calcite in the anorthosite has a relatively small range in both delta-O-18SMOW and delta-C-13PDB (8.6 to 14.9 parts-per-thousand and -4.1 to 0.4 parts-per-thousand, respectively), probably indicating that the hydrothermal fluids that precipitated that calcite had exchanged with a variety of crustal lithologies including marbles and orthogneisses, and that calcite was precipitated over a relatively narrow temperature interval. Values of delta-S-34CDT that range from 2.8 to 8.3 parts-per-thousand within the anorthosite can also be interpreted to reflect exchange between orthogneisses and metasediments. The recognition of retrograde fluid migration is particularly significant in granulite facies terranes because the controversy surrounding the origin of granulites arises in part from differing interpretations of fluid inclusion data, specifically, the timing of entrapment of high-density, CO-2-rich inclusions. Our results indicate that retrograde fluid migration, which in some samples may leave only cryptic petrographic evidence, is a process capable of producing high-density, CO2-rich fluid inclusions.