Pressure, Temperature and C-O-H Fluid Fugacities across the Amphibolite-Granulite Transition, Northwest Adirondack Mountains, New York

Abstract

Pressures, temperatures, water activities (aH2O) and fugacities of the other C-O-H fluid species have been estimated on a traverse across the amphibolite-granulite facies boundary in the Major Paragneiss, northwest Adirondacks, N.Y. Two-feldspar pairs give temperatures ranging from 650°C in the central portion of the unit to 760°C towards the northeast. Biotite-garnet pairs give erratic temperatures compared to two-feldspar temperatures. This discrepancy appears to be due to retrograde resetting as determined from compositional zoning patterns in biotites and garnets. Some of the discrepancy may also be due to non-ideality of pyrope-almandine mixing or to non-ideality from other components. Pressures ranging from 5·4 kb for the southwestern portion of the unit to 8·0 kb in the northeast were determined from anorthite-grossular-sillimanite-quartz barometry. Minimum pressures ij of 5·8 kb were also determined from coexisting garnet + rutile. Values of aH2O of 0·08-0·5 estimated from biotite and muscovite dehydration reactions show no correlation with grade. The variability in aH2O suggests that it is locally controlled and that a homogeneous, pervasive fluid was not present during high grade metamorphism. Graphite equilibria indicate that fO2 was less than 0·5 log units below QFM and that if a fluid was present, it was rich in CO2 and H2O. P-T-aH2O values suggest that partial melting did not occur during metamorphism. Pervasive flooding with CO2 does not appear to have occurred. The amphibolite-granulite transition at this locality is characterized by increasing temperature and pressure.