FLUID-ASSISTED MODIFICATION OF GARNET COMPOSITION ALONG RIMS, CRACKS,AND MINERAL INCLUSION BOUNDARIES IN SAMPLES OF AMPHIBOLITE FACIES SCHISTS

Compositional zoning patterns in garnet from schists in central Vermon t and western Connecticut suggest that prograde garnet growth was foll owed by an episode of compositional modification during fluid-assisted metasomatism. Prograde garnet zoning patterns are modified, with char acteristic depletion of X(Grs) and corresponding increases in X(Alm), X(Sps), and X(Prp), in a region 50-150 mum thick near the rim of garne t and along cracks and mineral inclusion boundaries. These regions of garnet compositional modification have a clouded appearance in thin se ction because they contain abundant secondary inclusions of both miner als and fluid 1-2 mum in size. Metasomatic reactions among the matrix mineral assemblages produced an assemblage more hydrous than the origi nal prograde assemblage. The dissolution and concomitant reprecipitati on of garnet in these samples occurred at lower amphibolite facies con ditions.