Modification of garnet by fluid infiltration during regional metamorphism in garnet through sillimanite-zone rocks, Dutchess County, New York

Metapelitic rocks from a progressive metamorphic sequence in Dutchess County, New York, record evidence for reaction between garnet and fluids associated with quartz (±plagioclase) veins and for net transfer among garnet interiors, mineral inclusions, and fluid. Garnet-fluid reaction resulted in modification of preexisting garnet textures, compositions, and growth-zoning patterns. The textural and compositional record of garnet-fluid interaction varies with grade and with proximity of garnet to quartz veins. Garnet-zone rocks do not contain evidence for extensive reaction with vein-forming fluids. In upper staurolite- and kyanite-zone rocks, garnet that is crosscut by quartz veins contains a fluid-inclusion-filled region near the garnet-vein interface. This region truncates growth-zoning patterns and quartz inclusions. Fluid inclusions in garnet decrease in abundance away from quartz veins. In some rocks lacking quartz veins, fluid inclusions are concentrated in garnet interiors, and in particular near fractures, and are notably lacking around mineral inclusions in garnet. Inferences from textural and compositional features combined with thermobarometric results indicate that preexisting garnet grains were modified at T ≈ 525-550 °C and >4 kbar by one or more reactions involving fluids. The fluid pathways included foliation-parallel channels in the rock as well as microcracks in mineral grains.