DETERMINATION OF THE DURATION OF RETROGRADE METAMORPHISM AT GORE MOUNTAIN, NY

Gore Mountain in New York state is well known for the large garnet porphyroblasts found there. The garnets at Gore Mountain are found in a narrow amphibolite body that defines the boundary between a metagabbroic body to the north and a quartz-syenite body to the south. The formation of the large-garnet amphibolite is believed to be due to ductile deformation of the quartz syenite body fracturing the rigid metagabbroic body and allowing limited fluid penetration into the metagabbro at amphibolite facies conditions.

The garnets present in the amphibolite display distinctive textures: thin inner plagioclase coronas (~1% of the garnet radius) surrounded by wider hornblende coronas (~10-30% of the garnet radius). In several of the specimens, between the garnet and the plagioclase corona a hornblende-plagioclase-orthopyroxene symplectite is present (0.5-3 mm wide). This symplectite is considered here to be a retrograde reaction texture that post-dates the end of fluid intrusion and represents a period of garnet resorption.

The resorption reaction was likely limited by intergranular diffusion, and thus can be modeled quantitatively. ICP-MS data suggested radial variation is limited to Ca and some REEs. Core-rim transects of the resorbed garnets for these elements show flat interiors with rimward variations, suggesting a homogenization of early growth zoning, followed by resorption zoning. In resorption zoning, elements partition between the surface of the reacting garnet and the local intergranular medium, and the surficial garnet composition propagates inward via intracrytalline diffusion. These zoning profiles were modeled using RESORB modeling software which allows one to constrain the temperature-time path given a set of diffusional input parameters and the observed zoning profile. Initial results suggest that the garnets were resorbed and achieved their observed zoning profiles within the first five million years, and likely within the first two million years.