Pressure-temperature conditions of kyanite-bearing migmatites at Ledge Mountain, central Adirondack Highlands

Sillimanite-bearing migmatites exposed at Ledge Mountain in the central Adirondack Highlands are the only known location in the Highlands where kyanite has been found. Phase equilibria modeling combined with geochemical and textural analysis suggests that migmatites at Ledge Mountain experienced peak P-T conditions at 12-16 kbar/900-1040° C followed by isothermal decompression into the sillimanite stability field, resulting in a clockwise P-T path. At these conditions, pseudosection models predict that the rock in the low-middle crust would have been -15-30% melt prior to exhumation. Presence of peritectic garnet with lobate quartz inclusions, pseudomorphed melt films, and biotitequartz symplectites indicates that garnet grew in the presence of melt following the dehydration melting of micas during prograde metamorphism. These results indicate that previous estimates of the P-T conditions of granulites in the Adirondack Highlands likely underestimate the peak metamorphic conditions in which they formed due to the inconsistencies of conventional thermobarometry at high-temperature conditions. During orogenic collapse, the low-middle crust would have been ~15-30% melt which is more than enough to cause channel flow and suggest that the Highlands formed as a gneiss dome, similar to what has been suggested in other orogens such as the Himalaya. These results support the conceptual model of the Grenville orogeny as a large hot orogen (LHO).