KYANITE-BEARING MIGMATITES IN THE CENTRAL ADIRONDACK MOUNTAINS: A NEW P-T-T PATH AND IMPLICATIONS FOR TECTONIC MODELS

Sillimanite-rich felsic migmatites exposed at Ledge Mountain represent the only location in the Adirondack Highlands where kyanite has been found. The texturally young kyanite is overprinted on sillimanite in largely undeformed pegmatitic leucosomes, suggesting a late episode of melting taking place deeper than previously thought, and requiring a counter-clockwise P-T path. A final phase of anatexis ca. 1050 Ma in the Eastern Adirondack Highlands is consistent with an influx of fluid or decompression from extension in sillimanite-bearing migmatites. Temperatures both from this study and previous work are consistent with granulite-facies metamorphism, however the presence of kyanite requires higher pressure conditions corresponding to deeper burial of these central Adirondack rocks.

The Adirondack Mountains are associated with the Grenville Province of eastern North America, that formed during four separate orogenic events (the Elzevirian, Shawinigan, Ottawan, and Rigolet orogenies). Preliminary U-Pb SHRIMP zircon ages from Ledge Mountain kyanite-bearing migmatites show anatexis continuing well after high-grade metamorphism is believed to have ceased in the range. A counter-clockwise P-T path is consistent with the mechanisms in the current model, and this study indicates anatectic melting persisted into the Rigolet phase. The Ledge Mountain migmatite may represent Lyon Mountain Gneiss that was metamorphosed to sillimanite grade and then overprinted by a higher pressure, lower temperature assemblage. These new data force us to re-evaluate tectonic models for the Adirondacks, for the entire Grenville Province of eastern North America, and for pre-Pangean tectonics. Further analysis of age, geochemical, and petrographic data will help develop a better-defined P-T-t path and may lead to the development of a new tectonic model with similarities to the high-grade rocks in the Himalaya or Bohemian Massif.