|
Department of Geology, University of California, Davis, CA 95616, USA
Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA
Isograds and bathograds define an apparent thermal and baric structure of the Appalachian metamorphic belt in northern New England. Peak metamorphism occurred primarily during the Devonian Acadian orogeny at about 390 Ma. Kyanite-sillimanite metamorphism took place along heating-decompression paths and andalusite-sillimanite metamorphism is consistent with isobaric heating or limited decompression during heating. Isograds were deformed by early nappes and late domes, and dip steeply in some areas. The aluminium silicate bathograd dips gently east towards lower preserved pressures, intersects the regional trends of isograds, and is deformed by late cross-folds. Argon ages of micas as young as 250 Ma in the highest grade rocks record a long postmetamorphic cooling history, whereas lower grade rocks record much older argon ages. Comparison of these observations with simple thermal models suggests that differences of uplift history did not dominate the prograde metamorphic history and that spatial variations of the pre-tectonic thermal regime or of thermophysical characteristics of the lithosphere were important influences on the observed metamorphism.
