CONSTRAINING THE TIMING AND CHARACTER OF EXTREME CRUSTAL MELTING IN THE ADIRONDACK MOUNTAINS USING MULTISCALE COMPOSITIONAL MAPPING AND IN-SITU GEOCHRONOLOGY

Migmatites are common in the hinterland of orogenic belts. The timing and character of melting is important for understanding crustal rheology, tectonic history, and orogenic processes in general. The Adirondack Highlands have been used as an analog for mid/deep crustal continental collisional tectonism. Migmatites are abundant, and previous workers have interpreted melting during one (or several) events, but questions remain about the timing, tectonic setting, and even the number of melting events. We use multiscale compositional mapping combined with in-situ geochronology and geochemistry of monazite to constrain the nature, timing, and character of melting reaction(s) in one locality from the eastern Adirondack Highlands. Full-section compositional maps show the mode and distribution of major phases (Grt, Bt, Feldspar). and the location of all monazite, zircon, and rutile. The rocks studied here are dominated by K-feldspar, interpreted to be the solid product of biotite dehydration melting (i.e. Bt + Pl + Als + Qtz = Grt + Kfs + melt). Most biotite occurs near garnet rims and reflects retrograde growth, but anhedral crystals partially surrounded by garnet are likely prograde. The complete lack of plagioclase suggests that depletion of plagioclase, not biotite, may have ultimately limited the melting reaction. Imbricated garnet crystals surrounded by leucosome suggest west-vergent shearing synchronous with partial melting. In-situ zircon analysis yielded ca. 1150 and 1050 Ma. Most monazite crystals contain one or more core and rim domains. Cores yield 1175-1150Ma and rims yield 1050-900Ma. Dramatic reduction in Y and Dy in monazite indicates garnet growth predominantly before 1150 Ma with some additional growth before 1050Ma. U depletion prior to 1050 Ma suggests that melting was mainly associated with AMCG magmatism overlapping with and the latest stages of the Shawinigan orogeny. Monazite compositions suggest little decompression between 1150 and 1050 Ma; instead, steady increase in Y and Dy in monazite after 1050Ma suggests exhumation (retrograde metamorphism) after 1050Ma. The data are consistent with a model involving tectonic burial during the Shawinigan orogeny, extensive melting during AMCG magmatism, possible reheating associated with the Ottawan event, and exhumation after 1050Ma.