PETROCHRONOLOGIC CONSTRAINTS ON ACCRETION, EXTRUSION, AND INVERTED METAMORPHISM IN THE GNEISS DOME BELT, APPALACHIAN OROGEN

Gneiss domes are an integral element of many orogenic belts and many provide a window into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapiric doming and interference folding, but recent work has suggested that central New England gneiss domes formed by ductile extrusion associated with collapse of an Acadian orogenic plateau (the Acadian altiplano). To better constrain the origin and evolution of domes in western New England, we constrained P-T-t paths by integrating in-situ U-Th-Pb monazite, xenotime, and zircon petrochronology with major and trace element thermobarometry. The domes consistent of a distinctive, layered tectonostratigraphy. The lowest layer of the Bristol dome, the Gondwana-derived Taine Mountain Formation, records Taconic (~0.6 GPa, 600°C; 450-440 Ma) and Acadian (~1.0 GPa, 650°C; ca. 400-370 Ma) metamorphism. Samples from the lower levels of the Waterbury dome record a similar P-T-t evolution at slightly higher P-T conditions. The overlying Collinsville Formation orthogneiss yielded a ca. 470 Ma crystallization age and records Taconic and Acadian metamorphism, with peak metamorphic conditions of ~1.2 GPa at ~800°C. These units record looping, overall counterclockwise P-T paths. The structurally higher Sweetheart Mountain unit records a clockwise P-T path, high-pressure granulite facies conditions (1.6-1.8 GPa, 800°C) and yields only Acadian dates. All units record garnet breakdown at ca. 340-330 Ma. Distinct P-T-t paths from each unit and increasing metamorphic pressure with from the lowest to highest levels of the domes suggest that these units were assembled tectonically and do not support diapiric models. Dome core rocks remained deep after the Taconic orogeny and underwent further burial in the Acadian orogeny. P-T-t paths of dome units and the Connecticut Valley belt cover rocks (e.g., The Straits Schist) converge by ca. 365, suggesting that they were juxtaposed in the mid-crust at this time. A second exhumation stage, 340-330 Ma, is interpreted to reflect plateau collapse. Orogen parallel fabrics and P-T paths show strong similarities to the North Himalayan and Pamir gneiss domes, suggesting that these domes formed by ductile flow/extrusion as syn-plateau shortening was balanced by horizontal flow.