PETROCHRONOLOGIC CONSTRAINTS ON INVERTED METAMORPHISM, TERRANE ACCRETION, THRUST STACKING, AND DUCTILE FLOW IN THE GNEISS DOME BELT, NORTHERN APPALACHIAN OROGEN

Gneiss domes, an integral element of the New England Appalachians, have been classically associated with diapirism and fold interference but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western CT with monazite, xenotime, zircon, and titanite petrochronology and thermobarometry. These data constrain distinct pressure-temperature-time (P-T-t) paths for each unit in the Gneiss Dome belt tectono-stratigraphy. The structurally lowest units, Laurentia-derived gneisses of the Waterbury dome, document 455-435 Ma and 400-370 Ma metamorphism with peak Acadian metamorphic conditions of ~1.0-1.2 GPa at 750-780°C at 391±7 to 386±4 Ma. The next structurally higher unit, the Gondwana-derived Taine Mountain Formation, records Taconic (0.6 GPa, 600°C at 441±4 Ma) and Acadian (0.8-1.0 GPa, 650°C at 377±4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473±5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436±4 Ma and 1.2-1.0 GPa, 750-775°C at 397±4 to 385±6 Ma. The overlying Sweetheart Mountain Member of the Collinsville Formation records high-pressure granulite facies metamorphism (1.8 GPa, 815°C) at ca. 380-375 Ma. The dome-mantling The Straits Schist records conditions of ~1 GPa, 700°C at 386±6 to 380±4 Ma. Garnet breakdown to monazite and/or xenotime occurred in two stages in all units at 375-360 Ma and 345-330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. We prefer a model involving ca. 380 Ma thrust stacking, followed by syn-collisional orogen parallel extension, ductile flow, and development of domes between 380 and 365 Ma. Garnet breakdown at ca. 345-330 Ma is interpreted to reflect further exhumation during collapse of the Acadian altiplano. Syn-convergent orogen parallel ductile flow dramatically modified earlier accretion-related structures here and throughout New England. These results highlight the power of pairing petrologic constraints with petrochronologic data from multiple chronometer phases to test structural and tectonic models.