PUSHING THE LIMITS OF PETROCHRONOLOGY TO CONSTRAIN POLYPHASE OROGENESIS IN NEW ENGLAND

Overprinting during multiple phases of high-grade metamorphism and regional variations in P, T, and deformation intensity stretch the limits of geochronologic tools, and have limited researchers’ ability to study rocks that have experienced multiple episodes of tectonism. To document distinct styles of tectonism associated with polyphase orogenesis in the Appalachian orogen of New England, we have utilized multi-scale compositional mapping, in-situ monazite and xenotime petrochronology, thermobarometry, and thermodynamic modeling. The data constrain distinct P-T-t paths from rocks across southern New England and show overprinting of progressively younger events from west to east. The earliest recognized garnet growth, ~470 Ma, is apparently limited and possibly associated with (soft?) accretion of the Moretown terrane. Widespread crustal thickening ~30 m.y. later may be related to late Taconic, Laramide-style tectonism and/or accretion of the Gander terrane. Thermal peaks ca. 400 and 380 Ma, associated with the Acadian and Neoacadian orogenies, respectively, are only recognized east of Cameron’s line. Abundant partial melting, high-pressure granulite facies metamorphism in gneiss domes, and very slow cooling associated with Neoacadian orogenesis provide further evidence for the existence of an orogenic plateau akin to the Pamir plateau. Garnet breakdown in all rocks east of Cameron’s line occurred at ca. 340-330 Ma and is interpreted to represent regional exhumation and plateau collapse. This may be associated with development of the 15 km high Moho step in western New England. Alleghanian-age tectonism is recognized in the vicinity of the Pelham dome with only minor and local reheating to the west. Together, these results provide quantitative constraints on polyphase orogenesis, define the spatial and temporal extent of overprinting, and track the evolution of eastern margin of Laurentia from terrane accretion to the rise and fall of a regional orogenic plateau.