The Distribution, Style, and Intensity of Alleghanian Metamorphism in South‐Central New England: Petrologic Evidence from the Pelham and Willimantic Domes

Abstract

Alleghanian metamorphism in south‐central New England is characterized by Late Pennsylvanian, $$\mathrm{kyanite}\,+\mathrm{muscovite}\,+\mathrm{staurolite}\,$$ grade, heterogeneous reconstitution of migmatitic, $$\mathrm{K\,feldspar}\,+\mathrm{sillimanite}\,$$ grade, pelitic gneisses previously metamorphosed during the Silurian to Devonian Acadian orogeny. Reconstitution resulted from strain within an approximately 5‐km‐thick zone of ductile deformation between the underlying Late Proterozoic Avalon Terrane, exposed in the cores of the domes, and cover rocks of the Putnam‐Nashoba, Merrimack, Kearsarge–Central Maine, and/or Bronson Hill Terranes. This style of metamorphism developed along the Avalon‐Acadian cover terrane boundary and is exposed in the Pelham and Willimantic Domes. Pressure‐temperature (PT) conditions in the cover rocks, based on conventional thermobarometry, oxygen isotope thermometry, and petrogenetic grid constraints, evolved from $$6\pm 1$$ kbar at 800°–825°C during Acadian metamorphism to $$6.5\pm 1$$ kbar at 600°–650°C during Alleghanian reconstitution. Based on the similarity in metamorphic style and intensity, structural setting, and timing of reconstitution, it may be inferred that the distribution of Alleghanian metamorphic mineral assemblages in south‐central New England outside the Avalon Composite Terrane is largely controlled by the proximity to a regional, essentially horizontal, midcrustal ductile shear zone. The formation of Alleghanian metamorphic mineral assemblages along the boundary zone accounts for the paucity of Alleghanian mineral assemblages in an area presently exposing mainly Acadian‐age assemblages. Prograde Alleghanian $$\mathrm{kyanite}\,+\mathrm{muscovite}\,+\mathrm{staurolite}\,$$ metamorphism within the boundary zone is contemporaneous with Late Pennsylvanian clastic sedimentation, and subsequent uplift and denudation is contemporaneous with prograde Barrovian metamorphism, in the Narragansett basin to the southeast. Metamorphism, denudation, and relatively rapid cooling documented for the Avalon Composite Terrane and Narragansett basin may thus be linked to Late Paleozoic tectonism in terranes to the northwest.