⁴⁰Ar/³⁹Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England.

⁴⁰Ar/³⁹Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as ⁴⁰Ar/³⁹Ar and K/Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in W New England occurred 465+ or -5Ma ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available ⁴⁰Ar/³⁹Ar and K/Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. -from Authors