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

Abstract

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 western New England occurred 465 ± 5 m.y. 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. 40Ar/39Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.