Geochronologic studies in central New England; I, Evidence for pre-Acadian metamorphism in eastern Vermont

(super 40) Ar/ (super 39) Ar geochronology on hornblendes from central New England reveals a Grenvillian cooling age (1026 + or -12 Ma) along the eastern margin of the Green Mountain massif, Acadian ages in east-central Vermont (350-397 Ma) and along the Bronson Hill anticlinorium in New Hampshire and Massachusetts (407-280 Ma), and Late Ordovician to Silurian ages (440-415 Ma) along a belt in eastern Vermont and central Massachusetts. These ages indicate a minimal Acadian overprint in eastern Vermont, the highest degree of Acadian overprint being located along the axis of the domes in the Connecticut Valley synclinorium. These data suggest that the formation of the domes occurred in the Acadian. Moreover, the Late Ordovician to Silurian cooling ages in eastern Vermont suggest that much of the metamorphic mineralogy and fabric observed in the Connecticut Valley synclinorium may be pre-Silurian in age.