Petrologic and 40Ar/39Ar geochronological constraints on the middle to late Paleozoic thermotectonic history of the southern Connecticut Valley zone, New England Appalachians

Abstract

New 40 Ar/ 39 Ar data from The Straits Schist and interlayered formations in the southernmost Connecticut Valley zone of the northern Appalachian orogen, together with published 40 Ar/ 39 Ar and U-Pb data, are used to account for relatively rapid Devonian (post-Acadian) cooling of gneisses and schists near the Waterbury dome and slower Devonian through Permian cooling in structurally higher schists south of the dome. Metamorphic hornblende 40 Ar/ 39 Ar cooling ages range from 374 Ma 10 km south of the Waterbury dome to 324 Ma 15 to 40 km south of the dome. Muscovite 40 Ar/ 39 Ar cooling ages also decrease to the south, from 318 to 254 Ma. The ages from the southernmost Connecticut Valley zone are as much as 50 and 100 m.y. younger than hornblende and muscovite, respectively, from the Waterbury dome. The difference in cooling histories between the Waterbury dome and southern Connecticut Valley zone is interpreted to be a result of relatively slower cooling in the southern Connecticut Valley zone following Acadian metamorphism and/or a static Pennsylvanian (Alleghanian) thermal event that reset the muscovite K/Ar system, promoted growth of fine-grained monazite, and resulted in partial melting of the crust beneath the southern Connecticut Valley zone. Such a thermal event would have been contemporaneous with Alleghanian orogenesis in Appalachian terranes in southeastern New England, but the mechanism for generating a Pennsylvanian thermal pulse is still unknown.