Isotopic ages from the Appalachians and their tectonic significance
Geological field relationships and previously reported isotopic age determinations througliout the Appalachians provided evidence for a pattern of Lower Paleozoic tectogenesis. The regional isotopic age pattern indicates that intrusion of ultrrtmafic and mafic igneous materials and major episodes of metamorphic recrystalliza~ion throughout the Appalachians were completed before the end of the Silurian Period. New K-Ar ages of metamorphic rocks fiom Vermont and adiacent regions in the Northern Appalachians are reported and confirm this pattern. Whole-rock ages of slates from the Taconic Klippe (460-445 m.y.) indicate that nletanlorphism and initial formation of slaty cleavage occurred during Middle to Late Ordovician times. Mica ages of Paleozoic schists exposed along the Green Mountain Anticlinoriuln (425-375 1n.y.) are indicative of prolonged posi-metamorphic uplift and cooling of this structure during the Silurian Period and Early Devonian Epoch. Local recrystalli~ation and consequent loss of argon from the Taconic slates also occurred during this time. Ages obtained from a variety of metamorphic rocks from eastern Vermont fall in a narrow range (345 i: 7 n1.y ), indicating that renewed uplift and rapid cooling of this area, east of a line oi'ultramafic intrusions, occurred during Late Devonian time. Con~parison of isotopic age data from the Appalachian and British Caledonian fold belts indicates that a broadly synchronous sequence of Lower Paleozoic tectonic and magmatic activity, lasting approximately 100 m.y., occurred in both regions.