Abstract

40Ar/39Ar dating of hornblende, muscovite, and orthoclase from southeastern Quebec provides new constraints on the timing, grade, and regional extent of Taconian and Acadian metamorphism in the Canadian Appalachians. Muscovite ages reveal that the Taconic orogeny occurred at ca. 463 Ma throughout the region. Lack of reset amphibole indicates that the metamorphic grade did not exceed greenschist facies. Acadian metamorphism, with a peak of <350 °C, is recorded by an orthoclase cooling age of 377 ± 4 Ma. This later metamorphism partially reset finer-grained muscovite toward the Vermont border. The calculated 300–340 °C closure temperature of orthoclase indicates that this age currently provides the closest estimate of the timing of peak Acadian metamorphism north of New England. Despite the lower grade of Taconian and Acadian metamorphism in Quebec compared to New England, peak temperatures appear to have been achieved synchronously. In addition, investigation of the timing of pre-Taconic decoupling of the Quebec oceanic fragments suggests a protracted obduction history. Obduction may have displaced slivers of both the subducting and overthrusting oceanic plates.

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