Abstract

Pennsylvanian sedimentary rocks record the effects of progressive Barrovian metamorphism and polydeformation in the Narragansett Basin of Rhode Island. Whole-rock phyllite samples from low-grade portions of this terrane display internally concordant 40Ar/39Ar age spectra with plateau ages of 248 to 258 m.y. (four samples). Biotite from higher-grade rocks records plateau ages of 240 to 248 m.y. (eight samples). Similar 40Ar/39Ar biotite ages are displayed westward across the southern Rhode Island basement terrane (239 to 250 m.y.; nine samples), although coexisting hornblende records slightly older plateau ages of 255 m.y. (two samples). The 40Ar/39Ar plateau ages are interpreted to date times of post-metamorphic cooling through temperatures required for argon retention in the various mineral phases examined. The ages suggest that a distinct late Paleozoic (Alleghenian) thermal event affected much of southern Rhode Island. Together with the depositional ages of the variably metamorphosed Pennsylvanian rocks (Westphalian B–Stephanian A or B), the 40Ar/39Ar data permit calibration of the sequence of Alleghenian tectonothermal events recorded in the Narragansett Basin. The controls suggest a Permian event lasting about 40 to 50 m.y. between initial Barrovian regional metamorphism and synchronous D1–D2 ductile deformation (between 295 and 275 m.y.) and post-D3 cooling below argon-retention temperatures (between 250 and 240 m.y.).

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