40Ar/39Ar ages from the Narragansett Basin and southern Rhode Island basement terrane: Their bearing on the extent and timing of Alleghenian tectonothermal events in New England

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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 D 1 –D 2 ductile deformation (between 295 and 275 m.y.) and post-D 3 cooling below argon-retention temperatures (between 250 and 240 m.y.).