Zircon crystallization ages, and subsequent isotopic disturbance events, in gneissic rocks of eastern Connecticut and western Rhode Island ( USA).

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We report on U-Th-Pb isotopic results for zircon from 13 gneissic rocks collected from the Waterford Group and the Sterling Plutonic Group located to the S and E of the Honey Hill-Lake Char fault system in a ductilely-deformed terrane of Connecticut, Rhode Island, and Massachusetts. Eleven of these rocks, including alaskite, granite gneiss, and plagioclase gneiss, are of Late Proterozoic age. Some of the granite gneiss, which previously was included as part of the Devonian Scituate Granite, is now considered to be a variant of the Late Proterozoic Hope Valley Alaskite Gneiss or Potter Hill Granite Gneiss of the Sterling Plutonic Group. The Late Proterozoic age of plagioclase gneisses south of the Honey Hill fault demonstrates that these rocks do not correlate with the Ordovician Monson Gneiss and supports recent mapping that shows the Bronson Hill zone to be separated by a fault(s) from Waterford Group rocks along coastal Connecticut. Moreover, the isotopic systems for all the Late Proterozoic rocks are disturbed and fall along a reference chord that has a lower intercept of 275 Ma. We attribute this disturbance to late Paleozoic metamorphism and deformation accompanying the Alleghanian orogeny; hence our information adds to the accumulating data that indicate the Alleghanian orogeny was more widespread and intense in the crystalline rocks of southeastern New England than was previously recognized.-from Authors

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American Journal of Science