40AR/39AR MINERAL AGES FROM THE SCITUATE GRANITE, RHODE-ISLAND - IMPLICATIONS FOR LATE PALEOZOIC TECTONOTHERMAL ACTIVITY IN NEW-ENGLAND

The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian (c. 370 Ma). Variably recrystallized amphibole (iron‐rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south‐westerly younging 40Ar/39Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron‐rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post‐metamorphic cooling. The 40Ar/39Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high‐grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system. Copyright © 1990, Wiley Blackwell. All rights reserved