40Ar/39Ar release spectrum ages for five hornblendes from two Acadian plutons in western Maine and petrologic observations for nearby metamorphic rocks are interpreted to indicate that high-grade K-feldspar + sillimanite metamorphism occurred during Carboniferous time, not Devonian time as previously believed. The metamorphism was closely associated in time with the emplacement of the Sebago batholith, 325 Ma. Petrologic data suggest that rocks as far as 30 km north of the Sebago batholith were affected. Release spectrum ages for hornblendes from the Songo pluton indicate that by 300 Ma high-grade metamorphic conditions had ended and cooling through 500 °C had occurred.

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