Abstract

New 40Ar/39Ar thermochronologic data from the internal zone of the Penokean orogen in east-central Minnesota indicate rapid cooling from mid-crustal temperatures. Early Proterozoic metamorphosed cover rocks yield concordant hornblende and biotite plateau ages of ~ 1755 Ma. Underlying Archean basement gneiss yields concordant muscovite and biotite plateau ages of ~ 1705 Ma. These are some of the oldest cooling ages recorded in rocks buried during the 1830–1860 Ma Penokean collisional orogeny. Cooling was coeval with regional rhyolitic volcanism and granite emplacement, suggesting a period of tectonic activity in this region during mid to late 1700 Ma. Based on similarities with Phanerozoic extensional regimes, we suggest, preliminarily, that these data might be explained by a previously unrecognized major extensional episode in the Lake Superior region.

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