POSTCOLLISIONAL COOLING OF THE PENOKEAN OROGEN IN EAST-CENTRAL MINNESOTA

New Ar-40/Ar-39 thermochronologic data from the internal zone of the P enokean orogen in east-central Minnesota indicate rapid cooling from m id-crustal temperatures. Early Proterozoic metamorphosed cover rocks y ield concordant hornblende and biotite plateau ages of approximately 1 755 Ma. Underlying Archean basement gneiss yields concordant muscovite and biotite plateau ages of approximately 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 rhyo litic volcanism and granite emplacement, suggesting a period of tecton ic activity in this region during mid to late 1700 Ma. Based on simila rities with Phanerozoic extensional regimes, we suggest, preliminarily , that these data might be explained by a previously unrecognized majo r extensional episode in the Lake Superior region.