WE have previously arrived at an age of 3,550 Myr for granitic1 gneiss in the vicinities of Morton and Montevideo in the Minnesota River valley, south-western Minnesota. We now report new Rb-Sr analyses (Table 1) and an age of 3,800 Myr for the fine grained foliated phase of the Montevideo Gneiss of Lund2. The rocks have undergone a complex history of metamorphism which remains to be deciphered, but the age determinations reveal that the geological mapping and previous interpretations did not provide a proper basis for sampling. ?? 1974 Nature Publishing Group.
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3,800-Myr granitic gneiss in South-Western Minnesota