Zircons from the Morton and Montevideo gneisses in southwestern Minnesota were found to have the oldest isotopic U-Pb ages so far determined for this continent. The results are discordant, but suggest a minimum crystallization age of 3300 m.y. A zircon sample from a granite pluton which intrudes the Montevideo gneiss has a probable age of 1850 m.y.
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E.J. Catanzaro (1963) studied this question.
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