Zircon ages in southwestern Minnesota

Abstract

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.