Absolute age of early Precambrian rocks in the Bighorn Basin of Wyoming and Montana, and southeastern Manitoba

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Abstract
Age determinations on nine samples from the southeastern part of the Beartooth Range, Montana, indicate that the absolute age of the pegmatites and granitic gneisses of this area is 2750 ± 150 m y. A single determination from the northern Bighorn Mountains, Wyoming, suggests a similar age for this portion of the Precambrian basement. Several samples from the Winnipeg River ‐ Johnston Lake area of Manitoba indicate an absolute age of 2650 ± 150 m y for both the pegmatites and gneisses. A similar age was found for a muscovite from the northwestern angle region of Minnesota. Rubidium‐strontium ages were found to be more consistent than potassium‐argon ages. Argon loss from micas was commonly as great as 15 pct.