Rb-Sr whole-rock and U-Pb zircon ages of granite and gneiss cores from three deep drill holes extend known occurrences of Archean rocks in the subsurface of northeastern Wyoming and southern Montana. Rb-Sr and K-Ar mineral ages are discordant and reflect early or middle Proterozoic disturbance. Highly altered rocks occur in a thin zone immediately below the sub-Cambrian unconformity. Samples from a few metres deeper in the basement are much fresher but show the effects of this alteration in filled fractures and thin adjacent alteration haloes. Whole-rock Rb-Sr systems have retained a fair degree of integrity in spite of increased susceptibility to modification because of the disturbed mineral systems. Interaction of the rocks with water a few metres below the sub-Cambrian unconformity probably occurred for only a relatively short time. Fractures filled rapidly with secondary minerals such as chlorite, anhydrite, and carbonate to maintain a relatively impermeable crystalline basement in which the silicates and their contained isotopic systems were preserved.

Ages of cores from two wells extend the Archean basement to the NACP anomaly — a possible suture related to early Proterozoic tectonism. These and published ages limit a possible ensimatic mobile belt to a zone ∼300 km wide beneath the Williston basin. However, the possibility remains that Archean crust was continuous between the Wyoming age province and the Superior Province.

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