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Granitic rocks ranging in composition from granite to tonalite and associated metavolcanic-metasedimentary rocks compose an east-trending belt as much as 180 km wide and 300 km long in northern Wisconsin. The granitic rocks have an initial 87Sr/86Sr of 0.7025 ± 0.0005 and a Rb-Sr whole-rock isochron age of 1,885 ± 65 m.y., which is interpreted as the time of crystallization of the granitic rocks. Rb-Sr whole-rock mineral secondary isochrons for two samples give ages of 1,655 ± 55 m.y. and 1,545 ± 55 m.y.; K-Ar ages of biotite from these samples are 1,615 ± 55 m.y. and 1,598 ± 54 m.y., respectively. These mineral ages are interpreted as resulting from isotopic resetting caused by a thermal event about 1,600 m.y. ago. The granitic rocks and associated metavolcanic-metasedimentary rocks constitute lower Proterozoic greenstone-granite complexes that are remarkably similar in pattern to the Archean greenstone-granite complexes in the Superior province of the Canadian Shield.

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