Archean (2500+ Ma old)

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Archean (2500+ Ma old)
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Archean (2500+ Ma old)

Wisconsin was assembled from three terranes: the Proterozoic Wausau-Pembine Terrane and two Archean continental terranes.

The Penokean orogeny was the first step in a series of continental and terrane collisions that accreted most of North America. The largest Archean block in North America is the Superior Province, which covers most of Quebec and Ontario and much of Manitoba in Canada, and parts of Minnesota, Wisconsin and Michigan in the U.S.

Archean continental terranes tend to be one of two types. Granite-Greenstone terranes consist of belts of low-grade mafic metavolcanic and metasedimentary rocks surrounded and intruded by granitic plutons. Most of the Superior Province consists of this type of terrane. Geologists are beginning to recognize that the Superior Province itself was accreted from many smaller terranes. Gneiss terranes consist of high-grade metamorphic rocks. The southern Marshfield Terrane and its probable equivalent in Minnesota are of this type.

As the diagram indicates, the actual positions of the three terranes are entirely unknown before their collision.

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Created 11 Sep 1997, Last Update 11 January 2020