Abstract

Rb-Sr geochronologic data have been obtained from a large variety of plutonic, metavolcanic, and metamorphic rocks that crop out throughout eastern and central Wisconsin. Most of the data are consistent with an apparent age of approximately 1,650 m.y., although for some samples the data suggest somewhat older ages. Comparison of the Rb-Sr results with other published geochronologic data and with recently determined U-Pb zircon ages for some of the same rocks indicates that the 1,650-m.y. apparent age obtained by Rb-Sr methods on rocks from this region does not represent their true age of extrusion or intrusion. Instead, U-Pb data for zircons from these rocks indicate that the true ages are in the range 1,800 to 1,900 m.y.

Rocks that yield Rb-Sr whole-rock or mineral isochrons having 1,650-m.y. apparent ages are also found throughout the western Great Lakes area (Michigan, Minnesota, and Ontario), indicating that a widespread, low-grade metamorphic event of some kind occurred throughout this region about 1,650 to 1,700 m.y. ago. As yet no major intrusive or extrusive rocks having true ages of 1,650 to 1,700 m.y. have been found in the region, so the exact cause of the alteration of the Rb-Sr geochronologic systems is still unknown.

One major result of these studies is the demonstration that most of the older Precambrian rocks in Wisconsin are in the age range of 1,800 to 1,900 m.y. old and represent extensive crustal development along the southern edge of the Superior Province during that time.

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