Two early Proterozoic successions in central Wisconsin and their tectonic significance

Abstract

Geological mapping of 7,500 km 2 of previously virtually unmapped Precambrian rocks in central Wisconsin suggests the presence of two early Proterozoic sedimentary-volcanic-plutonic successions. Evidence for the two successions includes structural, stratigraphic, metamorphic, and lithologic relations that are consistent throughout the area. The older succession unconformably overlies Archean rocks. It is characterized by amphibolite-facies metamorphism, folding about west- to northwest-plunging isoclinal (F-1) fold axes with more open, coaxial F-2 folding. Basal quartzite and pelitic sediments of the older succession are overlain by volcanic rocks and intruded by syntectonic tonalite, granodiorite, and trondhjemite plutons. The younger succession consists primarily of a greenschist-facies calc-alkaline basalt-rhyolite sequence that unconformably overlies the older amphibolite-facies rocks. Structures in the greenschist sequence trend northeasterly with fold axes that plunge easterly. The plutonic rocks are dominantly syntectonic to late tectonic granites and quartz monzonites with only minor tonalite. In most areas, the two successions have been juxtaposed along major faults. The older amphibolitic sequence may correlate with the Chocolay Group farther north in the Lake Superior region, and the younger green-schist rocks with the Menominee and/or Baraga Groups. If this correlation is correct, the deformation, metamorphism, and intrusion of the green-schist succession would have occurred during the Penokean Orogeny. Metamorphism to amphibole rank, isoclinal folding, and tonalite intrusion of the older succession would be a major pre-Penokean erogenic event within the early Proterozoic that may correlate with the post-Chocolay-pre-Menominee break in northern Michigan, Wisconsin, and Minnesota. We suggest that this earlier event has been unrecognized and that it may be a significant part of the early Proterozoic history of the Lake Superior region.