Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin

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Abstract
U-Pb analyses have been carried out on 36 zircon fractions representing 23 lower Proterozoic plutonic and volcanic units from throughout Wisconsin. These data, combined with published results from northeastern Wisconsin, allow detailed definition of the chronology of igneous events in Wisconsin associated with the Penokean orogeny. Two distinct pulses can be recognized. The older one began with mafic to felsic volcanism 1,859 ± 20 m.y. ago and was followed immediately by tonalitic to granitic plutonism 1,840 to 1,820 m.y. ago. Structural studies indicate that these rocks were emplaced during the main phase of the orogeny. The second pulse consisted predominantly of rhyolite and granophyric granite and occurred about 1,760 ± 10 m.y. ago. No Proterozoic igneous units with zircon ages in excess of 1,860 m.y. have been found to date in Wisconsin, nor have any been found with ages on the order of 1,615 to 1,630 m.y., the time of widespread alteration of Rb-Sr geochronologic systems in the region.

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