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Paper No. 53-6
Presentation Time: 2:30 PM-6:30 PM
PROTEROZOIC GEOLOGY OF THE DELLS OF THE EAU CLAIRE RIVER IN EASTERN MARATHON COUNTY, WISCONSIN, USA
The Dells of the Eau Claire River in eastern Marathon County, Wisconsin, provides an excellent exposure of deformed Proterozoic rocks of the southern Lake Superior region. Metamorphosed rhyolitic volcanic rocks, including garnet-bearing tuffs, occur about 5 km west of the western margin of the ~1480 Ma Wolf River batholith. These rocks have a strong NE striking, vertical planar fabric that has been variably interpreted as primary or mylonitic. Here we present new petrographic, geochronologic and paleomagnetic data for these metavolcanic rocks. Evidence for a mylonitic origin of the fabric is supported by the presence of recrystallized quartz ribbons and asymmetric sigmoidal porphyroclast tails and boudinaged garnet grains. U-Pb geochronology on zircons separated from metarhyolite was conducted at the University of Arizona Laserchron Center. Zircon yield was poor, and only 9 concordant ages were obtained. The youngest subset of four zircons yields a weighted mean age of 1483±18 Ma, which we interpret to be the age of the rocks. One geon 17 and three geon 27 zircon grains also are present, which we interpret to be inherited. Paleomagnetic investigations reveal a clean sub-vertical AMS fabric and a complex paleopole that is reversed (244°, -46°; upper hemisphere; AF demagnetization). Although geon 14 epizonal granitic rocks occur at the northern margin of the Wolf River batholith, the metarhyolite at the Dells of the Eau Claire River is the first extrusive igneous phase to be identified that is coeval with the emplacement of the batholith. Moreover, like the geon 14 Baldwin Conglomerate, a synorogenic metaconglomerate at the northeastern edge of the Wolf River Batholith, the Dells of the Eau Claire strata are interpreted to have been deformed and metamorphosed as part of the newly recognized Baraboo Orogeny, a major Mesoproterozoic tectonomagmatic event in the southern Lake Superior Province, whose age has not previously been well constrained.