SEDIMENTOLOGY, PALEOMAGNETISM AND DETRITAL ZIRCON PROVENANCE OF A PROTEROZOIC METAGRAYWACKE SUCCESSION IN MARATHON COUNTY, WISCONSIN, USA

A variety of Proterozoic sedimentary, volcanic, and plutonic rocks occurs in Marathon County, Central Wisconsin, USA. Exposed in the 3M quarry SSE of Brokaw is a succession of metaconglomerate, metagraywacke, and argillite, as much as 30 m in thickness, that overlies rhyolite and rhyolite agglomerate. The sedimentary rocks transition from dark gray metaconglomerate, which contains cobble- and pebble-size clasts of rhyolite and other aphanitic volcanic rocks, at the base of the section to interbedded greenish-gray metagraywacke and thinly bedded argillite above. These rocks dip gently to the northwest, commonly display spaced cleavage, and experienced geon 16 lower greenschist facies metamorphism, with chlorite, muscovite, epidote, and calcite occurring interstitially to the primary dominant quartz and feldspar. We interpret the metaconglomerate and graywacke to represent fluvial deposits, and the argillite to have been deposited in a lacustrine environment. Zircons were separated from a sample of metagraywacke and analyzed at the University of Arizona Laserchron Center, yielding a zircon age spectrum from 1798 to 3320 Ma, with a predominant unimodal age peak at 1848 Ma. Zircons were also separated from the underlying rhyolite and rhyolite agglomerate, but the ages obtained are inconclusive, ranging from 1900 to 3560 Ma and likely being inherited, so the rhyolite crystallization age remains unconstrained. AF demagnetization reveals a clean paleopole (275°, -22°; upper hemisphere), indicating reversed polarity during deposition or deposition in the southern hemisphere. Consequently, these strata were deposited following the conclusion of the Penokean orogeny and before deposition of the geon 16 Baraboo Interval supermature siliciclastic strata, possibly soon after the Penokean orogeny or during the geon 17 Yavapai event. These are the only sedimentary strata yet discovered in the southern Lake Superior region that are this age, and as such, provide evidence for another important episode in the Precambrian evolution of the midcontinent.