AN EARLY PENOKEAN AGE FOR THE MCCAULEY GNEISS IN THE MOUNTAIN SHEAR ZONE, NORTHEASTERN WISCONSIN, USA

The Mountain Shear Zone is a N60°E trending, 2 km wide, 10 km long belt of deformed metasedimentary, metavolcanic, and granitic rocks that occurs within the southern Superior Province northeastern Wisconsin, USA. The Mountain Shear Zone occurs within the Pembine-Wausau terrane that formed during the 18 Geon Penokean Orogeny. The Mountain Shear Zone is truncated to the south by rocks of the 1.48 Ga Wolf River Batholith. The previously undated McCauley gneiss, which has strongly and weakly foliated phases, occurs within the Shear zone. U-Pb geochronologic analysis on zircon was conducted on three phases of the McCauley gneiss using LA-ICPMS methods at the University of Arizona Laserchron Center. Thirty zircons were analyzed from each sample. Many zircons had high U concentrations (>1000 ppm) were dark colored and metamict. Of the 90 zircons analyzed from the three samples, more than 50 were rejected because of high 204Pb concentrations and strong discordance. The least foliated sample of the McCauley gneiss (n=22) has a concordia age of 1876.3±7.2 Ma. The two foliated samples (n=7 each) have concordia ages of 1894±30 and 1876±10, respectively. We interpret the crystallization age of the McCauley to be ~1875 Ma, which is early Penokean in age. Thus, much of the ductile deformation in the Mountain Shear Zone likely occurred during the main phase of the Penokean orogeny. Deformation was protracted as the Mountain Shear Zone was reactivated during the emplacement of the Wolf River Batholith and subsequent deposition and deformation of the <1.45 Ga Baldwin Conglomerate. The high 204Pb concentrations of many zircons, and the ~250 Ma lower concordia intercept of discordant zircons suggest a late Paleozoic hydrothermal event in the Pangean foreland.