PALEOPROTEROZOIC AGE FOR METASEDIMENTARY ROCKS IN JACKSON COUNTY, WISCONSIN, SOUTHERN LAKE SUPERIOR PROVINCE, USA

The Archean rocks of the Black River valley are part of the Marshfield Terrane, which accreted to the southern Superior Province during the 18 Geon Penokean Orogeny. The oldest known rocks in the Marshfield Terrane are the 29 Geon Hatfield Gneiss, which is intruded by younger granite and gabbro, and is overlain by a succession of metasedimentary and metavolcanic rocks that includes schist, amphibolite, iron formation and quartzite. The age of these supracrustal rocks was previously uncertain, and interpreted to be either Archean or early Proterozoic in age. Detrital zircon geochronology was conducted on a quartzite unit within the iron formation at the Jackson County Iron mine to determine the maximum deposition age and sedimentary provenance. Zircons were analyzed by LA-ICPMS methods at the University of Arizona Laserchron Center. Eighty zircons were analyzed, and ranged in age from 2437-3696 Ma. The maximum depositional age is 2475 Ma. Prominent age peaks occur at 2920, 2660, and 2575 Ma. The 2920 Ma zircons were likely derived from the Hatfield Gneiss, which occurs about 20 km to the north. The 2660 and 2575 Ma zircons may have been derived from undated granites that intrude the Hatfield Gneiss. The 2475 Ma maximum age of deposition indicates that the supracrustal rocks of the Marshfield terrane are Huronian rather than Animikean or Archean in age. These rocks are similar in age, lithology, and metamorphic grade to continental margin rocks in the Black Hills of South Dakota (near Nemo and may have been originally correlative. We interpret that the Marshfield Terrane supracrustal rocks were deposited as a continental margin succession that was locally sourced and was later deformed and metamorphosed during the Penokean Orogeny as the Marshfield Terrane collided with the southern Superior Province.