ZIRCON GEOCHRONOLOGY INTERPRETATION OF MAXIMUM DEPOSITIONAL AGE OF THE OGISHKEMUNCIE SEQUENCE, MINNESOTA, USA

Here we present LA-ICPMS U-Pb geochronology data of zircons separated from a diamictite unit in the Archean Ogishkemuncie Sequence of the Knife Lake Group near Alpine Lake in the Boundary Waters Canoe Area Wilderness of northeastern Minnesota, USA. The Ogishkemuncie unconformably overlies the 2689 ± 1 Ma Saganaga Tonalite batholith with a well-developed saprolite occurring at the top of the tonalite. The Ogishkemuncie folds are east-trending with vertical fold axes, and extend more than 80 km to the southwest. It contains alternating layers of polymict conglomerate, well-sorted channelized sandstone, and poorly sorted diamictite with boulders of the Saganaga Tonalite. These are interpreted to have been deposited in an alluvial fan/fluvial system in an isolated rift basin and then subsequently were deformed (the tonalite-Ogishkemuncie contact is vertical) and metamorphosed to lower greenschist facies. Zircons separated from each unit were zoned and no metamorphic overgrowths were noted. The detrital zircon age spectrum for the diamictite (n=48) ranged from 2629-2914 Ma, with a unimodal age peak of 2689 Ma and with a maximum age of deposition of 2643.9±8.2 Ma. The peak age is consistent with the bulk of the sediment being derived from the underlying Saganaga tonalite and is consistent with existing interpretations of the depositional environment. The maximum age of deposition, however is younger than the ~2680 Minnesotan orogeny, which is the last documented Archean deformational and metamorphic event in the region. We interpret that the Ogishkemuncie Sequence is latest Archean or perhaps even Paleoproterozoic in age. A Paleoproterozoic age would permit correlation to Huronian strata further to the east, which were deposited following late Archean rifting of the Superior province. Fabric orientation and the style of deformation and metamorphism is consistent with the ~1800-1900 Ma Penokean orogeny, which is well known in areas to the south and east.