PETROTECTONIC EVOLUTION OF PALEOPROTEROZOIC GRANITIC ROCKS ACROSS THE 1.8 GA CENTRAL PENOKEAN OROGEN, NORTHERN MI & WI

Recent U-Pb zircon geochronology of Paleoproterozoic post-Penokean granitic rocks of northern Michigan and Wisconsin, historically interpreted as an "anorogenic suite," has revealed a distinct age trend: magmatic pulses apparently migrated southward from ca. 1800 to 1750 Ma, after cessation of the Penokean orogeny. Yavapai-aged subduction slab rollback has recently been hypothesized to explain this magmatic pattern. Geon 17 granitoid bodies intrude the Archean gneissic basement, Proterozoic metasedimentary marginal sequences, and an accreted juvenile arc terrane rocks across the breadth of the orogen. Granite samples from eight localities were analyzed petrologically and geochemically, using major-, trace-, and rare-earth element analysis, to discriminate the tectonic setting into which they were emplaced. Major-element classification indicates a calc-alkaline to shoshonite trend for all localities sampled, likely indicating subduction-related genesis. Nevertheless, trace element tectonic discrimination results for the granitoids correlate to the local terrane lithology into which they were emplaced. Granites (Humboldt, Montello) intruding Archean gneissic basement are categorized as within-plate granite (WPG); bodies (Park Falls) intruding Proterozoic metasedimentary sequences are classified as collisional granite (COLG); and granites (e.g., Radisson, Lugerville) intruding the juvenile island arc terrane are classed as volcanic arc granite (VAG). Rare-earth element data show the COLG as the most evolved and WPG the least, highlighting relative continental crust evolution/component. These results are consistent with previous work on cogenetic rhyolites from Wisconsin. In this study, the geochemical analyses did not illustrate an age or geographical trend as expected, but rather a correlation of source area and/or relative crustal contribution interpreted as products of subduction induced melting across a variable source terrane.