PRELIMINARY PETROGRAPHIC AND GEOCHEMICAL INVESTIGATIONS OF (PRESUMED) ARCHEAN BASEMENT TO THE MARSHFIELD TERRANE EXPOSED IN THE WISCONSIN RIVER VALLEY (WOOD AND PORTAGE COUNTIES), WISCONSIN

The Penokean orogeny, located on the present-day upper Great Lakes region, is the oldest Paleoproterozoic orogenic belt preserved on the southern margin of the Archean Superior craton. It is often interpreted as a modern style accretionary orogen with an island arc(s) and a microcontinent colliding with and deforming a passive margin developed along the craton. The southernmost outboard colliding block, the Marshfield terrane, is a continental crustal fragment that includes Archean metamorphic rock, Paleoproterozoic metavolcanic units, and syn- to post-kinematic granitoid intrusions. This basement terrane is poorly exposed due to Phanerozoic cover, with most exposures found in river valleys. The Wisconsin River valley of Wood and Portage counties, between Stevens Point to just below Wisconsin Rapids, includes a variety of presumed Archean lithologies exposed for sampling. Previous work yielded Archean ages of ca. 2870 Ma at Point Edwards, to ca. 2780 Ma at the Linwood Township Quarry near Stevens Point, but lack data useful for tectonic discrimination. This study conducted petrographic and geochemical analyses of five metamorphic rock samples from Wood and Portage counties along the Wisconsin River. Three of the five samples are broadly granitoid gneisses, with one migmatitic biotite quartzofeldspathic gneiss and one biotite amphibolite. All five samples plot in the igneous protolith field of TiO₂ vs. SiO₂ of Tarney. Immobile element discrimination plots after Pearce et al. and Gorton and Schandl suggest that the rocks formed in volcanic arc, and possibly within plate, igneous environments. Due to the uncertainty in protolith and poorly constrained effects of weathering of the samples, these data are non-conclusive; however, they provide further insights into the possible origin of the Marshfield terrane basement and will contextualize the basement for future petrochronologic investigation.