THE OLDEST ROCK IN WISCONSIN? SINGLE GRAIN ZIRCON U/Pb AND Lu/Hf DATA FROM WISCONSIN RIVER VALLEY GNEISSES HELP CHARACTERIZE THE ANCIENT MARSHFIELD TERRANE, PENOKEAN OROGENY

The Penokean orogeny is a Paleoproterozoic orogeny preserved on the southern margin of the Superior craton. It is interpreted as an accretionary orogen, juxtaposing exotic terranes against the Paleoproterozoic continental margin units of the craton. The southernmost unit, the Marshfield terrane, is interpreted as a continental crustal fragment including Archean gneissic basement, Paleoproterozoic metavolcanic units, and syn- to post-tectonic granitoid intrusions. The terrane difficult to study due to widespread Phanerozoic cover, with most exposures limited to outcrop in river valleys. Satisfactory exposures of the eastern Marshfield terrane crop out in the Wisconsin River valley, between the cities of Stevens Point and Point Edwards. Previous geochronology of gneisses here yielded Archean ages of ca. 2.87 Ga at Point Edwards and ca. 2.78 Ga at a quarry near Stevens Point. Paleoproterozoic ages are recorded from deformed tonalites at the Conants Rapids and Biron Dam localities. Recent zircon geochronology from amphibolites in the Biron Dam locality yield ages at ca. 1815 Ma; however, recent work investigating the Archean basement in this area is lacking. This study expands upon these data utilizing single grain LA-ICP-MS U/Pb and Lu/Hf analyses to refine age control on the eastern Marshfield terrane, and investigate its crustal evolution. We present new U/Pb ages ranging from 3268 Ma for granodioritic gneiss at Wisconsin Rapids, to 2750 Ma from a mylonitic gneiss south of Stevens Point; in addition, a granitic gneiss from Biron Dam yields an age of 1853 Ma. Lu/Hf data reveal a complex crustal evolution history. Zircon from the Wisconsin Rapids gneiss reveals εHf_(i) values ranging from -7 to +9, with mean and median values near CHUR. Zircon from the Neoarchean gneisses are subchondritic with a variable spread in εHf_(i), with means around -2. These zircons may record reworking of older Marshfield crust, mixing juvenile material and ca. 3.3 Ga crust similar to the Wisconsin Rapids sample. Zircon from the Biron Dam gneiss are distributed evenly across CHUR, with εHf_(i) vales ranging from -10 to +7, again suggestive of reworked and juvenile inputs. As an interesting aside, the Wisconsin Rapids gneiss may be one of the oldest rocks in Wisconsin, providing a data point for testing reconstructions for the Penokean orogeny.