GEOCHRONOLOGIC AND MICROSTRUCTURAL CONSTRAINTS ON THE PROTEROZOIC EAU PLEINE SHEAR ZONE IN CENTRAL WISCONSIN

The Eau Pleine Shear Zone (EPSZ) is a WNW-ESE striking band of mylonitic rocks of Archean and Paleoproterozoic ages exposed mainly along the Eau Pleine River in Marathon County, central Wisconsin. The EPSZ has been interpreted as a fundamental tectonic suture formed in the ca. 1.85 Ga Penokean orogeny, which involved the accretion of an island arc complex and an older microcontinent fragment to the margin of the Archean Superior Craton. Geographically, the EPSZ is flanked to the north by the magmatic Pembine-Wausau Terrane (1.90 – 1.86 Ga) and on the south by the gneissic Archean Marshfield Terrane (2.8-2.5 Ga). The mylonites of the EPSZ formed from a wide variety of igneous and sedimentary protoliths including granitoids, pelites, and tuffs.

This study of the Eau Pleine Shear Zone included field mapping and sample collection, petrographic and microstructural analysis, and U-Pb dating of zircons. The orientation of feldspar porphyroblasts in one sample suggest that the EPSZ is a reverse fault with the south side acting as the hanging wall, indicating north-vergent Penokean transport. At one EPSZ outcrop, pseudotachylyte is found in a ca. 50 cm band with pervasive quartz veining, suggesting that seismic failure and related fluid pumping occurred at some stage along this mid-crustal shear zone. U-Pb ages of zircons from two samples of EPSZ mylonites are 2465± 24 Ma and 1877± 31 Ma, consistent with the late Archean and Penokean host rocks. Unexpectedly, both of these samples have lower concordia intercepts at ca. ~250 Ma. Unusually high ²⁰⁴Pb values in the zircons are evidence of an episode of fluid influx, possibly in late Paleozoic time. Collectively, these field and microstructural observations and U-Pb ages suggest that the EPSZ may have had a longer and more complicated history than previously thought.