The Eau Claire Volcanic Complex (ECVC) is located within the Marshfield Terrane of the Penokean Orogen. Although it represents a key component of the regional tectonic model, the ECVC remains poorly understood due to limited outcrop exposure due to Paleozoic sediments and glacial till. It has traditionally been interpreted as ~1.8-billion-year-old Penokean arc-related magmatic complex and has played an important role in defining the tectonic evolution of the Marshfield Terrane. This study integrates U–Pb zircon geochronology, whole-rock major and trace element geochemistry, petrography, and field-based mapping to refine the tectonic interpretation of the ECVC. U–Pb zircon ages from intrusions and associated supracrustal rocks yield crystallization ages of ~2.5 billion years old, indicating that portions of the ECVC previously mapped as Paleoproterozoic are instead Archean in age. Whole-rock geochemical signatures and petrographic observations support interpretation of these units as components of an Archean greenstone belt. These findings suggest a more complex magmatic and tectonic history for the Marshfield Terrane than previously recognized.
