A PALEOMAGNETIC AND STRUCTURAL STUDY OF THE PROTEROZOIC (1868 MA) GABBROIC XENOLITH, MARSHFIELD TERRANE, WISSOTA DAM, WISCONSIN

The mafic rock body exposed below the Wissota dam on the Chippewa river, western Wisconsin, was originally included in the “central Wisconsin dike swarm” and presumed to be a Keweenawan dike despite being folded, sheared, and having xenoliths. Our recent geochronology documents the Wissota gabbro (1868 Ma) instead an early Penokean xenolith with Archean xenocrysts (2602 Ma), rafted into a tonalite pluton at 1805 Ma, intruded by a Yavapai granite (1760 Ma) and uplifted during the Keweenaw-Grenville orogen at 1179 Ma. The host tonalite has a weak foliation that does not penetrate to older gabbroic xenolith. The folded gabbro is part of a larger swam of xenoliths with distinct aeromagnetic patterns. At Wissota, the gabbro includes brittle (folded and sheared joints) and ductile (S-C mylonites) kinematic fabrics that are margin-parallel and margin-normal. AMS and paleopole studies of the tonalite, north and south of the gabbroic xenolith, are very different suggesting shearing along the xenolith. AMS and paleopole studies of the gabbro indicate rotation along the length of the exposure (2 km).