AGE AND SIGNIFICANCE OF DEFORMED POST-PENOKEAN MAFIC DIKES FOR GROWTH AND OVERPRINTING OF PROTEROZOIC PROVINCES IN CENTRAL WISCONSIN, SOUTHERN LAKE SUPERIOR REGION

Strongly deformed and metamorphosed Archean and Penokean igneous rocks exposed in central Wisconsin (at Biron Dam and Conant’s Rapids) occur at the nexus of where the geon 14 Wolf River batholith intrudes the geon 17 Yavapai Spirit Lake tectonic zone. Determining the timing, nature, and relative contribution of post-Penokean overprinting events is critical for properly ascribing structures, strain features, and degree of metamorphism/reheating to their correct tectonomagmatic event. Following Penokean accretion, intrusion of east-northeast trending diabase dikes at 1817 ± 2 Ma coincided with a 30 m.y. gap in orogenic felsic magmatism at 1835-1805 Ma (between the Penokean and Yavapai orogenies) during which only more mafic magmatism is documented (1813 Ma Hines quartz diorite and Wissota Dam tonalite). Trace element data from the dikes exhibit a pronounced subduction signature reflecting derivation from mantle that was previously involved in Penokean subduction and arc accretion. The 1835-1805 Ma interval in the southern Lake Superior region thus represents a fundamental period of tectonic switching after the Penokean orogeny, when mafic magmatism was generated in an extensional back-arc setting during the initiation of north-directed Yavapai subduction. Subsequent 1805-1750 Ma metaluminous to peraluminous granitic magmatism could be related to a slab window or slab breakoff event during Yavapai subduction.

Geothermometry data document strong penetrative ductile deformation of the dikes and host rocks at temperatures of ~700 °C, previously attributed to Penokean deformation. Instead, published and new thermochronologic data indicate that the dikes and their Penokean and Archean host rocks were strongly ductilely deformed during 1750-1720 Ma accretion of the Yavapai arc onto Penokean/Archean rocks along the Spirit Lake tectonic suture, a Yavapai paleosuture. Geon 16 Mazatzal overprinting of the accreted Penokean and Yavapai provinces was widespread south of the Archean-Proterozoic suture zone, but of overall lower metamorphic grade (greenschist facies) in Wisconsin. Lastly, the thermal effects of the 1475 Ma Wolf River batholith was limited to a relatively narrow (10-15 km wide) contact zone surrounding the batholith, consistent with its shallow depth of intrusion.