RESULTS AND IMPLICATIONS OF MONAZITE GEOCHRONOLOGY FROM THE CENTRAL PENOKEAN OROGEN, WI & MI

Few reliable constraints exist on the timing of Paleoproterozoic metamorphism in the Lake Superior region, an area of classic and complex metamorphic map patterns. The metamorphic pattern within the northern deformed continental margin is represented by a narrow corridor (~100 km wide) of fault-bounded panels and concentric metamorphic isograds (nodes). Within the corridor, regional greenschist facies in the Michigamme terrane increase southward to kyanite and sillimanite grade within the panels, and also culminates to sillimanite and andalusite grade within the adjacent nodes. Immediately south of the main suture, the Wisconsin Magmatic Terrane (WMT) exhibits dominantly upper greenschist facies and localized amphibolite facies metamorphism. In order to determine the timing of metamorphism within these areas, we conducted U-Th-Pb geochronometry via ion microprobe analyses of in situ monazite grains from five samples across this central portion of the orogen. Two schist samples from the southern Peavy metamorphic node give metamorphic ages of 1832 ± 3 Ma and 1828 ± 6 Ma; both samples also yield evidence for a less pronounced metamorphic event at ca. 1795 Ma. To the west, a kyanite schist from the Watersmeet panel yields a dominant 1822 ± 4 Ma metamorphic age and a secondary metamorphic event at ca. 1795 Ma. The older dominant metamorphism in both areas is associated with a widespread thermal pulse formed in response to accretion-induced crustal thickening. Peak metamorphism was also accompanied by Penokean granitic magmatism; the ca. 1795 Ma metamorphism also coincides with a recently identified late Penokean magmatic pulse. A staurolite schist from the northern Republic node yields a notably younger 1760 ± 5 Ma metamorphic age, and a staurolite schist located near Merrill, WI from within the central WMT, gives a metamorphic age of 1744 ± 3 Ma. Similar geon 17 amphibolite facies metamorphism is well documented in the western Penokean orogen (MN) adjacent to the 1775 Ma East-central Minnesota batholith. The Republic and Merrill dates document geon 17 amphibolite facies metamorphism well away from known geon 17 plutons. This indicates the likelihood of a widespread Barrovian thermal event likely related to rapid orogenic collapse and crustal stabilization in the southern Lake Superior region.