Synchronous Deposition of Paleoproterozoic Superior-Type Banded Iron-Formations and Volcanogenic Massive Sulfides In the Lake Superior Region: Implications for the Tectonic Evolution of the Penokean Orogen

The tectonic setting of the Animikie basin in the Lake Superior region and Superior-type banded iron-formations within it have been variously interpreted as an intracratonic rift basin, passive continental margin, and collision-related foreland basin. This diversity of interpretations reflects in part past uncertainty in the age of the iron formations and in the relationship of the Animikie basin to the Penokean magmatic terranes immediately to the south in northern Wisconsin. New high-precision U-Pb zircon dating and the recent recognition of an ejecta layer related to the Sudbury impact event, which represents a precise 1850 Ma timeline, provide a new framework for interpreting Paleoproterozoic events in the region. In particular, data now show that the type occurrences of Superior-type banded iron-formations were deposited circa 1875 Ma in extensional basins along the southern margin of the Superior craton, now northern Michigan and northernmost Wisconsin. Immediately to the south, in northern Wisconsin, a prolifically-mineralized belt of volcanogenic massive sulfide (VMS) deposits was forming in the Pembine-Wausau magmatic terrane at this same time, based on new dating of rhyolites that host the deposits. Our tectonic reconstruction indicates that the Pembine-Wausau terrane was already accreted to the Superior Province by 1875 Ma, and that the VMS deposits were forming during calc-alkaline volcanism in a back-arc extensional basin as subduction and further accretion migrated to the south. At this same time, banded iron-formations were deposited on the immediately adjacent continental margin, also largely in extensional basins. The iron-formations are coeval and in part interfinger with volcanic rocks of continental affinity. These two contrasting types of mineralization are both linked to a back-arc setting in which extension affected both the accreted arc as well as the adjacent southern margin of the Superior Province.