PALEOPROTEROZOIC CONDITIONS INITIATING DEPOSITION OF IRON FORMATION

Following the rise of atmospheric oxygen, iron formations (>15 wt % iron) largely disappear from the geologic record, but reappear at ca. 1.8 Ga. Additionally, the distinctive morphologies and fossil assemblages in the ca. 1.8 Ga iron formations document a biotic world not found in older rocks. This co-evolution of life with unique Earth conditions suggests that the tectonic environment that allowed the reemergence of iron formation must also have been conducive to diverse microbial communities.

To identify the global and local tectonic setting that initiated these unique formations and the associated biotic assemblages, we carry out U/Pb zircon geochronology, geochemistry, and present new stratigraphic and mapping relationships of the Ironwood Iron Formation and the Emperor Volcanics located in the western Upper Peninsula of Michigan. This work is coupled with petrographic examinations to understand the original mineralogy, textures and possible biological influences on the local environment. We integrate the results from this locality into an updated geochemical and geochronological framework of the Penokean orogeny, specifically focusing on the relationship between basin formation within the Superior craton and the onset of calc-alkaline magmatism, further south, in the Pembine-Wausau terrane. Finally, the feasibility of a pure magmatic trigger for iron formation deposition is explored via basin box modeling that combines the tectonic model with magmatic constraints. These results have important implications for interpreting the atmospheric oxygen history and evolutionary trajectory of the Paleoproterozoic Era.