REINTERPRETATION OF PALEOPROTEROZOIC SEDIMENTATION AND DEFORMATION IN EAST-CENTRAL MINNESOTA

New discoveries from the Paleoproterozoic rock record in the Upper Great Lakes region have significantly modified our understanding of the geologic and tectonic history of the area. Recognition of abundant 1740-1815 Ma metamorphic and plutonic pulses and similar age detrital zircons in some Paleoproterozoic sedimentary sequences reveal an important Yavapai-age equivalent overprint previously attributed solely to Penokean (1875-1835 Ma) orogenesis. In e-c Minnesota, deformed Paleoproterozoic Animikie fore-arc basin sedimentary rocks show an increase in metamorphic grade and strain southward toward an exhumed plutonic-gneiss dome terrane. Holst (1984, Geology) recognized two distinct structural zones within this sedimentary sequence, a northern zone characterized by a single well-developed cleavage and a southern terrane containing two cleavages. Holst hypothesized that the map trace separating twice deformed rocks (south) from once deformed rocks (north) represented a tectonic/deformational front. Holst (1984) and more recently Sun et al. (1995, Tectonophysics), considered the entire deformed sedimentary sequence to have been deposited prior to both deformations, which they inferred to be Penokean in age. Alternatively, we propose that an angular unconformity may separate the once-deformed/twice-deformed units and that only the early deformation (involving formation of north-directed recumbent fold nappes) is Penokean in age. In this model, younger Yavapai-age equivalent exhumation of the plutonic-gneiss dome terrane in the south led to renewed sedimentation followed by folding of both sedimentary sequences. We note that this alternative interpretation is consistent with the gneiss dome being a post-Penokean structure and with a post-Penokean southerly source indicated by the detrital zircons from the upper portions of the Animikie basin. Geochemical data, petrographic analysis, and reinterpretation of detailed aeromagnetic/field data and cross-section construction can be used to test this alternative interpretation. Refining the Paleoproterozoic sedimentary/deformational history may help clarify how the Cuyuna Iron Range, which resides in the twice-deformed terrane, fits into the overall scheme of iron deposition in Minnesota, including the larger Mesabi Iron Range.