LATE PALEOPROTEROZOIC ANIMIKE BASIN DEPOSITS, UPPER GREAT LAKES REGION, REINTEREPRETED AS BOTH PENOKEAN AND YAVAPAI OROGENIC SEDIMENTATION

Late Paleoproterozoic Animikie Basin sedimentary rocks in the upper Great Lakes region are usually interpreted solely as Penokean (1875-1835 Ma) foredeep deposits. Much of the sequence is only very weakly metamorphosed and mildly deformed. However, along its southern margin in east-central Minnesota, the sequence is strongly deformed into what are interpreted to be refolded nappes. The twice deformed sequence occurs south of a similar sedimentary sequence that has undergone a single deformation; the contact between them interpreted to be a Penokean thrust fault and all of the deformation assumed to be Penokean in age (Holst, 1985). Given the wealth of new data which document Yavapai-age magmatism, metamorphism, and deformation overprinting the Penokean orogen, we reinterpret the contact instead to be an angular unconformity separating twice-deformed Penokean age sedimentary rocks (to the south) from once-deformed Yavapai age foredeep deposits (to the north).

Geochemical analyses of samples collected across the contact reveal a concentrated grouping of trace element data from the southern samples and a much larger spread from the northern samples, suggesting a more variable source for the northern sedimentary sequence. Nd isotopic data across the contact reveal a more juvenile source for rocks south (i.e., below) of the contact and an older more depleted Archean source directly north (i.e., above) of the contact. We interpret the twice-deformed southerly sequence to be Penokean in age and derived from the accreted Penokean magmatic arc terrain. However the once-deformed northerly sequence was likely deposited after the Penokean orogeny during emplacement of Archean cored gneiss domes and rapid exhumation of a Yavapai age mid-crustal batholith.

Our reinterpretation of a classic Penokean tectonic contact has important ramifications for interpreting the inventory of structures throughout the upper Great Lakes region. For instance, the late open upright folds and the sedimentary rocks that are folded at Thomson Dam, a popular fieldtrip site in Minnesota, both may be manifestations of Yavapai orogenesis and not classic features of Penokean orogenesis.