Timing of Penokean orogeny redefined: U-Pb ages from prekinematic monazite and xenotime in Lake Superior slates (midwestern USA) indicate deformation spanned 1835−1760 Ma

The southern margin of Laurentia records a prolonged history of accretion of juvenile volcanic arcs, oceanic terranes, and continental fragments beginning in the late Paleoproterozoic. In the southern Lake Superior region (midwestern USA), the onset of accretion has been linked to collision with the Pembine-Wausau and Marshfield terranes during the Penokean orogeny. Although Penokean structural fabrics have not been directly dated, the end of the orogeny is thought to coincide with the intrusion of “posttectonic” granites at 1835 Ma. Here we report U-Pb ages from prekinematic monazite and xenotime poikiloblasts in Paleoproterozoic slates and phyllites in the northern Penokean province, Michigan and Minnesota, United States, which constrain the timing of Penokean fabrics that formed during north-verging folding and thrusting. In the Marquette Range, monazite poikiloblasts wrapped by D1 fabrics in the Siamo and Wewe Slates formed at 1796 ± 5 Ma and 1799 ± 5 Ma, respectively, whereas in the Cuyuna Range, monazite in the Rabbit Lake Formation formed at 1817 ± 8 Ma, constraining the timing of early recumbent folding (F1). Immediately north of the collisional suture (Niagara fault zone), prekinematic monazite in phyllite (Michigamme Formation) from the Menominee Range constrains isoclinal folding to after 1833 ± 8 Ma. In Minnesota, xenotime (1772 ± 8 Ma) in cleaved (S2) shale (Virginia Formation; Mesabi Range) records the onset of F2 folding. The U-Pb monazite-xenotime ages (1833−1772 Ma) indicate that most structural fabrics attributed to Penokean deformation formed after 1835 Ma, suggesting that the Penokean orogeny is significantly younger than widely accepted.