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Reconstruction of the 1875–1835 Ma Penokean orogen reveals the presence of a narrow corridor of Archean basement-cored Paleoproterozoic gneiss domes on the southern continental margin of the Superior province between the Paleoproterozoic Niagara suture zone and the Archean Great Lakes tectonic zone just south of Lake Superior. Within the corridor, the gneiss domes are spatially associated with occurrences of amphibolite-facies metasedimentary cover rocks. Metamorphism is dominantly greenschist-facies away from gneiss domes within the corridor, and also outside of the corridor to the north and south. We conducted U-Pb sensitive high-resolution ion microprobe (SHRIMP II) and total-Pb electron microprobe analyzer (EMPA) geochronometry to obtain metamorphic timing constraints on distinct monazite mineral domains from amphibolite-facies rocks sampled across the entire length of the gneiss dome corridor. Based on metamorphic monazite crystallization ages, midcrustal amphibolite-facies metamorphism (M1) peaked around 1830 Ma and was concurrent with Penokean plutonism; subsequent thermal pulses are reliably recorded by monazite growth at ca. 1800 Ma (M2) and again at ca. 1765 Ma (M3), both also coeval with magmatic activity.

We interpret the 1830 Ma M1 metamorphism as recording the time of peak metamorphism after initial burial of the continental margin rocks beneath the accreted Wisconsin Magmatic terrane south of the Niagara fault zone. The 1800 Ma M2 metamorphism coincides with a pulse of late Penokean plutonism recently identified across the region. The youngest 1765 Ma M3 metamorphism immediately precedes widespread and rapid cooling of gneiss dome corridor rocks. We propose that M3 metamorphism occurred during initial exhumation of the tectonically buried continental margin rocks. In our conceptual model, tectonic extrusion of a midcrustal block containing the future gneiss dome corridor accommodated gravitational collapse of overthickened Penokean crust. Elevated country rock temperatures accompanied with profuse melting (i.e., intrusion of the 1775 Ma East-central Minnesota batholith) promoted doming of the lower density Archean basement into the more dense overlying Paleoproterozoic metasedimentary rocks, ultimately enabling its complete decoupling from the remaining lower crust.

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