253-1 Linking the Structural and Metamorphic Evolution of Paleoproterozoic Crust in Central Laurentia with Later Far-field Tectonic Events

Abstract:

The Penokean orogeny (1890-1830 Ma), best exposed in the Lake Superior region of Michigan, Minnesota, and Wisconsin, is a discrete event marking accretion of the Wisconsin Magmatic Terranes to the southern margin of Laurentia. However, new geochronology indicates deformation and metamorphism in the Penokean belt continued throughout the Paleo- and Mesoproterozoic, after the locus of accretionary activity in southern Laurentia had shifted hundreds of kilometers to the south. Whether this later period of deformation in the continental interior is coupled with these distal coeval accretionary events remains an outstanding, but central question, for understanding the drivers of intracratonic reworking in proto-North America. We test if and how far-field accretionary tectonics following the Penokean orogeny contributed to prolonged modification of crust in central Laurentia by integrating pressure-temperature-time-deformation (P-T-t-D) histories using high-precision and spatially resolved geochronology. In central Minnesota, detrital zircon U-Pb ages show protoliths of metasedimentary rocks were deposited by ~1820 Ma, and garnet Lu-Hf and monazite U-Pb geochronology indicates quick burial to amphibolite-facies conditions by ~1800 Ma. Microstructural relationships between garnet, monazite, and matrix fabrics indicate as many as four periods of fabric development, the last coinciding with uplift of cratonic basement in the McGrath Gneiss dome. This metamorphism and deformation immediately precede juvenile granitic magmatism in the adjacent East-Central Minnesota Batholith starting at ~1790 Ma. Farther east in the Penokean orogenic belt, kyanite-bearing rocks record higher grade metamorphic conditions slightly earlier at ~1820 Ma, followed by more protracted cooling through ~450°C by ~1740 Ma. These results, with previously published P-T-t-D data elsewhere in the Penokean belt, illustrate far-field accretionary tectonics following the Penokean orogeny played a major role in modifying the cratonic interior. The ages of prograde-to-peak metamorphism reveal crustal thickening lasting at least to ~1750 Ma, likely propagated from younger convergence far south of the Lake Superior region. Overlapping prograde and cooling ages between 1830-1750 Ma reflect exhumation during thickening, indicating crust modification via several cycles of synorogenic collapse. Additionally, the spatiotemporal relationship between magmatism and basement uplift in central Minnesota suggests orogenic collapse may have been aided by magmatic and/or thermal crustal weakening. Together, these processes reveal far-field stresses from continued convergence on Laurentia’s southern margin drove a dynamic orogenic system in the continental interior throughout the Paleoproterozoic.

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