THE GENERATION AND RAPID EVOLUTION OF NEOARCHEAN CRUST IN THE VERMILION GRANITIC COMPLEX, SUPERIOR PROVINCE OF NORTHERN MINNESOTA

The formation and evolution of Archean crust is debated in part because Archean lithospheric thermal and mechanical regimes are poorly known. Further, the conditions and tempo of metamorphism are poorly understood because Archean metamorphic rocks commonly lack the aluminous mineral assemblages useful for constraining P-T-t histories, thus hindering petrogenetic modeling. Metamorphic rocks in the Vermilion Granitic Complex (VGC) of the Quetico subprovince in the western Superior province locally contain a number of key aluminous phases, reactions, and accessory-mineral geochronometers. The VGC is composed of semi-pelitic metamorphic country rocks that host migmatite and multiple granitoid lithologies. New country rock detrital zircon ages from biotite schist show a provenance history dominated by 3200-2700 Ma sources, with age populations at ~2750, ~2715, ~2710, and ~2700 Ma. The youngest detrital zircon populations establish a maximum age of protolith deposition at ~2690 Ma. Sillimanite stability coexisting with muscovite, and garnet-biotite thermometry together indicate peak country rock P-T conditions in the middle amphibolite-facies, at ~620 ˚C and 4-5 kbar. Monazite U-Pb ages from the schists record peak metamorphism at ~2675 Ma. A new Lac La Croix Granite zircon age of 2668 ± 10 Ma constrains the timing of batholithic intrusions in the central VGC, but zircon ages of this and other granitic lithologies show complex U-Pb behavior resulting from both inheritance and post-crystallization Pb-loss. Field relationships indicate granitic intrusions are syn- to post-metamorphic. Together, these data demonstrate a temporally rapid evolution of the VGC, including consanguineous: (1) crystallization of plutonic source rocks; (2) exposure and erosion; (3) deposition of a semi-pelitic protolith; (4) metamorphism of supracrustal rocks to the middle amphibolite facies; and (5) emplacement of syn- to post-metamorphic granitoid intrusions. The provenance, metamorphic, and granitic age constraints indicate that the sequential formation of an igneous sediment source, development of depositional basins, metamorphism, melt intrusion, and migmatite formation occurred over a short time interval of as little as ~30 m.y.