Neoarchean Deposition, Metamorphism, And Intrusion In Rapid Succession, Vermilion Granitic Complex, Superior Province Of Northern Minnesota

Abstract

Alternating belts of high-grade granite-gneiss and low-grade metavolcanosedimentary granite-greenstone are characteristic of Archean crust (4 – 2.5 Ga). In the Superior province of northern Minnesota, these belts are represented as the Wabigoon, Quetico, and Wawa subprovinces. The formation of Archean metamorphic terrains is poorly understood, as estimates of Archean lithospheric thermal and mechanical conditions are debated, hindering consensus about petrogenetic models. Pressure-temperature-time (P-T-t) reconstructions are useful in exploring the evolution of metamorphic rocks present in such cases, and rely on aluminous minerals and accessory phases for (1) determining P-T paths, and (2) isotopic age dating. In many cases, the tempo and style of Archean crustal metamorphism are enigmatic as these terrains commonly lack the aluminous metamorphic assemblages useful for P-T-t reconstructions. Metamorphic rocks of the Vermilion Granitic Complex (VGC) in the Quetico subprovince locally host a number of key aluminous phases, reactions, and accessory minerals that facilitate study of the formation and metamorphic evolution of this terrain. Metamorphic assemblages, reactions, and garnet-biotite thermometry indicate peak P-T conditions in the middle amphibolite-faces, at ~ 620 °C and 4-5 kbar. New zircon and monazite U-Pb ages provide constraints on deposition, metamorphism, and the timing of granitic intrusion. Detrital zircon age populations in metamorphic country rock record mean igneous provenance ages of about 2.75 Ga and 2.72 Ga, establishing a maximum age of protolith deposition. Monazite ages document supracrustal metamorphism between about 2.69 Ga and 2.67 Ga. Zircon ages of crystallization for granitic lithologies show complex behavior, but suggest intrusions were 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 an interbedded greywacke-argillite 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 my.