Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution

Abstract

Metamorphism along the southern margin of the Archean Superior Province has been historically attributed to the Penokean orogeny. A narrow corridor of amphibolite facies rocks north of the main suture does record 1.83–1.80 Ga metamorphic monazite U–Th–Pb ages that mark the culmination of arc accretion. However, subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest Penokean crustal thickening: the tectonically buried Archean–Proterozoic continental margin. In Minnesota, new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the ∼1.775 Ga East-central Minnesota batholith at moderate depths. In northern Wisconsin and upper peninsula Michigan metamorphic monazite growth at 1.78–1.745 Ga (and far from geon 17 intrusions) reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca. 1.76 Ga Yavapai-interval accretion, not solely Penokean induced crustal collapse. South of the Penokean suture, Penokean terrane rocks were twice metamorphosed to upper greenschist facies; first during Yavapai accretion and again during geon 16 Mazatzal accretion. Geon 16 overprinting also affected a small part of the continental margin in the northeast orogen, the Peavy metamorphic node. South-directed basement thrusts there likely accommodated substantial Mazatzal foreland shortening, suggesting thick-skinned deformation. Mazatzal amphibolite facies metamorphism occurred throughout Iowa and southernmost Wisconsin (south of the Baraboo quartzite). 40Ar/ 39Ar mineral cooling ages from eastern Wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1.47 Ga Wolf River batholith, in part reflecting its rapid emplacement at shallow crustal levels. A local area of anomalously young <1.20 Ga 40Ar/ 39Ar mica ages away from known Midcontinent Rift exposures probably reflect shallow crustal level reheating during rifting, not exhumation. The pattern and degree of Proterozoic metamorphism preserved across the northern interior of the North American midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1.87 and 1.47 Ga. Collectively, the geo- and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of Proterozoic metamorphism in this region. These results demonstrate a changing tectonic environment from active margin to continental interior as Laurentia grew southward and stabilized over a few hundred million years.