Granite and rhyolite ranging in age from ∼1.8-1.34 m.y. are known to underlie much of mid-continental North America west of the Phanerozoic Michigan Basin. U-Pb zircon ages of similar rocks exposed adjacent to the Grenville Province in Ontario suggest that the mid-continental terranes extend northeast beneath and beyond the Michigan Basin. Prevalence of similar U-Pb zircon ages obtained from metaplutonic rocks southeast of the Grenville Front and affected by the 1.2-1.0 Ga Grenvillian Orogeny prompts the interpretation that deformed equivalents of the mid-continental Proterozoic rocks form the bulk of the crust in the Central Gneiss Belt of the Grenville Province east of Lake Huron. Farther east, pre-1.34 Ga terranes appear to be tectonically occluded by 1.28-1.22 Ga volcanic, sedimentary, and plutonic rocks of the Central Metasedimentary Belt, themselves deformed, metamorphosed, and intruded by younger Grenvillian plutons.