Minnesota lies astride two Precambrian W (lower Precambrian) terranes that differ in age, rock assemblages, metamorphic grade, and structural style. In northern Minnesota, greenstone-granite complexes 2,700 to 2,750 m.y. old, which are typical of the majority of the Canadian Shield, are exposed. These rocks trend northeastward, dip steeply, and typically produce narrow curvilinear aeromagnetic and gravity anomalies. In southwestern Minnesota, much older (3,550 m.y.) granulite-facies granitic and mafic gneisses, which are moderately flat-lying and produce relatively broad magnetic and gravity anomalies, are exposed through a window in the Phanerozoic cover in the Minnesota River valley. Similar gneiss, some of which has been dated radiometrically, is exposed sporadically in central Minnesota and is considered part of the same terrane as the gneiss in the Minnesota River valley. Judged from the outcrop pattern and available geophysical data, the boundary between the two terranes trends diagonally across central Minnesota, from approximately latitude 45°30′N at the western boundary to the vicinity of Duluth, on Lake Superior.
We postulate that the volcanic and sedimentary rocks in the greenstone terrane accumulated adjacent to the pre-existing gneiss terrane, which 2,700 m.y. ago was part of a sialic protocontinent of moderate size. There is no geologic or geochemical evidence that these rocks were deposited on a sialic crust. The tectonic environment that existed 2,700 m.y. ago, when the greenstone-granite complexes were formed, is not known; there is no compelling evidence that they were formed in a plate tectonic regime.