Precambrian rocks are exposed throughout an area of about 7500 square miles in northern Michigan. Of this area, approximately 3300 square miles is underlain by relatively unmetamorphosed Upper Precambrian (Keweenawan), about 2700 square miles by metamorphosed Middle Precambrian (Huronian), and about 1500 square miles by metamorphosed Lower Precambrian (“Archean”). Flat-lying or low-dipping strata of Paleozoic age overlap the Precambrian along the east margin of the area.
The metamorphism, which is shown most clearly by mineralogic changes in the Hu-ronian sedimentary rocks, especially in the iron formations and the graywackes and slates, and in the post-Huronian basic intrusive rocks, occurred during a post-Huronian-pre-Keweenawan interval of orogeny and minor granite intrusion. At least one earlier period of major, intense metamorphism of pre-Huronian age (Algoman?) is recognized. In near-by areas of Wisconsin and Minnesota, a later thermal metamorphism is related to the intrusion of the upper Precambrian Duluth gabbro and equivalent masses;it is restricted to the immediate vicinity of the gabbroic bodies.
Zones of metamorphic intensity delineated by chlorite, biotite, garnet, staurolite, and sillimanite isograds or their equivalents are partly or completely developed around four nodes, two of which lie in immediately adjacent parts of Wisconsin. The zones range in width from less than a mile to about 30 miles, and metamorphism of low grade is areally dominant over that of intermediate and high grade. The principal iron-ore deposits of the region—the “soft ores”—are restricted to the lower metamorphic zones, whereas the material considered suitable as low-grade concentrating ore (“taconite”) is restricted to the areas of high-grade metamorphism. The influence of metamorphism on grain size appears to be the controlling factor.
Analysis of thermal gradients inferred from the metamorphic zonation indicates that the heat required for metamorphism must have been derived from subjacent bodies of magma, but the metamorphic patterns show no clear relation to regional structure or to the exposed granitic intrusions of the same orogenic cycle. The granite is most abundant in the areas of strongest metamorphism, but field evidence shows that most of the masses were intruded after the metamorphism. Field evidence also shows that deformation and metamorphism are independent variables in the orogenic scheme for this particular region.
Retrograde metamorphism, not related to shearing, is widespread.