Geology and geochemistry of granitoid rocks in the Archean Northern complex, Michigan, U.S.A.

The Northern complex, located in the Upper Peninsula of Michigan, is an Archean greenstone–granite terrane that lies at the southern margin of the Superior Province. The origin of the plutonic suites in the Northern complex can be interpreted within a plate tectonic model proposed for the Superior Province and related to northward-directed subduction and subsequent collision along the Great Lakes tectonic zone. The following plutonic suites are recognized based on intrusive relationships, as well as textural and compositional differences: (i) gneissic tonalite suite; (ii) foliated tonalite suite; (iii) trondhjemite–granite suite; (iv) hornblendite–syenite suite; and (v) late granite dike suite. Rocks in the gneissic and foliated tonalite suites have lithologic and geochemical characteristics typical of Archean trondhjemite–tonalite–granodiorite assemblages exposed elsewhere in the Superior Province. They were emplaced during a primary deformation event and are interpreted to represent partial melts that formed during north-directed subduction of oceanic crust just prior to collision along the Great Lakes tectonic zone. During a second deformation event, stocks and plugs of the trondhjemite–granite suite, derived by intracrustal melting of amphibolite associated with collision and tectonic thickening, intruded both interior and exterior to a preexisting volcanic portion of the Northern complex. The hornblendite–syenite suite, composed of hornblende-rich syenites to monzodiorites with geochemical features that include high Mg numbers, and elevated Cr and Ni content, was derived from partial melting of the mantle during collision along the Great Lakes tectonic zone. The late granite dike suite, comprising late-stage, muscovite- and biotite-bearing quartz – alkali feldspar pegmatite and finer grained granitic lithologies, represents the last magmatic event in the Northern complex emplaced after collision.