Chapter 27 - Paleoarchean Gneisses in the Minnesota River Valley and Northern Michigan, USA

Abstract

Paleoarchean to Mesoarchean gneisses are exposed in the Minnesota River Valley (MRV) and in northern Michigan, USA. The MRV rocks include a complex of granitic migmatites, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton gneiss and the Montevideo gneiss. U–Pb geochronology for the MRV rocks shows that they have a long and complex history, with initial crystallization at c.3500Ma and other igneous events at c.3420Ma, c.3385Ma, and c.3140Ma. Many gneisses are clearly now mixtures of material formed, probably by both tectonic and magmatic processes, during at least the first three of these events. Hf and Nd isotopic systematics suggest derivation from precursor material that had resided in the crust for at least 100m.y. prior to 3500Ma and further demonstrate closed-system melting and remelting for about 1000m.y. The ancient gneisses were intruded by synkinematic granite at 2603Ma and by syenogranite and aplite at 2590Ma, igneous events likely related to granulite facies metamorphism recorded in paragneisses and events related to suturing of the MRV terrane to the Superior craton. Rocks exposed in northern Michigan include tonalitic gneiss and granite. Although they are less well-studied, the tonalitic gneiss is c.3600Ma and a late-granitic intrusion is c.2650Ma. Thus, the two terranes are similar and were probably once continuous, constituting a major feature on the southern margin of the Superior craton. Similarly ancient rocks are known on the northern margin of, and within, the Superior craton. Thus, the MRV rocks were likely a crustal component that was sutured to the Superior Province during the 2.6Ga Sacred Heart Event.