1. Paleoarchean Gneisses in the Minnesota River Valley and Northern Michigan, USA
- Author
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Joseph L. Wooden, Mark D. Schmitz, Aaron M. Satkoski, M. E. Bickford, Scott D. Samson, George D. Kamenov, Paul A. Mueller, and Robert L. Bauer
- Subjects
Craton ,geography ,geography.geographical_feature_category ,Paleoarchean ,Geochemistry ,Metamorphism ,Syenogranite ,Migmatite ,Granulite ,Geology ,Gneiss ,Terrane - 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.3500 Ma and other igneous events at c.3420 Ma, c.3385 Ma, and c.3140 Ma. 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 100 m.y. prior to 3500 Ma and further demonstrate closed-system melting and remelting for about 1000 m.y. The ancient gneisses were intruded by synkinematic granite at 2603 Ma and by syenogranite and aplite at 2590 Ma, 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.3600 Ma and a late-granitic intrusion is c.2650 Ma. 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 2.6 Ga Sacred Heart Event.
- Published
- 2019
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