RI-31 Pre-Penokean Igneous and Metamorphic Rocks, Benton and Stearns Counties, Central Minnesota

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RI-31 Pre-Penokean Igneous and Metamorphic Rocks, Benton and Stearns Counties, Central Minnesota

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1984

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Minnesota Geological Survey

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Report

Abstract

The pre-Penokean igneous and metamorphic rocks in Benton and Stearns Counties, central Minnesota, include the Richmond Gneiss and three units of formational status that were assigned to the Sauk Rapids Metamorphic Complex {Morey, 1978)--the Sartell Gneiss and mafic enclaves within it, the Watab Amphibolite, and the St. Wendel Metagabbro. Petrologic data reported here support the contention that both the Sartell Gneiss and the Richmond Gneiss may be northern extensions of lower and middle Archean gneisses typically developed in the Minnesota River Valley. The Richmond Gneiss is interpreted as a syntectonic pyroxenebearing quartz monzonite, with many similarities to charnockite, that underwent magmatic crystallization under conditions of the granulite grade. The Sartell Gneiss is a dominantly metasedimentary unit whose mineral assemblages are indicative of the granulite grade. In contrast, the mafic enclaves within the Sartell Gneiss have textures and mineral assemblages that are unequivocally igneous. Because they are intrusive into the gneiss, the enclaves may be considerably younger. It is proposed here that the descriptor Sauk Rapids t1etamorphic Complex be abandoned, and that the names Watab Amphibolite and St. Wendel Metagabbro be replaced by the more general and less formal terms Metadiabase near Watab and glartz Gabbro of St. Wendel Township. The stratigraphic position of the Quartz Gabbro of St. Wendel Township is uncertain. However, the Metadiabase near Wa tab is most likely correlative with mafic volcanic and hypabyssal rocks in the Animikie basin that are early Proterozic in age.

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Previously Published Citation

Dacre, G.A., Himmelberg, G.R. and Morey, G.B., 1984, Pre-Penokean Igneous and Metamorphic Rocks, Benton and Stearns Counties, Central Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 31, 16 p.

Suggested citation

Dacre, George A.; Himmelberg, Glen R.; Morey, G.B.. (1984). RI-31 Pre-Penokean Igneous and Metamorphic Rocks, Benton and Stearns Counties, Central Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/60725.

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