| Summary: | Prepared for the 85th Annual Meeting of THE GEOLOGICAL SOCIETY OF AMERICA and ASSOCIATED SOCIETIES Minneapolis, Minnesota, 1972. SPECIAL PAPERS: Precambrian geology of the Minnesota River Valley between Morton and Montevideo: Grant, J.A., Pt. 1— Geology and structure; and Pt. 2—Geochronology and geochemistry. The Minnesota River Valley provides a tantalizing window onto the Canadian Shield on the eastern margin of the Great Plains, tantalizing because of the high grade of the metamorphism, and especially because of the antiquity of the rocks there exposed. Essentially, this is a migmatitic terrane of granitic gneisses with lesser amphibolitic gneisses, commonly with pyroxene, and biotite-rich gneisses, which may contain garnet, cordierite, sillimanite, anthophyllite, or hypersthene. Some of the rocks are greater than 3.0 b.y. in age, and they have been involved in metamorphism and deformation at least 2.6 b.y. ago. These events left rocks with a metamorphic grade in the upper amphibolite or granulite facies, and with a major structure that is similar throughout most of the exposed area. Later minor intrusions, dominantly mafic, cut the older rocks, and conglomerate and quartzite of the Sioux Formation of Late Precambrian age locally overlie them. Deep weathering of the gneisses formed a regolith about 100 feet thick, a part of which was reworked in the formation of Cretaceous deposits of sand and clay. Over this came the glacial deposits of the Pleistocene. With the formation of Lake Agassiz, drainage via Glacial River Warren scoured out the precursor of the present valley leaving an underfit present-day Minnesota River and the glimpse of the Precambrian described in the following pages. The granitic gneisses in the vicinities of Morton, Granite Falls, and Montevideo are among the oldest known crustal rocks. Like very ancient rocks in other parts of the world the gneisses have had a complicated history, and metamorphic changes have masked their original characters and obscured their age. Conservatively the age may be given as 3200 or 3300 m. y. Goldich and others (1970) have attempted to probe the metamorphic history and concluded that the gneisses date back to 3550 m.y. ago. Similarly old, or older gneisses (3600 to 4000 m.y.) have been reported from the Godthaab district, West Greenland (Black and others, 1971). Field and more detailed geochronological and geochemical investigations are being continued, and the nature of this work is briefly indicated in following sections.
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