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    RI-69 Reexamination of the Minnesota River Valley Subprovince with Emphasis on Neoarchean and Paleoproterozoic Events
    (2014-03-06) Southwick, David
    The Minnesota River valley subprovince (MRV) is a fragment of Mesoarchean continental crust that was sutured to the southern margin of the Superior craton about 2,600 m.y. ago. The suturing event induced widespread regional metamorphism and local anatexis in a dominantly orthogneissic crust and ended with the emplacement of numerous granite plutons. In the Paleoproterozoic era the MRV was a tectonically rigid part of the cratonic foreland with respect to Penokean (geon 18), Yavapai (geon 17), and Mazatzal (geon 16) accretionary events. As such, it was affected by crustal extension and the emplacement of mafic dikes associated with the ca. 2,070 Ma opening of the pre-Penokean ocean. Subsequently, internal shear zones that had formed during Neoarchean docking of the MRV crustal block were reactivated in response to stresses applied during cycles of Paleoproterozoic stretching and subsequent compression from the south and southeast. Most of this reactivation is inferred to have taken place between 2,000 and 1,750 Ma. The Minnesota segment of the Great Lakes tectonic zone, the Neoarchean suture, was not significantly reactivated, whereas the Appleton shear zone and the Yellow Medicine shear zone both were. Six sets of mafic dikes were emplaced in the interval between 2,070 and ca. 1,750 Ma. Two sets that were emplaced early in the interval are the southwesternmost members of the pre-Penokean Kenora–Kabetogama/Fort Frances dike swarm. Two and perhaps four younger dike sets were emplaced during a period of vigorous crustal heating and magmatic activity that affected much of the MRV in early- to mid-geon 17. Numerous plugs and small plutons also were emplaced in early- to mid-geon 17. These intrusions range in composition from peridotite to granite and are comparable to rock types within and satellitic to the East-Central Minnesota batholith; they are most abundant in the eastern and southern parts of the MRV, relatively near the inferred Penokean and Yavapai tectonic fronts. Transtensional stress during the extensional stage of the Mazatzal orogenic cycle generated differential subsidence of crust south of the Yellow Medicine shear zone and produced en echelon fault-bounded depressions that became depocenters filled by supermature clastic sediment ancestral to the Sioux Quartzite. The Sioux Quartzite was deposited, lithified, and hydrothermally altered over a prolonged time interval that may have begun as early as ca. 1,730 and ended as recently as ca. 1,280 Ma.