Petrology of the precambrian igneous and metamorphic rocks in the vicinity of St. Cloud, Minnesota
Dittmann, Fred Melvin
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Igneous rocks in the vicinity of St. Cloud, Minnesota range in composition from tonalite to granite. The general order of intrusion of these rocks as determined from field studies, from oldest to youngest, is tonalite, granodiorite, and granite. Cutting the granite are dikes of granite porphyry which in turn are cut by dikes of basalt or diabase. The granitic rocks have been assigned to the Penokian orogeny, 1600-1900 m.y. ago, on the basis of K-Ar and Rb-Sr age determinations in biotite. The basalt dikes have been dated by the K-Ar method in the range from 1280 to 1570 m.y. ago. The present investigation provides petrographic background for radiometric studies in progress. The principal conclusions of this thesis follow: 1) Regional metamorphism of a graywacke-shale sequence developed the staurolite-almandine subfacies of the almandine-amphibolite facies. 2) Granitic intrusions related to the regional metamorphism locally affected the grade, but the intrusions are not the cause of the regional metamorphism. 3) inclusions of the Thomson Formation in tonalite near Hillman have been upgraded from the regional staurolite-almandine subfacies with the development of a quartz-andesine-cordierite-biotitegarnet assemblage which can be assigned to a higher level of cordierite-amphibolite facies or to a lower level of the granulite facies. In contrast with the regional metamorphism, the local metamorphism of the Thomson Formation inclusions in the tonalite is a high-temperature and low-pressure metamorphism. 4) The tonalites appear to be older than the granodiorite, but outcrops are few and small, and the great variety of rock types makes lithologic correlation more unreliable than normally expected. 5) Numerous examples of red granite cutting and including angular blocks of granodiorite indicate that the granite is younger. The red granite west of the Mississippi River appears to be less sheared than that east of the river.