Rb-Sr Whole-Rock Isotopic Geochemistry and Petrology of the Tonalitic-Trondhjemitic Phase of the Archean Quartzo-Feldspathic Gneiss, Sacred Heart-North Redwood Area, Minnesota River Valley
1977-06-11
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Rb-Sr Whole-Rock Isotopic Geochemistry and Petrology of the Tonalitic-Trondhjemitic Phase of the Archean Quartzo-Feldspathic Gneiss, Sacred Heart-North Redwood Area, Minnesota River Valley
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1977-06-11
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Abstract
Quartzo-feldspathic gneisses of Archean age crop out in southwestern Minnesota, within the Sacred Heart-North Redwood area of the Minnesota River Valley. In this area, three units of migmatitic quartzo-feldspathic gneiss exist in a grossly layered sequence with the uppermost unit being an interlayered unit of biotite-rich gneiss and amphibolite. Contained within the quartzo-feldspathic gneisses are layers and rafts of amphibolite. The units are several hundreds of meters thick, and have been folded into an easterly trending, open-fold system with a wave length of a little more than a kilometer. The gneisses are in the upper amphibolite facies, having been involved in a period of high-grade metamorphism prior to 2650 m.y., when they were intruded by quartz monzonite bodies. The composition of the quartzo-feldspathic gneiss varies from tonalitic to quartz monzonitic. Eleven whole-rock samples of the tonalitic phase, or gray gneiss, were sampled. The medium- to coarse-grained, granular samples of gray gneiss range in texture from homogeneous to banded and are composed primarily of plagioclases, with major amounts of quartz and biotite, and occasionally hornblende. Each sample has less than three percent of K-feldspar. The major element chemistry of the samples is similar to other Archean intrusive and gneissic rocks of tonalitic composition and K is less than 2.21 percent in each sample. In detail, the regular chemical variations of these samples allows them to be placed into two chemical groups, suggesting that the gneisses are not all cogenetic. Rb in the whole-rock samples varies from 15-88 ppm, and averages 58, while Sr varies from 191-944 ppm and averages 584. The abundance of Sr can be used to further subdivide the chemical groups. The eleven samples do not form an isochron. Model ages were calculated, assuming an initial 87Sr/86Sr ratio of 0.700, and show much variation. Three samples have model ages greater than 3400 m.y., while the bulk of the others are clustered at 2970 rn.y. and also 2700-2770 m.y. and may reflect metamorphic events. It is not clear from the limited amount of data to what degree metamorphic events have affected the istopic systems and model ages, hence the age or ages of the gray gneiss in this area cannot be uniquely resolved.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Randall Reid Seeling in partial fulfillment of the requirements for the degree of Master of Science, June 1977.
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Seeling, Randall Reid. (1977). Rb-Sr Whole-Rock Isotopic Geochemistry and Petrology of the Tonalitic-Trondhjemitic Phase of the Archean Quartzo-Feldspathic Gneiss, Sacred Heart-North Redwood Area, Minnesota River Valley. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/212478.
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