Montgomery, Carla W., 1951-
M.S. (Master of Science)
Department of Geology
Anorthosite--Wisconsin; Petrology--Wisconsin; Granite--Wisconsin
Petrologic, chemical, and isotopic analyses were performed on samples of the Tigerton Anorthosite and the Wolf River Granite from Northeast Wisconsin in an effort to determine magmatic and overprint ages for the anorthosite and how the two rock units are related. Thin section analysis of the anorthosite has shown that it was metamorphosed to the greenschist facies, resulting in the mineral assemblage chlorite - epidote - blue-green hornblende. Textural evidence indicates that the original mineral assemblage was anhydrous and the hydrous phases formed during metamorphism. The major and trace element whole-rock chemistry of the anorthosite is typical of other anorthosites and is characterized by high Al₂O₃, CaO, and K₂O contents, a low MgO/FeO ratio, high Sr/Ba and Ba/Rb ratios, and low Rb/Sr and K/Ba ratios. A comagmatic relationship between the anorthosite and the Wolf River Batholith is very unlikely based on chemical and isotopic data. Rb-Sr whole-rock analysis of the anorthosite has yielded two possible errorchrons depending on whether one particular point is excluded from the regression. The errorchron ages are 1615 ± 630 m.y. and 1825 ± 520 m.y., neither of which may have any geologic significance. The initial ⁸⁷Sr/⁸⁶Sr ratio in both cases is 0.7018. The low initial ratio suggests that the parent magma was derived by partial melting of a Rb-depleted mantle region and was not significantly contaminated during emplacement. Rb-Sr analysis of mineral separates from the anorthosite yielded an age of 396 ± 78 m.y., which may be due to partial isotopic reequilibration by local permeating fluids. The isochron age of the Wolf River Granite by Rb-Sr whole-rock analysis is 1380 ± 27 m.y. with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7056 ± .0032. Rb-Sr analysis of granite mineral separates yielded similar ages. Pb-Pb analysis of the anorthosite and granite samples yielded negative (future) ages. This indicates either the addition of a radiogenic contaminant or multistage growth of the Pb in two or more source regions with different values of ²³⁸U/²⁰⁴Pb. The results also indicate that the lead in the anorthosite spent some time evolving in a U-depleted mantle region, which is consistent with the Sr initial ratios.
Perkowitz, Gary R., "An isotopic, chemical, and petrologic study of the Tigerton Anorthosite and associated Wolf River Granite, Northeast Wisconsin" (1985). Graduate Research Theses & Dissertations. 1410.
x, 112 pages
Northern Illinois University
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