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Rodey Batiza, Larry A. Haskin, Jill D. Pasteris, Klaus J. Schulz (USGS)
Date of Award
Restricted Access Dissertation
Doctor of Philosophy (PhD)
Igneous and metamorphic mafic rocks form a volumetrically subordinate component of the 1480 Ma old granite-rhyolite terrane of Missouri. The igneous rocks are present in the St. Francois Mountains and a drillcore from Shannon county and can be subdivided into two groups. The Silver Mines Mafic Group exhibits some "calc-alkaline" chemical affinities that may be the result of crustal contamination by a source with a bulk intermediate to felsic composition. The contaminant probably is not the exposed granites or rhyolites of the St. Francois Mountains. The Skrainka Mafic Group exhibits a "transitional" chemistry. Major element modelling suggests that members of the group are related by (low pressure) fractional crystallization of the observed phenocryst phases—plagioclase + olivine + apatite—but trace element modelling indicates that the petrogenetic relationships are more complex. The metamorphic mafic rocks of the terrane are present as unexposed thick bodies. They are suggested to be lower crustal granulites that have been uplifted to upper crustal levels. The hypothesis that the 1480 Ma old granite-rhyolite terrane of Missouri is part of a broad, 1500 to 1400 Ma old granite-rhyolite belt that extends from at least Labrador to Missouri (e.g., Silver et al. 1977) is consistent with the results of this study. However, the belt may be characterized by bimodal felsic-mafic magmatism, including the Silver Mines Mafic Group in Missouri and the Bruce River mafic volcanics in Labrador. The bimodal magmatism possibly was the result of contemporaneous crustal and mantle melting. In Missouri, crustal melting may have been induced by mafic underplating. Although speculative, the mafic granulites could represent a portion of the mafic underplate. The 1500 to 1400 Ma old belt probably formed in an extensional tectonic setting. The Basin and Range province of the western United States may be the most similar modern analogue. Between 1400 to 1200 Ma ago, an incipient continental rift may have formed along the axis of the 1500 to 1400 Ma old magmatic belt. Mafic magma emplaced during the rifting event possibly included the Skrainka Mafic Group of Missouri, the Harp dikes and Seal Lake Group of Labrador, and some of the Gardar rocks of southwest Greenland.
Sylvester, Paul Joseph, "Geology, Petrology, and Tectonic Setting of the Mafic Rocks of the 1480 MA Old Granite-Rhyolite Terrane of Missouri, USA" (1984). Retrospective Theses and Dissertations. 62.