Publication Date


Document Type


First Advisor

Walker, Jim

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geology


Geochemistry--Missouri; Geology--Missouri


he Cope Hollow Formation (CHF) is the uppermost sequence of volcanic rocks exposed in the St. Francois Mountains (SFM). The formation consists mainly of >100 m of rhyolitic ash flow tuffs with interbedded basaltic lava flows. The CHF represents a marked change in the magmatism of the SFM, both chemically and mineralogically. The CHF rhyolites are higher in Na20, MgO, MnO, A1203, and Ti02 and lower in Si02 and K20, compared to underlying SFM rhyolites. The most notable mineralogic distinctions of the CHF rhyolites are their greater abundance of plagioclase phenocrysts and the lack of quartz phenocrysts, with the exception of the uppermost units. Within the CHF, four basaltic lava flows are present. These basalts and other basic rocks collected from several locations in the SFM help constrain the tectonic setting of the SFM which was dominated by acidic magmatism. There are essentially three groups of mafic rocks in the SFM, from oldest to youngest these are the SFM Andesite Suite, the Silvermines Mafic Suite and the Skrainka Mafic Suite, the latter two after Sylvester (1984). On AFM diagrams, all of these groups fall in the tholeiitic field. Incompatible element ratios of all three groups (NB/La, Hf/Ta, La/Ta, Ti/Y, Hf/Th, Th/Yb, Th/Nb, Zr/Y, and Ta/Yb) consistently show arc affinities. The ratios Zr/Y and Ta/Yb are greater than 3 and 0.1 respectively, suggesting an overall continental margin affinity. There are, however, some distinctions in incompatible element ratios among the 3 mafic suites, suggesting that the magmatic source, and perhaps some aspect of the tectonic setting, changed with time. For example, Zr/Nb ratios differ by a factor of three between the Silvermines and Skrainka groups, suggesting a shift towards a more depleted source for the younger Skrainka suite.


Includes bibliographical references (pages [155]-164)


xi, 182 pages




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