Montgomery, Carla W., 1951-
M.S. (Master of Science)
Department of Geology
Igneous rocks--Black Hills (S.D. and Wyo.); Geochemistry--Black Hills (S.D. and Wyo.); Geology--Black Hills (S.D. and Wyo.)
The phonolite series of Tertiary intrusives in the Black Hills contains relatively low SiO₂ (58.6-63.7%) and high Al₂O₃ (16.67-20.79%), total iron (as FeO, 2.10-4.26%), Na₂O (6.03-9.00%), K₂O (3.60-9.59%) and LOI (1.09-4.43%). The aegirine rhyolites differ from the calcalkaline rhyolites in their relatively lower silica (67.6-72.7 vs. 69.9-74.5%), CaO (.07-.53 vs. .6-1.46%), LOI (.74-1.09 vs. .77-3.63%) and higher Na₂O (4.88-6.72 vs. 1.8-4.85%). The major element concentrations of the calcalkaline series suggest a possible differentiation sequence of quartz monzonite porphyry - rhyolite porphyry - rhyolite. Normative calculations for this series indicate an anatectic origin. The phonolites contain relatively high concentrations of Ba (633-2,226 ppm) and Zr (301-733 ppm), like other phonolites. The aegirine rhyolites display trends typical of rhyolites, with the exception of Zr. The calcalkaline series also displays typical rhyolitic trends. The Ba and Zr concentrations fall within the lower range of typical rhyolites. Both Sr and Pb isotopic data for the phonolites are consistent with an enriched mantle source. The average initial Sr ratio is .7054±.004. Pb isotopic data resemble those of ocean island basalts, with ²⁰⁶Pb/²⁰⁴Pb = 18.60-19.03 , ²⁰⁷Pb/²⁰⁴Pb = 15.50-15.68 and ²⁰⁶Pb/²⁰⁴Pb = 38.21 -38.55. The low ²⁰⁷Pb/²⁰⁴Pb and high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb suggest a history of at least 3 stages for the source region. Rb-Sr whole rock analyses of the aegirine rhyolites yielded an isochron age of 49.2±2.7 Ma., with an initial ratio of .70751.0024. Both Pb and Sr isotopic analyses indicate a phonolitic/lower-crustal mix, with the lower crust being the dominant component. The calcalkaline series has initial ⁸⁷Sr/⁸⁶Sr ratios of .7063-.7101, higher than those for the phonolites and within the limits of the lower crust. The relatively unradiogenic lead (²⁰⁶Pb/²⁰⁴Pb = 17.02-18.86, ²⁰⁷Pb/²⁰⁴Pb = 15.20-15.83 and ²⁰⁸Pb/²⁰⁴Pb = 36.45-38.87) indicates a lower-crustal affinity. The Sr and Pb isotopic data for the monzonite porphyries indicate an enriched source similar to that of the phonolites. However, major and trace element data suggest a possible relationship to the calcalkaline series.
Beintema, Craig, "Geochemical and isotopic variation of the igneous instrusives of the northern Black Hills, South Dakota" (1986). Graduate Research Theses & Dissertations. 3268.
viii, 144 pages
Northern Illinois University
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