A Geochemical, Isotopic, and Geochronologic Study of Oligocene Magmatism, Never Summer Mountains, North-central Colorado

Jacob, Kristin Halberg

Graduate Thesis Or Dissertation

A Geochemical, Isotopic, and Geochronologic Study of Oligocene Magmatism, Never Summer Mountains, North-central Colorado Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/q524jn91z

Abstract

The processes by which silicic magmas are generated and how plutons evolve are fundamental to understanding the dynamics of Earth¡¦s magmatism. Entwined in these issues is the question of how volcanic rocks relate to contemporaneous epizonal plutons. Field observations, major- and trace-element geochemistry, Sr, Nd, and Pb isotopic data, and U-Pb zircon and 40Ar/39Ar geochronologic data were used to determine the petrogenesis of the intermediate Mt Richthofen stock (MRS) and the granitic Mt Cumulus stock (MCS) and to assess their relationship to the extrusive rocks at the ~28 Ma Never Summer igneous complex, north-central Colorado. Whole rock studies of the 28.892 +/- 0.014 Ma MRS reveal that it is compositionally zoned (55-67 wt % SiO2, fÕNd(T) -0.5 to -5.7, 87Sr/86Sr(T) 0.7049 to 0.7119), while the 28.020 +/- 0.011 Ma MCS has a uniform bulk composition, equivalent to that of high silica rhyolite (~77 wt % SiO2, fÕNd(T) ~ -6). Field observations combined with geothermobarometry of the most mafic rocks of the MRS reveal that this pluton was originally a shallowly intruded (fÕNd(T) -2.0 - -4.0, 87Sr/86Sr(T) 0.705 to 0.709) and culminating high-silica rhyolites (72-79 wt % SiO2, fÕNd(T) -3.6 to -5.1, 87Sr/86Sr(T) ~0.712). The emplacement of the MRS and MCS is contemporaneous with volcanic activity (biotite and sanadine 40Ar/39Ar ages ranging from 28.05 to 29.28 Ma); however, there is no evidence of a direct link between any of the melts or magmas associated with either of the plutons and any of the volcanic rocks.

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