A geochemical, isotopic, and geochronologic study of oligocene magmatism, Never Summer Mountians, north-central Colorado

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, ϵNd(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, ϵNd (T) ~ -6). Field observations combined with geothermobarometry of the most mafic rocks of the MRS reveal that this pluton was originally a shallowly intruded (<3 km), ~ 1 km thick, vertically zoned laccolith. The vertical zonation exhibited by the MRS was likely produced in situ in the upper few kilometers of the crust by the successive underplating followed by mixing of mafic magmas (as evidenced by enclaves in the structurally lowest parts of the MRS) with an earlier arriving silicic magma, (possibly a less differentiated magma derived from the same source as the MCS). The MCS's composition is identical to that of other high-silica rhyolites worldwide (low Sr, Ba, Eu and low LREE/HREE). Volcanic rocks consist of early andesites (~61 wt % SiO2, ϵNd(T) -2.0 - -4.0, 87 Sr/86Sr(T) 0.705 to 0.709) and culminating high-silica rhyolites (72-79 wt % SiO2, ϵNd(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.