Graduation date: 2013Silicic volcanism in the central Oregon Cascade range has decreased in both the size and frequency of eruptions from its initiation at ~40 Ma to present. The reasons for this reduction in silicic volcanism are poorly constrained. Studies of the petrogenesis of these magmas have the potential for addressing this question by providing insight into the processes responsible for producing and erupting silicic magmas. This study focuses on two extensive and well-preserved ash-flow tuffs from within the ~4-8 Ma Deschutes Formation of central Oregon, which formed after the transition from Western Cascade volcanism to the modern High Cascade. Documentation of outcrop extent, outcrop thickness, clast properties, and samples provide the means to estimate a source location, minimum erupted volumes, and to constrain eruptive processes. Major and trace element chemistry of glass and minerals constrain the petrogenesis and chemical evolution of the system.\ud \ud The tuffs selected for this study, the Lower Bridge and McKenzie Canyon, are the first known silicic units originating from the Cascade Arc following the reorganization from Western Cascade to High Cascade Volcanism at ~8 Ma. These eruptions were significant in producing a minimum of ~5 km³ DRE each within a relatively short timeframe. These tuffs are sourced from some vent or edifices related to the Three Sisters Volcanic Complex, and capture an early phase of the volcanic history of that region. The chemical composition of the tuffs indicates that the Lower Bridge erupted predominately rhyolitic magma with dacitic magma occurring only in small quantities in the latest stage of the eruption while McKenzie Canyon Tuff erupted first as a rhyolite and transitioned to a basaltic andesite with co-mingling and incomplete mixing of the two magma types. Major and trace element concentrations in minerals and glass indicate that the basaltic andesite and rhyolite of the McKenzie Canyon Tuff were well convected and stored in separate chambers. Geothermometry of the magmas indicate that the rhyolites are considerably warmer (~850°) than typical arc rhyolites. Trace element compositions indicate that both the Lower Bridge and McKenzie Canyon Tuff experienced mixing between a mantle derived basaltic melt and a rhyolitic partial melt derived from gabbroic crust. Rhyolites of the Lower Bridge Tuff incorporate 30-50% partial melt following 0->60% fractionation of mantle derived melts. The McKenzie Canyon Tuff incorporates 50-100% of a partial melt of a mafic crust with up to 15% post mixing fractionation.\ud \ud The results of this study suggest that production of voluminous silicic magmas within the Cascade Arc crust requires both fractionation of incoming melts from the mantle together with mixing with partial melts of the crust. This provides a potential explanation for the decrease in silicic melt production rates from the Western Cascades to the High Cascades related to declining subduction rate. As convergence along the Cascade margin became more oblique during the Neogene, the consequent slowing rate of mantle melt production will result in a net cooling of the crust, inhibiting the production of rhyolitic partial melts. Without these partial melts to provide the rhyolitic end member to the system, the system will evolve to the mafic melt and fractionation dominated regime that has existed along Cascadia throughout the Quaternary
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