Evolution of silicic volcanism following the transition to the modern High Cascades, Deschutes Formation, central Oregon

An understanding of the controls on silicic volcanism within convergent margin environments has important implications for crustal growth and modification during subduction. In the central Oregon Cascade range silicic volcanism has generally decreased in both size and frequency of eruptions over the last ~40 million years. Despite the general decrease, an increased abundance of silicic volcanism is observed from 5-8 Ma, corresponding to the transition from the Western Cascades to High Cascades volcanic regime. In order to constrain the processes that lead to formation of silicic magmas at this time we have studied the petrogenesis of two extensive and well-preserved ash-flow tuffs from this time period hosted within the Deschutes Formation of central Oregon. The Lower Bridge (LBT) and McKenzie Canyon Tuffs (MCT) produced ~5 km3 each of magma of predominantly rhyolitic and basaltic andesite composition. Both include large volumes of rhyolite, although the MCT also contains a significant mafic component. Both tuffs are normally zoned with mafic ejecta concentrated upsection. Geothermometry also shows that the rhyolitic component in both magmas was relatively hot (~830 degrees C). Distribution, thickness, welding facies, and paleoflow indications from imbricated pumice suggest that both eruptions derive from the same source region, probably near the present day Three Sisters complex, and were likely produced from the same magmatic system. Variations in major and trace element geochemistry also indicate that the magmas involved in both eruptions were produced through fractionation and mixing of mantle melts with a silicic partial melt derived from melting of mafic crust. Production of these voluminous silicic magmas required both crystal fractionation of incoming melts from the mantle, together with mixing with silicic partial melts derived from relatively hot mafic crust. This observation provides a potential explanation for the decrease in silicic melt production rates though time in the Cascades. As convergence along the Cascade margin became more oblique during the Neogene, the consequent slowing rates of mantle melt production resulted 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 evolved to the mafic melt dominated regime that has existed along Cascadia throughout the Quaternary. Rifting after the transition to the High Cascades probably increased crustal temperatures sufficiently to promote crustal melting in the period immediately following this transition.