Eruptive history of Middle Sister, Oregon Cascades-Product of a late Pleistocene eruptive episode

New mapping, geochemistry, and argon geochronology illuminate a brief, remarkably silicic episode set in a mafic segment of the Cascade arc. Middle Sister was constructed during a 35-k.y. episode in the late Pleistocene from mafic, intermediate, and silicic eruptions adjacent to the primarily rhyolitic South Sister. Eruptions in the Three Sisters volcanic cluster prior to 50 ka were exclusively mafic (<57 wt% SiO₂), and several basaltic andesite lava flows can be traced to Middle Sister or a predecessor volcano (prior to 150 ka). Lava flows erupted 50–37 ka at Middle Sister and on its periphery were chemically diverse, with abundant basaltic andesite, a high-silica rhyolite flow, and an andesite produced from mixing of a rhyolite and mafic magma. Abundant rhyolite and rhyodacite erupted in this interval also at South Sister. Eruptive activity paused at Middle Sister 37–27 ka but continued at South Sister with large volumes of dacite and andesite lavas. Middle Sister erupted mafic, intermediate, and silicic lava flows 27–15 ka and then ceased to erupt. Calculated eruptive rates for the entire Three Sisters volcanic cluster quadrupled from ∼0.2 to ∼0.8 km³/k.y. between 50 and 15 ka, largely owing to the eruptions focused at Middle and South Sisters, and the cluster has now returned to its modest eruptive output, mainly away from the stratovolcanoes. Time–volume results for the volcanic cluster are compared to studies of other well-mapped, well-dated stratovolcanoes. Nearly all centers record similar eruptive-volume behavior with long histories of relatively constant output punctuated by short episodes of voluminous eruptions. In addition to the Three Sisters, two of these centers (Mt. Mazama, Crater Lake, Oregon, and Puyehue/Cordon Caule in the southern Andes) record significant compositional changes associated with the voluminous eruptive episodes.