MIDDLE SISTER: THE YOUNGEST STRATOVOLCANO IN THE COMPOSITIONALLY-DIVERSE THREE SISTERS VOLCANIC CLUSTER, OR

The Three Sisters volcanic cluster in the central Oregon Cascades consists of stratovolcanoes with stark compositional and timing contrasts. Middle Sister eruptive products range in composition from basaltic andesite to dacite (67% SiO₂), with a high-silica rhyolite flank vent. Dacite and andesite lavas ranging from 48 to 27 ka form the base of Middle Sister; the most silicic unit—the high-silica rhyolite of Obsidian Cliff (76-77% SiO₂)—erupted 38 ka at its NW toe; and small-volume dacite flows and domes on the N, NW, W and S flanks erupted between 27 and 21 ka. The upper half of Middle Sister, a stack of thin, phenocryst-rich, vesicular basaltic andesite lavas (~52.5% SiO₂), erupted over a short time interval and was followed soon after by eruption of dacite lavas on the N and S flanks between 19 and 14 ka. Vesicular textures make dating the Middle Sister basaltic andesite difficult; however, precise dating of bracketing dacite units constrain the eruption as older than 19.1 ± 1.7 ka and younger than 21.4 ± 1.9 ka. Middle Sister appears old due to its glacially exposed eastern face, but since the eruption dates to the Last Glacial Maximum, this erosion must have been accomplished in a short time period. The adjacent South Sister volcano has a more symmetrical appearance and has long been considered to be the youngest of the Sisters; however, its youngest summit eruption occurred about 22 ka, earlier than Middle Sister's voluminous mafic eruption, and several thousand years before Middle Sister's last eruption. The two volcanoes began to grow at about the same time, with South Sister erupting rhyolite and dacite over the past 50 ka along with intervals of andesitic eruptions between ~35 and 22 ka (Fierstein et al., this meeting). North Sister is a long-lived mafic center (52-55% SiO₂), and its 55 ka summit unit is its youngest central-vent eruption. The abundance of rhyolite and dacite lava at the two youngest Sisters is unusual among Cascade stratovolcanoes. Abundant young mafic volcanism in this segment of the arc (e.g. Bachelor chain, McKenzie pass lavas) also suggests unusually high magmatic input, perhaps related to the influence of the adjacent Oregon High Lava Plains volcanism.