CONTRASTING MAGMATIC EVOLUTIONS OF THE THREE SISTER VOLCANOES, CENTRAL OREGON CASCADE ARC, USA

The Three Sisters stretch, one of the most magmatically active segments of the Oregon Cascade Arc, includes the Three Sisters volcanoes proper (North, Middle and South) as well as the shields and cinder cones of the Mount Bachelor Chain and McKenzie Pass. The Three Sisters record a magmatic transition from fundamentally basaltic andesitic activity in older North Sister products and the volcanoes of the Mount Bachelor Chain and those of McKenzie Pass to more diverse activity at Middle Sister, and unusually silicic activity for a Cascade volcano, including rhyolite lava, at South Sister.

North Sister magmas reflect magmatic evolution of basaltic andesite through crystal fractionation, mostly of olivine, and incorporation of crustal melts. Middle Sister, although mostly composed of basaltic andesite and andesite, produced dacitic magma through Assimilation Fractional Crystallization processes involving incorporation of silicic melts. South Sister developed a subjacent magma chamber under the southern half of the volcano where andesitic magmas evolved, dominantly through fractional crystallization, to dacite and rhyolite. A prominent compositional gap between dacite and rhyolite at South Sister and depletion of middle and heavy Rare Earth elements in rhyolite suggest that amphibole was prominently involved in fractionation processes involving separation of silicic magma from a gabbroic crystal mush. Silicic magma eventually formed a stagnant cap in the system, where it evolved through small scale fractional crystallization. Rhyolite magma was finally heated to elevated temperatures prior to eruption, presumably through influx of less evolved magma into the base of the stagnant silicic cap and was erupted in late rhyolite lava of Rock Mesa and the Devil’s Hill Chain.

South Sister lies along strike of the Northwest Rift of Newberry Volcano, which forms the northwestern terminus of the High Lava Plains trend of south-central Oregon. The increased magmatic activity of the Three Sisters stretch and unusual abundance of silicic products, including rhyolite, of South Sister may reflect the arrival and intersection of the High Plains magmatic anomaly with the Cascade Arc.