THE PERSISTENCE OF NORTH SISTER BASALTIC ANDESITE AND IMPLICATIONS FOR DEEP CRUSTAL PROCESSES IN THE CENTRAL OREGON HIGH CASCADES

North Sister Volcano of the Central Oregon High Cascades is of interest for its abundant, homogenous, and incompatible element-depleted basaltic andesite lavas, tephra, and dikes (53 to 55 wt% SiO₂, 0.62 to 0.78 wt% K₂O) that give insight to deep crustal processes in the Cascades Arc. North Sister, the oldest and most mafic of the Three Sisters, is located at the maximum extension of the Central Oregon High Cascade intra-arc graben and at its intersection with the Brothers Fault Zone/High Lava Plains trend. The fractured crust has provided conduits for multiple mafic melts, including diverse primitive mantle melts, as well as those that have reacted with the deep crust. The incompatible element concentrations of North Sister basaltic andesite are in fact lower than most primitive mantle melts in the region, except for depleted high alumina olivine tholeiite (HAOT). Fractionation alone of a HAOT can not produce a North Sister basaltic andesite because the 45 to 90% fractionation required for major element models produce excessively high incompatible trace elements (Rb, Zr, and REE). Partial melting of amphibolite (with or without garnet) is too enriched in incompatible trace elements to account for North Sister basaltic andesite. Instead, we propose a mantle-derived HAOT interacts with deep mafic crust by assimilation of a depleted tonalite produced by melting of a gabbro, leaving a pyroxenite residue. One thick, largely conformable stack of thin lavas on North Sister’s northeast ridge dates between 574.3 (±97.1) ka and 191.2 (±28.7) ka, indicating that North Sister’s distinctive composition is persistent. The proximal center Little Brother, overlaps in age with North Sister (an upper lava yielded 172.8 ± 29.0 ka), yet has a basaltic andesite more enriched in incompatible elements as is more typical of the High Cascades and is exemplified by Mount Washington (Hughes and Taylor, 1986, Conrey et al, 2000). In contrast, the Matthieu Lake Fissure, a series of vents, originating at North Sister and trending N10?E for about 8.5 km, is younger (15.1 ± 11.5 ka), but includes compositions very like North Sister and a compositional suite consistent with lower crustal fractionation of North Sister basaltic andesite.