Snow Peak, Oregon: Latest Miocene low-K tholeiite volcanism in the Cascadia forearc

Abstract

Snow Peak, Oregon, is a moderate size basaltic shield volcano (50–52wt.% SiO2, >7.4km3) located within the forearc of the Cascadia subduction zone, ~50km west of the current arc front. Herein we present new whole rock geochemistry, mineral chemistry and 11 new 40Ar/39Ar ages, together with petrologic modeling that allow us to constrain the timing and origin of volcanism.In contrast to previous K–Ar ages that suggested volcanism occurred at ~3Ma, our new 40Ar/39Ar ages show that Snow Peak formed between 5.3 and 6million years ago. The volcano lies unconformably on ~30Ma volcanic rocks of the Western Cascades. Volcanism occurred over a total duration of <0.5–1Ma, and at eruption rates (~0.008–0.013km3/ka), lower than those observed in large Cascade shield volcanoes. Snow Peak lavas derived from a single, or restricted set of primary magma compositions and evolved via crystal fractionation of olivine+pyroxene+plagioclase over a range of pressures equivalent to crustal depths of ~3–35km, consistent with fractionation occurring primarily during crustal transit or residence. The most evolved Snow Peak lava can be produced by ~50% crystallization from a primary magma with >14wt.% MgO. Snow Peak lavas have trace element characteristics transitional between the calc-alkaline basalt (CAB) and low-K tholeiite (LKT) primary magma types recognized throughout the Cascade Range, but are closer to LKT and are classified as such. Estimates based on phase equilibria models and plagioclase hygrometers suggest that the primary magmas contained moderate amounts of water (1.5–2wt.%), consistent with LILE/HFSE ratios that are greater than MORB values.Snow Peak is part of a widespread suite of LKT magmas that erupted between 5–8Ma throughout the central Oregon Cascade Range in response to intra-arc rifting, and Snow Peak shows that LKT magmatism at this time extended well into the forearc of the central Oregon Cascade Range. Overall LKT magmas of this age occur over a distance of ~120km normal to the arc front. We speculate that the presence of Snow Peak may relate to the western extension of the Brothers Fault Zone into the Cascade forearc.