Snow Peak, OR: late Miocene to early Pliocene volcanism in the central Cascadia forearc

Abstract

Snow Peak is a voluminous shield volcano located ~50 km west of the axis of the modern Cascade arc. A previously published K-Ar age estimate indicates that the Snow Peak lavas are ~3 Ma. Subsequently, the preliminary research framework for studying the Snow Peak lavas focused on constraining the petrogenesis of the primitive lava compositions, thereby addressing the chemical and thermal structure of the Cascadia forearc mantle. However, ⁴⁰Ar/³⁹Ar age dating conducted for this study indicates that the Snow Peak lavas are ~5.5 – 6 Ma. Consequently, the research framework undertaken for this thesis focused on constraining the petrogenetic pathway of the Snow Peak lavas, thereby addressing the volcano-tectonic development of the early High Cascades. ⁴⁰Ar/³⁹Ar age results for the spatial range of the Snow Peak volcanic center indicate that the volcano formed between ~5.5 and 6 Ma. Whole-rock major and trace element geochemical results indicate that Snow Peak is a monogenetic, basaltic volcanic center that unconformably overlies the Western Cascades. Petrographic analysis coupled with end-member mineral chemistry results indicate that phenocryst phases consist of olivine, pyroxene, and plagioclase. Decompression crystal fractionation of this phase assemblage explains the observed liquid line of descent. Phase equilibria modeling in COMAGMAT of a selected primitive, and a selected daughter sample, indicate that differentiation of the Snow Peak lavas began ~8-10 kb, and ~1150°C – 1200°C, and the Snow Peak daughter formed ~1-2 kb and ~1050°C – 1150°C. A plagioclase hygrometer indicates that the Snow Peak lavas contain ~2 wt % H₂O. These results are consistent with melting of moderately enriched mantle and addition of a relatively small amount of LILE-enriched slab-derived material. The timing of volcanism at Snow Peak correlates to the development of an intra-arc rift, marked by the eruption of voluminous mafic lava flows, such as the Deschutes Formation, and early Mt. Jefferson basalts. Mafic underplating may have played a greater role in Cascadia volcanism during such periods of intra-arc rifting, and such an extensional tectonic setting could facilitate the eruption of basaltic lavas. Additionally, Snow Peak aligns with the NW-trending Brother's Fault zone, although the Snow Peak center lies on the western side of the arc axis. Snow Peak magma may have formed in response to decompression associated with uplift along the Brother's Fault zone, with a fluid flux from the downgoing slab.