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.
