A TECTONIC MODEL FOR EARLY PLIOCENE MAGMATISM IN THE CENTRAL OREGON CASCADES: THE ERUPTION OF SNOW PEAK HAOT LAVAS

Snow Peak is a voluminous (~4.5 km³), dissected basaltic monogenetic shield volcano that sits within the modern Cascade forearc, 50 km west of the current High Cascades. This center represents the westernmost forearc mafic volcanism of the early (<5 Ma) High Cascades in Central Oregon, and predates the Boring Lavas erupted west of Mt. Hood.

New ⁴⁰Ar/³⁹Ar dating constrains the volcanic activity at Snow Peak between 6.05±0.12 Ma and 5.31±0.07 Ma. Snow Peak lavas unconformably overlie western Cascade lavas (~10-17 Ma). Previous K-Ar dating indicated that lavas from the southeast quadrant of Snow Peak were ~3 Ma. Compositionally, Snow Peak lavas consist of low-to-medium-K, high-alumina olivine tholeiite (HAOT) (SiO₂: 49.2-52.1 wt. %, MgO: 5.5-8.8 wt.%, K₂O: 0.12-0.66 wt.%, Al₂O₃: 16.4-18.7 wt.%) that ranges from primitive to moderately evolved (Mg# 51 - 62). Mantle-normalized multi-element plots indicate Snow Peak lavas are generally HFSE depleted and LILE enriched (Ba/Nb: 36.3 - 53.5), with LILE/HFSE enriched relative to High Cascades basalts. Geochemical trends suggest that the lava series may be derived from a single parent magma composition by polybaric fractional crystallization, and estimated depths of segregation of 27 km (~ 9 kb). Estimates of initial water content range up to ~1.8 wt %, based on plagioclase phase compositions.

The new ⁴⁰Ar/³⁹Ar results fundamentally change the tectonic framework of the Snow Peak volcanism. It now appears that they were contemporaneous to the basaltic volcanism of the Deschutes Formation, and initiation of intra-arc rifting of the modern High Cascade arc. In addition, the Snow Peak volcanic center is collinear with the Brother's Fault zone, a diffuse set of NW-trending faults marking the intersection of the extensional Basin and Range Province with the Cascade arc. Estimates of mantle temperature under Snow Peak at the time of magmatism (based on phase equilibria calculations using MELTS and COMAGMAT) are particularly high for the forearc (1270^oC at 27 km). This is consistent with extension and upwelling linked to the initiation of arc rifting – combined with the effects of a zone of weakness (Brothers Fault Zone), which propagated the extension into the western forearc.