EVIDENCE FOR RECENT H2O-RICH VIOLENT STROMBOLIAN ERUPTIONS IN THE CENTRAL OREGON HIGH CASCADES AT COLLIER, FOUR-IN-ONE, AND YAPOAH CONES

Cinder cone eruptions are the dominant form of Holocene volcanism in the Central Oregon segment of the High Cascades. We examine eruption products from three young (< 2300 yr b.p.) monogenetic vents (Collier, Four-in-One, and Yapoah) in Central Oregon and find significant compositional similarities to eruptions characterized as violent strombolian (e.g. Paricutin). Each eruption produced at least one scoria cone (≤ 185 m high), blocky lava flows (< 0.2 km³) and a tephra blanket extending up to several km away from the vent. Bulk tephra samples exhibit narrow ranges in composition (56-57, 53-55, and 54-56 wt% SiO₂ for Collier, Four-in-One and Yapoah, respectively). Bulk lavas are more silicic than associated tephra (55-65, 55-57, and 54-58 wt% SiO₂ respectively) and show compositional trends extending towards rhyolite xenoliths incorporated into erupted lavas. These observations suggest: 1) tephra production was restricted to the early phases of the eruptions and 2) eruptions occurred over time periods long enough to permit extensive crustal assimilation. Additionally, volatile contents in olivine-hosted (% Fo host: 77-85) melt inclusions from tephra associated with each eruption are high (≤ 4 wt% H₂O, ≤ 1200 ppm CO₂). High pre-eruptive volatile contents and extensive crustal assimilation recorded in lavas suggest that monogenetic eruptions in Central Oregon may be comparable (in both eruption style and duration) to Paricutin, Mexico: a 9-yr eruption (1943-1952) characterized by intermittent, ash-rich eruption columns (≤ 8 km high) and extensive lava and tephra production. The presence of H₂O-rich basaltic andesite magmas (and Paricutin-like eruptive activity) in Central Oregon is unexpected. Magmas in the Cascades arc (north of Mt. Shasta) have been considered H₂O-poor, a consequence of the elevated temperature and resulting shallow dehydration of the young, subducting slab. Our work shows that this view is erroneous and mafic eruptions in this region have the potential to be more explosive than previously realized.