HR: 0800h
AN: V41D-0807    [Abstracts]
TI: Snow Peak, OR: Miocene and Pliocene Tholeiitic Volcanism in the Cascadia Forearc
AU: * Hatfield, A K
EM: hatfiela@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Kent, A J
EM: adam.kent@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Nielsen, R L
EM: nielsenr@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Rowe, M C
EM: michael-rowe@uiowa.edu
AF: University of Iowa Department of Geosciences, 121 Trowbridge Hall, Iowa City, IA 52242, United States
AU: Duncan, R A
EM: rduncan@coas.oregonstate.edu
AF: Oregon State University College of Oceanic/Atmospheric Sciences, 104 COAS Administration Bldg, Corvallis, OR 97331-5503, United States
AB: Snow Peak is a voluminous (>150 km³), glacially dissected shield volcano located approximately 50 km southeast of Salem, OR, with a summit height of 1,310 m above sea level. Snow Peak lies approximately 60 km west of the current High Cascade arc axis. Lavas from the southeast face of Snow Peak have been previously dated using K-Ar at ~3 Ma. New Ar-Ar dating indicates that lavas from the northwest face are ~5.4 Ma, and the summit plug is ~6 Ma. Snow Peak volcanics unconformably overlie western Cascade volcanics aged from middle to late Miocene (~10- 17 Ma). The age of Snow Peak is broadly contemporaneous with the initiation of modern High Cascade volcanism. Snow Peak's location provides a rare opportunity to study magmas produced within the modern High Cascades forearc region. The goal of this investigation is to characterize the composition and timing of volcanism at Snow Peak and the role of volatiles in magma genesis. Hypotheses for the formation of Snow Peak include flux melting associated with the Cascadia subduction zone and/or decompression melting associated with extensional faulting. Preliminary geochemical data on the basalts from Snow Peak indicate that they are low-to-medium-K tholeiites (SiO₂ 47.9-51.7 wt.%, MgO 5.5- 8.3 wt.%, K₂O, 0.36-0.55 wt.%) and that they range from primitive to moderately evolved (Mg# 0.51-0.61). Common phenocryst phases are plagioclase, olivine, and clinopyroxene. Textures are typically hypocrystalline, and fine-grained to porphyritic. Mantle-normalized multi-element plots indicate Snow Peak lavas are generally HFSE depleted and LILE enriched. These data are consistent with a preliminary interpretation of a subduction zone signature, yet the major element composition most closely resembles high alumina olivine tholeiite (HAOT), more indicative of extensional environments. The degree of LILE enrichment is significantly lower than in calc alkaline lavas from the High Cascades and western Cascades. Determining the petrogenesis of this forearc center will include a comprehensive analysis of the volcano's major and trace element geochemistry, and additional age dating to constrain eruption rates. Direct measurement of volatiles in olivine-hosted melt inclusions will complement the major and trace element geochemistry in order to measure pre-eruptive water contents.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting