Honors College Thesis
 

Timing of Volcanism in Central Oregon: Dating Tam McArthur Rim Volcano

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https://ir.library.oregonstate.edu/concern/honors_college_theses/jd4732779

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  • The Tam McArthur Rim is a volcanic center located within the Tumalo Volcanic Field (TVF), approximately 25 km west of Bend, OR. The rim comprises of shallowly dipping bedded sequences of intermediate and silicic flows with minor tephra interbedding capped by a rhyodacite dome. Glacial activity has exposed significant portions of the flow stratigraphy. In this study we present new geochronology, petrography, mineral chemistry, and whole rock geochemistry of lava flows from the Tam McArthur Rim. Flow compositions vary significantly, ranging from basaltic andesites to trachyrhyolites. Some flows are aphyric with trachytic flow textures while others are crystal rich, lacking flow textures. Bulk rock geochemistry indicates that these rocks follow a more tholeiitic differentiation trend than typical arc rocks, implying the melt mechanism is not primarily from the introduction of water. Tholeiitic rocks in this geologic setting, like the High Lava Plains of Oregon, are likely due to corner flow of the mantle along the subducting Juan de Fuca plate slab causing decompression melting. We provide new ⁴⁰Ar/³⁹Ar geochronology which shows lower portions of the rim are ~185 ka, suggesting volcanism is younger than a previous K-Ar age. The new age data also suggest the Tam McArthur edifice was constructed relatively quickly, likely within a ~10 ka period. Constraining the timing of past volcanism associated with the TVF provides insights into potential future hazards, allowing for awareness and implementation of informed hazard mitigation strategies within Central Oregon communities.
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