Thesis
Geochemistry and petrogenesis of John Day ash flows near Prineville, Oregon
Washington State University
Master of Science (MS), Washington State University
2010
Handle:
https://hdl.handle.net/2376/103208
Abstract
The John Day rhyolite ash flows exposed near Prineville, Oregon are temporally equivalent to member G of the John Day Formation (JDF), a dominantly basalt-rhyolite bimodal province. Previous work has largely attributed John Day volcanism to subduction; however new data suggest this model needs revision. The rhyolites share affinities with A-type granitoids, and can be divided into two peralkaline groups; a less fractionated high-Fe group with less depleted Sr and Eu and less enriched Th, Rb, Nb, Hf, and LREE abundances and a more fractionated low-Fe group. The high-Fe group exhibits constant Zr/Hf ratios indicating no zircon fractionation, while the low-Fe group exhibits decreasing Zr/Hf ratios indicative of zircon fractionation. Thus, the high-Fe group was originally more peralkaline than the low-Fe group. The John Day rhyolites share striking similarities with the Trans-Pecos Volcanic Province (TPVP) rhyolites, which suggest they were derived by similar processes and their differences reflect variable source composition, degree of melting and fractionation. Viewed as a whole the TPVP displays a continual transition from calc-alkaline compositions to strongly peralkaline compositions as a consequence to changing stress regimes. These stress changes are similar to those postulated for the compositional changes observed in Clarno-John Day volcanism. Thus, the Clarno and John Day Formations should be viewed as one magmatic province that evolved as a result of changing stress regimes in Oregon. The nature of what caused the initial switch is still under debate, but coincident magmatism throughout much of west suggests it was related to post-Laramide removal of the Farallon slab.
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Details
- Title
- Geochemistry and petrogenesis of John Day ash flows near Prineville, Oregon
- Creators
- Karyn Ann Patridge
- Contributors
- John A. Wolff (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; Pullman, Wash. :
- Identifiers
- 99900525082301842
- Language
- English
- Resource Type
- Thesis