First Advisor

Martin J. Streck

Term of Graduation

Spring 2022

Date of Publication


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Earth, Environment, & Society


Earth, Environment, & Society





Physical Description

1 online resource (xvi, 231 pages)


Voluminous and widespread bimodal volcanism has significantly impacted the Pacific Northwest, USA, throughout the Miocene to present day. The two primary volcanic provinces of eastern Oregon include the Columbia River Basalt Group (CRBG) province and the High Lava Plains (HLP) trend. The magmatic and tectonic processes responsible for generating bimodal volcanism, and particularly rhyolites of the ~17-15 Ma CRBG and 12-0 Ma HLP provinces has recently been a popular topic of debate. Rhyolite volcanism of the HLP province has been ascribed to either buoyancy-driven westward plume spreading or to slab rollback and mantle convection spanning from southeast Oregon to Newberry volcano in the west. Numerous studies have focused on the mafic endmember of these bimodal provinces (e.g., Hooper et al., 2002; Camp et al., 2003; Cahoon et al., 2020), but until recently, few workers had investigated the rhyolite endmember. Rhyolites of bimodal systems can possess unique geochemical and petrologic signatures and can contain components of their associated mafic endmembers, thus providing workers with critical evidence necessary to understand pre-eruptive magma configurations, magma chamber evolution, and rhyolite petrogenesis.

Previous workers interpreted the co-CRBG and HLP rhyolite provinces as a product of separate and distinct magmatic and tectonic processes primarily due to the perceived ~15-12 Ma eruptive hiatus, but the relationship between the two provinces remained unclear with many of the rhyolites undated in the region where the two provinces overlap (117-119°W, 43-44°N). By dating nearly all undated or imprecisely dated regional rhyolites, I have refined the timeline of co-CRBG rhyolites and HLP volcanism, and I have uncovered two distinct rhyolite eruptive episodes within the HLP. I use these new data to develop a new model for post-17.5 Ma rhyolite volcanism that involves decreasing influence of the CRBG-related mantle plume upwelling and increasing influence of regional tectonic regimes. Rigorous geochemical investigation of unanalyzed rhyolites among the co-CRBG and HLP rhyolite provinces is critical in confirming my proposed model. Geochemical analysis of these rhyolites displays the extraordinary trace elemental variations among rhyolites of all ages, geographic locations, and degrees of differentiation. Rhyolite chemical affinity is intimately related to proximal, coeval mafic volcanism. The distribution of A- and I-type rhyolites aligns with our proposed model for rhyolite petrogenesis that involves decreasing influence of the CRBG mantle plume westward over time. Compositional heterogeneity among adjacent rhyolites is enhanced by localized variations in (1) the composition of the potential partial melt source (regional crust) and (2) petrogenetic processes like fractional crystallization and/or assimilation within an individual eruptive center.

Using the 7.1 Ma Rattlesnake Tuff (RST) as a case study, I investigate one of these heterogeneous, voluminous rhyolites to understand the complex pre-eruptive magma configurations associated with the HLP. I combined previous (Streck and Grunder, 1997) and new geochemical analysis of the strikingly banded high-silica rhyolite (HSR) pumices to confirm compositional Groups and gaps. Calculated density, viscosity, and storage temperature data highlight the importance of water content in generating a meromictic system of layered HSR magmas that are allowed to erupt simultaneously without mixing.

This work is driven by three main hypothesis: (1) Co-CRBG rhyolite volcanism is driven by the mantle upwelling associated with the CRBG flood basalts, initial HLP volcanism is a combined effect of enhanced regional tectonics in an area previously affected by CRBG volcanism, and HLP volcanism becomes progressively more tectonics and upper-mantle driven as it propagates westward, (2) Compositional variation among rhyolites of the co-CRBG and HLP provinces is primarily dictated by the composition and source of coeval mafic volcanism, with localized and intra-center petrogenetic processes responsible for extraordinary trace element variations among rhyolites of all ages and locations, and (3) Water contents, and thus, density variations allowed the five compositionally distinct HSR magmas of the RST to reside in a single, meromictic magma chamber prior to simultaneous evacuation. The broader significance of this research refines our understanding of bimodal volcanism, particularly within provinces also influenced by a proximal subduction zone. This work also underscores the complex petrogenetic and tectonic processes involved in generating both heterogeneous and homogeneous rhyolite magmas within the same region.


© 2022 Vanessa Marie Swenton

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