203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Contrasting source, accumulation, and storage to explain voluminous, compositionally diverse rhyolites related to flood basalt magmatism and High Lava Plains volcanism, eastern Oregon.

Paper number: 203-11
Program: Technical Sessions
Time: 4:15 PM – 4:30 PM

Authors

Abstract

Abundant mid- to late-Miocene rhyolites of eastern Oregon are cogenetic with flood basalts of the Columbia River Basalt Group or with the later High Lava Plains province. Here, we focus on rhyolite fields, single rhyolite lavas, and ignimbrites with volumes of ~100-300 km³.

The duration of peak activity of large fields of rhyolite lavas and tuffs is ~200-400 ka, such as the Mahogany Mtn.-Three Fingers rhyolite field (16–15.7 Ma), the Strawberry rhyolite field (15.3–14.8 Ma), and the Dooley Mtn. field (15.6–15.4 Ma). In all these cases, rhyolite batches with distinct, yet occasionally subtle compositional and/or mineralogical characteristics erupted in short succession, leading to a complex volcanic stratigraphy. While rhyolites can show evolution trends with time, they are unlikely the product of an evolving reservoir(s), but rather, rhyolite batches were individually delivered from a source region, accumulated, and erupted shortly thereafter.

            Conversely, the compositionally distinct Lower Littlefield (16.11 Ma; 100 km3) and Upper Littlefield (16.02 Ma; 150 km3) rhyolite lava flows represent single, voluminous, and homogenous rhyolite batches with virtually no variation in composition and mineralogy. These data demand a homogenous source, rapid accumulation, and eruption without significant shallow reservoir modifications.

            Voluminous rhyolite ignimbrites range from nearly homogenous to strongly compositionally zoned. Homogenous ignimbrites (e.g., the Dinner Creek tuffs) show compositional characteristics resembling units of the Littlefield Rhyolite and hence suggest similar source, accumulation, and storage conditions. In contrast, rhyolites of compositionally zoned tuffs (e.g., the Rattlesnake and Devine Canyon tuffs) resemble diverse rhyolites of rhyolite fields yet distinct rhyolites of tuffs erupted from a contiguous magma body as evidenced by rhyolites commingled in banded pumices. We propose that rhyolites of zoned tuffs progressively accumulated from distinct sources with a relatively prolonged storage accompanied by some shallow reservoir evolution prior to evacuation during caldera eruptions.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412604

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