GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 176-7
Presentation Time: 9:00 AM-6:30 PM


WALL, Kellie T., School of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, GUSEY, Daryl L., 12236 SE Bluff Drive, Clackamas, OR 97015, GRUNDER, Anita L., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis and HAMMOND, Paul E., 1305 SW Upland Dr, Portland, OR 97221,

Reconstructing arc-scale and volcano-scale processes requires detailed time-series petrologic and chemical data, which are challenging to obtain at locations where recent eruptions have buried older eruptive products. The recently extinct Goat Rocks Volcano offers a prime location to dig deeper into the recent history of Cascades volcanism, and to understand the life cycle of an andesitic-dacitic composite arc volcano. This inconspicuous center 35 km north of Mt. Adams is the northernmost major Quaternary volcano on the arc axis defined through Oregon. Deep glaciation has revealed an approximately 3-m.y. history of eruptions—longer than that exposed at neighboring Mt. Adams, Mt. Rainier and Mt. St. Helens. Activity at these centers increased as Goat Rocks waned and died.

Goat Rocks Volcano is built on the edge of a 3.2 Ma rhyolite caldera, and andesitic-dacitic activity continued past ~250 ka in four major pulses separated by major unconformities. Early lavas are intercalated with the ~3 Ma basalts of Devils Horn. The second pulse includes the Tieton Andesite, which comprises two andesite lava flows >50 km long—among the longest in the world (see Gusey et al., this volume). A dacite flow from the western flank of the volcano yields an age of 645 +/- 18 ka, confirming a previous age estimate by Swanson (1996) for the third eruptive pulse. Lavas of the most recent pulse fill glacially-carved valleys; their ages are constrained by underlying Hayden Creek Drift (250-275 ka; Evarts et al., 2003) and overlying Evans Creek Drift (~17-25 ka; Swanson, 1996).

Goat Rocks lavas range from 57-74 wt. % SiO2; pyroxene andesite-dacite dominates early units, with amphibole in younger units. Goat Rocks volcanics lie on the spectrum between a high-K suite like Mt. Adams and a low-K suite like Mt. St. Helens, with clear shift from high-K toward lower-K through time. Continued work will establish a timeline of volcanic productivity and erosion over the lifespan of this magmatic system, construct petrologic models based on temporal variations in composition and mineralogy, and assess mantle and crustal components contributing to magmatism. Our work supplements and would not be possible without the extensive geologic work conducted by Don Swanson between 1983 and 1996 and work by Clayton (1983). It is our honor to share and carry forward Don’s work.