THE YOUNG BUT EXTINCT GOAT ROCKS VOLCANIC COMPLEX: PERSPECTIVES ON CENTRAL CASCADE ARC MAGMATISM

The deeply eroded Goat Rocks volcanic complex was a major locus of andesitic volcanism in the Cascade arc in southwest Washington during the late Pliocene to Pleistocene. The volcanic complex includes the remnants of multiple andesitic edifices over an area of ~200 km², centered ~35 km north of Mount Adams on the arc axis. Goat Rocks volcanic complex was active for more than 2.5 million years, reaching its maximum volcanic output while early Mount Rainier and Mount Hood were built, and waning as Mount Adams and Mount St. Helens emerged. New ⁴⁰Ar/³⁹Ar and U/Pb ages constrain four eruptive stages: Tieton Peak, Bear Creek Mountain, Lake Creek, and Old Snowy Mountain, named for the major vent that was active during each stage. Tieton Peak stage is tentatively constrained between ~3.0 and 2.6 Ma based on magnetic polarity and one argon age on coeval Devils Washbasin basalt. Volcanism of Bear Creek Mountain stage occurred between >1.6 to 1.3 Ma, based on argon ages, geochemistry and field geology (Hammond, 2017; Gusey et al., 2018). Lake Creek stage occurred between 1.1 and 0.6 Ma, including activity at the Lake Creek volcano and Black Thumb. Lake Creek volcano was the most voluminous Goat Rocks edifice and is estimated to have been at least 3400 m (11,000 ft) high with a volume of ~60 km³, comparable to nearby active composite volcanoes. Old Snowy Mountain stage, from 0.4 to 0.1 Ma, included activity at Old Snowy Mountain, Goat Ridge, and Coyote Lake (source of the Clear Fork andesite). Compositional variability was greatest in the early and late eruptive stages, ranging from basaltic andesite to rhyolite, while the more voluminous middle stages were dominated by andesite to dacite. In addition, compositions shifted from high-K to medium-K with time, which mimics trench-ward variation across the modern arc; early eruptive products are similar in composition to those of Mount Adams, and Old Snowy Mountain stage compositions are more similar to Mount St. Helens. The life cycle of Goat Rocks volcanic complex provides new perspective on the longevity and evolution of major arc volcanoes, and on the complex distribution of magma in the Cascade arc at the latitudes of southern Washington and adjacent Oregon.