Multistage growth and compositional change at the Goat Rocks volcanic complex, a major Pliocene-Pleistocene andesite center in the southern Washington Cascades

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. This volcanic complex includes the remnants of multiple andesitic edifices over an area of approximately 200 km2, centered approximately 35 km north of Mount Adams on the arc axis. New 40Ar/39Ar ages for seven samples and U/Pb zircon ages for nine samples indicate a 2.5-2.9 m.y. eruptive history at Goat Rocks. Four eruptive stages are delineated: Tieton Peak (potentially 3.0-2.6 Ma), Bear Creek Mountain (>1.6-1.3 Ma), Lake Creek (1.1-0.6 Ma), and Old Snowy Mountain (0.4-0.1 Ma), each named for the major vent that was active during that time. Lake Creek volcano was the most voluminous of these edifices and probably rose at least 3400 m above sea level with a volume of approximately 60 km3, comparable to nearby active composite volcanoes. Thirty new bulk composition X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry analyses from the volcanic complex are presented, in addition to 54 previously unpublished XRF analyses for samples collected by Don Swanson. The compositional variability is greatest in the early and late stages, ranging from basaltic andesite to rhyolite, whereas the more voluminous middle stages are dominated by andesite to dacite. The middle eruptive stages are interpreted to have been a time of peak thermal energy with a mature subvolcanic plexus. In addition, compositions shift from high-K to medium-K compositions with time, which mimics variation across the arc; early eruptive products are similar in composition to those of Mount Adams, and Old Snowy Mountain stage compositions are more similar to those of 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.