THE CLEAR FORK LAVA: A VIGOROUS LAST GASP OF THE PLIOCENE-PLEISTOCENE GOAT ROCKS VOLCANIC COMPLEX, SOUTHERN WASHINGTON CASCADES

The voluminous, dacitic Clear Fork Lava is the youngest known remnant of the Goat Rocks volcanic complex, a deeply glaciated andesitic locus on the Cascade arc axis ~34 km north of Mount Adams in southwest Washington. If the Goat Rocks volcanic complex went extinct, it was only recently, based on the ⁴⁰Ar/³⁹Ar groundmass age of 115 ± 4 ka for the Clear Fork Lava. This low-silica dacite flow (63.1% SiO₂) has high Na₂O, low K₂O, and high Sr/Y, like other late-stage Goat Rocks eruptive products, and similar to Mount St. Helens and Mount Rainier. The Clear Fork Lava crops out along the White Pass Highway (Palisades viewpoint, milepost 141) as a spectacular ~70 m high colonnade of dacite columns that gradually thicken upward to as great as 2 m in diameter. The top of the lava flow is beveled by glaciation, and an upper colonnade is absent. Based on ten cross sections along the 12 km length of the lava flow, we estimate the present-day volume to be 0.37 km³. Assuming that ~1/3 of the flow (the upper colonnade) has been eroded away, the original volume may have been approximately 0.55 km³.

In hand sample, the Clear Fork Lava is uncommonly crystal poor, with 2-5 percent of mainly opacite-replaced euhedral prismatic to acicular amphibole, lesser, largely equant plagioclase, and lesser orthopyroxene that tends to occur in clusters. We infer that the dacite was water rich based on the crystallization of amphibole before (i.e., included within) plagioclase, consistent with its low crystal content. The opacite replacement of amphibole suggests that the ascent of the magma was not rapid enough to prevent amphibole breakdown, or that the magma was heated prior to eruption.

Zircon crystals in the Clear Fork Lava commonly have resorbed margins. They have the greatest range in age of any zircons analyzed from the Goat Rocks volcanic complex. The youngest zircon population is 107 ± 5 ka and within error of the argon age. Other populations as old as ~55 Ma and 177 Ma are evidence for entrainment of basement material. We conclude that the last gasp of the Goat Rocks volcanic complex was vigorous, erupting voluminous wet, hot, and contaminated dacite.