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