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Open-File Report O-13-23, Silicic volcanism in the Menagerie Wilderness and adjacent regions of the Western Cascades in Oregon, by Geoffrey W. Cook, Craig M. White, and James L. Crowley
Order publication CD, $15, from Nature of the Northwest. CD contains a 15-page text report and a 36- x 34-inch, 1:12,000-scale map plate, both in PDF format.
Geologic studies within and adjacent to the Menagerie Wilderness in the Western Cascade Range indicate this region was the site of several large silicic dome-building events in middle Miocene time. Rhyolites in this area occur as flow-banded, devitrified lava flows with well-developed basal vitrophyres, coarse block-and-ash breccias, and partially eroded endogeneous domes. Three petrographically distinctive units were recognized within the dome complex: a coarsely porphyritic hornblende rhyolite (rhyolite of Rooster Rock), a less crystal rich quartz-bearing rhyolite (rhyolite of Soda Fork and a related block and ash unit), and a biotite-bearing rhyolite with abundant large phenocrysts of quartz (rhyolite of Moose Mountain). The domes and lava flows overlie an older sequence of silicic ignimbrites and are themselves overlain by younger mafic lava flows. In addition to the field and petrographic descriptions, we report 10 bulk-rock chemical analyses of lavas and dome rocks and two new radiometric dates. All the chemically analyzed rhyolites display the relatively high contents of Al2O3, CaO, and Sr and low abundances of Nb typical of silicic rocks in volcanic arc settings. The analyses also plot within the compositional ranges for silicic rocks in the Three Sisters and Broken Top systems of the High Cascades. The dated specimens include a rhyolite lava flow from the Soda Fork unit and an ignimbrite exposed at low elevation along U.S. Highway 20. The lava flow yielded an 40Ar-39Ar plateau date on plagioclase of 13.15 ± 0.24 Ma, and the ignimbrite produced a high-precision CA-TIMS U-Pb zircon date of 16.27 ± 0.02 Ma. We present the following observations based on the results of this study: (a) effusive eruptions built several rhyolite domes in the area of the present-day Menagerie Wilderness during middle Miocene time; (b) prominent rock spires within the wilderness area are probably the erosional remnants of the largest of these domes; (c) the chemical similarity between the silicic magmas erupted in this part of the Western Cascades and those erupted in adjacent parts of the High Cascades suggests both suites were derived from similar source rocks and/or had similar petrogenetic histories; (d) the association of rhyolitic domes overlying an unknown thickness of silicic ignimbrites may indicate the presence of an eroded, large, previously unrecognized caldera.