The stratigraphy, geochemistry, and mineralogy of two ash-flow tuffs in the Deschutes Formation, central Oregon Public Deposited

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  • Two ash-flow tuff units of the late Miocene-early Pliocene Deschute Formation in central Oregon were studied in detail because of the widespread distribution, diverse compositions, and stratigraphic importance. The Lower Bridge tuff is a double-flow simple cooling unit that is poorly welded. The upper flow grades from rhyolite in the lower part to dacite in the upper part. A white 1.5 to 5-foot accretionary lapilli air-fall deposit often underlies the two ash-flow sequences. Phenocrysts in the pumice lumps are plagioclase (An 35-45), pargasite, hypersthene, augite, ilmenite, apatite, and magnetite. The compositional charge from rhyolite to dacite in the upper flow suggests that it was formed by eruption of successively lower parts of a zoned magma body. The McKenzie Canyon tuff is a multiple flow compound cooling unit that overlies the Lower Bridge tuff. It may have covered 160 square km and had a volume of 0.7 km3. It was erupted onto irregular terrain resulting in variable thicknesses. Up to three light-colored, rhyolitic ash-flow deposits are overlain by two red columnar-jointed units. The red color and welding of the upper two members are the distinguishing physical features of the McKenzie Canyon tuff. The lower nonresistant silicic flows are often absent in the northern part of the study area. The facts that the units decrease in thickness and in elevation northward and that the average pumice size becomes smaller suggests a source to the south. Another distinguishing feature of the upper red flow(s) is. the prevalence of white (rhyolite), black (andesite), banded (rhyolite and andesite), and collapsed pumices. A few dacite pumice clasts (mixed) are also present. In the lower silicic flows black or banded pumices are only found in minor amounts and collapsed pumices are absent. Collapsed pumices in the upper flow(s) only occur throughout the welded section in nearly horizontal orientations. The white pumice is a high-K rhyolite with phenocrysts of.oligoclase/andesine (An 29-31), hypersthene, augite, magnetite, ilmenite, and zircon. The black pumice is medium-K, hi-Ti and -Fe andesite that contains labradorite (An 60-65), olivine (Fo82), augite, hypersthene, and magnetite. The percentage of black pumice increases upward in the upper flow. Banded pumice is a combination of rhyolite and andesite magmas and represents the coeruption of these two compositions. Evidence of complete mixing of the magmas i.e., homogeneous dacite pumice, is minor, Collapsed pumices have the same composition as rhyolite or banded pumices. The McKenzie Canyon tuff was possibly derived from two separate magams. As mafic magma was injected into a silicic magma chamber, ensuing convection and vesiculation probably caused the formation of banded pumice. This hypothesis is based on the following relationships: 1) phenocrysts and bulk chemistry of rhyolite and andesitic pumices are of distinct compositions, 2) a paucity of phenocrysts occur in the andesitic pumices, and 3) Harker diagrams of major element chemistries show that the two magmas have divergent regression lines. The McKenzie Canyon tuff upper flows are unusual among banded pumice-bearing tuffs because of the aphyric nature of the andesite and the probability that the rhyolite and andesite magmas are not derived from the same magma chamber.
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