Comparison of Siliceous Volcanics from the Clarno and John Day Formations, Oregon
Atheim, Brian K.
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The Tertiary Clarno and John Day Formations occur in the Blue Mountains Region of north-eastern Oregon. The two formations are easily distinguishable overall, because the Clarno consists primarily of andesitic to dacitic flows, whereas the John Day consists primarily of intermediate to silicic air-fall tuffs. However, rhyolites which occur within the two formations are not easily distinguishable in the field or in thin section. Chemically, rhyolites from the Clarno and John Day formations are distinguishable. Variable degrees of alteration prevent the use of major element oxides and mobile trace elements, but incompatible trace elements, specifically Nb, Y, Mo and the REE's, do distinguish the two formations. These elements also help constrain the tectono-magmatic origin of the two formations. The REE composition of the rhyolites reveals a possible transitional character between the volcanic-arc produced rhyolites of the Clarno Formation and the within-plate produced rhyolites of the John Day Formations. Nb and Y discrimination plots further support this transitional nature. The transitional nature of certain rhyolites within the two formations allows temporal overlap to exist between the formations. This possible temporal overlap indicates a need for further study, particularly accurate age dating of transitional rocks from each formation. Kwywords: Pages: 68 Supervisor: Paul Robinson