OCCURRENCE AND DISTRIBUTION OF RHYOLITE MAGMA TYPES IN NE OREGON DURING JOHN DAY TIME

Historically, the John Day Formation was defined as formation comprising Oligocene to early Miocene (39–20 Ma) strata in NE Oregon and consisting primarily of fallout tuffs and ignimbrites of rhyolitic composition. Despite considerable work from the 1950’s to 1980’s, silicic magmas are not well understood due to patchy exposures and alteration processes that make correlation and petrological evaluations challenging. In addition, modern chemical and mineralogical data are lacking in most cases. We have focused in this study on silicic rocks in the greater area around the town of Unity where a particularly poorly understood part of the E. Oregon Oligocene-Early Miocene volcanic stratigraphy crops out. Also, we relate silicic rocks of the Unity area to lavas and tuffs of the Tower Mtn Caldera complex (~40 miles north) and to units of the greater John Day Fossil Beds area (~80 miles west).

Silicic rocks of the Unity area range from calc-alkaline (magnesian) to A-type (ferroan). Calc-alkaline rocks are typically phenocryst rich (~10-30%) and characterized by phenocrysts of biotite, amphibole and plagioclase. They range in composition from dacite (66% SiO2) to high-silica rhyolite (76% SiO2) and crop out as lava flows, ignimbrites and voluminous block & ash flow tuffs. On the other hand, A-type silicic rocks are always extremely crystal poor (~1 %) with dominantly feldspar and quartz as phenocyrsts and compositions range from low to high silica rhyolite with a possible dacitic component in some ignimbrites. Such volcanic rocks occur mostly as ignimbrites and some fallout tuffs. There are characteristic trace compositional distinctions between rhyolite types. Calc-alkaline rhyolites contain, for example, <200 ppm Zr, <40 ppm Y, Ba/Rb of <11 and La/Yb of 11-13 while A-type rhyolites contain 300-500 ppm Zr, 50-120 ppm Y, Ba/Rb of 19-30 and La/Yb of 5-8. Thicknesses, lithologies, stratigraphic constraints, and geochemical data suggest an earlier, northerly located calc-alkaline eruptive center was followed by activity from an eruptive center around Ironside Mountain producing several (3-4?) compositional similar outflow sheets that may become progressively less silicic upsection. Units of the Unity area have compositional counterparts at the Tower Mtn. Caldera complex and in outcrops at the John Day Fossil beds.