BIMODAL VOLCANISM IN UPPER OLIGOCENE-LOWER MIOCENE ROCKS OF SOUTHWEST OREGON

Rocks assigned to the Wasson Formation of the Western Cascade Volcanic Group are comprised of 23.3-22.9 Ma silicic air- and ash flow-tuffs interlayered with basaltic-andesite lavas and debris flows issued from local volcanic centers. Silicic volcanism north of 42° 15’ (dacite and rhyodacite flows and tuffs) is distinct from rhyolitic tuffs which crop out southward to the California border, reflecting different crystallization histories or substrates. The southern tuffs contain 4-5% phenocrysts of plagioclase, rare augite, and rare to absent quartz suspended in a devitrified microfelsic matrix. Basaltic-andesite lavas and debris flows contain an abundance of phenocrysts (25-37%) dominated by plagioclase. Early plagioclase crystals show plentiful inclusions of glass, opaques, dust, and pyroxene while inclusions are sparse in later-crystallized plagioclase. Lavas are strongly magnetic, augite is more abundant than hypersthene, and olivine is rare to absent. Zeolites, celadonite, calcite, and iron hydroxides are common diagenetic products in lavas, debris flows, and tuffs. Paleosols occur intermittently at the bottom of the unit and especially at the top. The latter combined with truncation of stratigraphic units indicate a period of local erosion prior to subsequent eruption of prolific andesitic debris flows and lava.

Petrographic evidence in lavas, including abundance of phenocrysts, two generations of plagioclase, zoning, and reaction rims as well as geochemical data suggests long residence time in magma chambers allowing for dominance of fractional crystallization and some assimilation of crustal material. Trace elements in the southern tuffs reflect strong fractional crystallization trends. A weak to moderate positive Eu anomaly suggests the tuffs, and a few lavas, are the products of plagioclase accumulation though assimilation is possible.