PETROCHEMICAL EVOLUTION OF HIGH CASCADE VOLCANIC ROCKS IN THE THREE SISTERS REGION, OREGON

Multi-element abundances and petrographic data are compiled for a suite of 50 volcanic rocks and selected mineral separates located within the Three Sisters area. Major element oxides, obtained by x-ray fluorescence and atomic absorption spectrophotometry, and trace element concentrations, obtained by sequential instrumental neutron activation analyses, enable classifications of normal basalts, divergent basalts, Mount Washington (MW) and North Sister (NS) type basaltic andesites, dacites and rhyodacites. Petrochemical types and geochemical models are evaluated in light of field studies (E. M. Taylor, Oregon State University), regional geologic environment and comparisons with similar systems.;A comprehensive model of High Cascades volcanic evolution is presented which incorporates a series of events in a subduction zone-mantle-crust system: Hydrous fluids, expelled from a dehydrating subducted slab, become enriched in incompatible elements through processes of liquid extraction and small amounts of partial melting. These fluids ascend through the overlying mantle wedge contaminating and catalyzing mafic melts which accumulate and fractionate in upper mantle and lower crust regions. Mafic magmas erupt as near-primary liquids or are intruded into upper crust regions where extensive fractionation produces siliceous dacites and rhyodacites. The magmatic events are conceptualized as vertical sequences of basalt-basaltic andesite-dacite-rhyodacite magmas produced in response to the advancing front of hydrous fluids and erupted during tensional readjustments of a thermally weakened crust.;High-alumina olivine tholeiites exhibit low Fe' values and fractionated abundances of K, Ba, Sr, REE and Sc which produce nonchondritic monotonic patterns relative to ionic radii. Primary basalts are modeled as 14% melts, with minor olivine crystallization, from a LIL element-enriched spinel lherzolite source. Divergent basalts represent contaminated magmas or different sources. Basaltic andesites having up to (TURN)62 wt.% SiO(,2) also exhibit low Fe' and fractionated monotonic trace element patterns and are modeled as 10% primary melts from similar source regions. Basalts and basaltic andesites contain source-equilibrated olivine and plagioclase phenocrysts attesting to their primary origin. One andesite sample (SiO(,2) = 58.6 wt.%) is derived by significant fractionation of a basalt parent and represents a low-SiO(,2) and high-TiO(,2) member of an early Pleistocene silicic system. Dacites and rhyodacites (SiO(,2) = 62.2-75.8 wt.%) have strongly fractionated non-monotonic trace element patterns and are derived by extensive (> 60%) fractionation from primary NS basaltic andesite magmas. A petrochemical hiatus is recognized between mafic and silicic compositions.