LITHOLOGY AND PETROGRAPHY OF THE WUKSI VOLCANIC CHAIN, CENTRAL OREGON CASCADES

The Wuksi Volcanic Chain is a N-S trending, ~10 km long magmatic lineament defined by scoria cones, tuff cones and two large (d = 0.5 km), lake-filled maars with associated tuff rings. The Wuksi Volcanic Chain is situated on the east flank of the High Cascades approximately 30 miles southwest of Bend, Oregon. Volcanic deposits of the Wuksi Chain overlie drift from the Suttle Lake glacial advance and are in turn overlain by Mazama ash constraining the eruption age to be between 18 and 7 ka. Lava flows that were preferentially issued from the northern and central vents are mostly pahoehoe flows and thickness is in the order of meters. Lava flows appear to break into two main lithologies. One has a coarser groundmass that appears in hand sample as medium gray while the other is finer-grained and darker. Olivine and plagioclase are the obvious phenocryst phases and occur often in glomerocrysts. Phenocryst abundances, although similar, indicate small variations between “light” and “dark” flows. “Dark” flows contain a smaller proportion of phenocrysts (~5%) and olivine is about equal in abundance to plagioclase. “Light” flows, on the other hand, have higher phenocryst abundances (~15%) and plagioclase occurs at a higher proportion than olivine. Lithological variations are independent of silica concentrations but seem to coincide with other geochemical parameters, such as Ni contents. Tuff cone and ring deposits are dominantly lapilli tuff with a lesser amount of tuff and lapillistone tuff. Lithic fragments in tuff deposits are subordinate and make up typically less than 10%. Juvenile clasts have a palagonized rind, but indicate interiors consisting of vesiculated fresh glass. Petrographic work is on going. We will present more complete results about modal proportions and chemical data of mineral and glass compositions of lavas flows, scoria and tuff samples. We will correlate our data on minerals and glasses with bulk geochemical data currently obtained by a companion study (Friberg et al., this volume). Our overall goal is to document the petrologic evolution of this very young volcanic chain.