MESSAGE FROM THE MOHO: PETROLOGIC CLUES TO THE ORGIN OF QUATERNARY BASALTIC LAVAS FROM OREGON'S HIGH LAVA PLAINS

Oregon’s High Lava Plains is among the most volcanically active areas of the continental U.S.. However, none of the accepted paradigms for crustal formation and magmatism, including back arc spreading, Basin and Range extension, or a migrating mantle plume, easily explain the volcanism in this intraplate setting. Here we present geochemical data and piston cylinder experiments that suggest Quaternary basalts, erupted at Jordan Valley Volcanic Center and Diamond Crater in easternmost Oregon, are the product of anhydrous mantle melting and minor subsequent fractionation. These mantle melts were last in equilibrium with a mantle assemblage at mantle potential temperatures of at least 1300°C and depths of 33-40 km, which is just below the Moho in this region based on recent seismic observations. The major and trace element abundances in the basalts do not suggest an OIB affiliation. Instead they are similar to high alumina olivine tholeiites found in other locations of the Cascadia back arc demonstrated to be the result of adiabatic decompression melting. This adiabatic decompression could have occurred because of mantle upwelling due to corner flow in the nearby Cascadia subduction zone, as previously documented for Medicine Lake volcano, or due to slab rollback, as recently documented in laboratory modeling of mantle flow below Oregon and the Mediterranean. In addition, the chemical data from our piston cylinder experiments can be used to expand the composition of basalts for which primary magma compositions can be predicted using the model of Kinzler and Grove (1992).