POTENTIAL FORMATION OF ‘HYBRID' ADAKITE MAGMAS WITHIN THE NORTHERN OREGON CASCADIA SUBDUCTION ZONE

Most theories for magma formation within subduction zones center on partial melting of the sub-arc mantle wedge during chemical interactions with hydrous fluids released from subducted oceanic lithosphere during the amphibolite-eclogite transition. However, these theories are augmented by research showing that magmas also may form by partial melting of young, warm subducted oceanic lithosphere. The potential for such adakite melts has been studied previously in the southern Washington Cascade Range at Mt. St. Helens (Defant and Drummond, 1993). Current research investigates whether lavas erupted at Quaternary volcanic centers in the northern Oregon Cascades (Mt. Defiance, Mt. Hood, Pinhead Buttes, Olallie Butte) also exhibit geochemical signatures characteristic of adakite melts. This region is appropriate for study because these volcanic centers are located along a north-south trend with similar distance and depth relationships to the Cascadia trench and the underlying subducted lithosphere as those of Mt. St. Helens. Most lavas erupted throughout the study area exhibit major and trace element concentrations similar to adakites identified elsewhere: SiO2 > 56 wt%, Al2O3 > 15 wt%, Na2O > 3.5 wt%, Sr > 400 ppm, Y < 18 ppm, Sr/Y > 20 - 40 ppm, Zr/Sm > 50 ppm. Furthermore, most lavas fall along a K2O-Na2O-CaO trend close to that of experimental melts representative of those derived by partial melting of subducted oceanic crust at the amphibolite-eclogite transition. However, certain lavas near Mt. Hood and at Pinhead Buttes exhibit relatively high MgO concentrations (~8 wt%), suggesting significant sub-arc mantle input. Additionally, positive correlation between compatible and incompatible trace elements in most lavas suggests metasomatic reactions between any slab-derived melts and the sub-arc mantle. Lavas from Mt. Defiance and Pinhead Buttes exhibit Sr/Y similar to typical arc volcanics. Accordingly, the hypothesis is that although certain northern Oregon Cascades Quaternary lavas do exhibit geochemical characteristics suggesting derivation by partial melting of subducted oceanic lithosphere, the region is largely characterized by the formation of ‘hybrid' adakite magmas containing melt contributions from both subducted oceanic lithosphere and the sub-arc mantle.