GEOCHEMICAL AND TECTONIC ATTRIBUTES FOR MELTING OF SUBDUCTED OCEANIC LITHOSPHERE, MT. HOOD VOLCANO, OREGON

The ~ 700,000 year history of Mt. Hood volcano, Oregon, is dominated by the eruption of andesitic lava flows and pyroclastic deposits. New geochemical studies of Mt. Hood andesites suggest magma formation by partial melting of subducted oceanic lithosphere (adakites). Most Mt. Hood andesites exhibit major and trace element characteristics similar to those of slab-derived melts identified elsewhere: SiO2 > 56.0 wt %, Al2O3 > 15.0 wt %, Na2O > 3.5 wt %, Sr > 400 ppm, Y < 18 ppm, Sr/Y > 40 ppm, Zr/Sm > 50 ppm. MgO concentrations in Mt. Hood andesites (> 2.5 wt % MgO) are characteristic of high-Mg adakites, suggesting chemical interactions between slab-derived melts and peridotite within the mantle wedge. Metosomatic reactions between slab-derived melts and the sub-arc mantle are indicated by positive correlation between compatible (e.g. Nb, Ta) and incompatible trace elements (e.g. La, Sr). The enrichment of high field-strength elements relative to large-ion lithophile elements in Mt. Hood andesites is a possible effect of metosomatic reactions within the sub-arc mantle. The melting of subducted oceanic lithosphere may occur beneath Mt. Hood due to shallow subduction and relative proximity of the volcano to the Cascadia trench, as compared to other volcanoes in the southern Washington - northern Oregon Cascades. A Mt. Hood slab depth of 75-85 km (the range of experimentally determined amphibolite-eclogite transition and dehydration melting) suggests low angle subduction, a tectonic attribute proposed for the generation of adakite melts elsewhere. Contrastingly, most Cascade Range volcanic centers in southern Washington to central Oregon do not exhibit strong geochemical evidence for melting of subducted oceanic lithosphere, suggesting deeper slab depths as a function of greater distance from the Cascadia trench. In this region of the Cascades, Mt. Hood appears most similar to Mt. St. Helens where slab depth is ~75-80 km and melting of subducted lithosphere also has been proposed.