POTENTIAL MAGMA FORMATION BY PARTIAL MELTING OF BOTH UPPER MANTLE AND SUBDUCTED LITHOSPHERE AT MT. HOOD VOLCANO, CASCADE RANGE VOLCANIC ARC

Mt. Hood, volcano, Oregon in the Cascade Range volcanic arc has erupted predominantly andesite lava and pyroclastic flow deposits over the last 700,000 years. Most lavas are medium-K, calc-alkaline two-pyroxene andesites showing a restricted range of composition (56 - 63 wt% SiO2). Past studies suggest that Mt. Hood magmas are generated by partial melting of an upper mantle source region, and that the restricted range of erupted compositions is due to repeated cycles of crystal fractionation combined with mixing of evolved and primitive magmas (Cribb and Barton, 1997). A new geochemical study is underway to investigate the possibility that formation of Mt. Hood magmas involves contribution of melts from subducted lithosphere (adakites). Some Mt. Hood lavas exhibit certain geochemical characteristics characteristic of adakites: SiO2 > 56 wt%, Al2O3 > 15 wt%, Na2O > 3.5 wt%, Sr > 400 ppm, Y < 18 ppm, Sr/Y > 40 ppm, Zr/Sm > 50 ppm. This new study involves analysis of lavas and pyroclastic deposits produced during each of the eruptive periods which dominate Mt. Hood’s history: Main Stage (700,000 - 29,000 yrs), Polallie (15,000 - 12,000 yrs), Timberline ( 1800 - 1500 yrs), Old Maid (250 - 176 yrs). Preliminary analysis shows that contribution of melts derived by partial melting of subducted lithosphere may occur over restricted intervals within individual eruptive periods, but is not continuous throughout the volcano’s history. However, the identification of slab-derived melts within individual eruptive periods is significant in that it suggests periodic tapping of independent magma source regions. Consequently, Mt. Hood may represent a ‘hybrid’ case between that of Mt. St. Helens, where magmas form by melting of warm subducted lithosphere at ~80 km, and Mt. Adams, where magmas form in the overlying mantle at ~135 km (Defant and Drummond, 1993).