A PETROLOGICAL AND GEOCHEMICAL INVESTIGATION OF TERTIARY TO QUATERNARY MAGMA EVOLUTION IN THE MOUNT HOOD REGION, CASCADE RANGE, NORTH AMERICA

Abstract

Over the last 700,000 years, Mount Hood, Oregon has erupted chemically consistent andesitic lavas and pyroclastic flow deposits. Lavas generally fall into the calc-alkaline range with a restricted composition of approximately 50-65 wt% SiO2. Major element geochemistry suggests Tertiary lavas have undergone more fractional crystallization and magmatic differentiation processes than those of the Quaternary. Evidence of magma mixing is also found in thin section analyses with inclusions, sieve textures, and resorbed phenocrysts. Trace element analyses indicate the absence of HSFE depletion relative to LILE in Quaternary samples, indicating Tertiary samples exhibit a closer relationship to arc magmas than what is seen in Quaternary samples. The depletion in Tertiary samples implies a modification of their upper-mantle source region by slab-derived fluids in the subduction zone to the west of the Cascades. The depletion also implies that Quaternary lavas underwent less intereaction with subducted sediment or less magmatic differentiation than Tertiary samples.