SIGNIFICANCE OF CRUSTAL AND SOURCE REGION PROCESSES ON THE EVOLUTION OF COMPOSITIONALLY SIMILAR CALC-ALKALINE LAVAS, MT HOOD, OREGON

Mt. Hood, Oregon, in the Cascade Range volcanic are has erupted predom inantly andesite lava and pyroclastic-flow deposits over the last 700, 000 years. Most lavas belong to the medium-K, calc-alkaline series and show a restricted range of composition (53-63 wt.% SiO2,). Least-squa res mixing calculations show that fractional crystallization of observ ed phenocryst phases can account for most major-oxide variation displa yed by Mt. Hood lavas. AFC modeling indicates that small amounts of as similation (M-a/M-c = 0.1-0.15) of high-K2O crustal rock occurred duri ng certain eruptive episodes, but did not have a significant effect on magma composition. Evidence of magma mixing (partially resorbed olivi ne, plagioclase and pyroxene phenocrysts, magmatic inclusions) is foun d in lavas erupted throughout the volcano's history. Trace-element mix ing calculations indicate that repeated cycles of mixing resulted in t he eruption of compositionally similar lavas throughout the history of the volcano. Mt. Hood lavas are unusual in that they do not exhibit d epletion of high field strength elements (HFSE) relative to large ion lithophile elements (LILE). Depletion of HFSE relative to LILE is cons idered a common geochemical characteristic of are lavas, and is usuall y attributed to modification of the upper mantle source region by inte raction with slab-derived fluids. Calculations of pre-eruptive water c ontents using an H2O-dependent plagioclase thermometer indicate that M t. Hood lavas contained up to 6 wt.% H2O prior to eruption. Therefore, the absence of HFSE depletions relative to LILE cannot be attributed to lack of interaction between slab-derived fluids and the source regi on. An alternate source of such depletions in are magmas is subducted sediment. Both pelagic and continental margin sediments potentially su bducted along the Cascades trench do exhibit depletion in certain HFSE (Nb, Zr) relative to LILE. The absence of HFSE depletions in lavas er upted at Mt. Hood, therefore, appears to reflect negligible subduction of sediment and/or negligible mixing between subducted sediment and t he upper mantle source region.