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 arc has erupted predominantly 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-squares mixing calculations show that fractional crystallization of observed phenocryst phases can account for most major-oxide variation displayed by Mt. Hood lavas. AFC modeling indicates that small amounts of assimilation (Ma/Mc = 0.1–0.15) of high-K2O crustal rock occurred during certain eruptive episodes, but did not have a significant effect on magma composition. Evidence of magma mixing (partially resorbed olivine, plagioclase and pyroxene phenocrysts, magmatic inclusions) is found in lavas erupted throughout the volcanoʹs history. Trace-element mixing calculations indicate that repeated cycles of mixing resulted in the eruption of compositionally similar lavas throughout the history of the volcano. Mt. Hood lavas are unusual in that they do not exhibit depletion of high field strength elements (HFSE) relative to large ion lithophile elements (LILE). Depletion of HFSE relative to LILE is considered a common geochemical characteristic of arc lavas, and is usually attributed to modification of the upper mantle source region by interaction with slab-derived fluids. Calculations of pre-eruptive water contents using an H2O-dependent plagioclase thermometer indicate that Mt. 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 region. An alternate source of such depletions in arc magmas is subducted sediment. Both pelagic and continental margin sediments potentially subducted along the Cascades trench do exhibit depletion in certain HFSE (Nb, Zr) relative to LILE. The absence of HFSE depletions in lavas erupted at Mt. Hood, therefore, appears to reflect negligible subduction of sediment and/or negligible mixing between subducted sediment and the upper mantle source region.