A petrologic and geochemical investigation of the evolutionary history of calc-alkaline magmas, Mt. Hood, Oregon

This thesis presents the results of a petrological and geochemical study of the calc-alkaline lava and pyroclastic deposits from Mt. Hood, Oregon. The study focuses on deposits erupted during the Main Stage (700,000-29,000 yrs), Cloud Cap (29,000-15,000 yrs) Polallie (15,000-12,000 yrs), Timberline (1800-1500 yrs) and Old Maid (250-176 yrs) eruptive periods.;The lava and pyroclastic deposits of Mt. Hood belong to the medium-K calc-alkaline suite and range in composition from basaltic andesite to dacite. Qualitative petrological and geochemical evidence shows that the lavas of Mt. Hood are mainly related by a process of fractional crystallization of the observed phenocryst phases, i.e. plagioclase, clinopyroxene, orthopyroxene, titanomagnetite, ilmenite, and hornblende. Calculated major and trace element models confirm the fractional crystallization hypothesis. Apart from fractional crystallization, there is qualitative and quantitative evidence that assimilation of volcanic crust occurred during the Main Stage and Timberline periods. Petrographic evidence suggests that magma mixing occurred during the Cloud Cap period.;Mt. Hood magmas appear to be derived from a MORB-like mantle source which was not depleted in high field strength elements (HFSE) prior to partial melting. The lack of HFSE depletions exhibited by Mt. Hood lavas is not necessarily related to the abundance of fluids in the mantle wedge.