Petrographic and multi-element geochemical data for a 37-sample suite representing Oregon's central High Cascade mafic platform allow the distinction of basalts (<53 wt % SiO2) from two types of basaltic andesites with associated high-silica basaltic andesites (53–62 wt % SiO2). All units yield fractionated REE and incompatible element-enriched patterns characteristic of continental tholeiitic magmas. Mount Washington (MW)-type basaltic andesites are distinguished from North Sister (NS) types by relatively higher total REE and other LILE abundances, although lithologic similarities prevail. Trace-element signatures of both types do not support their derivation from magmas represented by basalts, but represeint near-primary melts from regions apart from the sources of primary basalt magmas. Normal basalts are modeled with 14% incongruent partial melting and concomitant olivine fractionation of spinel Iherzolite upper mantle in the presence of water-rich fluids derived from a subducted oceanic slab. Basaltic andesites are modeled as 6% to 12% partial melting of fluid-enriched spinel Iherzolite accompanied by olivine (± clinopyroxene ± plagioclase) fractionation. Unusual units, including one with ground-mass biotite and rare quartz xenocrysts, probably resulted from divergent processes of crustal assimilation, crystal fractionation, or melting from alternate sources.

Early Pleistocene eruptions were largely diktytaxitic basalt outpourings in response to crustal readjustments and subsidence which occurred prior to 4.5 m.y. ago. The transformation to extensional tectonics abruptly reduced the amount of andesitic and silicic volcanism and allowed tapping of deeper, mafic magma source regions. Chemical and petrographic data support the concept of juxtaposed tectonomagmatic types such that, extension-related basalts erupted within the realm of calc-alkaline basaltic andesites and their silicic derivatives.

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