Evolution of calc-alkaline magmas at the early Quaternary Battle Ax volcano, Western Cascade Range, Oregon

Abstract

Battle Ax volcano is a dissected composite cone of early Quaternary age, located 33 km northwest of Mt. Jefferson in the Western Cascade Range of Oregon. It evolved during three major eruptive episodes, each of which produced lavas with a distinctive set of petrographic, geochemical and paleomagnetic features. The oldest rocks (Series I) consist of olivine-bearing basaltic andesites and two-pyroxene andesites, with the earliest erupted lavas generally being most mafic. The intermediate-age rocks (Series II) are a compositionally uniform suite of two-pyroxene andesites. The youngest rocks (Series III) consist mainly of hornblende andesite, but also include dacitic lavas and rhyolitic tuff. The first eruptions during the Series III episode produced the most silica-rich rocks and succeeding eruptions discharged progressively more mafic lavas. Rocks comprising one eruptive series cannot be related to those of another series by any simple fractionation or mixing process, owing mainly to intra-series differences in the abundances of incompatible trace elements. Compositional variations among the Series I lavas can be attributed primarily to fractional crystallization of plagioclase, olivine (or orthopyroxene), augite, and magnetite (POAM). The most mafic basaltic andesites in this series probably accumulated small amounts of olivine. The Series III magma evolved mainly by fractionation of plagioclase, amphibole, and orthopyroxene, ±magnetite, ±apatite. Compositional differences among the porphyritic andesites of Series III reflect different amounts and proportions of accumulated xenocrysts derived from hornblende-gabbro cumulates. The most silicic rocks in Series III are probably mixtures of fractionated magma and crustal components. Stratigraphic relations among the Series III lavas indicate they represent magmas erupted from progressively deeper levels in a compositionally zoned reservoir.