EXPLORING THE IN-BETWEEN: A STUDY OF DACITE PETROGENESIS AT MIDDLE SISTER OF THE THREE SISTERS VOLCANIC COMPLEX (TSVC) IN CENTRAL OREGON, USA

The Three Sisters (Klah Klahnee) Volcanic Complex (TSVC) in central Oregon has erupted mafic, intermediate, and silicic products. The Three Sisters comprise North (mafic, >50 ka), Middle (MS; mainly mafic-intermediate, 50-14 ka), and South Sister (SS; mainly intermediate-felsic, 50-2 ka). MS and SS experienced an overlapping eruptive history of alternating active periods, often with one volcano frequently erupting while the other is relatively inactive. MS erupted basaltic-andesites to rhyolites ~50-37 ka before experiencing a ~10 ka lull. Afterwards, MS largely erupted andesites, then shifted to exclusively basaltic-andesites and dacites ~22-14 ka. Although dacites are common eruptive products at both volcanoes, their origins are unconstrained at MS and debated at SS. To better explain intermediate magma origins at MS, the TSVC, and throughout the Cascades, this study focuses on three MS dacite eruptions—the pre-lull dacite of Linton Creek (dlc; oldest known MS/TSVC dacite, ~37 ka) and the post-lull dacites north of Separation Creek (dsn; ~19 ka) and of Irving Glacier (dig; youngest MS/TSVC dacite, ~14 ka). All three dacites are porphyritic, of low crystallinity, and contain individual phenocrysts and glomerocrysts. This study utilizes textural observations, major mineral compositions, and whole rock data to characterize co-crystallizing assemblages, investigate petrogenesis, and identify pre-eruption processes. Textural and geochemical evidence suggest multiple plagioclase and pyroxene populations in dlc, whereas dsn and dig exhibit singular populations of major phases. Unit dig has similar petrography to dsn, with larger average crystal sizes, and is in contact with dsn. Evidence for thermal and/or compositional disequilibrium (e.g., magma mixing, decompression) is preserved as extensive orthopyroxene rims surrounding clinopyroxene crystals and prevalent resorption textures throughout the three lavas, as well as amphibole pseudomorphs and reacted olivine crystals in dlc. Preliminary pyroxene and Fe-Ti oxide thermobarometry results for the dacites indicate storage at high temperature (>1000°C) and pressure (>7 kbar). Current interpretations include formation of dlc via multiple magmatic components, and that the post-lull units dsn and dig may have erupted from a shared reservoir.