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Evidence for adjacent yet discrete magma plumbing systems at Middle Sister and South Sister volcanoes, Oregon Cascades, USA

May (Mai) Sas 1, Sean Halstead1, Alex Newsom1, Madelyn Cook1, Olivia Ernst1, Emma Calvert1, James Peale1, Nathan L. Andersen2

  • Affiliations: 1Geology Department, Western Washington University, Bellingham WA 98225 USA; 2Cascades Volcano Observatory, United States Geological Survey, Vancouver WA 98683 USA 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 235

  • Programme No: 1.1.21

  • Theme 1 > Session 1

Abstract

Middle Sister (MS) and South Sister (SS) are two young (<50 ka), semi-coeval, and compositionally diverse (basaltic andesite to rhyolite) stratovolcanoes that are part of the Cascades Arc, USA. To understand how the plumbing systems of these geographically adjacent (within 5 km) volcanoes may or may not overlap, as well as illuminate magma origins, this study carried out a detailed investigation of mineral textures and compositions. A wide range of unit ages (ca. 47 to 14 ka), compositions (basaltic andesite to rhyolite), and localities (summit, east and west flanks) were selected in effort to identify any notable temporal, compositional, or spatial differences. An emphasis was placed on the overall more mafic MS system to better understand processes associated with mafic magmas and the deep crust, and to fill in existing knowledge gaps. Current findings suggest that mafic magmas at MS stall in the deep crust, where they interact with lower crustal melts, as has been interpreted for the older and monotonously mafic North Sister system. Mafic magmas at MS are also notable for their high-Ni olivine (in contrast to low-Ni olivine at SS), which is associated with crustal cumulates. Differences between the two systems extend into silicic magmatism, where MS magmas exhibit a higher alkalis trend (i.e., mostly trachytes at MS versus dacites at SS) and slightly more reduced conditions (i.e., ~∆NNO-0.5 at MS versus ~∆NNO+0.5 at SS). Cumulatively, these discrete characteristics support that separate processes are dominant at each volcano, as well as minimal interaction between the plumbing systems.