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Magmatic interactions at the Three Sisters (Klah Klahnee) volcanic complex, central Oregon Cascades, USA

Madelyn Cook1, Alex Newsom1, May (Mai) Sas 1, 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: 236

  • Programme No: 1.1.22

  • Theme 1 > Session 1

Abstract

This study utilizes compositions and textures of major mineral phases to identify distinct magmatic components in two lavas from the Three Sisters (Klah Klahnee) volcanic complex (TSVC) in Oregon, USA, to better constrain its plumbing system. The TSVC includes the adjacent Middle Sister (MS; primarily mafic-intermediate) and South Sister (SS; primarily intermediate-silicic), which are recently active (50--2 ka) Cascade Arc stratovolcanoes. The two lavas investigated here -- the MS-affiliated dacite of Linton Creek (dlc, ~37 ka) and the SS-affiliated rhyolite of Separation Creek (rsc, ~25 ka) -- present evidence for multi-magma interactions. Unit dlc hosts multiple populations of major mineral phases, as well as sparse amphibole pseudomorphs and reacted olivine; clotting relationships suggest three major magmatic components (mafic, intermediate, and silicic) and extensive compositional mixing between the mafic and intermediate components. Collectively, these components output a dacitic bulk composition. In contrast, rsc experienced two major recharge events, initially triggering partial mixing and then remobilization, recorded in multiple populations of major phases and extensively reacted olivine. Based on compositional differences recognized in MS-type magmas (i.e., more reduced conditions, high NiO versus forsterite trends in olivine, lack of amphibole) relative to SS-type magmas (i.e., more oxidized conditions, lower NiO versus forsterite trends, presence of amphibole), this work reveals that the MS-affiliated dlc is dominated by SS-type magmatic compositions, while the SS-affiliated rsc records recharge of a SS-type rhyolitic reservoir by MS-type mafic magma. These results are the first to demonstrate interactions between the MS and SS plumbing systems, which otherwise remain discrete.