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Middle Sister, South Sister, andesite, plagioclase, pyroxene


Klah Klanee (Three Points) is located in the central Cascades near Bend, Oregon. The Three Sisters Volcanic Complex (TSVC) lies at a tectonically complex intersection of the Cascade Subduction Zone, the Basin and Range Province, and the High Lava Plains. The TSVC is a compositionally diverse volcanic field consisting of four stratovolcanoes and numerous periphery cones and vents. Middle Sister and South Sister are the youngest of the stratovolcanoes and they share a largely contemporaneous and semi-alternating episodic eruptive history. These parallel histories could indicate a complex, interconnected magmatic root system within the crust below the TSVC and the surrounding area.

To determine the extent of interconnectivity between these two peaks, whole rock chemistry, mineral chemistry, and petrography were utilized to compare two temporally related andesites on the west flanks of Middle Sister and South Sister. The andesites, andesite of Lost Creek Glacier (alg) and andesite of Linton Creek (alc), erupted ca. 27 ka and have nearly identical whole rock chemistry, mineral types, and mineral abundances. Origins of these andesites were determined using mineral populations based on mineral textures and chemistry. South Sister unit alg contains three plagioclase populations, four clinopyroxene populations, five orthopyroxene populations, and one olivine population. Middle Sister unit alc contains two plagioclase populations, three clinopyroxene populations, three orthopyroxene populations, and two different types of enclaves. The enclaves identified consist of an olivine and plagioclase-bearing type and orthopyroxene and plagioclase bearing-type. Each of these types carries its own unique crystal cargo not found in the host, alc. Therefore, the enclaves were determined to be lithic fragments incorporated during the final stages before eruption. The andesites, alc and alg, appear to share two plagioclase populations, two clinopyroxene populations, and two orthopyroxene populations. Several mineral populations found in alg commonly have fine reaction rims that are not present in the equivalent alc populations, although other than this slight variation, these populations are interpreted to be the same.

Plagioclase mineral chemistry suggest that the andesites erupted on the west flanks of these two volcanoes share two sources, one at depth (higher An plagioclase, populations 1a and 1b) and a second, shallower source (moderate An plagioclase, population 2). Pyroxene mineral populations failed to clearly constrain potential sourcing due to significant overlap in compositions; further trace element analysis is required. Overall, alg and alc contain many of the same populations with similar chemistry and textures, indicating these magmas likely share a magmatic source(s). However, unit alg contains a much more complicated crystal cargo with more complex clots and increased mineral populations across every phase. It is likely that prior to eruption alg interacted with an additional magma reservoir(s) (e.g., mush) that alc did not encounter.


This paper is Emma Calvert's Senior Thesis for the Department of Geology, Western Washington University. Emma's advisor was Dr. Mai Sas.






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