MAGMA MINGLING IN THE RHYOLITE OF SEPARATION CREEK, THREE SISTERS/KLAH KLAHNEE VOLCANIC COMPLEX, OREGON CASCADES, USA

Middle Sister (MS) and South Sister (SS) are adjacent, recently active (50–2 ka) stratovolcanoes in the central Oregon Cascades. This area, the Three Sisters Volcanic Complex (TSVC), sits at the intersection of the Cascadia Subduction Zone, the Basin and Range extensional province, and the High Lava Plains. The convergence of these tectonic regimes has resulted in a high volume of volcanism with a diverse suite of eruptive products ranging from basalt to rhyolite. The abundance of rhyolites here is rare for the Quaternary Cascades; while only one rhyolite is associated with MS, many are associated with SS, including the rhyolite of Separation Creek (rsc). This unit erupted in the saddle between MS and SS during a transitional period (ca. 25 ka) when activity at SS was dwindling and activity at MS was restarting. Notably, rsc includes abundant mafic enclaves and crystal clots. These components offer crucial clues about the structure of transcrustal magma systems situated beneath volcanic edifices and provide information about how intrusions of magma interact with, and possibly catalyze the eruption of, shallow magma reservoirs. This study focuses on the major mineral phases in rsc, both within the silicic host and in the mafic enclaves and clots. We investigate the magmatic origins of each major phase using major and minor element concentrations, as well as compare mineral populations in rsc to other units from the TSVC to determine whether rsc is more similar to MS-type or SS-type magmas. The presence of amphibole in the rsc silicic host supports the association of the rhyolite with SS, as amphibole in the TSVC is almost entirely associated with SS rhyolitic eruptions. In contrast, olivine compositions from mafic enclaves and clots overlap with those of olivine found in basaltic andesites from MS, but not olivine found in the single basaltic andesite known to have erupted from the SS main vent. Our results thus far also reveal a broad array of plagioclase compositions that likely represent multiple magmatic and cumulate components. These findings suggest that a SS-type rhyolitic reservoir may have been recharged by a MS-type mafic magma.