CRYSTAL RECORDS AT THE THREE SISTERS VOLCANIC COMPLEX, OREGON

The Three Sisters Volcanic Complex (TSVC) is a bountiful volcanic focus that constitutes a triplet of composite cones known as the Three Sisters/Klah Klahnee that are surrounded by a mafic periphery. These sibling volcanoes, particularly Middle Sister (MS) and South Sister (SS), have produced a wide array of lava compositions that range from basaltic andesite to rhyolite. While silicic magmatism at SS has received growing attention in recent years, especially due to two decades of sustained inflation west of the SS summit vent, little is known about magma origins and processes at MS and little work has been done to assess potential reservoir connectivity between the SS and MS. Results presented here begin to tackle these questions through careful textural and compositional characterization of major mineral modes across a varied suite of lava compositions from both MS and SS, with a focus on intermediate to mafic volcanism. Several pairs of units were selected, with one unit from MS and another from SS, based on previous work that demonstrated contemporaneous ages (overlapping ages within uncertainty) and similar whole rock compositions, and additional units were chosen based on their age, composition, volumetric significance, and/or evidence for magma mingling. Most selected units erupted during a period of heightened activity at both edifices (ca. 27-21 ka). Findings thus far, based on mineral major and trace compositions, suggest that the MS and SS systems are fed via partial melts of a peridotite mantle with variable contributions from the lower crust. This agrees with the petrogenetic model for the older North Sister volcano, which implies extensional processes associated with the Basin and Range Province as its northern terminus intersects the TSVC. Despite similar magma origins and close proximity, distinct trace element concentrations of major phases suggest that the transcrustal magma systems feeding MS and SS are largely discrete (have little physical overlap or sharing of reservoirs), and the more complex crystal cargoes of SS lavas indicate the presence of a mush network beneath the SS edifice.