PRELIMINARY FIELD, PETROLOGIC, EXPERIMENTAL MIXING AND X-RAY COMPUTED MICROTOMOGRAPHY RESULTS FROM THE MIXED BASALT AND RHYOLITE COMPLEXES ALONG THE EAST GALLATIN-WASHBURN FAULT ZONE, YELLOWSTONE NATIONAL PARK, WY, USA

Magma mixing is one of the fundamental aspects of magmatic evolution and provides insight into volcanic hazards. The presence of at least four mixed magma complexes at Yellowstone National Park suggests that mingling is not due to random intersections of feeder dikes. Although these magmas visually appear to be commingled, there are several petrological evidences of chemical exchange (mixing) between the rhyolitic and basaltic magma. We propose a model that combines previous work on the Grizzly Lake, Gardner River, Crystal Spring, and Appolinaris Spring mixed magma complexes with results from new geochemical analyses, recent mixing experiments data, and regional structural geology studies. Coeval extensional tectonism, as seen in the East Gallatin-Washburn fault zone, is also present in other areas where basalt and rhyolite mixing/mingling occur. The central portions of the mixed complexes show a higher degree of mixing and development of complex structures with highly variable geochemistry. Phenocrysts transfer between mafic and felsic portions of the complexes suggest that these mixed magmas did not have enough time, or energy, to thoroughly mix into homogeneous hybrid magma. Mixing of these two extreme end-members may have required decompression of the lower basaltic magma chamber as a consequence of the eruption of the overlying rhyolitic magma into through structurally weakened zones of the upper crust. The decompression induced mixing was facilitated by coeval extensional tectonics and structures in the Norris-Mammoth Corridor. Petrologic experiments and X-ray computed microtomography are used to explore the time scale of mixing and address if mixing triggered the eruption of volcanic magma-mixing complexes in the Yellowstone volcanic field, or vice versa.