The Shevlin Park Tuff, Central Oregon Cascade Range: Magmatic Processes Recorded in an Arc-Related Ash-Flow Tuff

The circa 260 ka Shevlin Park Tuff is found throughout an area of some 400 square km west of the city of Bend, OR. The tuff is composed of several flow units, the lowest of which was mapped separately in the past as the Century Drive Tuff. We have found the Century Drive to be chemically and paleomagnetically similar to the Shevlin Park. The spatial distribution and pumice imbrication of the Shevlin Park suggest a source at an elevation near 2000 m on the Bend Highland 5-6 km east of Broken Top volcano. Deposition of the Shevlin Park may have been preceded by a Plinian airfall eruption, now mainly preserved in the Columbia Canal irrigation ditch, which is likely equivalent in the distal tephra record to the Summer Lake NN layer. Despite our extensive database of bulk pumice and glass geochemistry, we cannot corroborate an earlier correlation of the Shevlin Park with the Summer Lake JJ tephra. The Shevlin Park Tuff is compositionally bimodal, with black pumice ranging from 55-62% silica, and commonly paler silicic pumice from 64-68%. Lower flow units appear to contain proportionally more silicic pumice and slightly more fractionated (lower MgO) mafic pumice. Mafic pumice is much more heterogeneous for a given silica percentage than silicic pumice, especially in P, Fe/Mg, and Sr. Both types of pumice are crystal-poor, and thus the bulk pumice and glass compositions are similar. Phenocrysts present in pumice include plagioclase (dominantly reversely zoned An30-40, but ranging up to skeletal An82), two pyroxenes (typically reversely zoned), olivine (Fo71-76), magnetite, and ilmenite. The phenocryst assemblage and mineral chemistry of the Columbia Canal pumice are similar, with the exception of slightly more Fe-rich opx. Mixing of mafic and silicic magma appears to be the dominant process in the generation of the wide compositional range within the Shevlin Park. A simple mixing model can account for most of the major and trace element, and Sr isotopic variations. The wider range in Fe/Mg and Sr in the mafic pumices can be modeled with approximately 20% crystal fractionation, but variations in P, and in P/Zr ratio, cannot be explained by a fractionation model. The P variations suggest a three component mixing model of silicic magma and two mafic magmas, with the most fractionated mafic magma having the highest P and P/Zr. The latter magma may have resorbed apatite during its evolution to account for its higher P/Zr ratio.