PETROGRAPHIC AND GEOCHEMICAL INVESTIGATION OF MAGMATIC PROCESSES AT OLALLIE, PINHEAD AND CLEAR LAKE BUTTES, NORTHERN OREGON CASCADE RANGE

Quaternary volcanism in the northern Oregon Cascade Range is dominated by andesite to rhyodacite lavas at Mt. Jefferson (Conrey, 1991) and at Mt. Hood (Cribb and Barton, 1996). Open-system magma mixing is proposed as a cause of eruptions at both Mt. Hood (Kent et al., 2010) and at Mt. Jefferson (Ferrell et al., 2015), and repeated cycles of magma mixing–fractionation are suggested as a cause of the narrow compositional range of lavas erupted at Mt. Hood (Cribb and Barton, 1996). This research examines the petrographic, major element and trace element characteristics of three small Quaternary buttes located between Mt. Jefferson and Mt. Hood: Clear Lake Butte (CLB), Pinhead Buttes (PB), and Olallie Butte (OB). The research investigates whether CLB, PB, and OB exhibit geochemical and/or petrographic evidence of open-system magma mixing similar to that at Mt. Hood and Mt. Jefferson, or whether the smaller CLB, PB and OB magmatic systems were closed and dominated by fractional crystallization. The research has potential to illustrate similarities and differences between adjacent magmatic systems of similar age, yet different scale. Petrographic evidence for magma mixing (plagioclase and pyroxene zoning, sieving, reaction rims) is clearly exhibited by OB samples, while both CLB and PB samples lack widespread disequilibrium textures. Major element oxide trends are consistent with fractional crystallization, but show narrow ranges of concentrations and non-overlapping compositions between PB, OB and CLB. All three buttes are characterized by narrow ranges of incompatible and compatible trace element concentrations. CLB, PB and OB all exhibit LREE enrichment and lack significant HFSE depletions, with PB exhibiting greatest enrichment in REE. These results suggest that CLB, PB and OB magmas were derived from compositionally similar, but discrete mantle sources, and that their magmas were not co-genetic. Geochemical data is consistent with fractionation of observed mineral phases as the dominant magmatic process at each butte, although disequilibria textures indicate a history of magma mixing at OB, and all three buttes exhibit narrow major and trace element compositional ranges such as those attributed to magma mixing at Mt. Hood.