Constraints on the nature of rhyolite genesis at South Sister volcano, Oregon through 238U-230Th zircon ages and 230Th-226Ra plagioclase ages

I present in situ 238U-230Th ages for 10 zircons and 230Th -226Ra ages for four plagioclase size fractions along with in situ major and trace-element data for plagioclase and zircon from the Rock Mesa rhyolite flow at South Sister volcano, Oregon. Zircons yield ages from ~11ka-55ka with one grain >350ka, whereas all bulk plagioclase separates yield ages from 3800-7300yBP. Saturation and dissolution calculations indicate that the Rock Mesa magma was undersaturated in zircon at magmatic temperatures and zircon would dissolve in ~3200yrs. This observation along with Ti-in-zircon temperatures and large intra-grain variations in trace-elements implies that most zircons in the Rock Mesa lava flow are antecrystic (i.e., recycled from genetically related but chemically distinct magmas), with one xenocryst (i.e., genetically unrelated) originating from country rock.;Plagioclase average ages are young suggesting that some plagioclase is near eruption age; however, in situ LA-ICPMS trace-element data show that there is more variability in the trace-element data at a given anorthite content than can be explained by partitioning behavior. This observation precludes simple crystallization from a single source and implies that some plagioclase in the Rock Mesa is antecrystic and came from a source similar in composition to the Rock Mesa magma. Modeling of plagioclase Th-Ra data demonstrates that ~75% of the plagioclase could be >10ka and thus similar in age to the zircons; whereas, ~25% must be <10ka and different in age than the zircons.;The presence of antecrystic zircon whose ages overlap with periods of basaltic to rhyolitic volcanism at South Sister and antecrystic plagioclase similar in composition to younger (likely autocrystic -- i.e., crystallized from the host magma) plagioclase implies that antecrystic grains are largely recycled from previous magma batches from the South Sister system. This favors rhyolite genesis driven by recycling of material from earlier South Sister magmatism. Furthermore, these data indicate that plagioclase and zircon record different information about the South Sister magmatic system and demonstrate that a more complete understanding of magmatic systems can be derived from dating multiple phases and placing the ages into context using corroborating evidence than by dating either major or accessory phases.