GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION OF A SUBDUCTION-RELATED, REAR-ARC CONTINENTAL MAGMA SYSTEM: NEWBERRY VOLCANO, CENTRAL OREGON

Ash-flow tuffs from Newberry Volcano, located ~50 km east of the Cascade axis, provide insight into the evolution of a subduction-related, rear-arc continental magma system. Whole-rock major and trace element data were obtained from the following Pleistocene ash-flow tuffs exposed on the east side of Newberry: tuff of Tepee Draw (Qtp), dacitic tuff (Qdt), tuff of Orphan Butte (Qto), and tuff of lower east flank (QTae) (designations after MacLeod et al., 1995). Qtp pumices (n=31) are andesitic, dacitic, and rhyolitic (62.3-72.6 wt. % SiO₂), Qdt and Qto pumices (n=13) are dacitic (65.6-66.5 wt. % SiO₂), and QTae pumices (n=12) are dacitic to rhyolitic (66.0-70.8 wt. % SiO₂). Major and trace element variations with increasing SiO₂ are consistent with fractionation of the observed phenocryst phases, in particular plagioclase, augite, orthopyroxene, Fe-Ti oxides, and fayalite. On spider diagrams, Newberry pumices show enrichment in LIL elements relative to HFS elements, and on REE plots, samples display Eu anomalies, LREE enrichment, and HREEs are flat to slightly concave up. Overall, these trends are similar to other continental volcanic arcs, reflecting the influence of Cascadia subduction on Newberry magmatism.

The ash-flow tuffs record at least three samplings of the Newberry magma system at discrete times. Qtp, the oldest of the three (~0.5 Ma), represents eruption of a chamber that was zoned from andesitic to rhyolitic magma. Compositionally, pumices from Qdt and Qto are nearly identical and represent tapping of a homogeneous dacitic magma chamber. This result is consistent with previous interpretations that these units are from a single eruption or several co-magmatic eruptions. On many variation diagrams, Qdt and Qto pumices lie along the trend between the least evolved and most evolved Qtp samples, but a petrogenetic linkage via fractionation and/or mixing is not supported by the available data. QTae also records eruption of a zoned chamber but with a more limited compositional range and comparatively lower amounts of Ba, Rb, Nb, Ta, and Th. This suggests that QTae is not genetically related to the other tuffs. The lack of direct petrologic relationships between the ash-flow tuffs in this study indicates that the Newberry magma system consisted of transient magma bodies rather than a prolonged silicic magma chamber.