THE INFLUENCE OF CASCADIA SUBDUCTION AND THE HIGH LAVA PLAINS ON SILICIC MAGMATISM AT NEWBERRY VOLCANO, CENTRAL OREGON

Newberry Volcano is located at the west end of the High Lava Plains (HLP) ~60 km east of the Cascades. To evaluate the influence of Cascadia subduction and the HLP on silicic volcanism in this rear-arc setting, Pleistocene ash-flow tuffs related to the earliest caldera-forming eruption were analyzed, including the Tepee Draw tuff (Qtp), dacitic tuff (Qdt), and tuff of lower east flank (QTae). Most Qtp pumices are rhyolite (70.1-72.6 wt. % SiO₂) and homogeneous in hand sample, though some are less evolved (62.3-68.7 wt. % SiO₂) and banded. Qdt pumices are dacite with minimal SiO₂ variation (avg. 66.3 wt. %), and QTae pumices vary from dacite to rhyolite (66.0-70.8 wt. % SiO₂). Mineral composition data from representative pumices are used to calculate pre-eruption temperatures for each unit. Two-pyroxene geothermometry using the Putirka (2008) model is applied to Qtp and Qdt, which have both augite and enstatite. For pairs that pass the K_D(Fe-Mg) equilibrium test, average temperatures are 1049°C and 994°C for Qtp and Qdt, respectively. Fe-Ti oxide geothemometry via ILMAT is applied to Qdt and QTae, which contain both magnetite and ilmentite. Using coexisting pairs that satisfy the Bacon and Hirschman (1988) Mg/Mn equilibrium test, average temperatures are 929°C for Qdt and 1091°C for QTae. For both units, oxygen fugacity plots along the NNO buffer.

The temperature results are higher than those from calc-alkaline systems in western North America and more akin to Snake River Plain (SRP) rhyolites. Additional similarities between SRP and Newberry rhyolites include higher FeO/(FeO+MgO) ratios and lower Sr contents. However, the Newberry tuffs also share characteristics with Great Basin (GB) subduction-related dacites and rhyolites, including lower Nb, Ce, Ba, and Zr contents compared to SRP rhyolites. Further, Newberry samples are enriched in LILs relative to HFSEs. These trends are analogous to other continental volcanic arcs and document the influence of Cascadia subduction on Newberry magmatism. Conversely, higher temperatures and other parallels with SRP rhyolites indicate that crustal melting was driven by hot, mantle-derived mafic magmas. The compositional affinity of Newberry tuffs to SRP and GB silicic rocks suggests that both subduction and HLP volcanism played a role in generating rhyolitic magmas at Newberry.