DISTRIBUTED VOLCANISM IN THE CENTRAL OREGON CASCADES: INSIGHTS INTO ERUPTION STYLES, DURATIONS, AND MAGMA COMPOSITIONS FROM THE TWIN LAKES-WUKSI BUTTE CHAIN

Eruptions from distributed volcanic fields can exhibit variable eruption styles and diverse magma compositions. The north-south trending, 10-km long Twin Lakes-Wuksi Butte (TLWB) chain (central Oregon, USA) likely erupted in the late Pleistocene (~14–20 ka). During this time window, numerous other mafic eruptions occurred near TLWB, including the roughly N-S aligned Mount Bachelor volcanic chain. The TLWB chain contains at least 10 centers, ranging from maars in the south to tuff rings and cinder cones in the north. We combine field mapping; whole-rock, mineral, and glass geochemistry; and paleomagnetic data to evaluate spatial and temporal changes in eruption styles and magma compositions, and compare the TLWB to other eruptive centers in the region.

Volcanism in the TLWB chain progressed from south to north over time. Initial eruptions were dominantly phreatomagmatic and produced maar craters and surge deposits. Vents in the middle part of the chain exhibit variable eruption styles, and produced a spatter vent, a large tuff cone with thick (up to 20 m) surge deposits, and a lava lake. The northern and youngest eruptions appear to have been dominantly magmatic, producing cinder cones and lava flows. Paleomagnetic directions from the four northernmost vents overlap, suggesting all were emplaced in less than a few decades. The TLWB directions also overlap with the earliest phase of activity in the Mount Bachelor chain, indicating that basaltic activity was widespread at this time.

Magmas of the TLWB chain are basaltic with high TiO₂ (~1.7 wt%) and overlap in composition with other Ti-rich basalts in the region. Olivine-hosted melt inclusions contain low H₂O (<1.5 wt%) and variable CO₂ (<2000 ppm), suggesting entrapment pressures <350 MPa. Glass and mineral data also indicate that one of the central vents erupted a slightly different, hotter, magma batch (higher SiO₂, lower TiO₂ and more Mg-rich olivine). TLWB trace element data, including Nb/Zr ratios, indicate that magma batches originate from variably enriched mantle, similar to other basalts erupted in the region. This work highlights the diverse eruption styles and magma compositions of the TLWB chain over short temporal and spatial scales, and expands our understanding of the history of volcanic activity from distributed volcanic fields in central Oregon.