Newberry Volcano, Oregon: No traveling hot spot is needed

Newberry Volcano (NV) has been interpreted as forming the end of the traveling “Newberry Hot Spot” responsible for producing progressively younger rhyolites as it passed westward under the High Lava Plains (HLP). However, Newberry rhyolites are unlike those to the east in the HLP. HLP rhyolites are characterized by high silica (>74%) and high FeO (Ford et al., 2009 GSA abs.). None of the Holocene Newberry rhyolites and only a few of its older rhyolites have such high silica contents. The NV rhyolites have low FeO contents comparable to those of rhyolites to the west near the Cascades axis. NV is situated at the western edge of the HLP (a subprovince of the Basin & Range) east of the Cascades arc axis, its broad shield shape in striking contrast to typical Cascades stratocones. Isotopic and petrologic evidence (Graham et al., JVGR 2009; Carlson et al., 2008 Goldschmidt abs.; Grove et al, 2009 GSA abs.) indicate that Newberry lava compositions are unlike HLP lavas, and instead include a strong input of subduction fluid. The shape, size, and rear-arc position of NV are like those of Medicine Lake volcano (MLV) east of Mount Shasta in northern California, also often misinterpreted as a Basin & Range volcano. Newberry is even larger than the 2000 km2 MLV. In fewer than half a million years, NV has built a km-high edifice, suffered at least 3 caldera collapses, and its lavas have covered approximately 3000 km2, making it the largest of any Cascade volcano. At both NV and MLV, the central caldera is situated over the intersection of major tectonic trends. The widespread distribution of vents and their dominant NE to NW trends at both volcanoes attest to tectonic control of eruptive pathways and to broad magmatic foci. Fluids are required to generate the calcalkaline basalts present at both volcanoes (especially NV) in addition to tholeiitic basalts. Hydrous arc magmatic inputs also play a key role at both NV and MLV in generation of significant volumes of rhyolite and coeval shallow granitic intrusive rocks. Focused extension, subduction fluids, and shallow hot mantle are the essential factors that combine to create NV and MLV. Plumes are unnecessary.