Newberry Volcano (Oregon, USA) Revised

Abstract:

Newberry Volcano (NV) located E. of the Cascades arc axis is often interpreted as (1) a High Lava Plains (NW Basin & Range -- B&R) volcano hosting rhyolites generated by a traveling plume, (2) a shield volcano built of basalt, or (3) an enigma unrelated to the adjacent High Cascades. Recent work shows that these interpretations are incorrect. Petrologic, geochemical, isotopic, drill hole, & seismic data indicate that the NV magma system results from arc-related processes at the NW corner of the B&R, where this major extensional province impinges on the Cascades arc. NV rhyolites are geochemically distinct and lower in SiO₂ than those to the east where a general NW-younging trend of rhyolite ages has suggested a traveling hotspot -- a consequence instead of propagation of B&R extension. NV lies ~90 km above the downgoing slab based on seismic evidence (McCrory et al. 2012), ~15 km deeper than under the Three Sisters (TS) volcanic complex 60 km to the NW on the arc axis. NV & TS exhibit a range of compositions and both have generated rhyodacite with unusually high Na₂O contents (~7 wt. %; Mandler et al. 2014), exhibiting similar petrogenetic processes. Silicic lavas and tuffs of the caldera-centric NV make up a significant component (~20% of drill core) of its 600 km³, although basaltic andesite is the dominant composition. Basalts of calcalkaline affinity erupted on the edifice as recently as early Holocene time. These basalts contain petrologic evidence for high pre-eruptive H₂O contents, have strong arc-like trace element signatures, and are isotopically Cascadian and distinct from basalts to the east in the B&R that have much higher ^3/4He (Graham et al. 2009). NV is one variety of Cascades arc volcano among which are a range of stratovolcanoes including Mt. Baker (15 km³) and Mt. Shasta (500 km³), a Holocene caldera (Crater Lake), and the many basaltic andesite shield volcanoes that make up most of the Oregon High Cascades.