IGNIMBRITES OF THE HIGH LAVA PLAINS, OREGON: IMPLICATIONS FOR PERIODICITY AND EVOLUTION OF IGNIMBRITES IN WESTWARD MIGRATING SILICIC VOLCANISM

Bimodal volcanism of the High Lava Plains consists mainly of rhyolitic domes, ignimbrites and ash-flow sheets associated with widespread tholeiitic basalt. Ignimbrites represent the dominant volume of rhyolite in this region and include the Devine Canyon Tuff (9.74 Ma), the Prater Creek Tuff (8.4 Ma), and the Rattlesnake Tuff (7.1 Ma); each of which represents 100–300 km³ of erupted material. We add dates for two ignimbrites that crop out west of these three large ignimbrites. The newly identified tuff of Potato Lake has an age of 5.1 ± 0.02 Ma (2σ), similar to that of the Glass Buttes dome complex, a likely source area. The Hampton Tuff has an age of 3.91 ± 0.02 Ma (2σ) and has its source near Frederick Butte Volcanic Center, a 3.9 m.y. dome complex. There is a ~1.4 m.y. recurrence interval of major ignimbrites with westward migration of silicic volcanism on the High Lava Plains.

The majority of outcrops of the Hampton Tuff exist to the north, within ~ 35 km of the Frederick Butte Volcanic Center. Thickness of the tuff is typically ~8 m, but varies from 6.5 m up to ~46 m. These are minimum thicknesses as outcrop bases are rarely exposed and the nonwelded top is ubiquitously removed. Assuming a thickness of 8 m and a defined outcrop area of 2418 km², the estimated eruption volume is ~20 km³, dense rock equivalent (DRE). The volume is likely to be twice that, given the lack of a known caldera and no accounting of potential fall deposits. The volume of the tuff of Potato Lake is unknown, but were it large, it would presumably have been discovered sooner.

The Hampton Tuff and the tuff of Potato Lake bear the high-iron signature (up to 3.9 wt% FeO*) that is characteristic of regional rhyolites and sparse dacites (up to 6.4 wt% FeO*). Microprobe analysis of glass shards from the Hampton Tuff reveals at least four compositional clusters with distinct ranges of silica that vary inversely with iron content. Although silica content of rhyolite compositions range from 73–77.5 wt% SiO₂ only ~25% of analyses are high-silica rhyolite (>75 wt% SiO₂), suggesting that the magma chamber is an example of “arrested development” of a magmatic system that could evolve a voluminous high-silica rhyolite like the Rattlesnake Tuff, given the opportunity to stage in the crust and enough thermal input.