The Hampton Tuff, High Lava Plains, Oregon : Implications for Westward Migrating Silicic Volcanism

The Hampton Tuff is a 3.9 ± .02 Ma (2σ) ignimbrite sheet from the High Lava Plains of central Oregon. The majority of known outcrops exist to the north, within ~22 mi (~35 km) of the Frederick Butte Volcanic Center, the proposed source of the tuff. Thickness of the tuff is typically ~8 m (~25 ft), but varies from 6.5 m (20 ft) up to ~46 m (150 ft). In nearly all cases, these are minimum thicknesses as the bases of outcrops are rarely exposed and the tops are eroded. Assuming a constant thickness of 8 m and a defined area of 2418 km², the estimated eruption volume is ~20 km³, dense rock equivalent (DRE). Welding facies varies from nonwelded with pumice to densely welded with fiamme. Westward migration of bimodal volcanism of the High Lava Plains consists mainly of rhyolitic domes, ignimbrites and ash-flow sheets, and is associated with widespread tholeiitic basalt. Ignimbrites represent the dominant volume of erupted high-silica rhyolite in this region and include the Devine Canyon Tuff (9.7 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. A westward younging trend of rhyolitic volcanism of the High Lava Plains represents a coarse mirror of the northeast trending Yellowstone hotspot track from ~12 Ma to present. The Hampton Tuff is the youngest and westernmost tuff of the mapped ignimbrites within this westward younging trend. Although less voluminous than other High Lava Plains ignimbrites, the Hampton Tuff bears 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 indicates 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 more voluminous high-silica rhyolite like the Rattlesnake Tuff, given the opportunity to stage in the crust and enough thermal input. Field mapping and new ⁴⁰Ar/³⁹Ar age dates correlate units previously mapped as the Tuff of Espeland Draw by Johnson (1998), the ash-flow tuff west of Hampton Butte (Walker, 1970) and the Hampton Tuff of Iademarco (2009); also included are outcrops previously considered to be the Buckaroo Lake tuff by Streck (unpublished data). Field mapping also leads to the exclusion of several outcrops near Wagontire Mountain thought to be the Hampton Tuff and (or) the Tuff of Buckaroo Lake (MacLean, 1994). Major element analysis of the Buckaroo Lake tuff (6.85 Ma), sample HP-91-9, (Jordan, et al., 2004), confirms that the Buckaroo Lake tuff and the Hampton Tuff have distinct geochemical compositions. Presented herein is the discovery of a previously unknown ignimbrite of the High Lava Plains. The ignimbrite is informally named the Potato Lake tuff. It has an age of 5.13 ± 0.02 Ma (2σ) and has a chemical composition that is distinct from the Hampton Tuff, as well as the Buckaroo Lake tuff. Chemical analysis, outcrop descriptions, and age data are presented