THE GEOLOGY OF THE HAWKS VALLEY — LONE MOUNTAIN VOLCANIC CENTER: THE OLDEST KNOWN CALDERAS ASSOCIATED WITH THE YELLOWSTONE HOTSPOT

Benson et al. (2017) interpret the time-space progression of Mid-Miocene silicic calderas along the Oregon/Nevada border in the High Rock Caldera Complex (HRCC) and McDermitt Volcanic Field (MVF) as marking the propagation of massive flood basalt dike swarms away from the initial impingement of the Yellowstone plume near Steens Mountain. New geological mapping at the Hawks Valley— Lone Mountain (HVLM) volcanic center north of HRCC, coupled with new 40Ar/39Ar geochronology, has identified two previously unknown calderas, the Hawks Mountain and Buck Buttes Calderas, as sources for the oldest ignimbrites in the region: the 16.51 Ma (FCs = 28.02 Ma) lithic-rich trachytic Tuff of Juniper Spring and the 16.48 Ma pumice-rich low-silica alkali rhyolite Tuff of Robbers Roost. These tuffs are easily distinguished from the younger, mostly peralkaline ignimbrites of HRRC and MVF by their metaluminous compositions and conspicuous biotite phenocrysts.

Eruption of the 16.51 Ma Tuff of Juniper Spring truncated trachytic and low-silica alkali rhyolite lavas, and resulted in collapse of the 9-km-diameter Hawks Mountain Caldera. Nested within it is the slightly younger 7-km-diameter Buck Buttes Caldera, which collapsed on eruption of the 16.48 Ma Tuff of Robbers Roost. Locally preserved within the Buck Buttes Caldera are diatomaceous lake sediments intruded by perlitized crystal-rich rhyolitic lava domes at ~16.46 Ma.

The 40Ar/39Ar ages of the Tuffs of Juniper Spring and Robbers Roost are consistent with their stratigraphic position below the oldest ignimbrite of the MVF, the 16.47 Ma Tuff of Oregon Canyon, in exposures on the western flank of the southern Pueblo Mountains (Mahood and Benson, 2017). In addition, the oldest ignimbrite from HRCC, the 16.38 Ma Idaho Canyon Tuff, laps post-caldera lavas associated with the Juniper Spring and Buck Buttes Calderas, confirming their designation as the oldest known calderas associated with the Yellowstone plume. The location of the two calderas ~14 km north of the northernmost and oldest caldera of HRCC, the Virgin Valley Caldera, supports the conclusions of Coble and Mahood (2016) and Benson et al. (2017) that the younging of silicic calderas away from Steens Mountain reflects crustal melting caused by the intrusion of dike swarms of flood basalt that propagated southward with time from a focus beneath Steens Mountain.