NEWLY DELINEATED CALDERAS AT THE HAWKS VALLEY-LONE MOUNTAIN CENTER IN SOUTHEASTERN OREGON ARE THE OLDEST KNOWN ASSOCIATED WITH STEENS FLOOD BASALTS AND THE IMPINGEMENT OF THE YELLOWSTONE PLUME

Benson et al. (2017, GSA Bull) interpret the time-space progression of Middle 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 volcanic center north of HRCC, coupled with new ⁴⁰Ar/³⁹Ar geochronology, has identified two previously unknown calderas, the Juniper Springs and Buck Buttes Calderas, as sources for the oldest ignimbrites in the region: the 16.50 Ma (FCs = 28.02 Ma) lithic-rich trachydacitic Tuff of East Creek and the 16.48 Ma pumice-rich trachyrhyolitic Tuff of Monument Basin. These tuffs are easily distinguished from the younger peralkaline ignimbrites of HRRC and MVF by their metaluminous compositions and conspicuous biotite phenocrysts.

Eruption of the 16.50 Ma Tuff of East Creek truncated 16.53 Ma trachydacite lavas, and resulted in collapse of the 9-km Juniper Springs Caldera. Nested within it is the slightly younger ~7 km Buck Buttes Caldera, which collapsed on eruption of the 16.48 Ma Tuff of Monument Basin. Locally preserved within the Buck Buttes Caldera are diatomaceous lake sediments intruded by perlitized crystal-rich rhyolitic lava domes and associated minor ignimbrite erupted ~16.44 Ma.

The ⁴⁰Ar/³⁹Ar ages of the Tuffs of East Creek and Monument Basin are consistent with their stratigraphic position below the oldest ignimbrite of the MVF, the 16.47 Ma Tuff of Oregon Canyon, on the western flank of the southern Pueblo Mountains (Mahood and Benson, 2016, EPSL). In addition, the oldest ignimbrite from HRCC, the 16.38 Ma Idaho Canyon Tuff, laps post-caldera lavas associated with the Juniper Springs 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, Geosphere) 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.