Peralkaline ash flow tuffs and calderas of the McDermitt Volcanic Field, southeast Oregon and north central Nevada

The McDermitt volcanic field covers an area of 20,000 km² in southeastern Oregon and northwestern Nevada and consists of seven large-volume ash flow sheets that vented from 16.1 to 15 Ma ago. The volcanic field is characterized by peralkaline, high-silica rhyolite, and all but one of the sheets are comendites. The ash flow field overlies a 1.5-km-thick section of Miocene Steens volcanic series and Steens Basalt, indicating a minimum age of 16.1 Ma for the Steens Basalt. The Orevada rift, a 200-km-long NNW-striking structure, developed just prior to eruption of the mafic lavas. Each ash flow sheet resulted in the formation of a large collapse caldera along the Orevada rift. The center of the volcanic field is dominated by four overlapping and nested calderas; satellitic to these are three additional calderas. The areas adjacent to the incipient calderas subsided during ash flow eruption. Thickening of the ash flow sheets, monoclinal warping outside the caldera ring fault, and tilting in toward the caldera of blocks bounded by curvilinear faults all indicate regional subsidence prior to caldera collapse. The McDermitt caldera complex is highly mineralized; it contains ore deposits of Hg, Sb, Cs, Li, and U. The peralkaline tuffs have high contents of these elements and were the source rocks from which metals were leached by hydrothermal systems developed during the last stage of caldera-related volcanism.