VOLCANISM ASSOCIATED WITH ERUPTION OF THE STEENS BASALT AND INCEPTION OF THE YELLOWSTONE HOT SPOT

Silicic volcanism associated with eruption of the Steens Basalt and attributed to inception of the Yellowstone Hot Spot extends beyond the proposed area of inception at the McDermitt caldera. Prior to eruption of the Steens Basalt, calc-alkalic rocks exposed near the base of Steens Mountain consisted of rhyolite dome fields (21.1 to 24.3 Ma) and andesite flows (17.2 to 20.8 Ma). Age and stratigraphic relations indicate an abrupt transition from calc-alkalic rocks to flood-basalt tholeiites. N-NNE-trending feeder dike systems for the calc-alkalic rocks and the Steens Basalt have similar orientations indicating no change in the stress orientation during this transition. Volcanic rocks overlying the Steens Basalt in the Trout Creek Mts. consist of basaltic-andesite flows with disequilibrium phenocryst assemblages that reflect contamination of Steens Basalt by lower crustal melts. After eruption of the Steens Basalt, large-volume peralkaline ignimbrites from the McDermitt (15.5 to 16.1 Ma) and the Lake Owyhee volcanic fields (15.0 15.5 Ma) in eastern Oregon, were erupted. Steens-type mafic melt inclusions occur in some of the ignimbrites, such as the Spring Creek tuff (15% by volume). The two silicic volcanic fields and associated calderas define two maxima in crustal melting within the broader thermal anomaly indicated by the extent of the Steens Basalt. Outside the silicic volcanic fields, such as along the northern Nevada rift, eruptions of Steens-type basalts continued and locally interfingered with flows composed of variable mixtures of tholeiitic and lower crustal melts. The age range of the Steens Basalt, 15.3 to 16.6 Ma, and the ignimbrites, 15.0 to 16.1 Ma, overlap, but stratigraphic relations indicate that Steens-type basalt eruptions ended with eruption of the ignimbrites in both silicic volcanic fields. Emplacement of large silicic magma chambers in the upper crust precluded the continued ascent and eruption of tholeiite flood-basalt within the volcanic fields. Continued eruption of small volumes of Steens type basalt occurred on the periphery of the volcanic fields, but the main locus of voluminous flood-basalt tholeiites moved northward to the Columbia plateau. Inception of the Yellowstone Hot Spot near the McDermitt caldera does not adequately explain these major middle-Miocene volcanic events.