MORE THAN JUST MCDERMITT AND STEENS MOUNTAIN: TEMPORAL-SPATIAL RELATIONSHIPS OF MID-MIOCENE MAGMATISM ON THE OREGON PLATEAU

Central to understanding how the Yellowstone hotspot has interacted with the North American lithosphere is its mid-Miocene initiation. Nowhere was this interaction more diverse than on and adjacent to the Oregon Plateau, the southern portion of the ~17-14 Ma Columbia River-Steens flood basalt province. Here, four phenomena define the onset of the Yellowstone hotspot: [1] the eruption of dominantly tholeiitic Steens flood basalt magmas from locations across the Oregon Plateau (OP); [2] the region-wide development of focused zones of extensional tectonism ± eruption of flood basalt magmas (e.g. the northern Nevada rift and Oregon-Idaho graben); [3] the development of numerous dominantly silicic magmatic systems (e.g. the McDermitt and Santa Rosa-Calico volcanic fields); and [4] abundant epithermal Au-Ag mineralization (e.g. Buckskin-National, Sleeper, DeLamar). Recent geochronologic results suggest that the main phase of Steens volcanism occurred from 16.5 - 15.5 Ma, with most activity occurring between 16.5-16 Ma. Because of these new age constraints, the temporal pattern of OP silicic magma generation and volcanism can be refined. New work in the Santa Rosa-Calico volcanic field (SC) and in other OP locations helps clarify the details of OP silicic magmatism. In the SC, magmatism occurred from 16.7 to ~14 Ma and was characterized by local Steens volcanism, abundant intermediate magma generation, and a silicic assemblage that formed due to crustal melting, stimulated by Steens magmatism. Local silicic volcanism was dominantly effusive and occurred primarily from 16.5 to 16 Ma. Additionally, a younger pulse of caldera-forming silicic activity occurred at ~15.5 Ma, coincident with younger Steens eruptions in the vicinity of Steens Mountain. Chronologic results from other regional silicic volcanic systems appear to mirror the temporal relationships exhibited by the SC and suggest that most OP mid-Miocene silicic activity occurred within the abrupt time-period that marked the main phase of Steens flood basalt volcanism (16.5-16 Ma). More work is needed to verify this temporal relationship and also document any spatial trends, both of which ultimately have direct implications for the models used to explain the nature of the Yellowstone hotspot and how it interacted with the North American lithosphere.