WIDESPREAD MID-MIOCENE SILICIC VOLCANISM CONTEMPORANEOUS WITH COLUMBIA RIVER FLOOD BASALTS IN THE WESTERN USA

Silicic volcanism is an under-appreciated component of many flood basalt provinces, including the Columbia River basalts in the northwestern United States. We report new geologic and geochronologic evidence for silicic volcanism at the High Rock caldera complex (HRCC) in northwest Nevada, which is contemporaneous with the main phase of Columbia River flood basalt volcanism. The HRCC consists of the products of four major overlapping calderas, 14- to 40-km in diameter, that were active 16.4-15.7 Ma, shortly after initiation of flood basalt eruptions at nearby Steens Mountain at ~16.7 Ma. Regionally, silicic volcanism that was contemporaneous with flood basalt eruptions 16.7-15.0 Ma was more voluminous and occurred over a wider area than previously recognized. The most voluminous silicic eruptions were early and peralkaline, occurring at multiple calderas at HRCC and McDermitt caldera. Silicic eruptions at Lake Owyhee volcanic field in eastern Oregon are slightly younger and less peralkaline. The largest calderas are located along a narrow NNE trend near the transition between continental and accreted oceanic crust (based on the projection of the ⁸⁷Sr/⁸⁶Sr_i = 0.704 isopleth), and occur peripheral to zones of intense basaltic diking. We suggest that the pattern of 16.5-15.0 Ma silicic centers reflects the initial spreading of the Yellowstone plume head at the base of the North American lithosphere. At a critical distance from the foci of flood basalt intrusion, accumulations of crustal melts became density barriers to the rise of later basaltic dikes, the stagnation of which further augmented crustal melting and the growth of silicic magma chambers. In our model, contemporaneous silicic magmatism at McDermitt caldera and the HRCC do not mark the center of the impinging plume head; rather, they mark the edge of the plume head where it encountered thicker and more felsic transitional continental crust.