NEW GEOLOGICAL MAPPING AND 40AR/39AR GEOCHRONOLOGY DEMONSTRATE THAT RHYOLITE VOLCANISM MIRRORS NORTHWARD PROPAGATION OF MID-MIOCENE COLUMBIA RIVER FLOOD BASALTS

It is well-documented that the locus of mid-Miocene Columbia River flood basalt volcanism migrated northwards from the Nevada-Oregon border (Steens Basalt; ~16.8-16.5 Ma) to the Imnaha (~16.6-16.2 Ma), Grande Ronde (~16.3-15.9 Ma), and Wanapum (~15.9-15.7 Ma) members in eastern Oregon and southern Washington. New detailed mapping and high-precision ⁴⁰Ar/³⁹Ar measurements on sanidine phenocrysts from >100 samples of rhyolite lavas and tuffs contemporaneous with the flood basalts demonstrate that the spatiotemporal pattern of silicic volcanism mirrors that of the flood basalts. Weakly peralkaline caldera-forming eruptions and related lavas at McDermitt Volcanic Field and High Rock Caldera Complex near the Nevada-Oregon border began ~16.6 Ma, a few hundred thousand years after the onset of Steens Basalt volcanism. Further north, caldera forming eruptions began around ~16 Ma in the Lake Owyhee Volcanic Field. This succeeded the onset of Grande Ronde Basalt volcanism in the area by ca. 200-300 Ka. While flood basalts ceased to erupt in caldera complexes once silicic magma chambers coalesced to form a “shadow zone”, continued intrusion of basalt into the upper crust fueled caldera-related activity at each center for nearly 1 My. At the periphery of the main S-N eruptive axis defined by flood basalt dikes and caldera complexes, local intrusion of basalt caused some metaluminous silicic lavas to erupt sporadically between 16.5 and 15.5 Ma (e.g., Strawberry Volcanics, Silver City Rhyolite), though the total volume of these lavas is minor (<5%) compared to the volume of rhyolite formed at the caldera centers.

The ~ 1 My S to N propagation of both Columbia River flood basalt and rhyolite magmatism (ca. 0.45 m/year) was an order of magnitude faster than the ca. 0.03 m/year rate of rhyolite magmatism that occurred after 15 Ma from SW to NE along the Snake River Plain/Yellowstone trend. The data are thus consistent with a model in which relatively fast, northerly progression of magmatism >15 Ma resulted from the lateral spread of a plume head along the edge of the North American crustal margin while the subsequent slower, NE progression of magmatism reflects movement of the North American plate over a relatively stable plume tail.