IMPLICATIONS OF SILICIC VOLCANISM OF THE SNAKE RIVER PLAIN – YELLOWSTONE (SRPY) PROVINCE

The SRPY province is characterized by large volume bimodal basalt-rhyolite volcanism. Inception of silicic magmatism generally migrated from SW Idaho to Yellowstone between 15 – 2 Ma. Silicic activity typically lasted ca. 2-3 Ma at individual eruptive centers, after which basaltic effusions became dominant. This pattern (main trend) is consistent with migration of N. America over a deep thermal source (plume?) now located beneath the Yellowstone region. However, rejuvination of silicic magmatism in the west and central SRP requires modification of this model.

Rhyolite bulk compositions mimic that of average crust. Pb isotopic data are consistent with an ancient source, but Sr and Nd isotopic data preclude direct melting of the Precambrian basement underlying much of the province. Moreover, sharp isotopic discontinuities across pre-existing structures (e.g., W. Idaho suture zone) imply lateral variation in source lithology. SRPY rhyolites likely contain a significant juvenile component - either younger crustal source material (e.g., Idaho batholith) or inputs of SRP basalt (or both). Most ‘main trend' rhyolites are anhydrous, pyroxene-bearing, metaluminous with moderate silica (ca. 70-73%), and yield T estimates approaching 900°C or higher; such magmas likely form by H2O-undersaturated melting of calcalkalic crustal protoliths. Anomalously young rhyolites in the central SRP become progressively cooler and more hydrated (amph- ± bi-bearing) as they evolve to higher silica (>74%). Variations on this theme can account for different patterns of compositional evolution for individual SRPY centers.

Basaltic magmatism is invoked as the impetus for large-scale crustal anatexis to produce the rhyolites. Considering the volumes of main trend rhyolites produced and the energetics of magma production, basaltic input at rates comparable to those estimated for oceanic hot spot tracks must predate and accompany rhyolitic activity. Lithospheric extension is required to generate these mafic inputs, to accommodate the volume of intrusive material required, and to explain the overall duration and spatial distribution of bimodal magmatism. Associated thermal and material exchanges likely caused profound modification of the crust.