ORIGINS OF CENOZOIC SILICIC VOLCANIC ROCKS OF THE SNAKE RIVER PLAIN AND GREAT BASIN, WESTERN UNITED STATES

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite were erupted in the Oligocene in what is now the Great Basin and contrast with the voluminous rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkalic, magnesian, oxidized, wet, cool (<850 degrees), Sr- and Al-rich, and Fe-poor. The silicic rocks are contemporaneous with basaltic andesite and andesite. This is interpreted to reflect the derivation of their mafic parents by slab dehydration related to a middle Cenozoic subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates which also have low Fe/Mg ratios and relatively low temperatures. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating the variety of silicic magmas erupted during this Oligocene magmatic episode.

On the other hand, most of the rhyolites of the Snake River Plain crystallized at low fO2 (near QFM), are relatively hot, dry, Al- and Sr-poor, and Nb- and Fe-rich. They are part of a distinctly bimodal sequence. These characteristics were largely imposed by the derivation of parental basalts with low fH2O and low fO2 by partial melting in or above a mantle plume causing the early precipitation of plagioclase and retarding crystallization of Fe-Ti oxides. These characteristics were passed on to their silicic partial melts which were caused by renewed intrusions of basalt in a middle crustal density trap. Partial melting of relatively dry gabbro required high temperatures. Subsequent fractionation of the rhyolitic partial melt led to the array of rhyolites found along the Snake River Plain trend. Minor assimilation of continental crust also occurred during rhyolite genesis and differentiation, as indicated by moderately lower epsilon Nd and higher initial Sr isotope ratios in the most evolved rhyolites. Only a very small volume of distinctive rhyolite is derived by fractional crystallization of Fe-rich intermediate rocks of the Craters of the Moon trend.