Late Cenozoic volcanism at Twin Peaks, Utah: Geology and petrology

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Abstract
Bimodal volcanism that formed in the Twin Peaks volcanic complex, west‐central Utah, spanned a period from 2.7 to 10 km3 of aphyric lavas and pyroclastic deposits and resulted in a local subsidence of approximately 100 m. Tuffaceous lake deposits followed by voluminous olivine tholeiites subsequently filled the depression. Eruption of the second sequence of silicic rocks spanned a short time at 2.4 m.y. and produced a total volume of 0.7 km3 of strongly porphyritic felsites. At least two subsequent episodes of basaltic volcanism followed. Chemical variations displayed by the early differentiation sequence of silicic rocks are characterized by increases in volatiles, SiO2, Na2O, heavy rare‐earths, Rb, and highly charged cations and decreases in all other major components, light rare‐earths, Sr, and Ba. These variations in time closely resemble spatial variations in compositionally zoned ash flow sheets and are incompatible with crystal fractionation. The early differentiation sequence resulted in the development of a highly evolved, compositionally stratified roof zone, from which the ash and aphyric lavas were tapped. Conductive cooling models for the silicic magma chamber require thermal input into the system, probably by underplating a large volume of basalt, during the early eruptive sequence to sustain it beyond 50,000 years after initial emplacement. Chemical variations displayed by the second eruptive sequence exhibit an increase in K and decrease in Na and are consistent with crystal fractionation as the dominant control. Crystal fractionation and a short eruptive history indicate rapid cooling and a cessation of thermal input for the second sequence.