Petrology, geochronology, and chemical evolution of the Twin Peaks Rhyolite Domes, Utah

Two distinct sequences of silicic volcanism at the Twin Peaks volcanic field, Millard County, Utah, spanned periods from 2.74 +- .10 to 2.54 +- .09 m.y. and 2.43 +- .08 to 2.35 +- .08 m.y., and produced a total exposed volume of 4 km/sup 3/ of rhyolites and volcanoclastics. Wet chemical, x-ray fluorescence, microprobe, atomic absorption, and neutron activation methods have been employed to obtain a wide range of chemical data on whole rock and mineral separate samples. Calculated distribution coefficients are comparable to previously published values for high silica rocks. Each sequence is characterized by a systematic trend from < 72% to > 76% SiO/sub 2/. Accompanying this increase in SiO/sub 2/ over time are increases in Rb, Y, Sb, Cs, U, Th, HREE and decreases in Mg, Co, Fe, Sr, Ba, and LREE. Decreases in temperature and fO/sub 2/ and an increase in fH/sub 2/O are also indicated. These trends are very similar to gradients observed in ash flow tuffs erupted instantaneously from compositionally zoned magma chambers. Chemical evolution at Twin Peaks was dominated by the same mechanism of liquid state differentiation which produce the compositional zonation in larger silicic magma chambers