Late Cenozoic Volcanism at Twin Peaks, Utah: Geology and Petrology

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