Structure and Petrology of Tertiary Volcanic Rocks Near Etna, Utah

Three volcanic domes and related volcanic rocks of Tertiary age are located near Etna, Utah, in Box Elder County. The domes follow a north-south trend and are fault controlled. Flow structure indicates a change from a less viscous, flow-forming lava which produced an exogenous dome to a more viscous lava which formed endogenous domes. Associated pyroclastic deposits are negligible. The volcanic rocks are composed of porphyritic rhyolite and rhyolitic vitrophyre having phenocrysts of quartz, sanidine, plagioclase and biotite with minor amounts of Fe-Ti oxides, hypersthene, allanite and calcic amphibole. Quartz and sanidine phenocrysts are generally embayed whereas plagioclase phenocrysts are euhedral and extensively zoned. Average whole-rock chemical analyses yield: SiO2, 77.13; TiO2, 0.12; Al2O3, 11.01; Fe2O3, 0.9; FeO, 0.35; MnO, 0.02; MgO, 0.19; CaO, 0.82; Na2O, 2.93; K2O, 4.99; P2O5, 0.03; H2O+, 1.17; H2O-, 0.22; total, 99.94 weight percent. Coexisting Fe-Ti oxide microphenocrysts yield equilibration temperatures ranging from 872° to 684°C while respective log f0 2 values range from -13.5 to -19.5. These temperatures are comparable to temperatures obtained using the plagioclase-glass geothermometer at a water pressure of 1 kb. Mineral buffer reactions yield water fugacities with corresponding water pressures up to 4.9 kb. Assuming water pressure equals total pressure, calculated depths of approximately 18 km are obtained indicating an origin within the crust. High silica values and high alkali to calcium ratios indicate that ix the lavas are chemically similar to bimodal rhyolite-basalt assemblages located in other areas of the western United States. Small outcrops of basalt, located west of the Etna area, also suggest a bimodal assemblage. Viscosity values (log n) for the south dome range from 7.05 to 10.35 suggesting that there was a change from a less viscous to a more viscous lava. Comparisons between hydrous and dry calculations indicate that falling water content as well as decreasing temperature were responsible for the change in viscosity and resulting structural changes