Structure and Petrography of the Tertiary Volcanic Rocks Between Death Creek and Dairy Valley Creek (Box Elder Co.), Utah

Several volcanic flows lie between Death Creek and Dairy Valley Creek, near Etna, Utah. The major, central portion of the volcanic flows is composed of dacite and dacite vitrophyre. An elongate ridge in the southeastern corner of the study area and several small outcrops in Death Creek Valley are composed of rhyolite and rhyolite vitrophyre. Additional rock types include conglomerate, volcanic ash, and tuffaceous sedimentary rock of the Tertiary Salt Lake Formation, Paleozoic limestone, and Tertiary basalt.Dacite and dacite vitrophyre samples are porphyritic, containing phenocrysts of plagioclase, quartz, biotite, amphibole, orthopyroxene, and iron-titanium oxides. Rhyolite and rhyolite vitrophyre samples are porphyritic, containing phenocrysts of plagioclase, alkali feldspar, quartz, and iron-titanium oxides, with minor amounts of biotite and amphibole. Plagioclase phenocrysts are complexly twinned, zoned, and corroded in dacitic samples; whereas, they are simply twinned, unzoned, and uncorroded in rhyolitic samples . Quartz and alkali feldspar phenocrysts contain glass-filled inclusions and are subrounded in rhyolitic samples.Average chemical analyses of five dacitic sample yield weight percent: SiO2, 69.11; TiO2, 0.31; Al2O3, 14.18; Fe2O3, 1.26; FeO, 1.23; MnO, 0.04; MgO, 0.54; CaO, 2.27; Na2O, 3.30; K2O, 4.07; P2O5, 1.23; H2O+, 1.89; H2O-, 0.46; total, 99.27. Average chemical analyses of three rhyolitic samples yield weight percent: SiO2, 76.66; TiO2, 0.12; Al2O3, 11.80; Fe2O3, 0.58; FeO, 0.43; MnO, 0.01; MgO, 0.09; CaO, 0.78; Na2O, 2.69; K2O, 5.26; P2O5, 0.02; H2O+, 1.17; H2O-, 0.22; total, 99.85.The volcanic flows of the study area are believed to be derived from fusion of sialic material within the crust which differentiated to form dacite and rhyolite. The dacite is believed to be the first magma extruded in the study area; whereas, the rhyolite represents the later extruded magma. Evidence supporting this relative emplacement is the restriction of rhyolite to the southeastern corner of the study area and the intrusion of rhyolite into dacite in the narrows of Death Creek Valley.The volcanic flows of the study area are believed to have been derived during the second stage of Basin and Range volcanism beginning approximately 14 million years ago. The study area rhyolite has chemical compositions similar to those rhyolites of bimodal basalt-rhyolite fields formed during the second stage of Basin and Range volcanism which includes high silica contents, higher alkali to calcium ratios, and greater sodium contents compared with rhyolite of calc-alkalic fields.