Paper No. 76
Presentation Time: 9:00 AM-6:00 PM


GENTRY, Amanda, Geoscience Department, Weber State University, 2507 University Circle, Ogden, UT 84408 and BIEK, Robert F., Utah Geol Survey, PO Box 146100, Salt Lake City, UT 84114-6100,

Geochemical and petrologic analysis of early phases of volcanism of the Marysvale volcanic field of south central Utah has revealed several distinct volcanic units in the Bear Valley area. Extensive volcanic activity began in the middle Cenozoic during a transition between prior regional mountain building of the Laramide orogeny (~60-40 Ma) and subsequent Basin and Range extension (~17 Ma to present). Regionally, middle Cenozoic ash flow tuffs, lava flows and brecciated mudflows with a southwestern source area dominate the stratigraphy, while a chemically distinct interval associated with the Brian Head Formation records a local source. Initial volcanism began about 36.5 +/- 1.7 Ma based on a U-Pb zircon age from near the base of the formation. Although previous geologic mapping had combined multiple volcanics of this interval into a single unit, new petrographic and scanning electron microscope analyses have revealed significant variations in mineralogic and chemical compositions. Whole-rock geochemical analysis indicate calc-alkaline rock types including andesite, trachyte, dacite, rhyolite and trachyandesite. Variations in major and trace element compositions are consistent with differentiation by crystal fractionation and variable crustal contamination. Complexly zoned plagioclase grains are common in the volcanic rocks and likely record changes in fluids and temperature in the underlying magma chamber. Interlayered sedimentary rocks contain reworked volcanic material and rounded quartz grains that may represent recycled sedimentary material. Based on new mapping and geochemical analysis four distinct volcanic units are recognized: a pyroxene-plagioclase-bearing trachyandesite flow, a hornblende-plagioclase-pyroxene-bearing dacite flow, a plagioclase-rich dacite flow, and a pyroxene-plagioclase-bearing welded andesite tuff. Early phase volcanism in the Marysvale area is interpreted to record part of a north to south temporal sweep in magmatic activity related to changes in subduction dynamics of the Farallon plate and changes in tectonic style to extension along the western Cordillera during the middle Cenozoic.