2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 208-9
Presentation Time: 11:00 AM

INVESTIGATION OF A PRE-SUPERERUPTION IGNIMBRITE: PETROLOGY OF THE MIOCENE COOK CANYON TUFF, BLACK MOUNTAINS, ARIZONA


PRATT, Ricky Daniel1, CLAIBORNE, Lily L.2, MILLER, Calvin F.3, FERGUSON, Charles A.4, CRIBB, J. Warner5 and SZRAMEK, Lindsay A.1, (1)Geosciences, Austin Peay State University, Clarksville, TN 37042, (2)Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, (3)Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, (4)Arizona Geological Survey, 416 West Congress, Suite 100, Tucson, AZ 85719, (5)Geosciences, Middle Tennessee State Univ, Murfreesboro, TN 37132, rpratt3@my.apsu.edu

The Cook Canyon Tuff (CCT) is a Miocene ignimbrite of the Colorado River Extensional Corridor. This enigmatic tuff precedes the better known caldera forming supereruption of the Peach Spring Tuff (PST 18.8 Ma). The goal of this study was to obtain geochemical and petrologic data for interpretation of magmatic history and comparison to adjacent PST (Foley et al., 2014, GSA abst) and older trachyte lavas (Rentz et al., Rice et al., GSA absts, 2014). We collected samples of glassy fiamme, vitrophyre and pumice from the Black and Cerbat Mountains in northwestern Arizona. SEM and bulk XRF analysis were used to characterize CCT composition and mineralogy and for comparison with previous CCT and PST data.

The CCT has a phenocryst abundance of ~25% , dominated by ~20% plagioclase and ~5% biotite. Much less abundant Cpx, magnetite, zircon, apatite and rare sanidine are evident in thin section and by SEM. The mean bulk pumice and fiamme composition determined by XRF (4 samples) is SiO2 65.7 wt%, Al2O3 17.3%, Fe2O, 3.7%, MgO 1.5%, CaO 2.9%, Na2O 2.7%, K2O 5.5%, TiO2 0.5%, P2O5 0.2%; Rb 125 ppm, Sr 450 ppm, Ba 440 ppm. Two distinct pumice compositions are present, suggested by variation in color in hand samples and corroborated by SEM glass analysis: the lighter pumice (orange-white in color) with an average of 73.2 wt.% SiO2, Al2O3 15.2%, FeO 1.4%, MgO 0.3%, CaO 0.7%, Na2O 2.5%, K2O 6.46%, TiO2 0.4%, and a dark grey pumice with an average of 70.4 wt.% SiO2, Al2O3 15.5%, FeO 1.9%, MgO 0.4%, CaO 1.4%, Na2O 3.2%, K­2O 6.7%, and TiO2 0.5%. Average glass composition by SEM of fiamme gave a composition of 70.5% SiO2, 15.4% Al2O3, 1.5% FeO, 0.4% MgO, 1.2% CaO, 3.5% Na2O, 7.0% K2O, and 0.5% TiO2. Apatite and zircon saturation calculations suggests magma temperatures of ~875 ºC prior to eruption. TAS plotting of XRF values show that the CCT is trachytic in compostition.