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