Mineralogy, Petrology, and Magmatic Conditions from the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado

The Fish Canyon Tuff is one of the largest currently recognized ash-flow tuffs (> 3000 km3). It is a crystal-rich quartz latite containing about 40 per cent phenocrysts of plagioclase, sanidine, biotite, hornblende, quartz, magnetite, sphene, and ilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene, and hornblende. The consistency of mineralogy and whole rock chemistry confirms that the Fish Canyon tuff is remarkably homogeneous. Most chemical variations can be accounted for by phenocryst-matrix fractionation, probably due to glass winnowing during eruption and emplacement. The composition of the parent magma, corrected for such winnowing, is very similar to that of calc-alkaline batholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ± 30 °C for most of the outflow tuff. No evidence for a regular thermal gradient in the magma chamber could be detected. Two feldspar and Fe-Ti oxide equilibria indicate that the magma developed at depths of 25 to 30 km (about 9 kb pressure), and was erupted without time for phenocryst re-equilibration. The reconstructed composition of the liquid in equilibrium with the phenocrysts also suggests a deep source for this ash flow. A late, upper package of flow units have mineralogical characteristics which may reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO2 = — 11.5 ±0.3) but are similar to those obtained from other continental calc-alkaline ash-flow tuffs. The water fugacity is limited by calculations using biotite equilibria and experimental work relating to the stability of the phenocryst assemblage. Best estimates are that water fugacity was 2000 ± 1000 bars. The activities of sulphurous gases are estimated at fSO2 = 2 to 4 bars, fso2 = 150 to 200 bars, fH2S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous, granitic magma body of batholithic proportions. Calculations of its probable viscosity, density, and size indicate that the system should convect with any reasonable thermal gradient. Convective mixing may account for the homogeneity of the parent magma body.