Magmatic processes at Cotopaxi Volcano, Ecuador: Geochemical and petrological constraints, and inferences for continental arc volcanoes

Cotopaxi Volcano, Ecuador is one of many active volcanoes in a chain of continental arc composite cones, that together comprise the Northern Volcanic Zone (NVZ) of the South American Andes. Pyroclastic deposits and lava flows comprise andesites and rhyolites that were erupted between at least 7.2 ka and 125 years. Major and trace element data, combined with U-series and other isotope data reflect lower and upper crustal AFC processes. Geochemical data from andesites and rhyolites reflect assimilation of lower crust, followed by fractionation of plagioclase and pyroxene at higher levels in the crust. The U-series data support this model, and trace element models of lower crustal melting and fractional crystallization processes show that the observed Th-excesses of 1--5% in the Cotopaxi andesites can be produced by melting of lower crust, whereas the 14% U-excesses observed in the Cotopaxi rhyolites can be produced by a low percentage ∼2% of apatite fractionation. These data suggest that process effects, not source effects dominate variations in U-series isotopes in the Northern volcanic zone, and that polybaric AFC processes were important in magma evolution at Cotopaxi volcano. The data also suggest that the timeframe for fractionation of rhyolite is on the order of ∼75 ka. It has been argued in recent years that melting of the subducting slab is an important process in the NVZ, however a careful examination of regional geochemical trends is not consistent with this model, and I conclude that although slab melting probably does occur in some arcs, regional geochemical trends in the Northern volcanic zone and their relationship to the subduction-zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge-derived basaltic magmas.