The Volcanic Evolution of Cerro Uturuncu: A High-K, Composite Volcano in the Back-Arc of the Central Andes of SW Bolivia

Abstract

Cerro Uturuncu, southwest Bolivia, is a high-K, calc-alkaline, composite volcano constructed upon extremely thick continental crust approximately 125 km behind the arc-front of the Andean Central Volcanic Zone (CVZ). Eruptive activity occurred between 890 - 271 ka in a single phase of volcanism lasting ~620,000 years. The edifice consists of a central cone and several flank vents where dacitic and andesitic lava flows and domes erupted. Volumes of individual eruptive units range from 0.1 to ~10 km 3 ; the composite volume of Uturuncu is ~89 km 3 . In this paper, we present new field, petrographic, and geochemical data in an effort to understand the volcanic and magmatic evolution of Uturuncu. Lava flows and domes have a restricted range in whole rock compositions ranging from 61 wt% - 67 wt% SiO 2 ; magmatic inclusions contained within these units have a larger range from 53 wt% - 64 wt% SiO 2 . Typical phenocryst assemblages are plagioclase > orthopyroxene > biotite >> quartz and Fe-Ti oxides. Pb isotope ratios are characteristic of the southern CVZ by containing high 207 Pb/ 204 Pb and 206 Pb/ 204 Pb and moderate to high 208 Pb/ 204 Pb. Sr and Nd isotope ratios indicate that Uturuncu magmas were modified by high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd felsic basement lithology during magma migration and differentiation. In all eruptive units, there is petrographic and geochemical evidence for magma mixing and mingling. In this regard, magma mixing and mingling is considered to be responsible for the small range in lava flow and dome compositions throughout the eruptive history of the center.