TITLE:
The Volcanic Evolution of Cerro Uturuncu: A High-K, Composite Volcano in the Back-Arc of the Central Andes of SW Bolivia
AUTHORS:
Gary S. Michelfelder, Todd C. Feeley, Alicia D. Wilder
KEYWORDS:
Utrunucu, Volcanic Evolution, Central Volcanic Zone, Magmatic Inclusion
JOURNAL NAME:
International Journal of Geosciences,
Vol.5 No.11,
October
13,
2014
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 km3; the composite volume of
Uturuncu is ~89 km3. 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% SiO2; magmatic
inclusions contained within these units have a larger range from 53 wt% - 64 wt%
SiO2. Typical phenocryst assemblages are plagioclase >
orthopyroxene > biotite >> quartz and Fe-Ti oxides. Pb isotope ratios
are characteristic of the southern CVZ by containing high 207Pb/204Pb
and 206Pb/204Pb and moderate to high 208Pb/204Pb.
Sr and Nd isotope ratios indicate that Uturuncu magmas were modified by high 87Sr/86Sr
and low 143Nd/144Nd 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.