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.
Highlights
Volcanism in the Central Volcanic Zone (CVZ) of the Andes is a prime example of active continental arc magmatism
Previous studies of CVZ volcanic rocks have greatly contributed to our understanding of the regional setting of the magmas [4] [5]; and references therein in addition to ignimbrite forming eruptions [6]-[9]; among others and volcanism along the modern arc-front [3] [10]
Long periods of quiescence are common at composite volcanoes as they been observed at other intermediate composition systems in the CVZ and elsewhere such as at Soufrière Hills Volcano, Montserrat [3] [17] [18]
Summary
Volcanism in the Central Volcanic Zone (CVZ) of the Andes is a prime example of active continental arc magmatism. The lack of comprehensive studies on individual back-arc composite volcanoes in the CVZ prompted a detailed field, petrologic and geochemical study of the back-arc center Cerro Uturuncu (6010 m; 22 ̊15'S, Figure 1). This study of Uturuncu results from a reconnaissance petrologic and geophysical investigation by Sparks et al [16]. These authors concluded Uturuncu exhibits stages of volcanism separated by periods of quiescence common at other intermediate composite centers in the CVZ. We build on the reconnaissance investigation of Sparks et al [16] by furthering discussion of the volcanic history, field relationships, and petrography of volcanic rocks at Uturuncu. More detailed discussion of the petrology and petrogenesis of Uturuncu magmas will ensue in a forth-coming paper
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