Magma accumulation and evolution in the Andean Central Volcanic Zone: Insight into Magma Hybridization at Continental Arc Volcanoes

Volcanic systems in the Andean Central Volcanic Zone (CVZ) erupt compositionally homogeneous monotonous intermediate magmas over timescales ranging from thousands to hundreds of thousands of years. This monotonous activity reflects the high degree of homogeneity in the magmatic system in whole rock compositions but is not necessarily reflected at the crystal-scale. Many composite volcanoes along and behind the arc-front of the CVZ contain plagioclase crystal loads that reflect initial heterogeneity, which become more homogeneous over time. Within the CVZ, the Altiplano-Puna Magma Body (APMB) is the largest imaged magma reservoir on Earth with an estimated volume of over 500,000 km3. Although the APMB is a first-order geologic feature, its role in controlling magma composition is poorly understood. Studies of pre-eruptive processes at volcanic systems within the geophysical boundary of the APMB provide precise petrologic constraints on the influence and interaction between volcano plumbing systems and regional magma bodies. Here, we present new petrologic and geochemical data, U-Th/U-Pb zircon geochronology, and diffusion timescales from Uturuncu volcano, Bolivia, as a case study of crustal hybridization at a continental arc volcano. We compare these results with similar data from Aucanquilcha and Lascar volcanoes. On an arc-wide scale, lavas from CVZ volcanoes display systematically higher K2O contents, 18O values, and 87Sr/86Sr ratios towards the interior of the APMB. These eastward compositional changes are interpreted to reflect increased interaction with the APMB. The oldest Uturuncu (>500ka) lavas have bimodal plagioclase populations, which become more homogeneous with time. In contrast, outside the geophysical boundary of the APMB, magmas tend to be more primitive with shorter storage time scales and contain only one plagioclase composition. We suggest magma recharge causes mush remobilization and crystal hybridization over timescales ranging from near-instantaneous to millennia preserving a complex record of pre-eruptive processes within the APMB.