Magmatic Flux Influences Chemical Heterogeneity of Continental Silicic Volcanic Systems

Abstract

Compositional diversity and architecture of magmatic systems can be related to magmatic flux as a driver of magma dynamics. We explore these relationships through an investigation of magmatic diversity of the Chaxas Volcanic Complex in the Altiplano-Puna Volcanic Complex (APVC) of the northern Chilean Andes using whole rock geochemistry, mineral chemistry, and radiogenic isotopic data. The high-flux Pre-Chaxas stage of the Chaxas Volcanic Complex produced small-volume rhyolitic magmas erupted from shallow reservoirs along with the catastrophic supereruption of the Puripicar Ignimbrite (PPI) at 4.18 Ma. The monotonous dacitic PPI eruption carries crystal cargo derived from a well-mixed reservoir of co-existing melt-rich domains and relatively cool heterogeneous mush. Pressure estimates made via amphibole and plagioclase equilibrium along with geochemistry indicate the PPI eruption tapped magma that contained crystal cargo equilibrated at depths from 15 – 6 km. Heterogenous radiogenic isotope data of Pre-Chaxas rhyolitic eruptions and the PPI indicate that the system was isotopically heterogenous likely due to assimilation of multiple crustal lithologies that were incompletely hybridized during early stages of the reservoir construction. Eruptions during the low-flux Chaxas Complex stage of the Chaxas Volcanic Complex were produced from small, ephemeral magma batches emplaced into a cooling mush column. Petrologic data derived from in-situ analysis of plagioclase indicate crystallization from isotopically homogenous melt batches that were distinct in major element geochemistry, contrasting with Pre-Chaxas eruptions. Together, these data reveal that the monotonous character of voluminous dacitic eruptions like the PPI only extends as far as major element geochemistry, whereas the crystal-scale records a diverse magmatic system. Smaller eruptions produced from discrete magmatic reservoirs may have more heterogeneity between eruptions but are internally more homogenous on a micro-scale. Heterogenous geochemical and petrologic signatures within and between volcanic eruptions of the Chaxas Volcanic Complex is attributed to the geographic location of this volcanic system at the margin of the APVC. Regional synthesis of marginal APVC volcanic systems with the Chaxas Volcanic Complex indicate that magmatic heterogeneity at the margins of continental magmatic flare-ups is likely systematic.

Keywords: Monotonous intermediate, Geochemical Heterogeneity, Polybaric Assimilation, Regional Geochemical Variation