RECENT EXPLOSIVE ERUPTIONS OF DISTINCT DACITIC MAGMA GENERATED IN THE DEEP CRUST: HUAYNAPUTINA AND TICSANI, SOUTHERN PERU

The products of explosive silicic volcanism in the Central Andes are common but are dominated by supereruptions during the Neogene ignimbrite flare-up. Since transitioning to a steady state arc in the late Pleistocene, explosive silicic eruptions are rare and small. Peruvian volcanoes Huaynaputina and Ticsani are two exceptions that have both produced large (VEI 6 and 4 respectively) Holocene eruptions and are categorized as high to very-high risk by the Geophysics Institute of Peru. Located in a very restricted area of southern Peru behind the main Central Volcanic Zone (CVZ) arc, previous studies (Adams et al., 2001; Lavallée et al., 2009) have proposed a structural and geochemical link between Huaynaputina and Ticsani, yet the hypothesis of a common magma genesis has not been rigorously tested. Herein we explore the proposed connection between Huaynaputina and Ticsani using geochemistry, petrology, and isotopes to constrain the source and evolution of dacitic magmas at both sites. Erupted compositions are predominantly dacitic with Na₂O/K₂O higher than typical CVZ dacites. A phenocryst assemblage of plagioclase>hornblende>biotite>Fe–Ti oxides±apatite is set in rhyolitic matrix glass (73-76 wt.% SiO₂). Elevated Sr/Y (>60) and La/Yb (>20) ratios together with the absence of a negative Eu anomaly suggest that the initial stage of differentiation for the Huaynaputina and Ticsani dacites occurred at deep crustal levels where garnet is stable, and plagioclase is not. Geobarometry and rhyolite-MELTS modeling indicates that magmas were stored at ~200-300 MPa prior to eruption. Restricted isotopic ratios (⁸⁷Sr/⁸⁶Sr: 0.706-0.707, ¹⁴³Nd/¹⁴⁴Nd: 0.5122-0.5123) indicate that negligible upper crustal assimilation occurred during shallow residence. The petrochemical data connotes a two-stage evolution of the Huaynaputina and Ticsani dacites: initial magma generation occurred deep within the crust whereas the final stages of magmatic evolution occurred within the upper crust as closed-system. This short intense episode of explosive volcanism in southern Peru appears to have erupted rare deep-crust derived dacitic magma in the CVZ where most of the dacites have an upper-crustal origin. This suggests a clear petrochemical distinction between steady-state arc related dacites and those that erupted as supereruptions of the ignimbrite flare-up in the Central Andes.