Guallatiri, an andesitic-to-dacitic composite volcano, records a complex magmatic evolution during the ascent of magmas through the crust, which is divided into two main eruptive stages. The Early Stage (∼130 ka) is dominated by effusive activity and characterized by amphibole showing breakdown textures, with fine- (∼5-30 μm) and coarse- (∼30-200 μm) replacement rims constituted by clinopyroxene, orthopyroxene, plagioclase and opaque minerals. In contrast, the Late Stage (∼5 ka) also has explosive activity, with scarce amphibole crystals showing breakdown textures. Whole-rock geochemistry shows an increase in SiO2 contents from the Early (57-64 wt%) to the Late Stage (63-75 wt%). The Late Stage mainly has higher 87Sr/86Sr ratios (0.7067-0.7069) than the Early Stage (0.7066-0.7068), and Sr-isotope ratios of both stages slightly correlate positively with SiO2 and inversely with Sr concentration (276-1036 ppm). Finally, U-series disequilibria reveal higher initial (230Th/232Th) activity ratios in the Late Stage (0.785-0.845) compared to the Early Stage (0.707-0.859), with data from the Late Stage lying near the equiline and showing lower radioactive decay.
The geochemical and isotopic results indicate that both stages evolved through a magmatic evolution on which: i) assimilation and fractionation crystallization (AFC) process, with up to ∼20% vol of assimilation in a garnet-bearing lower crustal environment, generating a parental magma with high Sr concentrations; and ii) fractional crystallization at upper crustal levels under plagioclase-stable conditions, with low assimilation (≤5% vol) of continental crust by the early generated parental magma. Moreover, the amphibole textures and the geochemical difference between both stages are related to variations in the evolution of Guallatiri during its lifespan. Thus, magmas of the Early Stage are less evolved and contaminated, for which development of reaction rims in amphiboles and Th radioactive decay suggest higher stagnation times at upper crustal levels. For the Late Stage, magmas underwent more intense differentiation and crustal assimilation, which occurred mainly at shallower crustal levels, with lower stagnation times which enhanced the explosive activity of the volcano. This evolution reflects the increasing influence of upper crustal interaction over time for Guallatiri, highlighting the role of this interaction in controlling the geochemical composition and the eruptive dynamics of this volcanic structure.
