Abstract

La Poruña is a monogenetic volcano located within the Altiplano-Puna Volcanic Complex (21°-24°S) in the Central Andean subduction zone. Since crustal contamination of Andean magmas is ubiquitous, and because extensive geochemical data exist for La Poruña, we employ this volcano as a case study to examine the behavior of boron isotopes during crustal assimilation. We present whole-rock boron concentration and 11B/10B ratios (as ?11B values) for La Poruña lava samples that were prepared as nano-particulate pressed pellets. La Poruña B contents range from 14 to 20 ?g/g and ?11B values range from ?1.39 ± 0.54 ‰ (2?) to +0.94 ± 0.30 ‰ (2?), which overlap with the range of available whole-rock data for Central Andean lavas. Moreover, La Poruña ?11B values correlate negatively with 87Sr/86Sr ratios from the same samples. Since 87Sr/86Sr is a proxy for crustal contamination at La Poruña, the data lead us to suggest that La Poruña magmas assimilated a low-?11B, high 87Sr/86Sr component such as Andean continental crust. Mixing models based on B and Sr isotopes support a broadly two-step magma evolution for La Poruña. In step 1, mantle-derived primary melts interacted with boron-rich slab-derived fluids with high ?11B values, which yielded subduction-modified parental magmas with ca. 3 ?g/g B and relatively high ?11B values. In step 2, the high ?11B parental magmas ascended through the crust where they assimilated up to 20% crustal material, which further modified their ?11B values and 87Sr/86Sr ratios. In comparison to available regional values for B and ?11B, it appears that La Poruña and nearby volcanic centers shared a similar source and magmatic history, whereas volcanoes south of 23°S differ. We stress, however, that deconvolving the roles of various subduction and crustal inputs in the Central Andes would require further studies on individual volcanoes along the arc. © 2023 The Authors