Abstract

La Porun similar to a is a monogenetic volcano located within the Altiplano-Puna Volcanic Complex (21 degrees-24 degrees S) in the Central Andean subduction zone. Since crustal contamination of Andean magmas is ubiquitous, and because extensive geochemical data exist for La Porun similar to a, 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 811B values) for La Porun similar to a lava samples that were prepared as nano-particulate pressed pellets. La Porun similar to a B contents range from 14 to 20 mu g/g and 811B values range from -1.39 +/- 0.54 %o (20) to +0.94 +/- 0.30 %o (20), which overlap with the range of available whole-rock data for Central Andean lavas. Moreover, La Porun similar to a 811B values correlate negatively with 87Sr/86Sr ratios from the same samples. Since 87Sr/86Sr is a proxy for crustal contamination at La Porun similar to a, the data lead us to suggest that La Porun similar to a magmas assimilated a low-811B, 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 Porun similar to a. In step 1, mantle-derived primary melts interacted with boronrich slab-derived fluids with high 811B values, which yielded subduction-modified parental magmas with ca. 3 mu g/g B and relatively high 811B values. In step 2, the high 811B parental magmas ascended through the crust where they assimilated up to 20% crustal material, which further modified their 811B values and 87Sr/86Sr ratios. In comparison to available regional values for B and 811B, it appears that La Porun similar to a and nearby volcanic centers shared a similar source and magmatic history, whereas volcanoes south of 23 degrees 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.