TRANSCRUSTAL MAGMATIC PROCESSES REVEALED THROUGH AN INTEGRATED STUDY OF AMPHIBOLE CRYSTAL CARGOES AT THE QUILLACAS VOLCANIC CENTER, BOLIVIA

Amphibole exerts a fundamental control on arc magma petrogenesis, differentiation, and the long-term evolution of the mid-lower arc crust. While often referred to as a “cryptic” fractionating phase, we find multiple amphibole populations within andesitic lavas and entrained hornblendite cumulates at the Quillacas monogenetic volcanic center on the Eastern Altiplano, Bolivia. Through a detailed textural, mineralogical, chemical, and geothermometry study, five distinct amphibole populations were identified. Phenocrystic amphiboles in the hornblendites are large (≤800 μm) and pargasitic with thick (avg. 27 μm), coarse-grained, equant reaction rims. In contrast, phenocrystic pargasites in the andesites are smaller (250-400 μm) with thin (avg. 7-9 μm), fine-grained reaction rims. Within the hornblendite cumulates, intercumulus amphibole microphenocrysts are characterized by their small size, lack of reaction rims, and true hornblende compositions (Si apfu ≥6.5-7.5≤, Na + K apfu ≤0.5 at low Al₂O₃). In the andesites, we find both fine-grained rimmed and unrimmed microphenocrystic amphiboles with true hornblende compositions. Amphibole thermometry suggests that both the cumulate phenocrystic pargasites and the host andesitic lava pargasites crystallized at high T: 945-991 ± 22°C and 828-1004 ± 22°C, respectively. In contrast, the cumulate microphenocrystic hornblendes crystallized at relatively cooler T (as low as 556°C) while the andesite hornblendes crystallized at higher T (850-990 ± 22°C). The presence of multiple amphibole populations is explained through a multi-stage trans crustal magmatic system at Quillacas. In this scenario, a hot, lower crustal, amphibole-dominated cumulate reservoir underlies a more chemically evolved mid-crustal reservoir within the arc crust. A shallow reservoir (<10 MPa) is then tapped prior to eruption. This model is supported by amphibole reaction rim textures, grain size and shape variations, distinct chemical populations, and amphibole thermometry. This study supports the presence of amphibole-dominated crystal mush filters in the lower arc crust and advances our understanding of the role of amphibole in the evolution of arc magmatic systems.