Trans-crustal magmatic processes revealed by amphibole breakdown textures at the Quillacas monogenetic volcanic center, Bolivia

Abstract

Amphibole exerts a fundamental control on arc magma petrogenesis, differentiation, and the long-term evolution of the arc crust. This study identifies two texturally distinct amphibole populations within andesitic lavas and entrained hornblendite cumulates at the Quillacas monogenetic volcanic center in the Eastern Altiplano, Bolivia. Within the hornblendites, all amphiboles are tschermakitic, large (≤800 μm) with thick, granular reaction rims (avg. 27 μm thickness). In the host andesites, tschermakites are also the dominant amphibole species but are smaller (250–400 μm) with thin, symplectic reaction rims (avg. 7–9 μm thickness). An intergrowth of symplectic and granular reaction rims is also observed in this population. Collectively, the amphibole populations within the Quillacas magmatic system also record irregular volumetric decomposition where amphibole is replaced by mineral aggregates of plagioclase, pyroxene, and oxide within the crystal. This supports a relatively slow reaction of amphibole with melt trapped in fractures and cleavages during decompression-induced degassing. Geothermobarometry suggests that the hornblendite cumulate tschermakites crystallized at P-T conditions ranging from 453 to 598 ± 12 % MPa and 928–1004 ± 22 °C. The host andesite tschermakites crystallized at P-T conditions ranging from 430 to 617 ± 12 % MPa and 928–1004 ± 22 °C. These geothermobarometric constraints correspond to depths of 16–24 km which, within this region of the Central Andean crust, also corresponds to a regionally extensive low-seismic velocity zone. The texturally distinct amphibole populations imply a multi-stage trans-crustal magmatic system likely exists beneath the Quillacas volcanic center. In this scenario, a crystal mush zone exists at upper crustal depths where cumulate amphiboles initially crystallized. Magma recharge into this mush zone initiated a reaction between hornblendite cumulates and the melt which formed the amphibole granular rims. This recharge event also transported the host andesite amphiboles that subsequently developed symplectic rims due to heating and ascent-driven decompression. This study supports the presence of amphibole-dominated mush filters in the upper crust of the Central Andean arc and advances our understanding of amphibole's role in the evolution of arc magmatic systems.