Monogenetic volcanism fed by complex magmatic processes: El Negrillar volcanic field (Central Andes, Chile)

Abstract

Gaining insight into the feeding systems and pre-eruptive processes of monogenetic volcanic fields is crucial to better understand the genesis and potential hazards of monogenetic volcanoes. This study examines pre-eruptive processes at El Negrillar volcanic field in Chile through the analysis of mineral cargo in time-constrained (982 ± 8 to 141 ± 72 ka) samples. With 51 eruptive centers, 98 lava flows, and minor phreatomagmatic deposits distributed into three clusters aligned NE-SW, El Negrillar encompasses the largest volume of erupted magma among the Pleistocene monogenetic volcanoes in the Central Andes. The samples range from basaltic andesite to dacite, and have microporphyritic textures with microphenocrysts of olivine, pyroxene, plagioclase and amphibole embedded in a hypocrystalline to holocrystalline groundmass. Based on the analysis of mineral compositions and their textures, fractional crystallization represents the primary differentiation process of El Negrillar magmas during their ascent to the surface. In addition, mineral disequilibrium textures further record evidence of magma recharge, mixing, assimilation, and recycling processes in the thickened crust context of the Central Andes, leading to the eruption of evolved antecrysts and xenocrysts within relatively primitive magmas. The crystallization sequence begins with the appearance of olivine (1180–1110 °C), followed by plagioclase (1120–1030 °C), pyroxene (1110–1010 °C), and in a later stage, amphibole (960–930 °C) microphenocrysts, within a main stagnation zone located in the upper mid-crust (2.5–5.0 kbar; ∼ 9–19 km depth). A compositional evolution from the southwest to the northeast across El Negrillar is evident through whole-rock and mineral chemistry, indicating the involvement of several, similar parental magmas that stalled in a network of small sills within the main stagnation zone, where microphenocrysts grew before ascent to the surface. These findings challenge the traditional view of monogenetic systems as simple source-to-surface magma channels, instead revealing a surprising level of petrological complexity marked by limited stagnation and intricate compositional changes tied to the pre-eruptive history.