Pre-eruptive conditions and pyroclastic emplacement of the last known vulcanian eruption of Azufral Volcano, SW Colombia

Abstract

Azufral is one of the most touristic, but also one of the most explosive active volcanoes of SW-Colombia. Although no historical eruptions have occurred, the potential thread is inferred from the predominance of dacitic pyroclastic units and lava domes since the Holocene. This paper presents new stratigraphical and compositional data of the uppermost and thickest proximal pyroclastic deposits exposed on the current inner and outer SE crater rim. The studied eruption units correspond to parallel and low-angle cross-bedded vitric-crystalline to lithic-crystalline tuffs. The absence of interbedded paleosols or epiclastic deposits indicates continuous eruptive activity throughout the studied succession. Criteria for stratigraphic package division at proximal sites includes erosive contacts, marked changes in grain sizes, and ballistic-bearing horizons. Each package comprises several bedsets consisting of two to three beds of distinctive lithofacies associations suggesting deposition from diluted pyroclastic density currents. Temporary partial or total conduit obstruction conditions could also be inferred from the presence of ballistic horizons.Petrographical and geochemical analyses from a type-bedset with fresh, poorly to moderately vesicular rhyolitic glassy particles and the mineral association Pl + Amp + Bt + Fe-Ti oxides ± Qz microcrysts suggest the presence of juvenile material. Textures and mineralogy of glassy ash particles are the same as those in large ballistic blocks, which are dacitic in whole-rock composition. Geochemical analyses suggest that magma evolution was mainly controlled by Pl + Amp + Bt + Py + Fe-Ti oxides fractionation and magma recharge events. All Pl and Amp microcrysts in the studied juvenile ash and ballistics can be grouped into two populations: (i) unzoned microcrysts and (ii) zoned microcrysts. Zoned microcrysts have evolved and partially corroded cores, coated by a more Mg-rich (Amp) or An-rich (Pl) mantle, and followed by an outermost Fe-rich (Amp) or Ab-rich (Pl) rim that broadly overlaps with the composition of the unzoned microcrysts.Textures, mineral composition, and pre-eruptive conditions obtained from similar-sized Amp, Pl and Fe-Ti oxide microcrysts within the juvenile ash and a juvenile ballistic provide evidence of complex crystallization paths before final decompression and eruption.