Stratigraphy and dynamics of the 6.4 ka explosive rhyolitic eruption at Colli dome, La Primavera caldera, Mexico

Abstract The La Primavera caldera, located in western Mexico on the outskirts of the Guadalajara metropolitan area, Mexico’s third-largest city, comprises lavas, domes, and volcanic centers emplaced between 143.5 ka and the present, both within and beyond the caldera rim. Colli dome (29–31.9 ka) represents the youngest volcanic structure of the post-caldera activity. Based on detailed field descriptions, stratigraphic correlations, grain size and componentry analyses, juvenile-clast density measurements, and two new AMS 14 C radiocarbon ages, we reconstruct the eruptive history and stratigraphy of a previously undocumented explosive rhyolitic eruption sourced from Colli dome, designated as Unit O. The eruption is younger than a paleosol dated at 6,400 ± 30 years BP. The spatial distribution of Unit O deposits, the orientation and length of dune bedforms near Colli dome, and the geochemical composition of pumice clasts collectively identify Colli dome as the likely source vent. The proximal stratigraphy of Unit O records four distinct eruptive phases: Phase 1 (Layer A), characterized by partial column collapse of an eruptive column generated from repeated phreatomagmatic explosions; Phase 2 (Layer B), reflects the development of an unsteady eruptive column with repeated partial collapses and simultaneous phreatomagmatic explosions; Phase 3 (Layers C–D), marked by repeated column collapses and simultaneous phreatomagmatic explosions; and Phase 4 (Layer E), associated with the development of an eruptive column from repeated phreatomagmatic explosions with partial collapses. Juvenile products are rhyolitic in composition (76.4–77.8 wt.% SiO 2 ) with trace element concentrations of 32–32.8 ppm La and 164–195 ppm Zr. Highlights • An explosive rhyolitic eruption occurred in La Primavera some 6.4 ka BP. • The chemistry and orientation of the dune bedforms indicate the Colli dome as the source of the eruption. • Field evidence and juvenile-clast characteristics indicate magma-water interaction. • We propose a model for the evolution of the eruption.