THE 6.4 KA PHREATOMAGMATIC ERUPTION OF COLLI DOME, LA PRIMAVERA CALDERA, MEXICO
La Primavera is a Quaternary volcanic field located in the occidental part of the Trans-Mexican-Volcanic-Belt. La Primavera was active between 143.5 ka and the present. After the formation of the caldera ~95 ka ago, several domes and stratovolcanoes were emplaced either, on the rim, inside, and outside the caldera. This activity was accompanied by an intense explosive activity that deposited fifteen pyroclastic deposits (named units GP and A to N) between 86.4 and 25.6 ka. Recently, we identified a new and young pyroclastic unit, named “O” produced by a phreatomagmatic eruption, described less than 1 km north of the Colli dome in the northwestern outskirts of the densely populated metropolitan area of Guadalajara (5 million 268 inhabitants, INEGI 2020). The deposits of unit O are dominated by wet and dry dilute and dense pyroclastic density currents (PDC) that rest on a paleosol dated at 6,400 +/- 30 years BP using the AMS 14C method. Detailed stratigraphic descriptions coupled with grain size, componentry, and whole-rock geochemistry revealed that unit O is the youngest pyroclastic eruption of La Primavera. The distribution of the deposits, the length and orientation of the pyroclastic dunes, and impact sags near the Colli dome suggest this volcanic structure as the vent-source of this phreatomagmatic eruption. The general abundance of dilute and dense PDCs, accretionary lapilli, and soft deformation of the wet dilute PDCs suggest that magma-water interaction took place during the eruption. The proximal stratigraphic record of the O phreatomagmatic eruption is divided into five phases: (Am), an initial phreatomagmatic phase caused by the interaction between rising magma and groundwater that formed a phreatomagmatic eruptive column; (Bm), generation of an unsteady column that partially collapsed with simultaneous phreatomagmatic explosions; (Cm and Dm), collapse of the eruptive column with simultaneous phreatomagmatic explosions; and (Em), a closing phase with the generation of a phreatomagmatic eruptive column.