Cyclical dome formation and destruction leading to highly explosive PDCs at the Late Pleistocene Tlaloc volcano, Mexico

Abstract

The age, eruptive dynamics, and magmatic processes driving the explosive eruptions of the Tlaloc stratovolcano remain poorly understood. These eruptions may reflect a cyclical phase of dome growth and destruction, as suggested by lithic content variations within the Late Pleistocene units of Tlaloc. In this study, we present stratigraphic, petrological, and compositional analyses of three major units: Xichimanla (unknown age), La Joya (between 38 and 43.5 ka), and Tlaminca (∼34 ka). These units, consisting of pyroclastic density current (PDC) deposits emplaced along the NW gullies of the volcano, capture a significant part of its explosive eruptive history during the Late Pleistocene. The Xichimanla, La Joya, and Tlaminca units represent vent-opening events initiated by dome explosions, followed by boil-over-type eruptions. A distinct compositional transition from rhyodacite to rhyolite (64.16–69.25 wt% SiO₂) is evident in the units. Textural and geochemical evidence, including banded pumice, disequilibrium textures in plagioclase, amphibole, and pyroxene, as well as the presence of xenoliths, xenocrystals and mineral relics, suggest that the explosive eruptions at Tlaloc were driven by magma mixing and crustal assimilation. The interaction between a mafic recharge magma and a resident felsic reservoir generated a hybridized melt, incorporating remobilized crystal mush parts from felsic reservoirs. These processes, along with volatile input from crustal assimilation, contributed to overpressure buildup and ultimately triggered explosive activity. Amphibole thermobarometry (100–500 MPa) from the La Joya PDC points to the presence of multiple magma reservoirs beneath Tlaloc, potentially explaining the compositional and textural variability. Moreover, the shift from effusive to explosive behavior may have been triggered by the intrusion of fresh, hot magmas into deeper reservoirs, introducing volatiles and heat that facilitated melt migration toward shallower chambers before eruption. These processes could explain the explosive eruptive dynamics of other stratovolcanoes in the Sierra Nevada of Mexico.