The Acoculco Caldera Complex magmas: Genesis, evolution and relation with the Acoculco geothermal system

Abstract

The Acoculco Caldera Complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt, México. The ACC was formed 2.7 Ma, and since then, volcanic activity has persisted until 0.06 Ma, through the emission of domes, cinder cones, fissure lava flows and two ignimbrite eruptions dated at 1.2 and 0.65 Ma. The pervasive hydrothermal alteration in the central part of the ACC has motivated considerable geothermal exploration, however, none of it assessed the relation between the magmatic heat supply and the evolution of magmas, including their reservoir depth.Magmatic processes were investigated with petrography, major oxides-trace-element geochemistry, mineral chemistry and isotopic analysis. In addition, we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1.2 Ma during a reactivation of the caldera. We investigated the origin of magmas tapped during the eruptive history of the ACC, and the magmatic processes (magma mixing, assimilation and crystallization) that modified these magmas.After the caldera collapse 2.7 Ma, the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems. Such magmas mixed with calc-alkaline magmas and formed the post-caldera volcanism. Influx of new magma is considered as the probable heat supply that maintains the active magmatic system, whereas assimilation has been negligible, but yields insights about the depth at which processes have occurred. Our work suggests that there are either shallow storage zones (200–500 bars) where magmas crystallize and either erupt or stall and cool, or there are intrusive magmas unrelated to the ACC suite that have accumulated nearby and heat the reservoir magmas.