40Ar/39Ar geochronology of Volcan Tepetiltic, western Mexico: Implications for the origin of zoned rhyodacite-rhyolite liquid erupted explosively from an andesite stratovolcano after a prolonged hiatus

Abstract

40 Ar/ 39 Ar geochronology is used to determine the eruptive history of Volcan Tepetiltic, a predominantly andesitic stratovolcano that underwent caldera collapse during explosive eruption of zoned rhyodacite–rhyolite. The main edifice was largely constructed between 560 and 450 ka, but it was not complete until ca. 416 ka, during which time ∼42 km 3 of phenocryst–rich (25–40 vol%) lavas ranging from 57 to 69 wt% SiO 2 were erupted. After a hiatus of ∼180 k.y., there was a climactic Plinian eruption of ∼4–8 km 3 of zoned magma (68–75 wt% SiO 2 ). Afterward, a small rhyodacite dome (69 wt% SiO 2 ; 190 ± 22 ka) and an andesite dome were emplaced on the caldera floor. There has been no subsequent volcanic activity. The crystal–poor (0–5 vol%) Plinian pumice could not be dated directly, owing to the hydration of glass and the absence of a K–rich mineral phase. Instead, the age of the climactic eruption was bracketed to be ca. 236 ± 52 ka from 40 Ar/ 39 Ar dates (±2δ) on peripherally erupted basaltic andesite flows, which are found underneath (241 ± 47 ka) and overlying (220 ± 36 ka) the associated fallout deposits. A volume of ∼9 km 3 of basaltic andesite was erupted from three peripheral vents surrounding Volcan Tepetiltic, most of it from a shield volcano that was active before, during, and after the Plinian event. Thermal models indicate that the magma chamber beneath Volcan Tepetiltic would have cooled below its solidus within 36 k.y. in the absence of new injections of magma. Given the long hiatus (∼180 k.y.) between the cone–building episode that built the andesite stratovolcano and the explosive eruption of rhyodacite–rhyolite, it is proposed that the influx of voluminous basaltic andesite into the upper crust drove partial melting of the subsolidus magma chamber beneath Volcan Tepetiltic, which explains the synchronicity of the basaltic andesite volcanism with the Plinian eruption of zoned, crystal–poor rhyolitic melt.