Production of andesite in the western Mexican arc: Constraints from argon chronology, volume analysis, and geochemistry

40Ar/39Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, ortho-airphotos and Geographic Information System software) to constrain magma eruption rates at the Ceboruco-San Pedro and the Volcan Tepetiltic volcanic fields in the western Trans-Mexican arc. Each volcanic field is dominated by an intermediate composition stratovolcano (∼51 and ∼42 km 3, respectively) that was built in tens of kyrs. The smaller volume peripheral lavas (<1 km3) are more geochemically diverse than the large volume flows associated with the main edifices. The peripheral flows tend to be crystal-poor (<5% phenocrysts) and show little evidence for magma mingling or disequilibria. Based on silica content, there is no systematic progression to the composition of magma type (basalt, andesite, dacite, rhyolite) erupted with time, at either the individual stratocones or across the entire volcanic field. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, and geochemistry, the lavas in the Ceboruco-San Pedro volcanic field and the lavas at Volcan Tepetiltic do not each reflect the differentiation of a single parental liquid in a long-lived magma chamber. The distinct geochemical signatures were present prior to magma emplacement in the upper crust, whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages.; The crystal-poor lavas peripheral to the central stratocones provide insight into the generation of magmas at volcanic arcs. The composition of plagioclase is strongly dependent on the water concentration of the melt from which it crystallized. This relationship is used to develop a new plagioclase hygrometer to estimate water content in the melt. The plagioclase in the crystal-poor lavas span a broad range in composition and reflect crystallization from magmas with different amounts of dissolved water. Textures in the plagioclase (abundant melt inclusions) reflect rapid crystallization. These two separate lines of evidence suggest that crystallization is degassing induced.; The broader tectonic evolution of western Mexico is explored with a detailed geochronology study of the silicic volcanism over the last ∼80 Myr. A previously unrecognized volcanic flare-up in the Pliocene suggests that western Mexico may have been part of an aborted rifting event.