Abstract

40Ar/39Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, orthophotos, and geographic information system [GIS] software) to constrain magma eruption rates at the Ceboruco–San Pedro volcanic field (1600 km2) in the western Trans-Mexican arc. Ages are reported for 40 volcanic units, including Volcán Ceboruco (an active, andesitic stratovolcano), peripheral domes, shields, cinder cones, and fissure-fed flows. After a hiatus of ∼3 m.y., volcanic activity recommenced to produce 80.5 ± 3.5 km3 of magma at a rate of 63 m3/km2 per year over the past 0.8 m.y. However, 75% of this volume erupted in the past 100 k.y., including the 51 ± 2.5 km3 of Volcán Ceboruco, equivalent to an eruption rate of ∼377 m3/km2 per year. There have been at least two stages of cone-building activity at Volcán Ceboruco. The main edifice is composed of ∼38 km3 of precaldera andesites, the youngest dated at 45 ± 8 ka. Their eruption was followed by a hiatus, interrupted by a Plinian eruption at 1 ka. The Plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute ∼13 km3 of the total volume of Volcán Ceboruco. Overall, the relative proportions of lava types erupted in the past 0.8 m.y. are 18%–19% basaltic andesite, 56%–60% andesite, 18%–22% dacite, and 3% rhyolite. The peripheral lavas are each of small volume, geochemically diverse, and show little evidence of prior storage in an upper-crustal chamber. The eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and geochemistry imply that the lavas do not 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.

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