The eruptive history of the Tequila volcanic field, western Mexico: ages, volumes, and relative proportions of lava types

Abstract

The eruptive history of the Tequila volcanic field (1600 km2) in the western Trans-Mexican Volcanic Belt is based on 40Ar/39Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units ≤1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km3) and high-Ti basalt (~39 km3) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km3 of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km3 of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km3 of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km3). Over the last 1 Myr, a total of 128±22 km3 of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km3/kyr. This volume erupted over ~1600 km2, leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.