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

The eruptive history of the Tequila volcanic field ( 1600 km(2)) 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 Volcan 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 (similar to 35 km(3)) and high-Ti basalt (similar to 39 km(3)) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km(3) 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, similar to 31 km(3) of andesite erupted to form the stratocone of Volcan Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at similar to 2 - 3 km depth. After a hiatus of similar to 110 kyrs, similar to 15 km(3) of andesite erupted along the W and SE flanks of Volcan Tequila at similar to 90 ka, most likely from a second, discrete magma chamber located at similar to 5 - 6 km depth. The youngest volcanic feature (similar to 60 ka) is the small andesitic volcano Cerro Tomasillo (similar to 2 km(3)). Over the last 1 Myr, a total of 128+/-22 km(3) of lava erupted in the Tequila volcanic field, leading to an average eruption rate of similar to 0.13 km(3)/ kyr. This volume erupted over similar to 1600 km(2), leading to an average lava accumulation rate of similar to 8 cm/kyr. The relative proportions of lava types are similar to 22 - 43% basalt, similar to 0.4 - 1% basaltic andesite, similar to 29 - 54% andesite, similar to 2 - 3% dacite, and similar to 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 similar to 1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.