Abstract

Volcan Ceboruco is a moderately sized stratovolcanc located in the northwestern part of the Mexican Volcanic Belt. The bulk of the volcano consists of relatively alkali and incompatible element—poor hypersthene andesites erupted prior to the formation of two concentric calderas. These precaldera andesites were followed in the eruptive sequence by the eruption 1,000 yr ago of a white rhyodacite pumice, termed the Jala pumice. The eruption apparently caused the formation of Ceboruco's outer caldera. A low-silica dacite dome and lava flow were then erupted and partially filled this caldera. These dacites contain xenoliths of high-Al basalt that were apparently partially molten at the time of their inclusion, suggesting that the dacite magma formed as a result of magma mixing involving Jala pumice and basaltic magmas. The dacite dome later collapsed to form the inner caldera, and Ceboruco again erupted andesites. These postcaldera andesites contain both augite and hypersthene and are enriched in alkalis and incompatible elements relative to the precaldera andesites. Finally, between 1870 and 1875, rhyodacite lava flows were erupted during Ceboruco's only historic activity.

Crystal fractionation models using both major and trace elements suggest that the 1870 dacites and the Jala pumice could have been derived from magma with postcaldera andesite composition. To determine the origin of the Ceboruco andesites, trace-element partial-melting models tested three possible sources: subducted oceanic crust in the eclogite facies and the amphibolite facies and mantle peridotite. All models were found unreliable because of the many assumptions involved. It is concluded that a complex process involving partial melting, fractional crystalization, magma mixing, and, perhaps, crustal assimilation was responsible for the generation of the andesites. Such a model cannot be tested, however, because there are at present no viable constraints that can be placed on this complex sequence of events.

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