Zoneamiento composicional y mineralógico en los productos eruptivos del centro volcánico de los Humeros, Puebla, México

Abstract

Los Humeros volcanic center, located 180 km east of Mexico City, is one of several silicic volcanic centers located in the "back-arc" portion of the Mexican Neovolcanic Belt. During the last 460 000 years, three major plinian eruptions and two major episodes of lava flow emplacement periodically "sampled" the magma chamber. The eruptive products of these events arecompositionally zoned and indicate that the magma chamber was zoned from rhyolitic uppermost levels to andesitic or perhaps even basaltic lower levels. Strong compositional zonation persisted throughout the lifetime of the system. In general, the most silicic products of each plinian eruption are either aphyric or are characterized by the lowest phenocryst contents and simplest phenocryst assemblages (plagioclase, Fe-Ti oxides, and biotite or orthopyroxene). Clinopyroxene and olivine make their appearance in progressively more mafic products of each eruption. Ca/Na ratios of plagioclase and Mg/Fe ratios of the mafic phenocrysts increase as the eruptive products become more mafic. Phenocryst content increases progressively, reaches a maximum in rhyodacitic and andesitic compositions, and then decreases in basaltic andesite compositions. Changes in phenocryst assemblages and abundances are probably a consequence of superimposed gradients in composition, temperature, and volatile content in the magma chamber. Temperatures calculated from Fe-Ti-oxide geothermometry range from 800 to 875ºC for the uppermost rhyolitic levels of the chamber, 860 to 940°C for the rhyodacitic portions, and 920 to lO00ºC for the andesitic portions.