Strong Compositional Zonation in a Silicic Magmatic System: Los Humeros, Mexican Neovolcanic Belt

Abstract

The Los Humeros volcanic center, located 180 km east of Mexico City, is one of several silicic centers in the ‘back-arc’ portion of the Mexican Neovolcanic Belt. Eruptive products span the compositional range from high-silica rhyolite to basalt. During the last 0·46 Ma, three major explosive eruptions and an extended period of lava flow emplacement periodically sampled an integrated magma reservoir that was initially zoned from rhyolitic uppermost levels to andesitic and perhaps basaltic lower levels, with compositional gaps in the ranges 63–67 and 72–75 per cent SiO2. The compositional zonation can largely be explained by fractional crystallization, but must be accompanied by assimilation to explain the range of Sr and Nd isotopic ratios. Higher than predicted concentrations of Ni, Cr, and strongly incompatible elements such as Rb and Ba in andesites suggest continuous replenishment of a fractionating chamber by mantle-derived basalts. The volumetric predominance of rhyolite in the early history of the center points to a long period of accumulation of differentiates without eruptive withdrawal. Once volcanic activity started tapping the chamber, eruptive rates seem to have exceeded the rate of regeneration of differentiated magma. Although there is overlap in the compositional zonation of the products of successive eruptive events, the dominant volume of each is always more mafic than that of the preceding eruption, indicating only limited regeneration of differentiated magmas during repose periods. This seems to have been a consequence of the chamber remaining in approximate thermal balance during the last 0·46 Ma, as shown by similar Fe-Ti oxide temperatures for given magma compositions, regardless of age. Calculations suggest that the chamber received thermal input through the injection of basalt at an average rate of 0·1 km3 per thousand years. Apparently this thermal input was too small to generate differentiated magma by partial melting of the wall rocks of the chamber, but large enough to offset conductive or hydrothermal cooling that would promote differentiation by extensive crystallization.