Contemporaneous eruptions of calc-alkaline and alkaline magmas along the volcanic front of the Mexican Volcanic Belt

Abstract

According to general models for subduction-related volcanic ares, magma compositions become progressively richer in K20 and incompatible elements with increasing distance from the trench and volcanic front and with increasing depth to the Benioff zone. Arc-type magmas are typic-ally hypersthene-and quartz-normative. Nepheline-normative alkali basalts are expected to erupt, if at ali, only at great distan ces from the tren ch. Along the volcanic front of the Mexican Volcanic Belt, however, hydrous, nepheline-normative, basic alkalic magmashave erupted at two different locations during the late-Quaternary Period: in the southern Colima graben and in the · southern Michoacán-Guanajuato Volcanic Field. In both places, hypersthene-and quartz-nor-mative calc-alkaline magmas with typical 'volcanic front' characteristics were erupting contem-poraneously. These contrasting magma suites can not be related by any simple mechanism; they appear to record distinct melting events within the upper mande.