Contemporaneous eruptions of calc-alkaline and alkaline magmas along the volcanic front of the mexican volcanic belt

TLDR: According to general models for subduction-related volcanic arcs, magma cornpositions 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.

Abstract: According to general models for subduction-related volcanic arcs, magma cornpositions 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 typically hypersthene- and quartz-normative. Nepheline-normative alkali basalts are expected to erupt, if at ah, only at great distances from the trench. Along the volcanic front of the Mexican Volcanic Belt, however, hydrous, nepheline-normative, basic alkalic magmas have erupted at two different locations during the late-Quaternary Period: in the southern Colima graben and in the southern Michoacan-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 mantle.