Beyond subduction and plumes: A unified tectonic-petrogenetic model for the Mexican volcanic belt

The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to presen t-day volcanism in southern Mexico. Its origin has been controversial, alth ough the majority opinion views it as a volcanic are related to the subduct ion of the Cocos plate under the North American plate. Both calc-alkaline a nd alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objecti ons to these explanations, and conclude on the basis of geological, geochem ical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active exte nsion. Calc-alkaline or alkaline geochemistry of magmas is not useful for i nferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contami nation, and the GIB-like geochemistry suggests an enriched mantle source. O ur proposal of a heterogeneous mantle beneath the MVB comprising "normal" m antle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt .