Alkalic (ocean-island basalt type) and calc-alkalic volcanism in the Mexican volcanic belt: A case for plume-related magmatism and propagating rifting at an active margin?

The Mexican volcanic belt has been traditionally regarded as a classic case of subduction-related calc-alkalic volcanism, However, a series of geologi c, geophysical, and petrological arguments makes this simple relationship d oubtful,A seismic gap beneath the belt, a large-scale mantle anomaly, a gra ben triple-junction domain, and the presence of volumetrically important oc eanic-island basalt (OIB) volcanism throughout the belt suggest a more comp lex tectonic scenario involving plume- and subduction-related processes. We here propose a model involving the development of a propagating rift openi ng from west to east in response to plume activity. The process started in Miocene time within the western sector of the belt (Guadalajara) and gave r ise to a graben triple junction and OIB-type and calc-alkalic volcanism, Ex tension and volcanism proceeded to the east, giving rise to progressively y ounger ages for the initiation of OIB-type volcanism: (1) Miocene in the we st (e.g., Guadalajara), (2) Pliocene in the central zone (e,g,, Michoacan-G uanajuato), and (3) Quaternary farther east (e.g., Chichinautzin), Geochemi cal evidence suggests that part of the modern calc-alkalic volcanism (e,g,, Chichinautzin) may be derived from magma mixing between the OIB mafic magm as and silicic, crust-derived magmas, However, we do not preclude some infl uence of the subducting slab in the generation of other (e,g,, Jorullo) cal c-alkalic volcanic rocks, Our model suggests a currently unrooted upper plu me attached to the subcontinental lithosphere, which defines a hot zone ben eath the Mexican volcanic belt.