Geology

The relationship between volcanism and subduction of the Cocos plate is examined on the basis of new as well as published Sr, Nd, and Ph isotopic and geochemical data on late Miocene to Holocene (ca. 9-0 Ma) basic volcanic rocks from southern Mexico and Central America. Basic rocks (with SiO2 < 52%) were chosen in order to minimize the effects of crustal-assimilation processes and, therefore, to test the contribution from deeper sources: the subducted Cocos plate and the mantle wedge. By using fluid-mobile to relatively fluid-immobile elements and radiogenic isotope ratios for such rocks, I show that the subduction of the Cocos plate does not contribute to the basic volcanism in all of southern Mexico as opposed to that in Central America (from Guatemala to northwestern Costa Rica). South Mexican volcanism is related to ongoing rifting processes, inferred from field geology, seismology, gravity, tectonics, and volcano alignments. This lack of subduction relationship probably represents the first case on Earth where the ongoing subduction of an oceanic plate does not give rise to basic volcanism, such as is present throughout southern Mexico