Isotopic and trace element constraints on the origin and evolution of alkaline and calc‐alkaline magmas in the Northwestern Mexican Volcanic Belt

Abstract

A wide variety of rock types has been produced by Pliocene to Recent volcanism in the northwestern portion of the Mexican Volcanic Belt. Composite cones erupted rocks typical of calc‐alkaline volcanic belts associated with subduction. Monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks. One larger center, Las Navajas, produced basalts, trachytes, and peralkaline rhyolites. Such alkaline rocks are typically associated with crustal extension related to rifting. Both subduction and rifting appear to be taking place in western Mexico. The most basic calc‐alkaline rocks, although somewhat fractionated, show depletion in high field strength elements (HFSE) and enrichment in alkaline earth elements relative to other incompatible elements, a characteristic typical of magmas in subduction‐related volcanic arcs. The basic alkaline rocks show no depletion in HFSE and show higher concentrations of incompatible elements than the basic calc‐alkaline rocks. The calc‐alkaline rocks show a tight cluster of 87Sr/86Sr and 143Nd/144Nd ratios, both of which correlate weakly with SiO2. Basic alkaline rocks have lower Sr and higher Nd isotopic ratios, whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc‐alkaline rocks. Trace element data indicate that calc‐alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas. The evolution of both calc‐alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation (AFC) of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite. Assuming a contaminant located in the lower or middle crust, AFC modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc‐alkaline magmas. Calc‐alkaline and alkaline rocks appear to have resulted from melting of two distinct sources: alkaline from an OIB‐type source, and calc‐alkaline from a mixed mantle and slab‐derived source.