Geochemistry of evolved magmas and their relationship to subduction-unrelated mafic volcanism at the volcanic front of the central Mexican Volcanic Belt

This study reports new geochemical and radiogenic isotope data on hy-normat ive Pleistocene to recent (<40,000 years) andesitic to dacitic volcanic roc ks from Sierra de Chichinautzin (SCN), located south of Mexico City, in the central part of the Mexican Volcanic Belt (MVB). Their rare-earth element, Nb, Zr, and Y concentrations are generally lower than in mafic rocks from this area, which have been shown to be subduction-unrelated. The isotopic r atios of these evolved magmas show the following ranges: Sr-87/Sr-86 0.7037 0-0.70469, Nd-143/Nd-144 0.51278-0.51289, Pb-206/Pb-204 18.64-18.72, Pb-207 /Pb-204 15.59-15.62, and Pb-208/Pb-204 38.39-38.52. These isotopic ratios a re generally similar to those for the associated mafic rocks, although some differences do exist between them, particularly for the Sr-87/Sr-86 ratios that are somewhat higher and Nd-143/Nd-144 that are slightly lower for the evolved rocks. The available geochemical evidence does not support the gen eration of the SCN evolved magmas by simple fractional crystallization (FC) or even by assimilation coupled with fractional crystallization (AFC) of s uch mafic magmas, nor by direct (slab melting) or indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The viable mech anism seems to be that the most evolved andesitic and dacitic magmas in the SCN were produced by partial melting of a heterogeneous mafic granulite so urce in the lower crust. The magmas of intermediate compositions may reflec t magma mixing processes between the most evolved andesitic and dacitic mag mas generated in the lower crust and the mantle-derived mafic magmas (basal t and basaltic trachy-andesite) at the volcanic front of the central part o f the MVB. These results have direct bearing to other volcanic areas with c omplex tectonic setting. (C) 1999 Elsevier Science B.V. All rights reserved .