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

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This study reports new geochemical and radiogenic isotope data on hy-normative Pleistocene to recent (<40,000 years) andesitic to dacitic volcanic rocks 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 ratios of these evolved magmas show the following ranges: 87 Sr/ 86 Sr 0.70370–0.70469, 143 Nd/ 144 Nd 0.51278–0.51289, 206 Pb/ 204 Pb 18.64–18.72, 207 Pb/ 204 Pb 15.59–15.62, and 208 Pb/ 204 Pb 38.39–38.52. These isotopic ratios are generally similar to those for the associated mafic rocks, although some differences do exist between them, particularly for the 87 Sr/ 86 Sr ratios that are somewhat higher and 143 Nd/ 144 Nd that are slightly lower for the evolved rocks. The available geochemical evidence does not support the generation of the SCN evolved magmas by simple fractional crystallization (FC) or even by assimilation coupled with fractional crystallization (AFC) of such mafic magmas, nor by direct (slab melting) or indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The viable mechanism seems to be that the most evolved andesitic and dacitic magmas in the SCN were produced by partial melting of a heterogeneous mafic granulite source in the lower crust. The magmas of intermediate compositions may reflect magma mixing processes between the most evolved andesitic and dacitic magmas generated in the lower crust and the mantle-derived mafic magmas (basalt and basaltic trachy-andesite) at the volcanic front of the central part of the MVB. These results have direct bearing to other volcanic areas with complex tectonic setting.