Miocene andesitic lavas of Sierra de Angangueo: a petrological, geochemical, and geochronological approach to arc magmatism in Central Mexico

Lead isotope values 206 Pb/ 204 Pb vary between 18.68 and 18.83, 207 Pb/ 204 Pb from 15.57 to 15.65, and 208 Pb/ 204 Pb from 38.39 to 38.67, suggesting contamination of magmas by juvenile crust. Thermodynamic calculations indicate equilibrium conditions at ≤900°C and ~2 kbar and an average log ƒO 2 ≈ −10. Ar–Ar and K–Ar dating carried out on whole rock, matrix, amphibole, and plagioclase phenocrysts yielded ages between 13.0 ± 0.5 and 23.9 ± 0.3 Ma.

These dates indicate a series of pulses with maximum magmatic activity between 24 and 18 Ma. The geochemical and petrologic data suggest partial melting processes in the lower or middle crust associated with possible magma mixing during its ascent; such a mechanism could explain a hybrid mantle-lower crust origin of this large volume of andesites. The MALSA, as well as the early to middle Miocene magmatism from Western Mexico, could represent two coeval and independent magmatic arcs prior to the present Trans-Mexican Volcanic Belt (TMVB) in Central Mexico.