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

摘要

The early and middle Miocene andesitic lavas of the Sierra de Angangueo (MALSA) represent one of the most prominent landforms of intermediate magmatism in Central Mexico. In this paper, we present new petrological, geochemical, thermobarometric, and geochronological data in order to discuss the conditions of MALSA's emplacement in the lithosphere.MALSA comprises a voluminous eroded early Miocene andesitic structure, emplaced on a Mesozoic basement. MALSA shows a wide variety of textures, from glassy and aphanitic to porphyritic, mainly composed of plagioclase (An(10-55)), clinopyroxene (Wo(60-65); En(35-40)), amphibole (Mg-hornblende/edenite), and rarely olivine (Fo(75-86)) or orthopyroxene (En(72-80)). Major and trace elements plots follow a typical calc-alkaline trend with relatively increasing fractionation.The Sr-87/Sr-86 isotopic signatures range between 0.703343 and 0.704459 and epsilon Nd values from +1.37 to +4.84; apparently without a significant contribution of an old, thick, or highly radiogenic continental crust. Lead isotope values Pb-206/Pb-204 vary between 18.68 and 18.83, Pb-207/Pb-204 from 15.57 to 15.65, and Pb-208/Pb-204 from 38.39 to 38.67, suggesting contamination of magmas by juvenile crust. Thermodynamic calculations indicate equilibrium conditions at 900 degrees C and similar to 2 kbar and an average log integral O-2 approximate to -10.Ar-Ar and K-Ar dating carried out on whole rock, matrix, amphibole, and plagioclase phenocrysts yielded ages between 13.00.5 and 23.9 +/- 0.3Ma. These dates indicate a series of pulses with maximum magmatic activity between 24 and 18Ma.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.