ABSTRACT

This study reports new geochemical and radiogenic isotope data for Pliocene to Holocene (<2.8 Ma) alkaline (trachybasalt, basaltic trachyandesite, and trachyandesite) as well as subalkaline (basalt to dacite) volcanic rocks from the Michoacán-Guanajuato volcanic field (MGVF), located in the west-central part of the Mexican Volcanic Belt (MVB). There is no clear correlation of most geochemical parameters with differentiation indicators such as SiO₂. The rare-earth elements show light-REE enrichment, flat heavy-REE pattern, and absence of Eu anomaly. Depletion of Nb as compared to large ion lithophile elements such as Ba is observed for most rocks, probably suggesting involvement of subducted Cocos or Rivera plate. However, other HFSE such as Zr and Ti and key trace elements such as B and Be, and isotopic data do not support this conclusion. In most binary plots, the MGVF data lie at mantle compositions toward the lower end of subduction-input parameters, as is the case of magmas from well-known rifts; this contrasts with the data for subduction-related Central American Volcanic Arc (CAVA) rocks that clearly show high values of subduction input. The average isotopic ratios of the MGVF rocks show the following ranges: ⁸⁷Sr/⁸⁶Sr 0.70320-0.70439, ¹⁴³Nd/¹⁴⁴Nd 0.51273-0.51298, ²⁰⁶Pb/²⁰⁴Pb 18.62-18.88, ²⁰⁷Pb/²⁰⁴Pb 15.57-15.62, and ²⁰⁸Pb/²⁰⁴Pb 38.32-38.66. There are apparently no significant differences between the isotopic ratios for alkaline and subalkaline rocks, although the Sr and Pb isotopic ratios for alkaline rocks are somewhat higher and Nd isotopic ratios lower than those for subalkaline rocks. The available geochemical and isotopic evidence does not support the origin and evolution of the MGVF magmas by a simple model such as simple fractional crystallization (FC), nor by direct (slab melting) or indirect (fluid transport to the mantle) participation of the subducted Cocos plate. Instead, it appears that the MGVF basic magmas were generated in a heterogeneously veined-mantle source enriched in LILE, HFSE, and REE, but the intermediate and acid magmas could also contain a crustal component.