Lower-crustal xenoliths from the Valle de Santiago maar field, Michoacan-Guanajuato volcanic field, central Mexico

We report on a recent finding of granulitic and gabbroic xenoliths in the R incon de Parangueo and La Cintura maars, within the Valle de Santiago volca nic field. The field is formed by several maars, cinder cones, and shield v olcanoes and is located at the northern sector of the extensive Michoacan-G uanajuato volcanic field, characterized by abundant Neogene cinder cones an d medium-sized shield volcanoes. The maars are distributed along a belt elo ngated NW-SE, apparently related to a regional fault system that has been d ocumented in the west-central sector of the Trans-Mexican volcanic belt (TM VB). Two types of xenoliths have been distinguished-i.e., hypersthene-diopside g ranulites and gabbroic xenoliths-which are present in the Rincon de Parangu eo and La Cintura maars, respectively. The silica content in the granulites has a narrow range between 46.9 and 50.9 wt%, plagioclase crystals are fra ctured and display reaction borders, and crystals appear to have grown slow ly and show no compositional zoning. Silica in the gabbroic xenoliths range s from 45.7 to 47.5 wt%. Chemical composition is similar to mafic and inter mediate orthogneiss xenoliths from the San Luis Potosi maars of the Central Mexican Altiplano. An Sm/Nd model age for the granulitic xenoliths from Rincon de Parangueo ma ar of 1.5 Ga supports the presence of a Precambrian lower crust. This findi ng challenges current models of crustal structure in central and western Me xico. The Valle de Santiago field represents the western-most lower-crustal xenolith locality reported thus far. It lies within the Guerrero terrane t hat has been considered as a collage of island arcs built on oceanic lithos phere and accreted to the North American plate during late Mesozoic time. O ur results support a composite nature for the Guerrero terrane and extend t he limit of possible granulitic lower continental crust farther to the west of the Central Altiplano province, beneath the TMVB, than currently consid ered (i.e., to the east of the Guerrero terrane).