Petrology, geochemistry and geochronology of lower-crustal xenoliths, Central Mexico

Lower-crustal xenoliths from Quaternary-age volcanic vents in central Mexico have diverse lithologies. Their petrographic and geochemical characteristics are similar to other xenoliths collected in Mexico and a similarity in lithology probably exists over a large portion of the Mexican lower crust. Meta-igneous granulites range widely in composition and may have been formed in an arc-setting as cumulates or liquids, since most retain trace element concentrations that are similar to those found in typical arc-related rocks. Meta-sedimentary granulites are similar to depleted shales and may represent either the basement that existed before arc volcanism created the meta-igneous granulites, or alternatively, sediments that were obducted from a subducting slab. U-Pb zircon geochronology suggests that the protoliths for metaigneous xenoliths either formed from subduction-related magmas in Paleozoic to Precambrian time, or alternatively, formed in the mid-Tertiary from Sierra Madre Occidental magmas that assimilated older, zircon-bearing crust.