Young high-temperature granulites from the base of the crust in central Mexico

Characterization of the lower continental crust is essential to understanding processes of crustal formation and evolution. Many workers have concluded that the lower crust is composed of granulite-facies rocks (see, for example, refs 1, 2), but many exposed granulite-facies terranes seem to have mid-crustal rather than lower-crustal origins^3-5. Here we present new data from direct samples of the lower continental crust, in the form of metapelitic rock fragments brought to the surface by Quaternary volcanism in central Mexico. These xenoliths contain exsolved ternary feldspars that record extremely high minimum metamorphic temperatures of 950-1,100 °C, the highest yet known to be preserved in deep-seated metamorphic rocks. The metamorphism seems to be the result of underplating by basaltic magma and to be regional in extent. Our barometry on these samples demonstrates that granulite-facies metamorphic rocks are present in the lower crust, and provides important constraints on tectonic models and interpretations of geophysical data. Chemical and text lira I evidence suggest that the xenoliths were not able to cool to below 850-900 °C before being entrained in their host magmas, indicating that the granulite-facies metamorphism occurred no more than ∼30 Myr ago.