CRUSTAL XENOLITHS AS POSSIBLE REMNANTS OF RELAMINATED SUDUCTED SEDIMENTS BENEATH THE TRANSMEXICAN VOLCANIC BELT

The Chalcatzingo trondhjemitic domes (CTD) are a remarkable set of almost pristine slab-derived melts emplaced during the early evolution of the TMVB (~21 Ma) (Gómez-Tuena, et. al., 2008). They carried up granulitic xenoliths, mostly high-Al metapelites from the lower crust, which experienced a clockwise metamorphic path reaching nearly UHT. It has been proposed that these xenoliths represent Proterozoic (Ortega-Gutiérrez, et. al., 2012) rocks probably from Oaxaquia, but no dating had been done. Here we present LA-ICP-MS U-Pb zircon ages from one orthogneiss and five metapelites, coupled with Pb, Sr, Nd and Hf isotopes from four metapelites. Many analyzed zircons are highly discordant, but concordant and nearly concordant zircons show that that these xenoliths are not part of Oaxaquia, as their ages range from Precambrian to early Oligocene with a large population in the Jurassic. The isotopic compositions of the xenoliths are also different from a typical Oaxaquia crust. Zircon REE patterns and textural relationships indicate an Eocene-Early Oligocene age for the UHT metamorphism. The maximum depositional age of the protolith could be Danian, Campanian or Turonian, ages that are younger than the Albian-Cenomanian platform intruded by the CTD. We propose that the only mechanism capable to place post-Albian sedimentary rocks in the lower crust of southern Mexico is relamination of subducted sediments. However, because these young ages are constrained by a few crystals, and many zircons are highly discordant suggesting some Pb-loss during high-T metamorphism, the sediments could instead represent basement rocks beneath the platform. Nevertheless, the high-strain metamorphism evident in the xenoliths would require a deep-rooted Eocene-Early Oligocene orogeny, which never occurred in Mexico, thus making relamination of young subducted sediments a more plausible mechanism. Hence, the Chalcatzingo xenoliths suggest that substantial portions of the continental lower crust may be composed of relaminated subducted sediments, making it more silicic than previously thought. These xenoliths would also provide additional evidence that buoyant sediments can rise through the mantle without completely melting in route to the lower crust.