U-Pb age of a late Cenozoic ultra-high temperature metamorphic event under Central Mexico, as inferred from granulite xenoliths from Cerro El Toro, Mexico

Abstract

ABSTRACT Cerro El Toro, a Pliocene scoria cone that carried abundant lower crust xenoliths to the surface, is located in the Mesa Central (MC), Mexico, where mafic alkalic intra-plate volcanism has occurred since the early Miocene (end of the Sierra Madre Occidental volcanism). Early studies of El Toro xenoliths showed that the feldspathic granulite equilibrated at P = 0.9–1.4 GPa and T = 900−1100°C under anhydrous conditions (ternary feldspars calculations). A young (Oligocene-Quaternary?) pulse of ultra-high temperature (UHT) metamorphism was thus proposed for the region, without a strict age constrain. Zircon crystals recovered from a set of nine xenoliths (7 Grt-Sil bearing metapelites, a Px-bearing meta-quartz diorite, and a Grt-Opx bearing orthogneiss) were selected for LA-(MC)-ICPMS U-Pb geochronology. Palaeozoic to Neoproterozoic zircon crystals are scant, whereas Mesozoic to Cenozoic ages are more abundant. Based on their chondrite-normalized REEs all Neoproterozoic to Mesozoic zircons are interpreted as igneous, whereas those of late Oligocene to late Miocene age (ca. 27–6 Ma) are mostly metamorphic and grew during a protracted pulse of UHT metamorphism in the lower crust. The presence of Cenozoic metasediments in the lower crust under Cerro El Toro is indicative of the action of the subducted Farallon plate, coupled with tectonic erosion of continentally derived sediments, either from a forearc basin and/or an accretionary prism that were relaminated to the lower crust by sediment diapirism. Similarities among the xenolith zircon ages with those from modern sediments belonging to Central Mexico Pacific coast point towards a NW-SE 100 km long coast stretch across Zihuatanejo as a possible sediment source. The paucity of Grenville-age detrital zircon grains in the recovered xenoliths suggests that the El Toro area is not underlain by a Proterozoic basement, thus implying a substantial reduction of Oaxaquia extension beneath Central Mexico and presence of the younger Guerrero terrane.