U–Pb and Lu–Hf isotope systematics of lower crust from central-southern Mexico – Geodynamic significance of Oaxaquia in a Rodinia Realm

Combined U–Pb ages and Lu–Hf isotope systematics of individual zircon grains from lower crustal rocks of late Mesoproterozoic (Grenville) age, reveal a long geologic evolution of the Oaxaquia terrane (also Oaxaquia) of E and S Mexico. Zircon grains from migmatites, which are host rocks of the AMCG (anorthosite-mangerite-charnockite-granite) suite, have complex internal structures. They contain zoned cores, surrounded by high-U zones, and low-U metamorphic overgrowths. Most of these grains crystallized at ∼1.2 Ga, but some cores are as old as ∼1.35 Ga. Igneous crystallization of the protoliths was followed by migmatization and by granulite facies metamorphism at ∼1.0 Ga, and late stage retrogression. Lutetium–hafnium isotope systematics reveal that both AMCG and host rocks have a limited range of 176Hf/177Hf(i) (0.28215–0.28224) with ɛHf(1.2 Ga) ≈ +5 to +7 indicating crust formation from a depleted source throughout the Oaxaquia terrane in the early Mesoproterozoic. Zircon from one particular exposure that contains inherited components as old as 1.8-1.5 Ga, has lower 176Hf/177Hf(i) (0.28207–0.28209) with ɛHf(1.2 Ga) ≈ +2. The Hf isotope ratios of these zircons are comparable with zircon from the Garzón Massif (Colombian Andes). The data indicate that Oaxaquia comprises (1) a crustal precursor that was formed probably as an oceanic arc at 1.4-1.2 Ga, here referred to as “typical Oaxaquia”, and (2) distinct crustal slices with influence from older continental crust that were probably thrusted over “typical Oaxaquia” during the 1.0 Ga orogeny. Migmatization is interpreted to have occurred during the accretion of “typical Oaxaquia” to Amazonia. This event was followed by extension and AMCG intrusion at ∼1.01 Ga, and then the collision of Amazonia with Baltica (assemblage within Rodinia), which caused granulite facies metamorphism elsewhere in the Oaxaquia terrane at ∼0.99 Ga.