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 e{open}_{Hf(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 e{open}_{Hf(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.