U-Pb zircon evidence for a pan-african granulite facies metamorphism in the central african republic. a new interpretation of the high-grade series of the northern border of the congo craton

Abstract

Abstract The results of U-Pb zircon and Sm-Nd whole-rock analyses are reported on three meta-igneous granulitic samples from the Central African Republic basement, formerly ascribed to an Archaean complex. UPb data point to late Proterozoic ages (833+/−66 Ma, 652+/−19 Ma and 639+/−3 Ma), and Sm-Nd model ages in the 1700–2200 Ma range preclude an exclusive derivation from an Archaean source. Whatever their precise geologic significance (either first crystallization of zircons at the time of igneous emplacement or complete resetting during metamorphic recrystallization), the U-Pb data clearly rule out an Archaean age for the high grade event, in contrast with previous opinion. Indeed, the granulite facies metamorphism cannot be older than late Proterozoic. This is in good agreement with the recent recognition of a major collision type orogen (Oubanguides belt) of Pan-African age in this area. It is proposed that two main generations of granulitic rocks occur at the northern margin of the Congo craton: an Archaean one in the foreland of the Oubanguides belt (e.g., Ntem complex, Cameroon), and a Pan-African one in the internal zones of this orogen.