U–Pb chronology and geochemistry of detrital monazites from major African rivers: Constraints on the timing and nature of the Pan-African Orogeny

Abstract

The African continent consists of several cratons rimmed by orogenic belts, which were mainly formed by collisions of the cratons during the Gondwana supercontinent assembly. Understanding of the timing and nature of the Pan-African Orogeny is essential to decipher the development of the African continent and Gondwana supercontinent. We carried out U–Pb dating and geochemical analyses of approximately 500 detrital monazites from the Nile, Niger, Congo, Zambezi and Orange Rivers. The U–Pb dating defined age peaks at ∼600Ma for the Nile, ∼580Ma for the Niger, ∼630, ∼610 and ∼550Ma for the Congo, ∼500Ma for the Zambezi and 1200–1000Ma for the Orange. All but the Orange age peaks correspond to the period of the Pan-African Orogeny. In the Niger, Congo and Zambezi, the Pan-African age peaks are 20–40Myr younger than those defined by the detrital zircons from the same rivers. In the Nile, however, the detrital monazite and zircon age peaks are contemporaneous. The geochemical analyses revealed significant variations in Eu/Eu∗, Gd/Lu and Th/U ratios among the detrital monazites, which can be attributed to elemental partitioning with the co-existing feldspars, garnet, and zircon/rutile, respectively. Considering that typical abundances of these minerals vary among igneous and metamorphic rocks, these geochemical data can be potentially linked to the genesis of the detrital monazites. The geochemical systematics, together with the discrepancy between the detrital monazite and zircon ages, suggest that the Pan-African monazite age peaks essentially reflect the timing of syn- to post-collisional metamorphic events in the source terranes, whereas the detrital zircons provide a more representative record of magmatic events. The results imply that the Pan-African Orogeny was promoted by pre- to syn-collisional felsic magmatism during the early stages and, 20–40Myr later, it was dominated by syn- and/or post-collisional metamorphism.