Petrographic and Geochemical Characterization of the Metasedimentary rocks in the southern part of Ouaddaï (eastern Chad): Source, Provenance and Tectonic Setting.

Résumé

The southern part of Ouaddaï (eastern Chad), is part of the Central African Orogenic Belt (CAOB), composed mainly of metasedimentary rocks affected by low (green schist) to medium (amphibolite) grade metamorphism. Petrographic and geochemical data are used to understand their origin and tectonic environment. Paragneisses, metapelites and phengite quartzites are mostly derived from greywackes and arkoses. CIA and A1₂O₃-CaO + Na₂O-K₂O values indicate a low to intermediate degree of weathering. Th/U, PIA and CIW values suggest a low to intense weathering degree. LREE enrichment, the negative Eu anomaly and the La/Sc ratio show that these metasedimentary rocks are originated from a felsic source rock. The geochemical characteristics of these metasedimentary rocks are similar to those of Archean to post-Archean upper continental crust. The original sedimentary rocks were probably deposited in an active to passive continental margin tectonic environment.

Keywords: Metasedimentary rocks, Ouaddaï massif, Central African Orogenic Belt, post-Archean upper continental crust.