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Croissance et différenciation crustales au Néoprotérozoique : exemple du domaine panafricain du Mayo Kebbi au Sud-Ouest du Tchad

Abstract : The Mayo Kebbi massif (south-western Chad) is located between the Congo craton, the West African craton and the Saharan Metacraton. It consists of two greenstone belts (Zalbi and Goueygoudoum), three magmatic complexes (Gauthiot falls, Lere, Figuil) and post-tectonic intrusions distinguished on the basis of their structural, petrological, geochemical and geochronological characteristics. The geodynamic evolution of this massif includes the following phases: Phase 1: Emplacement of a Mafic to Intermediate Plutonic (MIP) complex. Boloro metadiorite, which belongs to this complex, is dated at 748 ± 4 Ma (U-Pb zircon age). This metadiorite is enriched in REE and characterized by LaN/YbN ~ 12, Sr/Y > 32, high LILE, Cr and Ni contents but negative anomalies in Nb-Ta. These features are attributed to partial melting of the slab followed by interaction of the produced magmas with the mantle wedge during their ascent. Phase 2: Emplacement of metagabbros and metabasalts (700 ± 10 Ma: U-Pb zircon age) of the Zalbi metavolcanic-sedimentary group. These rocks are characterized by a decoupling of LILE and HFSE, negative Nb-Ta anomalies, weak to moderate LREE fractionation relative to HREE. In particular, their geochemical characteristics are similar to modern back-arc basins. The isotopic compositions of Sr and Nd of these rocks preclude contamination by old continental crust of the related magmas during their emplacement. Accordingly, the MIP complex and the Zalbi metavolcanic-sedimentary group are associated to juvenile accretion in an island arc/back-arc basin tectonic setting. Phase 3: The syntectonic quartz metadiorite of Gauthiot Falls magmatic complex (665 ± 1 Ma: U-Pb zircon age, Penaye et al., 2006) is emplaced during a first collision event, which involves the Mayo Kebbi massif and the Adamaoua-Yade domain to the east. This event marks the beginning of the closure of the Zalbi back-arc basin and crustal thickening. Phase 4: The thickening is responsible of intra-crustal differentiation by partial melting of rocks accreted during the previous phases at the base of the arc. During this phase, several tonalitic intrusions are emplaced, including hornblende-biotite tonalites of Gauthiot Falls and Guegou tonalite (Lere magmatic complex). The latter is dated at 647 ± 5 Ma (U-Pb zircon age). The produced magmas have typical features of TTG magmas, leaving a garnet bearing residue at the base of the continental crust. Phase 5: The syntectonic tonalite of Figuil magmatic complex dated at 618 ± 6 Ma (U-Pb zircon age), is characterized by initial ?Nd = -3 and initial 87Sr/86Sr = 0.7073 attesting for the involvement of pre-Neoproterozoic crust on its origin. It marks a second collision event between the Mayo Kebbi massif and the Western domain of the Central African Orogenic Belt to the west. Phase 6: The Zabili A-type granite emplaced at 567 ± 10 Ma (U-Pb zircon age) and is related to the last magmatic events of the Pan-African orogenic cycle (post-tectonic intrusions). The geochemical (low Sr, Eu, Ca, Mg, Ni) and isotopic compositions (initial ?Nd = +3 à +7) of this granite point to an origin involving extreme fractionation of mantle-derived magmas which interacted with an old crustal component during their emplacement in the upper continental crust
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Moussa Isseini. Croissance et différenciation crustales au Néoprotérozoique : exemple du domaine panafricain du Mayo Kebbi au Sud-Ouest du Tchad. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2011. Français. ⟨NNT : 2011NAN10035⟩. ⟨tel-01746184⟩

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