This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the southern Ouaddaï massif, located at the southern margin of the Saharan metacraton. This area is made of low-grade metasediments intruded by large felsic batholiths. The oldest but not dated magmatic activity is represented by amphibolites intercalated within silicilastic metasediments. They are metagabbros characterized by high Mg#, flat, LREE-depleted patterns, and superchondritic εNd, suggesting derivation of the parental basaltic magma from a depleted, probably asthenospheric mantle source. Subsequently, sediments of southern Ouaddaï were intruded by large composite peraluminous meta-granites. U-Pb zircon dating yield ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite and a muscovite-leucogranite, respectively. These plutonic rocks contain recycled Archean and Paleoproterozoic zircon cores and are interpreted as the result of partial melting of metasediments. The last magmatic event is marked by the emplacement of granodiorite, biotite-granite and pyroxene-monzonite defining a metaluminous, high-K calc-alkaline to shoshonitic suite with a trace-element signature similar to calc-alkaline post-collisional series. A biotite-granite of this suite contains zircon grains that yield an age of 540 ± 5 Ma with inherited cores crystallized around 1050 Ma. These intermediate to felsic rocks are interpreted to derive from the differentiation of basic to intermediate magmas generated in an enriched mantle that has been affected by partial melting at the end of the Pan-African orogeny. The Mesoproterozoic zircon cores might reflect the presence of a previously unrecognized Mesoproterozoic basement near southern Ouaddaï.
These new data show that the southern Ouaddaï is dominated by Ediacarian to lowermost Cambrian magmatic suites intrusive in a siliciclastic metasedimentary sequence. Amphibolites probably record Rodinia breakup or the development of a back-arc basin before 635 Ma. Early Ediacarian (635-613 Ma) deformed anatectic leucogranites correlate to contemporaneous migmatites and leucogranites in Cameroon and Sudan emplaced during the main Pan-African tectonic-metamorphic event leading to pervasive melting in the deep crust. The end of the Pan-African orogeny is marked, as in the whole Central Africa Orogenic belt, by a high-K magmatic event corresponding to the post-collisional Pan-African stage.
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