Tectonic setting and petrogenesis of Alibori and Yarra plutons in the Kandi Shear Zone, Pan-African Dahomeyide orogen (Northern Benin Republic)

The Alibori and Yarra plutons, located in the Sinende area, are spatially associated with the Kandi shear zone in the Pan-African Dahomeyide orogenic belt of northern Benin. This study aims to characterize the tectonic setting and magmatic processes of these plutons. The methodological approach integrates structural investigations of the host rocks, and petrographic, U–Pb dating and whole-rock geochemical analyses of igneous rocks. The Alibori pluton is formed of sheared diorite and granodiorite while the Yarra pluton consists of porphyritic, coarse-grained biotite and biotite-hornblende granites with magmatic texture. The Alibori pluton yielded a U-Pb zircon age of 661 ± 1.66 Ma, interpreted as the emplacement age. Geochemical investigations reveal that the igneous rocks of the Alibori and Yarra plutons are metaluminous. The Alibori pluton is magnesian, calcic to calc-alkalic, while the Yarra pluton belongs to the ferroan, alkali-calcic, strongly potassic series. Trace and rare earth elements (REE) investigations reveal that the Alibori igneous rocks originate from mantle magma with crustal contamination in a forearc environment, while the Yarra granitoids originate from the mixing and mingling of crustal and mantle magmas in a post-collision setting. The combination of these data and the existing data suggest that the magmatic event in the Sinende area can be divided into two episodes, which can be temporally located relative to the main regional shearing. The first one emplaced during the convergent tectonism is calcic to calk-alkalic (c. 660Ma) and derived from mantle magma with crustal contamination is interpreted as pre-shearing magmatism. The second one is alkali-calcic strongly potassic (c. 545 Ma) and intruded during the Kandi shearing activity towards the waning stage of the orogeny and involving both crustal and mantle magmas.