Relationships between deformation and magmatism in the Pan-African Kandi Shear Zone: Microstructural and AMS studies of Ediacaran granitoid intrusions in central Bénin (West Africa)

• Time and space relationships between magmatism and tectonics are observed in the KSZ.
• High-K calk-alkaline magmatic event (Dassa) predates the transcurrent tectonics.
• Most of the KSZ intrusions (Tré, Gobada, Tchetti and Fita) are syn-kinematic.
• The succession of high-K calk-alkaline evolves toward alkaline magmas (Fita).
• The KSZ in Bénin presents similarities with the Northeast Brazil shear zones.

Relationships between the metamorphic basement, granitic intrusions and the Kandi Shear Zone (KSZ) in central Bénin have been investigated using petrological and structural approaches, in order to better understand the space and time parameters of the Pan-African shear deformation and the Ediacaran magmatism. In central Bénin, metamorphic rocks from the KSZ display a steep to vertical N–S trending foliation, a sub-horizontal mineral lineation together with kinematic indicators in agreement with a dextral transcurrent mega-shear zone. Four granitic intrusions (Dassa, Tré, Gobada and Tchetti) show many petrological similarities. They are biotite ± amphibole – ilmenite ± magnetite monzogranites with ferrous and metaluminous I-type features derived from high-K calk-alkaline magma. A fifth intrusion (Fita) is an alkali-feldspar, biotite, magnetite and ilmenite bearing granite crystallized from an alkaline magma. Moreover, high K2O, Zr, Y, Nb and low CaO, MgO and Al2O3 contents together with high (FeOt/MgO) and low LIL/HFS elements ratios suggesting an A-type granite affinity.Microstructural and AMS investigations presented in this paper show (i) solid-state deformation evidence for Dassa pluton and (ii) a magmatic deformation for the Tré, Tchetti, Gobada and Fita granitoids. Foliation in Dassa is parallel to the mesoscopic planar mylonitic foliation of the metamorphic basement. In the Tré, Tchetti, Gobada and Fita granitoids, magmatic textures and magnetic fabrics are coherent with the KSZ activity. These data suggest (i) a syn-kinematic nature for most of the intrusions (Tré, Gobada, Tchetti and Fita), except Dassa which correspond to an earlier event (ii) the succession of high-K calk-alkaline (Dassa, Tré, Gobada, Tchetti) evolves toward alkaline magmas (Fita) during the KSZ strike-slip tectonics. These observations highlight the changing nature of magma composition, magmatic processes and the different sources during KSZ activity in the Bénin Nigerian Shield. These new results suggest that the previous geodynamic interpretations, which assume a post-tectonic emplacement for the Gobada intrusion or an active continental margin setting for most of the late Pan-African (Ediacaran) granites intruded in central Bénin, has to be re-examined.