Structural analysis of the Banyo syenitic pluton: Implication for its synkinematic emplacement and correlations to Godé Gormaya and Rocher du Loup shear zones – Related Pan-African N–S syenites

Abstract

The Banyo Syenitic Pluton (BSP) is located on the south western extension of the Mayo Nolti Shear Zone (MNSZ). It is a NNE-SSW elliptical pluton. On the petrographic view, the BSP displays two rock types namely: hornblende-pyroxene-quartz syenite (HPQS) and hornblende-biotite granite (HBG), intruded in a plutono-metamorphic basement rock consists of biotite granite (BG) and hornblende-biotite gneiss (HBGn). Structural investigations indicate that the study area recorded three deformation phases: D₁, D₂ and D₃. D₁ is a flattening phase characterized by WNW-ESE to NNW-SSE (N110°E to N160°E) trending metamorphic foliation (S₁) with moderate (50°–60°) dips toward NNE to ENE. D₂ trends N–S and is characterized by crenulation cleavages (S₂), N–S sinistral shear and coeval S₂ foliation in hornblende-biotite gneiss. The emplacement of the HPQS and (HBG) took place during this phase considering the N–S global trend of the entire pluton and the NNE-SSW shape of the HPQS in the one hand and sinistral shear deformation microstructures display by plagioclase and K-feldspar crystals in HPQS. D₃ displays NE–SW (N30E to N45E) trend in HBGn and HPQS. Magnetic data indicate an inward-dipping NNE-SSW concentric pattern around station N32. The NNE-SSW elliptic shape and concentric magnetic foliation trajectories displayed by the BSP indicate its synkinematic emplacement during the D₂ N–S sinistral activation of the MNSZ with the feeder zone (station N32) located on the north eastern border. The sinistral activation is related to the Saharan Metacraton convergence over the Cameroon northern margin. This emplacement was disturbed by an overprinting E–W to NNW-SSE dextral syn-D₃ shear phase probably due to the dominant convergence (during its late stage) of the West African Craton over the Cameroon western border. Structural field data and magnetic fabrics infer that the Banyo syenitic pluton was emplaced in a N–S to NNE-SSW oriented fracture initiated during the transcurrent strike-slip MNSZ. The BSP is intruded in HBGn basement rock whose deformation ages are bracketed between 600 Ma (for the early syn-D₁ deformation structures) and 550 (for the late syn-D₃ deformation structures). The location of the BSP on the N–S to NNE-SSW sinistral MNSZ, between the GGSZ to the north and the RLSZ to the south, displaying similar structural features and N–S syn-to late kinematic syenitic plutons respectively dated at 593 Ma and 590 Ma help in dating the BSP pluton at 593 - 590 Ma. This age range dates several synkinematic emplacements of Pan-African younger syenite and granitoids along N–S and NE–SW shear zones during the western Gondwana post-collisional history.