Petrogaphic and structural analyses of high-grade amphibolites from Fotouni-Kékem and Nyakong-Manyi shear zones: implications for the geodynamic significance of the Central Cameroon Shear Zone

Fotouni-Kékem shear zone (FKSZ) and the Nyakong-Manyi shear zone (NMSZ) are respectively located to southwest and northwest of the N50E branch of the central Cameroon shear zone (CCSZ). Three deformation phases are recorded in these shear zones including, D₁, D₂ and D₃. The D₁ phase, with σ₁ applied in the NE-SW direction, is remnant and poorly represented, whose structures (NW–SE S₁ foliation) were transposed by the late D₂ and D₃ phases related structures. The D₂ phase is an early sinistral shear phase, with σ₁ applied in the WNW-ESE direction, which developed NNW-SSE to NNE-SSW S₂ foliation, B₂ shear band boudins, F₂ knee-like folds and asymmetric fish-like structures. The D₃ phase is a NE-SW dextral mylonitic shear phase, with σ₁ applied in the NW–SE direction, responsible the development of S₃ foliation, P₃ recumbent and overturn folds, B₃ shear band boudins, σ-type sigmoids and asymmetric amphibole fishes. Pyroxene amphibolite (PA) occurs as slab stones, banded to lens-like, egg-like enclaves, folded bands, sheared and/or boudinaged green to dark green rocks displaying NE-SW preferred orientation. It displays heterogranular nematoblastic texture marked by amphibole (60%, hornblende), plagioclase (≈20%) and clinopyroxene (15 à 20%) porphyroblasts dispersed in between medium-grained mineral showing preferred orientation. Under microscope, PA evidenced a polyphasic prograde-peak-retrograde high-grade regional metamorphism. Prograde-peak phase is evidenced by primary mineral paragenesis (stable amphibole + pyroxene + plagioclase + K-feldspar) and microstructures, which indicate granulite facies. This occurs during the D₁ deformation phase. retromorphic relic-like pyroxene crystals displaying amphibole-plagioclase-quartz-opaque minerals assemblage, which follows the peak metamorphism, related to relaxation during D₂-D₃, evidence retrograde phase. Early sinistral syn-D₂ and late dextral syn-D₃ mylonitic events, whose microstructures evidence high-grade deformation setting, overprinted this regional metamorphism. This polyphasic activation of the CCSZ during these mylonitic events (D₂-D₃) stretched, sheared, folded dismembered and scattered amphibolites along the Pan-African mobile zone during late phases of the Pan-African orogeny. Geochemical data indicate that PA derives from mafic protoliths originating from a garnet lherzolite reservoir melting that was contaminated by both subducted sediment and slab-derived fluids as evidenced by the slightly positive ƐNd_600Ma (+ 1.27). The model age (T_DM = 1.25 Ga) with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70488 suggest an ancient Mesoproterozoic crust that underwent metamorphic transformation during the collisional (burial) and post collisional (exhumation) stapes of the Pan-African orogeny.