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, D 1 , D 2 and D 3 . The D 1 phase, with σ 1 applied in the NE-SW direction, is remnant and poorly represented, whose structures (NW–SE S 1 foliation) were transposed by the late D 2 and D 3 phases related structures. The D 2 phase is an early sinistral shear phase, with σ 1 applied in the WNW-ESE direction, which developed NNW-SSE to NNE-SSW S 2 foliation, B 2 shear band boudins, F 2 knee-like folds and asymmetric fish-like structures. The D 3 phase is a NE-SW dextral mylonitic shear phase, with σ 1 applied in the NW–SE direction, responsible the development of S 3 foliation, P 3 recumbent and overturn folds, B 3 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 D 1 deformation phase. retromorphic relic-like pyroxene crystals displaying amphibole-plagioclase-quartz-opaque minerals assemblage, which follows the peak metamorphism, related to relaxation during D 2 - D 3 , evidence retrograde phase. Early sinistral syn- D 2 and late dextral syn- D 3 mylonitic events, whose microstructures evidence high-grade deformation setting, overprinted this regional metamorphism. This polyphasic activation of the CCSZ during these mylonitic events ( D 2 - D 3 ) 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