Calc-alkaline orthogneisses of the FoumbanBankim shear zone: witnesses of an internal zone of a Pan-African active margin

The mylonitic rocks of the Central Cameroonian Shear Zone (Foumban-Bankim sector) are mainly ancient magmatic rocks emplaced in an internal zone of a Pan-African active margin. They display calc-alkaline affinities with granitoids of northern Cameroon, but differ by their higher K contents and shoshonitic nature. This spatial distinction in pre- to syn-orogenic magmatism permits to define a north to south potassium increase trend, compatible with the existence of a northern Pan-African subduction zone. The shearing evolution of this margin is marked by the superposition of two mylonitic foliations and the occurrence of unusual delta-type porphyroclasts. This suggests the interference of two shearing phases operating in opposing sense at a constant direction, under deep and shallow metamorphic conditions, respectively.