Tectonique, géochronologie et pétrogenèse du massif de Bapé et de son encaissant (Bafia, centre - Cameroun) : implications géodynamiques pour l’evolution crustale de la bordure sud du bloc Adamaoua-Yadé

Abstract

The present work focuses on the crustal evolution of the Bafia Sector, located in the southern part of the Central Domain or Adamawa-Yadé bloc in Cameroon. To achieve this goal, a multidisciplinary approach, combining petrographic, structural, geochemical and geochronological (U/Pb on zircon, Sm/Nd, Rb/Sr) methods, are used to study the Bape massif and its host basement. The Bafia area corresponds to an orthoderived Archean (2.9-2.5 Ga) cratonic nucleus that consist of migmatitic gneiss having TTG composition, and which undergone a polycyclic and polyphased tectono-metamorphic evolution. This nucleus was affected by a Paleoproterozoic high grade metamorphism (2.08-2.07 Ga), followed by sedimentation, then magmatism and metamorphism with the emplacement of metasediments and the Bape massif. It is partially, covered by a Neoproterozoic metasedimentary sequence, which correspond to a mixture of arkose, with intercalated metagranites and rhyolitic volcanites, deposited in an active continental margin environment. The set was intruded between 0.64 and 0.63 Ga by the Bapé Massif, which emplaced under high-grade amphibolite facies metamorphism, leading to hyperpotassic and shoshonitic orthogneissified calc alkaline, alcali-calcic granitoids of type I, ferriferous, metaluminous to peraluminous (syenites, monzonites, quartz monzodiorites, quartz monzonites and granodiorites) associated with gabbro and granites veins. Isotopic data Rb-Sr, Sm/Nd and U-Pb on zircon revealed that the granitoids derived from a mantle origin and generated by partial melting of (i) a subducting crust associated with sediments and (ii) a metasomatised pinched mantle, in a subduction context that culminated in collision. The associated gabbros are magnesian and rich in alkaline elements, corresponding to orogenic calco-alkaline mantellic rocks. Four deformational phases D1, D2, D3 and D4 are identified in the pluton and its immediate bedrock. D1, identified only in the Neoproterozoic host rock, has transposed the original metasedimentary stratification S0 into a S0/S1 foliation. The internal structures of the massif, in disharmony with those of the host country rocks, suggest that the granitoids emplaced during D2, which corresponds to a horizontal general flattening in a pure shear regime. It is caracterised by a S1/S2 foliation in the bedrock and a primary penetrative and non-composite S2 foliation in the granitoids; stretching (Le2) and mineral (Lm2) lineations mainly plugging towards the NNE or SSW direction; isoclinal asymmetric or symmetric intrafolial P2 folds; a double perpendicular boudinage displaying a NE-SW maximum longitudinal stretching and a NW-SE minimal transversal elongation. D3 is a constriction resulting from a declining tectonic regime. This transcurrent regime is characterized by (i) transversal (P3t) et longitudianal (P3l) folds which result from a double perpendicular and horizontal E-W and N-S shortening that reflect a radial disposition of main stresses (ϭ1 and ϭ3 are horizontal, and ϭ2 vertical); (ii) dextral and senestral C3 shearing planes and (iii) F3 declining faults, compatible with the Riedel system where the main ENE-WSW direction is parallel to the Sanaga fault. This kinematic, associated with C/S structures, indicates a syn-shearing emplacement context. D4 is a relaxation phase of the stresses giving rise to gabbroic and granitic veins intrusions, which could constitute potential metallotects.These results highlight (1) a monoclyclic tecto-metamorphic evolution (Panafrican) for the Bape massif compared to its metamorphic polycyclic basement (Archean, Paleoproterozoic, Mesoproterozoic and Panafrican). (2) They also indicated that the Bafia group (whose Eastern, Western and Southern boundaries remain to be determined) belongs to the Adamawa-Yadé domain. (3) The latter suggests that the Adamawa-Yadé domain and the Archean Congo craton formed a single block (same Archean and Paleoproterozoic crust), which dislocated during Neoproterozoic (1100 -625 Ma) during the formation of the Yaoundé proto-oceanic basin, followed by its subduction beneath the Adamawa-Yadé cratonic bloc. (4) This subduction-collision induced the emplacement of neoproterozoic calco-alkaline, hyperpotassic to shoshonitic magmatic intrusions along the subduction front, from Bafia right to the Adamawa. It also explains the coexistence of Archean, Paleoproterozoic and Neoproterozoic tectono-metamorphic structures in the Adamawa-Yadé domaine. (5) Overall, this complex evolution of the CAFB in Cameroon, similar to that of the Zambezi and Lufilian belts and the Borborema province in NE Brazil spotlights the geodynamic implication of the main results on the chronostratigraphy of the crust north of the Congo craton, from Archean up to the neoproterozoic time.