Evolution lithostructurale et géochronologie de la chaine panafricaine du nord Cameroun : cas du secteur de Boula Ibi

Abstract

The work presented in this thesis is based on petrography, structural analysis, geochemistry and geochronology. It aims to characterize the geodynamic evolution in the Boula Ibi area, in the Northern Domain of the Central African Pan-African Belt (CAPB) in Cameroon. This area is mainly constituted of interbedded metamorphic formations, deformed and undeformed plutonic formations.The metamorphic formations consist of various gneisses, metadiorites and amphibolites. The mineralogical parageneses with Amp+Px+Pl+Bt+Qtz in the metadiorites, Amp+Px+Pl+Bt+Grt+Qtz in the amphibolites, Amp+Pl+Fk+Qtz±Grt+Bt and Ep+Qtz+Chlin the biotite gneisses show that these rocks have undergone HT amphibolite metamorphic facies and retrograde greenschist metamorphic facies. The deformed plutonic formations are represented by monzonites and quartz monzonites. They have an oriented heterogranular texture with a Qtz+Fk+Pl+Bt+Amp mineralogical assemblage, while the undeformed plutonic formations have a coarse grain to porphyroid coarse grain texture with Qtz+Fk+Amp+Bt+Pl±Chl±Cpx mineralogical assemblages for monzodiorites and Qtz+Fk+Pl+Amp+Bt for granites and syenites. The deformed rocks bear the marks of three phases of ductile deformation noted D1, D2 and D3. D1 can only be observed on a few outcrops of the host rocks. D2, the most marked, is regional in extent and can be observed on host rocks and deformed plutonic rocks. Field observations and data analysis show that this phase is dextral transpressive. The convergence of the geometry and kinematics of the D2 structures in the metamorphic formations and the deformed plutonic rocks argues for a synchronous setting of the latter with D2. D3 phase is characterized by the presence of C3 shears and P3 folds which, intersect the D2 structures and whose manifestations are not observed in undeformed plutonic rocks. The plutonic rocks studied are calc-alkaline, hyper potassic, metaluminous, I-type and magnesian dominant. They are formed by fractional crystallization and partial melting from a meta-igneous to intermediate source, and belong to the syn-collisional granitoids and volcanic arc granitoids domains. U/Pb geochronological data on zircon give ages of 682±10 Ma for deformed plutonic rocks, 625±36 Ma for monzodiorites, and 581±15 Ma for undeformed granites. These ages interpreted as emplacement ages suggest that D2 was already active around 682 Ma in the Boula Ibi area.