Evolution tectono-métamorphique des formations précambriennes du secteur Eseka -Minlongo (région du sud-Cameroun)

Abstract

Eseka-Minlongo area belongs to the cratonic domain (Nyong Unit). Detailed petrography, geochemistry, geochronology, structural analysis and a regional correlation with the Ogooué belt in Gabon, CAR and the São Francisco craton highlight its main distinctive features and its tectono-metamorphic evolution. The study area consists of: (i) meta-ultramafic composed of serpentinites with olivine and pyroxene remnants, tremolite serpentinites, pyroxenites and talcshists; (ii) metamafiques constituted of coronitic, symplectic and pyrigarnites sensu stricto; and (iii) metafelsics composed of garnet pyroxene gneisses, biotite epidote gneisses, garnet biotite gneisses and epidote amphibolites. Zircon from meta-ultramafic rocks yielded Archean age of 2752 ± 26 Ma involved in metamorphic event of Paleoproterozoic age (1738 ± 170 Ma). Zircons from metamafic rocks yielded Paleoproterozoic age (2086 ± 26Ma) of tholeiitic affinity compatible with a distensive environment and Archean metafelsiques (2695 ± 10Ma) of calc-alkaline affinity and characteristic of compressive organic domains. The study area shows a polycyclic, plurifacial, polyphase tectonic and metamorphic evolution marked by two main deformation phases. These events are associated with recrystallizations phases pre-D1 and D1. The –pre- D1 phases are Archean and correspond to the relics of schistosity prior to S1 schistosity. They are marked in pyrigarnites by the oldest associations that are preserved as inclusions in garnet and pyroxene porphyroblasts; while in serpentinites, they are found in primary minerals (olivine and pyroxene). Tectonic event D1 corresponds to a tangential tectonic phase which would have started during the Paleoproterozoic. It includes a sub-horizontal mylonitic foliation S1 with NE-SW to NNESSW direction. This orientation is similar to the rocher du Loup shearing (NNE-SSW)s. The second deformation phase D2 is shearing. The pre-D1 and D1 recrystallizations show that rocks from Eseka-Minlongo area have marks of an ancient (Archean) history, corresponding to a complete metamorphic cycle pre-D1, underlined by a Paleoproterozoic granulitic - eclogitic metamorphism (942-1002°C, 15-18 Kbar). The high TiO2 and Cr contents suggest magma generated in a subduction environment. Thus, the Nyong unit in Cameroon represents a Paleoproterozoic suture zone between the Congo craton and the São Francisco craton.