Petrology and geochemistry of the Meshi rocks, eastern margin of the Central Cameroon Shear Zone

The Meshi area is located at eastern margin of the Central Cameroon Shear zone. Its petrography and petrogenesis is up to date not well known compared to the central part of the Central Cameroon Shear Zone. It is made up of amphibolite and granites host. Whole rock geochemical and electron microprobe analyses indicate that amphibolites dykes display a chemical characteristic of gabbroic diorite and diorite while granites present the chemical composition of granite. The silica content varies from 53.64 to56.94 wt% for amphibolites, and 69.00 to 73.87 wt% for granites. The amphibolites display moderate to high Mg# value (46–63), Ni (5.32–71.60 ppm), and variable Ba (701–1169 ppm) contents compared to the granites that present low Ni (4.32– 10.71 ppm) and Mg# value (35–45) and variable Ba (109–3685 ppm) content. The amphibolites and the host granites are high-k to calc-alkaline in nature and present negative to positive Eu anomalies (Eu/Eu* = 0.68–1.32 for amphibolite and 0.48–1.08 for the granites). The studied amphibolite dykes and granite hosts display enrichment in large ion lithophile elements (LILEs) such as Rb, Ba, Th and Pb and depletion in high field strength elements (HFSEs) such Nb, Ta and Ti. The trace element ratios Ba/Rb, Nb/Ta, Rb/Sr along with the amphibole chemistry, point to a felsic-mafic mixed source for the Meshi rocks. The partial melting of the subducting oceanic slab (amphibolite) and the mafic rocks from the lower part of the thickened crust (granite) perhaps triggered by slab break-up and upwelling of the asthenospheric heat may have produced ultrapotassic magmas forming the Meshi zone as it is the case in the neighboring areas in the Central Cameroon Shear Zone. The amphibolites melt may have followed the shear axes and sub-vertical faults to crystallize in the granite host already emplaced in a shallow crustal level at approximately 3.9 to 11.3 km depth at temperatures ranging from 800 to 914 °C. The variable water content (4.3 to 5.5%) of the magma may be due to incorporation of the slab with the overlaying sediments in the magma.