Petrography and Geochemistry of the Letta Pan-African Plutonic and Metamorphic Rocks in Eastern Part of the Central African Fold Belt in Cameroon

This paper presents petrographic, structural, and geochemical data on metamorphic and magmatic rocks of the Letta area (eastern Cameroon), in the Central Cameroon domain, that lead to discuss the petrogenesis of these rocks in the context of the Pan-African orogeny. The study area is made of amphibolite, orthogneiss, and dioritic gneiss, and they are related to a various facies of plutons including granodiorite, biotite-granite, and alkali-feldspar granite. Field observations, structure, and texture support that amphibolite is an orthoderived rock, issued from the metamorphism of gabbro. Field relationships, microstructures, and geochemical signatures suggest that orthogneissess correspond to syntectonic intrusions transposed in the shear foliation, and their genesis involve a metasomatized mantle and/or crustal contamination. By contrast, the protolith of dioritic gneiss involves juvenile Neoproterozoic and reworked Archaean/Palaeoproterozoic materials, with the possible contribution of a Mesoproterozoic source. The high-K (7.31 wt%), shoshonitic affinity, and the high LILE/HFSE ratios and negative Nb, Ta, and Ti anomalies of dioritic gneiss are interpreted as indicating an origin of the protolith of this rock through interaction between the Archaean TTG continental crust and magmas derived from a LILE-enriched mantle. Granodiorite, by their medium Mg# (average = 37.54), geochemical features [(high Th/La (0.10–0.37), K/Rb and Rb/Sr (0.12–0.78)], and the presence of mafic enclave, indicate derivation of this magmatic rocks through partial melting of metamorphozed mafic rock (amphibolite) issued from the melting of metasomatized spinel lherzolite with a contribution of crustal material. Biotite granite is characterized by (a) the presence of enclave with mafic and felsic composition, (b) high-temperature zircon saturation, (c) I-type signature, (d) and high Zr/Sm ratio (37.34–60.29) point to an origin through partial melting of amphibolite and dioritic gneiss with contribution of sediments. Alkali-feldspar granite has features (positive Eu anomalies, enclave of felsic rock, and REE patterns) indicating that the magma of this rock is issued from the partial melting of dioritic gneiss with contribution of mantle-derived magma and was contaminated by sediments. The presence of the granodiorite enclave in alkali-feldspar granite suggests influence from magma mixing in their generation. The geochemical signature of studied rocks indicates that they were formed in magmatic arc developed further to subduction of the hot Yaounde oceanic crust beneath the Adamawa-Yade domain.