Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon

Abstract

Ophiolites are the remnants of ancient oceanic crust that can serve as tools to reconstruct the evolutionary history of fossil oceans. At Boumnyebel, the western part of the Yaoundé group, the sole of the lower unit (magnesian mica-schists and gneiss) of the Pan-African nappes is made up of ophiolitic rocks (talc schists, chlorite schists, serpentinites, amphibolites, pyroxenites, hornblendites). Structurally, the lower unit has a flat-lying schistosity with some recumbent folds while the underlying TTG gneisses are marked by a high-dipping foliation at Mapan. The contact between the Pan-African mica schists and the basement formations (Congo craton) is therefore discordant. In ophiolitic rocks, amphibole is actinolite, tremolite or magnesio-riebeckite. Chlorite compositions correspond to clinochlore. Talc shows notable substitution of Mg by Fe whereas substitution of Mg by Fe and Al prevails in serpentine. Spinel compositions are either Cr-rich with high FeO/Fe2O3 values (peridotitic spinels) or Cr-poor with low FeO/Fe2O3 values (melt-derived or magmatic spinels). Main mineral features indicate a metamorphic retromorphosis of the original lherzolites, harzburgites, dunites and pyroxenites to hornblendites and serpentinites. This is followed by hydrothermal alteration (at various temperatures: 250–570 °C and low pressure: < 2 kbar) that yielded chlorite schists and talc schists. Chemically, talc schists are made up of SiO2, MgO and minor FeO and Al2O3, while chlorite schists and hornblendites contain notable amounts of Al2O3 and CaO, respectively. All the ultramafic rocks display the same range of high Ni, Cr, Co contents with various LREE enrichments but flat HREE patterns. Multi-element patterns show that most talc schist are depleted in almost all elements. All the ultramafic rocks are marked by deep Nb, Ta, K, Pb, Sr, Ti, Zr and Hf troughs suggesting that the ultramafic rocks possibly derived from heterogeneous mantle sources varying from fertile to refractory peridotites. Amphibolites are generated from N-MORB, IAT or CA geotectonic settings. The compositions of spinel and ultramafic whole rocks reveal an arc setting that has evolved from divergent plate setting (oceanisation, N-MORB) to convergent plate (IAT, CA, SSZ D-MORB) settings. In summary, ophiolites emplaced in various geotectonic settings and were amalgamated during the Pan-African collision.