Origin and evolution of the late precambrian high-grade Yaounde Gneisses (Cameroon)

The Yaoundé series is composed of low to high-grade garnet-bearing schists and gneisses belonging to the Pan-African North-equatorial fold belt. The high-grade rocks consist of kyanite-garnet gneisses and garnet-plagioclase gneisses containing layers of calcsilicate rocks, marbles, quartzites and magnetite-rich orthopyroxenites. The chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite-rocks, dolomitic marls, evaporitic beds, quartzites and iron-rich sediments. Some volcanogenic greywackes may have been of alkaline affinity. This sequence was probably deposited in shallow-water, near-shore, semi-barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the Congo craton. The Yaoundé gneisses were deformed during two main tectonic phases: the first phase (D1) is marked by a layering (S0-S1) resulting from tectonic transposition; the second one (D2) corresponds to tangential tectonics with isoclinal folds and flat-lying shear zones associated with a S2 schistosity and a L2 lineation; a third phase (late-D2) is marked by the development of low temperature mylonitic shears. Scarce wrench faults indicate later E-W compression. The D1-D2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high-pressure granulite facies conditions (T=750-800°C, P=10-12 kbar). These peak metamorphic conditions triggered a widespread migmatization dated at 565±22 Ma and coeval with the D2 event. The gradual decrease of the metamorphic conditions toward the south and the existence of a syn-D2, retrograde, inverse metamorphic gradient, suggest that the overall structure of the Yaoundé series is a large tectonic nappe thrust onto the Congo craton. A geotectonic scheme for the North-equatorial fold belt is discussed. © 1988.