HIGH-PRESSURE DEHYDRATION MELTING OF METAPELITES - EVIDENCE FROM THE MIGMATITES OF YAOUNDE (CAMEROON)

The migmatites of Yaoundé consist essentially of anatectic metapelitic kyanite-garnet gneisses characterized by granulite-facies mineral assemblages. Several types of migmatitic rocks have been recognized: (1) leucosomes associated with garnet-rich melanosomes, conformable with the regional metamorphic layering; some leucosomes are granitic in composition whereas some others are granodioritic and characterized by low K and Rb and by the lack of HREE fractionation; (2) quartzo-feldspathic differentiations without the related melanosomes, occurring as veins conformable with or cross-cutting the regional metamorphic layering or along shear-zones, and corresponding mineralogi-cally to granitic or quartz-rich véins; (3) garnet-rocks mainly composed of garnet with abundant accessories, occurring as intrusive bodies within the migmatitic series.Structural and petrographic data suggest that the migmatites are not derived from the surrounding granulite-facies gneisses but that both types of rock result from a single dehydration melting event. The formation of migmatites or gneisses, interpreted in terms either of absence of melt extraction or of shear-induced melt segregation, is ascribed to variations in strain distribution within the metamorphic pile.The chemical characteristics of the rocks and petrogenetic modelling suggest that the migmatites of Yaounde arose from the superimposition of the following events: (1) subsolidus differentiation of biotite-gneisses; (2) dehydration melting of biotite-gneisses at temperatures around 800°C (P=10-12 kbX leading to low amounts of melt (F<0·2), which was either tectonically segregated (migmatites) or not (granulite-facies gneisses); (3) injection of anatectic material comprising both partial melts and garnet-rich residues, corresponding to high melt fractions (F>0·5) and probably formed at higher temperatures (850°C) and at deeper structural levels. The REE signature of equilibrium partial melts (9·3<CeN/YbN78; l·2<YbN<5·4) indicates that granitic magmas cannot be derived from dehydration melting of biotite-bearing metapelites only. Several other possibilities are discussed. © 1990 Oxford University Press.