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High-Pressure Dehydration Melting of Metapelites: Evidence from the Migmatites of Yaounde (Cameroon) |
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| Authors: | BARBEY P; MACAUDIERE J; NZENTI J P |
| Institution: | 1Laboratoire de P?trologie, Universit? de Nancy I BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France 2Centre de Recherches P?trographiques et G?ochimiques BP 20, 54501 Vandoeuvre-les-Nancy Cedex, France 3Ecole Nati?nale Sup?rieure de G?ologie BP 452, 54001 Nancy Cedex, France 4Centre de Recherche G?ologique et Mini?re BP 333, Garoua, Cameroun |
| Abstract: | The migmatites of Yaound? consist essentially of anatectic metapelitickyanite-garnet gneisses characterized by granulite-facies mineralassemblages. Several types of migmatitic rocks have been recognized:(1) leucosomes associated with garnet-rich melanosomes, conformablewith the regional metamorphic layering; some leucosomes aregranitic in composition whereas some others are granodioriticand characterized by low K and Rb and by the lack of HREE fractionation;(2) quartzo-feldspathic differentiations without the relatedmelanosomes, occurring as veins conformable with or cross-cuttingthe regional metamorphic layering or along shear-zones, andcorresponding mineralogi-cally to granitic or quartz-rich v?ins;(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 migmatitesare not derived from the surrounding granulite-facies gneissesbut that both types of rock result from a single dehydrationmelting event. The formation of migmatites or gneisses, interpretedin terms either of absence of melt extraction or of shear-inducedmelt segregation, is ascribed to variations in strain distributionwithin the metamorphic pile. The chemical characteristics of the rocks and petrogenetic modellingsuggest that the migmatites of Yaounde arose from the superimpositionof the following events: (1) subsolidus differentiation of biotite-gneisses;(2) dehydration melting of biotite-gneisses at temperaturesaround 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 anatecticmaterial comprising both partial melts and garnet-rich residues,corresponding to high melt fractions (F>0?5) and probablyformed at higher temperatures (850?C) and at deeper structurallevels. The REE signature of equilibrium partial melts (9?3<CeN/YbN78;l?2<YbN<5?4) indicates that granitic magmas cannot bederived from dehydration melting of biotite-bearing metapelitesonly. Several other possibilities are discussed. |
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