GEOCHEMISTRY OF BIMODAL AMPHIBOLITIC-FELSIC GNEISS COMPLEXES FROM EASTERN MASSIF-CENTRAL, FRANCE

High-grade basic and acidic meta-igneous rocks are widespread in the b imodal amphibolitic-felsic gneiss complexes, which are characteristic formations of the 'Middle Allochthonous Unit' from eastern and souther n French Massif Central. The metabasites from the Lyonnais and Doux co mplexes are chemically diverse and range from N-MORB type tholeiitic t o transitional types. The two populations are not related by fractiona l crystallization or crustal contamination processes and their chemica l characteristics reflect differences in their mantle sources. An ensi alic setting is supported by the crustally-derived character of some o f the associated felsic rocks, but the presence of N-MORB-type metabas ites argues for an extensional environment. This bimodal association c ompares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic c rust formation during the Cambro-Ordovician spreading event.