Low-potassium vaugnerites from Guéret (Massif Central, France). Mafic magma evolution influenced by contemporaneous granitoidsVaugnérites á faible teneur en potassium du massif de Guéret (Massif central français). Evolution de magmas mafiques influencée par granitoöes contemporains

The Guéret massif (Massif Central, France) is mainly composed of granitoids that define an alumino-potassic association. An age of 356 ± 10 Ma is accepted for the intrusion of these granitoids for which a pure crustal origin has been suggested. However, the presence of mafic magmatic enclaves and of scattered stocks and dykes of amphibole-biotite mafic rocks also point to contribution of mantle-derived magmas in their genesis. From field relationships and radiometric data, the intrusion of the basic magmas is deduced to be quasi-simultaneous with the emplacement of granitoids. Petrologic, mineralogical and geochemical characteristics of the mafic rocks correspond to low-potassium vaugnerites or redwitzites. They are derived from a basic magma evolving by contamination with crustal components. However, pure magma mixing or assimilation by fractional crystallisation between mafic magmas and Guéret granitoids does not fit with the geochemical trends observed in both types of rocks. The positive correlation of M index [100 * MgO/(MgO +FeOt)] and K index [100 * K2O/(K2O + Na2O)] with respect to SiO2 in vaugnerites together with the positive correlation between compatible and incompatible trace-elements, rather favour the contamination of the basic magma by a wall rock assimilation process. However, dissimilar trends of vaugnerites and Guéret granitoids prevent to ascertain the role of basic magmas in granitoid genesis.