Geochemical and isotopic study of a norite-eclogite transition in the European Variscan belt: Implications for UPb zircon systematics in metabasic rocks

Three different geochronological techniques (UPb conventional multigrain, single zircon stepwise Pb-evaporation, and SmNd on minerals) were applied to rocks from a norite-eclogite transition observed in the West European Variscan belt and combined with major and trace element (including REE) geochemistry. The studied rocks belong to a cogenetic tholeiitic suite which originated in an immature back-arc basin or within-plate magmatic environment. The initial ϵNdT=480 Ma values around +1.2 imply that the noritic protoliths were emplaced in a geotectonic setting in close association with the continental crust. UPb mulfgrain and the SmNd dating of a norite sample indicate an age of 481 Ma for the basic magmatism. SmNd dating on two whole-rock-garnet pairs yields an age of 408 Ma for the eclogite-facies metamorphism. These two ages are consistent with the present understanding of the European Variscan belt. Finally, the single zircon Pb-evaporation study of an eclogite sample demonstrates the co-existence of several types of zircons. Each zircon population yields a particular type of geochronological information, related to the contrasting behaviours of the UPb isotopic system during the eclogite-facies tectonometamorphic event. These analytical results suggest that complex and heterogeneous behaviour within the UPb zircon system may also be recorded in numerous other samples. This last point raises the question of the significance of results obtained by UPb conventional multigrain dating in metabasic rocks.