Pierre Monchoux

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

Abstract 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.

First field-scale occurrence of Si-Al-Na–rich low-degree partial melts from the upper mantle

Felsic dikes intruding an orogenic lherzolite body from the North Pyrenean zone (France) show great compositional similarities to some of the Si-Al-Na(K)–rich glasses found as microscopic inclusions in peridotite xenoliths. As shown by Sr-Nd isotopes, these silicic magmas originated from a mantle reservoir that was moderately depleted on a time- integrated basis. Their Si-Al-Na–rich bulk compositions match closely those predicted by experimental studies relevant to near-solidus melting of mantle peridotite. Specifically, the dikes are closely similar to low-degree melts from sodic harzburgite. On this basis, an olivine-orthopyroxene source that had been metasomatized by addition of sodic-carbonatite melts is inferred, as also suggested by peculiar trace element features (extreme fractionation of the light rare earth elements, high Nb contents). This new occurrence lends support to the existence of high-silica liquids in the mantle and provides evidence for their efficient extraction to form relatively large batches.

Métagabbros amphiboliques et mise en place crustale des lherzolites des Pyrénées (France)

Slices of ultramafic rocks from the subcontinental Pyrenean mantle, emplaced in the north Pyrenean zone, mainly consist of layered lherzolites and anhydrous pyroxenites. In the lherzolite massifs of the Ariege, the layering is cross-cut by dykes of amphibole-pyroxenites and hornblendites, interpreted as mineral segregates of Cretaceous alkaline magmas under mantle conditions. In the central and western Pyrenees, the lherzolites contain dykes of amphibole metagabbros. Petrological and geochronological characteristics of these rocks show that they crystallized from alkaline liquids, possibly linked to the Cretaceous magmatism. These metagabbros therefore represent a stage in the emplacement of the ultramafic bodies within the crust.