Application of in situ zircon geochronology and accessory phase chemistry to constraining basin development during post-collisional extension:A case study from the French Massif Central

International audienceA series of five volcanic ash layers interbedded in Late Carboniferous sedimentary basins from the southern part of the French Massif Central (FMC, France) have been studied by ion-microprobe analyses of zircons in order to constrain the age of basin formation and sedimentation. Weighted mean 206Pb/238U ages for the five studied tuffs are indistinguishable at the 95% confidence level and range from 295.5±5.1 Ma (Graissessac) to 297.9±5.1 Ma (Roujan–Neffies). These U–Pb ages support the argument for intense magmatic activity in the southern part of the French Massif Central during the period 295–300 Ma. Inherited zircons were identified in two out of the five dated tuff horizons and indicate a anatexis of basement source rocks with ages of ca. 2400 (Jaujac basin), 1900 and 600 Ma (Graissessac basin). The Proterozoic components suggest a Gondwanan affinity for the deep-seated material. Chemical compositions of apatites and of one single zircon grain from the Roujan–Neffies bentonite further indicate magma generation mainly from anatexis of the continental crust and a rhyolitic affiliation. Conversely, the same minerals extracted from the Jaujac bentonite indicate involvement of a mantle component in the source of the magmas and a trachytic affiliation. The 295–300 Ma volcanic episode in the French Massif Central is contemporaneous with volcanic events identified in other parts of the Variscan Belt which suggests it was triggered by orogen-wide processes. Contemporaneous eruption of trachytic and rhyolitic magmas may be related to replenishment of magma chambers at depth by influx of mantle-derived magmas triggering the Late Carboniferous flare-up

Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U-Pb dates from migmatite-hosted eclogites? (Montagne Noire, France)

International audienceEclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) have an oceanic protolith and recorded a prograde P–T path from ~19.5 kbar, 700°C to the pressure peak at ~21 kbar, 750°C (pseudosection modelling), suggesting metamorphism in a subductional setting. Subsequent exhumation to low-pressure high-temperature (LP–HT) conditions (~6 kbar, 730°C) is constrained by the equilibration conditions of the embedding migmatite. In samples of a fresh and a retrogressed eclogite, all zircon crystals but one display similar REE patterns (no Eu anomaly, flat HREE), usually ascribed to crystallisation under eclogite-facies conditions. Yet, the U-Pb apparent ages of zircon crystals from both eclogites spread from c. 360 Ma to a dominant data cluster at c. 315 Ma. The c. 315-310 Ma zircon U-Pb dates obtained from the embedding migmatite are interpreted as the age of crystallisation of the partial melt during the LP–HT metamorphic stage. First-order geological evidence, in particular the sedimentary record, excludes the existence of a subduction zone in the region at this period. Unless calling upon a major reappraisal of the tectonics of the European Variscan orogen, this suggests an ambiguous relation between REE patterns and U-Pb dates in the zircon population. Various scenarios that could account for the observations are discussed. Confronting our data with the results of a previously published Sm-Nd dating of garnet, and regional considerations, we consider that 360 Ma is the best approximation of the minimum age of the eclogite-facies event. We hypothesise that the eclogites formed farther north and were transferred to their present location by lower-crustal flow. It is inferred that during the subsequent exhumation, eclogite-facies zircon grains recrystallised and underwent partial to total resetting of their U-Pb system, whereas the REE system remained mostly unmodified. These results caution against the use of REE patterns as the only criterion to associate a specific zircon age with HP metamorphism in eclogites occurring in migmatitic domes