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

A 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.