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 Pb-206/U-238 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 (Janjac 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. (C) 2003 Elsevier B.V. All rights reserved.