Upper Stephanian volcanism and sedimentation in the French Massif Central (France): A high resolution ion microprobe U-Th-Pb study of volcanic tuffs and its tectonic significance

Post-convergence evolution of the Variscan belt is characterized by the development of numerous intramontane coal-bearing basins containing volcano-sedimentary successions. As these basins are widely distributed in the whole Variscan belt, they represent important tectonic markers that can be used to bracket the phases of extensional tectonics affecting basement country rocks. A series of five volcanic ash layers interbedded in stephanian sedimentary basins from the southern part of the French Massif Central (France) have been studied by high resolution 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 which is contemporaneous with volcanic events identified in other parts of the Variscan Belt. This suggests magma generation by orogenic belt scale phenomenoms. Inherited zircons were identified in two out of the five dated tuff horizons and support a model involving a anatexis of basement source rocks with ages of ca. 2400 Ma (Jaujac basin), 1900 and 340 Ma (Graissessac basin). One possible xenocrystic grain, ca. 600 Ma old was also detected in zircons from the Graissessac bentonite. The Proterozoic components indicate a Gondwanan affinity for the deep seated material. Apatite concentrates and single zircon grains, analysed for their chemical composition, further indicate magma generation from the continental crust and, at least for the Jaujac basin, with participation of a mafic component. Contemporaneous eruption of mafic and silicic magmas is thought to be related to replenishment of magmatic chambers at depth by influx of mafic, mantle-derived, magmas triggering the upper Stephanian flare-up. This may be consistent with a model of delamination of the lithospheric mantle and a possible slab break-off during large scale strike-slip faulting accompanying clokwise rotation of Gondwana at the Carboniferous/Permian boundary.