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 O. Bruguier a, * , J.F. Becq-Giraudon b , M. Champenois c , E. Deloule c , J. Ludden c , D. Mangin c a Service ICP-MS, cc 056, ISTEEM, Universite ´ de Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France b BRGM, 3 Avenue C. Guillemin, BP 6009, 45 060 Orle ´ans, France c CRPG, 15 rue Notre Dame des Pauvres, 54 000 Vandoeuvre-les-Nancy, France Accepted 7 August 2003 Abstract 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 206 Pb/ 238 U ages for the five studied tuffs are indistinguishable at the 95% confidence level and range from 295.5 F 5.1 Ma (Graissessac) to 297.9 F 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. D 2003 Elsevier B.V. All rights reserved. Keywords: Stephanian basins; Ash-fall tuffs; Zircon; Apatite; French Massif Central; Variscan orogen 1. Introduction Extensional tectonics is preferentially located along orogenic belts with a thickened crust and is an important feature of post-collisional orogenic 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.08.005 * Corresponding author. E-mail address: bruguier@dstu.univ-montp2.fr (O. Bruguier). www.elsevier.com/locate/chemgeo Chemical Geology 201 (2003) 319 – 336