François Guillot

Âge Paléozoïque inférieur (U—Pb sur zircon) de métagranophyres de la nappe du Grand-Saint-Bernard (zona interna, vallée d'Aoste, Italie)

Abstract Early Paleozoic U—Pb age of zircons from metagranophyres of the Grand-Saint-Bernard Nappe (zona interna, Aosta Valley, Italy). The U—Pb age of a metagranophyre from the Val de Rhemes (Brianconnais basement of the Aosta valley) of 511 ± 9 Ma suggests that the micaschists which dominate the pre-alpine basement of the Grand-Saint-Bernard Nappe are, in part, of Early Paleozoic age. The Brianconnais domain is characterized by a Cambrian to Ordovician alkaline to subalkaline (monzonitic) magmatism, similar in age to orthogneisses known in other parts of the Variscan belt and generally interpreted as related to an extensional event.

Late- and post-Variscan evolution of the Ardennes in France and Belgium: constraints from apatite fission-track data

Abstract Apatite fission-track (AFT) analyses were performed on 13 Late Palaeozoic samples in order to unravel the late- to post-Variscan evolution of the Ardennes. The dated AFT ages cover a range from 290±33 Ma to 168±12 Ma, and the mean confined track lengths correspond to a unimodal distribution, with means varying between 13.1±0.1 µm and 11.7±0.3 µm. These ages for the sedimentary rocks are clearly younger than the respective stratigraphic ages, indicative of a cooling through the apatite partial annealing zone after post-depositional complete annealing. All available AFT data (290–146 Ma) from this region might be classified as three groups, that is 290–229 Ma, 218–198 Ma and 190–146 Ma, at least in correlation with three exhumation events. Using an inverse model, four major cooling episodes are identified from the modelled temperature–time (T–t) paths. The first rapid cooling (4.2–5.4 °C Ma−1, 320–300 Ma) corresponds to the late-Variscan rapid thrusting that ceased at about 300 Ma. The second cooling episode (0.2–4.0 °C Ma−1, up to 230 Ma) activated differentially, and was probably controlled by the post-Variscan transtension. The third cooling regime (0.1–0.3 °C Ma−1, 230–45 Ma) in the Ardennes Allochthon is slow, and represents a long-term and slow exhumation. In the Brabant Parautochthon, however, it is subdivided into 0.7 °C Ma−1 (225–110 Ma) and 0.2 °C Ma−1 (110–45 Ma). The last accelerated cooling (0.7–1.1 °C Ma−1, since 45 Ma) that affected the whole Ardennes is associated with a south–north compression during the Pyrenean phase.