Céline Ducassou

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

Relief variation and erosion of the Variscan belt: detrital geochronology of the Palaeozoic sediments from the Mauges Unit (Armorican Massif, France)

Abstract The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying the evolution of relief during Palaeozoic time. U–Pb dating on zircon and 39Ar/40Ar on white mica are used to constrain the age and nature of the sources. The first relief identified is marked by an Early Devonian unconformity interpreted as the opening of a northern back-arc basin. Detrital minerals are first reworked from underlying layers indicating a local supply. Magmatic zircons at c. 400 Ma then record the emergence of a magmatic arc. During the Middle Devonian, the gap in the sedimentary record is attributed to an emersion followed by the disappearance of the relief during the Late Devonian. At the Devonian–Carboniferous boundary, the main collision is followed by the onset of a relief. The continental sedimentation in the Ancenis Basin (late Tournaisian–Viséan) is a coarsening-upwards megasequence indicating an increasing and/or approaching relief. The detrital minerals record the progressive exhumation of Variscan metamorphic (mica at c. 350 Ma) and magmatic rocks (zircons at c. 390–340 Ma). The Serpukhovian–Bashkirian sedimentation records the erosion of a proximal metamorphic source (Champtoceaux with micas at c. 350–340 Ma) showing a much shorter drainage system. Supplementary material: Sample coordinates and U–Pb on zircon and 39Ar/40Ar on white mica analyses are available at http://www.geolsoc.org.uk/SUP18730.

The early land plants from the Armorican Massif: sedimentological and palynological considerations on age and environment

The Châteaupanne Unit belongs to the South Armorican domain of the Armorican Massif (France), which is part of the Variscan belt. This unit includes two Lower Devonian plant levels and one of them corresponds to the Basal Member of the Chalonnes Formation. A sedimentological and palaeontological analysis of these fossiliferous deposits from the Châteaupanne quarry (Montjean/Loire, Maine et Loire, France) is presented here for the first time. The age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the Armorican Massif. Their presence suggests an emergence event that has never been described before. Our study highlights the promising potential of the Basal Member of the Chalonnes Formation to aid in understanding these occurrences, and provides new insights into the history of the Variscan belt.