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Notice de la Carte géologique de France au 1/50000 : feuille de Fontenay le Comte (Vendée)
Abstract
carte géologique - publisher: Service de la Carte Géologique du BRGM
... La plupart des protolithes des orthogneiss des unités fortement déformées et métamorphiques ont été datés aux environs de 480 Ma (par ex. Peucat et al., 1978 ; Paquette et al., 1984 ; Jégouzo et al., 1986 ; Guerrot et al., 1997 ; Janjou et al., 1998 ; Béchennec et al., 2001 ; Ballèvre et al., 2002 ; Diot et al., 2007 ). De même, les porphyroïdes, ces métavolcanites porphyriques intensément schistosées connues de la Vendée au Morbihan (Belle-Île), que l'on croyait être siluriennes (Peucat et al., 1986), se sont avérées être elles aussi émises aux environs de 480 Ma (Bouton et Branger, 2007 ; Diot et al., 2007 ; Béchennec et al., 2008 Ballèvre et al., 2012). ...
... Peucat et al., 1978 ; Paquette et al., 1984 ; Jégouzo et al., 1986 ; Guerrot et al., 1997 ; Janjou et al., 1998 ; Béchennec et al., 2001 ; Ballèvre et al., 2002 ; Diot et al., 2007 ). De même, les porphyroïdes, ces métavolcanites porphyriques intensément schistosées connues de la Vendée au Morbihan (Belle-Île), que l'on croyait être siluriennes (Peucat et al., 1986), se sont avérées être elles aussi émises aux environs de 480 Ma (Bouton et Branger, 2007 ; Diot et al., 2007 ; Béchennec et al., 2008 Ballèvre et al., 2012). La diversité géochimique de ce magmatisme ordovicien (association de magmas calcoalcalins , alcalins, voire peralumineux) pourrait être source de confusion, induisant une multiplicité de modèles ou suggérant une multiplicité de sites géodynamiques contemporains, mais peut aussi relever d'une interprétation unitaire, selon laquelle la fusion mantellique induirait au sein de la croûte continentale des fusions étagées, lesquelles contamineraient à des degrés divers les magmas mantelliques. ...
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.
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.
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