GEOLOGICAL HISTORY OF THE MORVAN
Structure of the crystallophyllian basement in the northeastern part of the Massif Central at the end of the Medio-Variscan period
See geologic maps 1/80 000 Avallon, Château-Chinon, Charolles and Autun; 1/50 000 Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Autun and Le Creusot; explanatory notes Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Autun and Le Creusot.
About 400 Ma - Early Devonian
In this northern part of the Arverno-vosgian or Morvano-vosgian zone, the crystallophyllian basement structured at the end of the Medio-Variscan period, is composed of gneisses and migmatites. These metamorphic rocks are distributed in several nappes whose boundaries will be likely the preferential sites of the large crustal wrench faults later took by Early and Middle Carboniferous granitic intrusions. The eclogitic relics in Monts du Lyonnais [REF] and in Autunois [REF] allow to link these two domains to the Upper gneissic unit [REF]. Auxerrois, Chastellux and Affoux gneisses can belong to the Lower gneissic unit.
In the northern part of Morvan, crystallophyllian basement occurences are distributed in two structural units:
Chastellux gneisses extend between the Settons and Pierre-qui-Vire granitic massif and Sincey-lès-Rouvray stephanian syncline and are mostly of uniform composition, biotite gneiss ; anatectic gneisses are locally encountered , and also greenstones (roches vertes) (metamorphosed granodiorites) and in the Bussière area, leptynites intercalations .
granutitized gneisses [REF] extending between the Sincey-lès-Rouvray stephanian syncline and two micas Avallon granite to the North. Metamorphosed porphyroïd granite or orthogneiss occurs in this unit .
In the southern part of Morvan, the Autunois, the cristallophyllian basement occurs in several terranes enclosed in Mesvres leucogranite, the larger being biotite gneisses Montjeu .
Two km north from Marmagne, extends a succession of lenses of metamorphosed basic and ultrabasic rocks containing eclogitic relics .
On the southern edge of Autun basin occurs Morlet gneisses .
Sources for this diagrammatic representation of the gneissic basement are the 1/1 000 000 scale geologic map of France [REF] and the geophysical interpretation of the basement lying under the Bassin de Paris [REF]. Nappes disposition is took from deep structures disclosed along geophysic profils across the German Saxothuringian zone [REF] and from the Arverno-vosgian zone cross-section [REF].