GEOLOGICAL HISTORY OF THE MORVAN

Late Tournaisian - Early Visean

SUB-ALKALINE GRANITIC INTRUSIONS


See geologic maps 1/80 000 Avallon, Château-Chinon, Charolles and Autun; 1/50 000 Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Autun, Dompierre-sur-Besbre, Bourbon-Lancy, Fours and Lucenay-l'Evêque; explanatory notes Avallon, Quarré-les-Tombes, Sémur-en-Auxois, Corbigny, Autun, Le Creusot, Dompierre-sur-Besbre, Bourbon-Lancy, Fours and Lucenay-l'Evêque.


During this period large bodies of sub-alkaline granites emplace preferentially between the gneissic basement and the overlying Upper Devonian to Lower Carboniferous volcano-sedimentary cover. Arising from continental crust melting the sub-alkaline granitic magmas (gg) intrude in the weakness zones of the crust: large lateral wrench-faulting and overthrust planes generated during the Medio-Variscan period [REF].

The northern Settons batholith and the southern Luzy batholith show similar structure [REF], [REF]. The Settons batholith core is composed of porphyritic sub-alcaline biotite granite (Settons , Dommartin ), gradding northward and southward to aluminous biotite and cordierite granites (Lormes , Sermages). The Saint-Didier-sur-Arroux granodiorite extends in the Luzy batholith core; it is surrounded by the porphyritic biotite and amphibole Grury granite that grades to aluminous (biotite and cordierite) granites (Saint-Léger-sous-Beuvray,Uchon ). The sub-alcaline granites yield radiometric ages around 350-345 My [REF], [REF], [REF]. .A similar disposition occurs in the Saint-Léon horst where the Chatelperron granodiorite intrusion extends between Coulon granite and Sorbier granite [REF].

The granodiorite (gg) with abundant granophyric textures (indicating a shallow intrusion), describes in the Gien-sur-Cure type-locality [REF], extends in the central part of the northern Morvan belt in two intrusive massifs, the Gien-sur-Cure pluton to the East and Arleuf pluton to the West , separated by the Anost sedimentary and volcanic rocks (Tournaisian to Middle Visean). An age of 346+/-7 Ma was mesured by the Rb/Sr method on biotites of the granodiorite cropping out near Château-Chinon [REF], [REF].

At the end of granites emplacement, occurs intrusions of lamprophyre dikes (minette type) . But these dikes do not seems close the Tournaisian - Early Visean magmatic cycle, being found enclosed in the Picampoix microgranite that yield a similar dating to the granites [REF].

Granites

Microgranites (gg) follow and cape the granite massifs roofs . These are porphyritic textured rocks except along their edges where fine grained and flow banded structures occur giving way to a rhyolite facies .


Tournaisian-Lower Visean HOMEHOME Intra-Visean phase