Conditions et ages 40 Ar/ 39 Ar de mise en place des granitoides de la zone externe sud du Massif central francais; exemple des granodiorites de St-Guiral et du Liron (Cevennes, France)

2000 ◽  
Vol 171 (5) ◽  
pp. 495-510 ◽  
Author(s):  
Khalid Najoui ◽  
Andre Francois Leyreloup ◽  
Patrick Monie
Keyword(s):  
Gravitational Collapse ◽  
Regional Metamorphism ◽  
Low Grade ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Laser Probe ◽  
Thermal Structures ◽  
Minimum Age ◽  
Contact Aureoles

Abstract The St-Guiral and the Liron laccolithic granodiorites outcropping in the southeastern area of the French Massif Central intrude the Cambro-Ordovician low grade series of the western Cevennes. The petrological study of their contact aureoles shows that these plutons emplaced at low depth (1-2 kb; 690-700 degrees C). The laser probe 40 Ar/ 39 Ar allows us to suggest a minimum age of 324 Ma for the emplacement of the St-Guiral granodiorite. This emplacement seems synchronous with the LP-HT regional metamorphism in the western Cevennes for which the cooling ages are identical. The Liron granodiorite (310 Ma) is younger and crosscuts the thermal structures of the LP-HT regional metamorphism. Accordingly these plutons emplaced at the Namurian-early Westphalian during the extensional phases related to the generalised gravitational collapse of the previously thickened Hercynian belt. The detachment of the subducted lithosphere (slab detachment) could be related to these phenomena and could explain the granitization of the south external area of the Hercynian belt as well as the migration of the magmatism towards the south of the belt.

Geothermal Survey of the South-East Margin of the French Massif-Central

1980 ◽  
pp. 202-212
Author(s):  
B. Bosch ◽  
P. Degranges ◽  
J. Demange ◽  
M. Leleu ◽  
A. Marce ◽  
...  
Keyword(s):  
French Massif Central ◽  
The South ◽  
Massif Central

scholarly journals The South Millevaches Middle Carboniferous crustal melting and its place in the French Variscan belt

2009 ◽  
Vol 180 (6) ◽  
pp. 473-481 ◽  
Author(s):  
Michel Faure ◽  
Eugène Be Mezeme ◽  
Alain Cocherie ◽  
Jérémie Melleton ◽  
Philippe Rossi
Keyword(s):  
Crustal Melting ◽  
Variscan Belt ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Rock Types ◽  
Fold And Thrust Belt ◽  
Three Samples ◽  
Middle Carboniferous ◽  
Montagne Noire

AbstractSeveral episodes of crustal melting are now well identified in the Variscan French Massif Central. Middle Devonian (ca 385-375 Ma) migmatites are recognized in the Upper and Lower Gneiss Units involved in the stack of nappes. Late Carboniferous migmatites (ca 300 Ma) are exposed in the Velay Massif only and Middle Carboniferous migmatites crop out in the Para-autochthonous Unit and southern Fold-and-Thrust Belt. In the SW part of the Massif Central, the South Millevaches massif exposes migmatites developed at the expense of ortho- and paragneiss. They form kilometer-sized septa within the foliated Goulles leucogranitic pluton, which is in turn intruded by the non-foliated Glény two micas granite pluton. Monazite grains extracted from these three rock-types have been dated by the EPMA chemical method. Three samples of migmatite yield a late Visean age (ca 337-328 Ma), the Goulles and Glény granitic plutons yield ages at 324-323 Ma and 324-318 Ma, respectively. These new results enlarge the evidence of a Middle Carboniferous crustal melting imprint that up to now was only reported in the eastern part of the French Massif Central, in the northern Cévennes and in the Montagne Noire axial zone. At the scale of the French Variscan massifs, the Visean crustal melting event is conspicuously developed since it is recognized from the Massif Armoricain (Vendée and south coast of Brittany) to the Central Vosges. This episode is synchronous with the huge thermal event responsible for the “Tuffs anthracifères” magmatism of the northern Massif Central and Vosges, and took place immediately after the last thickening phase recorded both in Montagne Noire and Ardennes, that is on the southern and northern outer zones of the Variscan Belt, respectively. However, the geodynamic significance of this major event is not fully understood yet.

La tectonique cisaillante polyphasee du Sud Limousin (Massif central francais) et son interpretation dans un modele d'evolution polycyclique de la chaine hercynienne

2000 ◽  
Vol 171 (3) ◽  
pp. 295-307 ◽  
Author(s):  
Jean-Yves Roig ◽  
Michel Faure
Keyword(s):  
Middle Devonian ◽  
Early Carboniferous ◽  
Evolution Model ◽  
Ductile Deformation ◽  
Late Devonian ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Mafic Rocks

Abstract Structural, kinematics and thermo-barometric analyses of the ductile deformation of the south-Limousin metamorphic formations show a polyphase shear tectonics corresponding to two different thrusting events. The older one, is a to the top-to-the-SW thrusting during middle Devonian. This deformation occurs under minimum PT conditions of 7 Kbar/700 degrees C simultaneously to anatexis. The second event is a top-to-the-NW shearing which occurred in late Devonian-early Carboniferous under Barrovian conditions (5 kbar/600 degrees C). Diorites bodies and non-eclogitized mafic rocks allow us to argue for an extensional phase between the two thrusting events. These two ductile and syn-metamorphic deformations take place in a polycyclic evolution model of the Hercynian belt of the French Massif Central.

The petrochemistry of the basic volcanic rocks of the South Connemara Group (Ordovician), western Ireland

1983 ◽  
Vol 120 (2) ◽  
pp. 141-152 ◽  
Author(s):  
P. D. Ryan ◽  
M. D. Max ◽  
T. Kelly
Keyword(s):  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Major And Trace Elements ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
The South ◽  
Northern Margin ◽  
Southern Side ◽  
Western Ireland ◽  
Basic Volcanic

Summary16 samples of Ordovician basic volcanic rocks of the South Connemara Group, which abut the southern side of the metamorphic rocks of the Connemara massif in western Ireland, have been analysed for both major and trace elements. Although subject to low grade regional metamorphism and subsequently hornfelsed by the Galway Granite (400 Ma), their immobile element contents do not appear to be significantly disturbed. These elements characterise the metabasites of the South Connemara Group as ocean floor basalts having their origins in a marginal basin. The Skird Rocks Fault, separating the South Connemara Group from high grade metamorphic rocks of the Connemara massif, is consequently regarded as the northern margin of the vestiges of the lapetus Ocean which can be traced into, and along, the Southern Uplands Fault.

The Wenlock Strata of South Mayo

1960 ◽  
Vol 97 (4) ◽  
pp. 265-275 ◽  
Author(s):  
J. G. C. Anderson
Keyword(s):  
Regional Metamorphism ◽  
Low Grade ◽  
The South ◽  
North West ◽  
The North

AbstractSouth of Clew Bay, Co. Mayo, Wenlock strata, cut off to the north by a belt of serpentine marking the Highland Boundary, are divisible into three stratigraphical groups. The succession is repeated by a syncline overturned towards the south, which is modified by north-west cross-folds. The Wenlock strata have undergone low-grade “regional” metamorphism, in spite of which fossils have been preserved at a few localities.

Occupation and land-use history of a medium mountain from the Mid-Holocene: A multidisciplinary study performed in the South Cantal (French Massif Central)

2009 ◽  
Vol 8 (8) ◽  
pp. 737-748 ◽  
Author(s):  
Frédéric Surmely ◽  
Yannick Miras ◽  
Pascal Guenet ◽  
Violaine Nicolas ◽  
Aurélie Savignat ◽  
...  
Keyword(s):  
Land Use ◽  
Land Use History ◽  
French Massif Central ◽  
The South ◽  
Massif Central ◽  
Multidisciplinary Study ◽  
History Of

scholarly journals Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

2020 ◽  
Vol 191 ◽  
pp. 25 ◽  
Author(s):  
Olivier Vanderhaeghe ◽  
Oscar Laurent ◽  
Véronique Gardien ◽  
Jean-François Moyen ◽  
Aude Gébelin ◽  
...  
Keyword(s):  
Partial Melting ◽  
Gravitational Collapse ◽  
Late Devonian ◽  
Variscan Belt ◽  
French Massif Central ◽  
Massif Central ◽  
Facies Metamorphism ◽  
Orogenic Plateau ◽  
Subducting Slab ◽  
Mechanical Decoupling

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

Successive shearing tectonics during the Hercynian collisional evolution of the southwestern French Massif Central

2004 ◽  
Vol 175 (1) ◽  
pp. 49-59 ◽  
Author(s):  
Manuel Duguet ◽  
Michel Faure
Keyword(s):  
Early Carboniferous ◽  
French Massif Central ◽  
The South ◽  
Medium Temperature ◽  
Massif Central ◽  
Ductile Shearing ◽  
Pressure Medium ◽  
Dextral Strike Slip Fault ◽  
Montagne Noire ◽  
Dextral Strike Slip

Abstract In the French Massif Central, the Devonian-Carboniferous tectonic evolution of the Rouergue-Albigeois area is characterized by three phases of low-angle ductile shearing. The first event D1, which occurred probably in the Lower Devonian, is responsible for the south-westward thrusting of the high metamorphic Lévézou nappe which belongs to the Upper Gneiss Unit above the Lower Gneiss Unit overlying itself the Para-autochthonous Unit, (locally called the St-Serninsur-Rance nappe). In the late Devonian-early Carboniferous, this stack of nappes is reworked by a second event, D2, characterized by a top-to-the-NW shearing of the Para-autochthonous Unit upon the Lower Gneiss Unit developed under medium pressure/medium temperature metamorphism. The contact between the Lower Gneiss Unit and the Para-autochthonous Unit is a top-to-the NW low-angle fault that progressively evolues into a dextral strike-slip fault from west to east. The D2 event is followed by a top-to-the south D3 thrusting dated around 330–340 Ma. The main feature of the D3 compressional stage is the emplacement of the whole stack of nappes previously structured by D1 and D2 events upon the Albigeois series. The D3 event produced south-verging recumbent folds in the Albigeois, Mont-de-Lacaune and Montagne Noire. The significance of the D2 event either as thrusting or normal faulting is discussed.

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