Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

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.

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Middle Carboniferous intracontinental subduction in the Outer Zone of the Variscan Belt (Montagne Noire Axial Zone, French Massif Central): multimethod geochronological approach of polyphase metamorphism

Geological Society London Special Publications ◽

10.1144/sp405.2 ◽

2014 ◽

Vol 405 (1) ◽

pp. 289-311 ◽

Cited By ~ 19

Author(s):

Michel Faure ◽

Alain Cocherie ◽

Julien Gaché ◽

Chloé Esnault ◽

Catherine Guerrot ◽

...

Keyword(s):

Outer Zone ◽

Variscan Belt ◽

French Massif Central ◽

Axial Zone ◽

Massif Central ◽

Middle Carboniferous ◽

Montagne Noire ◽

Intracontinental Subduction ◽

Polyphase Metamorphism

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Detrital zircon age distribution from Devonian and Carboniferous sandstone in the Southern Variscan Fold-and-Thrust belt (Montagne Noire, French Massif Central), and their bearings on the Variscan belt evolution

Tectonophysics ◽

10.1016/j.tecto.2016.03.032 ◽

2016 ◽

Vol 677-678 ◽

pp. 1-33 ◽

Cited By ~ 5

Author(s):

Wei Lin ◽

Michel Faure ◽

Xian-hua Li ◽

Yang Chu ◽

Wenbin Ji ◽

...

Keyword(s):

Detrital Zircon ◽

Age Distribution ◽

Variscan Belt ◽

Thrust Belt ◽

French Massif Central ◽

Massif Central ◽

Zircon Age ◽

Fold And Thrust Belt ◽

Montagne Noire

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Coupling RSCM paleothermometry with 40Ar/39Ar analysis in the south Variscan belt (Cévennes, France): new constraints on the late-orogenic metamorphic gradient in an orogen outer domain.

10.5194/egusphere-egu2020-20140 ◽

2020 ◽

Author(s):

Clément Montmartin ◽

Michel Faure ◽

Stéphane Scaillet ◽

Hugues Raimbourg

Keyword(s):

Cross Sections ◽

Crustal Thickening ◽

Early Event ◽

Variscan Belt ◽

French Massif Central ◽

Raman Spectrometry ◽

Massif Central ◽

Temperature Jumps ◽

Outer Domain ◽

The One

In the SE part of the Variscan French Massif Central, the C&#233;vennes area belongs to the para-autochthonous unit of the southern Variscan belt. This area underwent three metamorphic events (Faure et al., 2001). &#160;I) A green schist to low amphibolite facies one (500&#176;C, 4.5Kb Arnaud, 1997) developed in micaschists and quartzites. These rocks were stacked as south-directed nappes during the final stage of the Variscan crustal thickening dated at ca 340 Ma by 40Ar/39Ar on biotite (Caron, 1994). This early event was responsible for the flat-lying foliation, the N-S striking stretching lineation, and intrafolial foliation. II) A high temperature event (680&#176;C, 4.5kb Rakib, 1996) dated at ca 325 Ma (40Ar/39Ar on two biotites, Najoui et al, 2000) overprinted the early one. On the basis of the mineral assemblages of this event, a NE-ward increase of the T conditions was interpreted as a remote effect of the Velay Dome (Rakib, 1996). III) Finally, the Mt-Loz&#232;re and Aigoual-St-Guiral-Liron monzogranitic plutons intruded the C&#233;vennes para-autochthonous unit. Monazite and biotite yield U-Pb, and 40Ar/39Ar ages at 315-303Ma and 306 Ma , respectively (Brichaud et al. 2008). The pluton emplacement conditions are determined at 695&#176;C, 1.5Kb (Najoui et al, 2000).We report Raman Spectrometry of Carbonaceous Matter (RSCM) paleotemperature data acquired on more than 100 samples throughout the entire C&#233;vennes area. These show a regional homogeneous thermal distribution with a 535 &#177; 50 &#176;C mean temperature without any geometric correlation with the nappes structure, nor the granitic intrusions. Moreover, no thermal increase towards the NE can be documented. SW of the Aigoual-St-Guiral-Liron massif, our RSCM data document a temperature jumps between the overlying C&#233;vennes micaschists and the underlying epimetamorphic rocks belonging the the Fold-and-Thust belt unit of the French Massif Central.In order to constrain the age of this regional thermal event, we 40Ar/39Ar dated 25 new regionally-distributed syn- and post-folial muscovites by step heating along two N-S cross sections within the C&#233;vennes micaschists series. In areas distant from the plutons, the muscovite yields a ca 325 Ma age interpreted as the one of the HT event recorded by the RSCM measurements. However, young muscovite ages at ca 305Ma are observed around the plutons. We assume that the heat supplied by the plutons reset these muscovites at around 400&#176;C while the organic matter cannot record the contact metamorphic peak lower than the regional one. Moreover, 40Ar/39Ar in-situ analyses carried out on 5 mm-sized post folial (but deformed) biotites in the central part of the micaschist series provide ages around 320Ma. The presence of a hidden dome, underneath the C&#233;vennes micaschists, similar to the pre-Velay migmatites exposed in the northern part of the C&#233;vennes area (Faure et al., 2001, Be et al., 2006) is discussed.

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French and Belgian Uplands

The Physical Geography of Western Europe ◽

10.1093/oso/9780199277759.003.0022 ◽

2005 ◽

Author(s):

Bernard Etlicher

Keyword(s):

Continental Crust ◽

Rift Valley ◽

Sedimentary Basins ◽

Shear Zones ◽

Metamorphic Complex ◽

French Massif Central ◽

Massif Central ◽

Montagne Noire ◽

Oceanic And Continental Crust ◽

Moderate Elevation

The French Uplands were built by the Hercynian orogenesis. The French Massif Central occupies one-sixth of the area of France and shows various landscapes. It is the highest upland, 1,886 m at the Sancy, and the most complex. The Vosges massif is a small massif, quite similar to the Schwarzwald in Germany, from which it is separated by the Rhine Rift Valley. Near the border of France, Belgium, and Germany, the Ardennes upland has a very moderate elevation. The largest part of this massif lies in Belgium. Though Brittany is partly made up of igneous and metamorphic rocks, it cannot be truly considered as an upland; in the main parts of Brittany, altitudes are lower than in the Parisian basin. Similarities of the landscape in the French and Belgian Uplands derive from two major events: the Oligocene rifting event and the Alpine tectonic phase. The Vosges and the Massif Central are located on the collision zone of the Variscan orogen. In contrast, the Ardennes is in a marginal position where primary sediments cover the igneous basement. Four main periods are defined during the Hercynian orogenesis (Bard et al. 1980; Autran 1984; Ledru et al. 1989; Faure et al. 1997). The early Variscan period corresponds to a subduction of oceanic and continental crust and a highpressure metamorphism (450–400 Ma) The medio- Variscan period corresponds to a continent–continent collision of the chain (400–340 Ma). Metamorphism under middle pressure conditions took place and controlled the formation of many granite plutons: e.g. red granites (granites rouges), porphyroid granite, and granodiorite incorporated in a metamorphic complex basement of various rocks. The neo-Variscan period (340–320 Ma) is characterized by a strong folding event: transcurrent shear zones affected the units of the previous periods and the first sedimentary basins appeared. At the end of this period, late-Variscan (330–280 Ma), autochthonous granites crystallized under low-pressure conditions related to a post-collision thinning of the crust. Velay and Montagne Noire granites are the main massifs generated by this event. Sediment deposition in tectonic basins during Carboniferous and Permian times occurred in the Massif Central and the Vosges: facies are sandstone (Vosges), shale, coal, and sandstone in several Stephanian basins of the Massif Central, with red shale and clay ‘Rougier’ in the south-western part of the Massif Central.

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Monazite U-Th/Pb chemical dating of the Early Carboniferous syn-kinematic MP/MT metamorphism in the Variscan French Massif Central

BSGF - Earth Sciences Bulletin ◽

10.2113/gssgfbull.180.3.283 ◽

2009 ◽

Vol 180 (3) ◽

pp. 283-292 ◽

Cited By ~ 10

Author(s):

Jérémie Melleton ◽

Michel Faure ◽

Alain Cocherie

Keyword(s):

Electron Probe ◽

Early Carboniferous ◽

Variscan Belt ◽

French Massif Central ◽

Massif Central ◽

Close Range ◽

Chemical Dating

AbstractIn situ U-Th-Pb geochronology on monazite using Electron Probe Micro Analyser, constrained by structural and textural observations, has been performed on four samples from the Limousin area (northwest part of the French Massif Central) in order to date the syn-kinematic MP/MT metamorphism related to the top-to-the-NW shearing that deformed the stack of nappes in this zone of the Variscan belt. All the analyzed samples lead to a mean age at 360 ± 4 Ma. The close range of ages obtained during this study (360 Ma) and with the previous 40Ar-39Ar ones (360–350 Ma) suggests fast processes of cooling and exhumation during the Early Carboniferous in internal zones of the Variscan belt. The geodynamic significance of this Early Carboniferous event is discussed at the scale of the Ibero-Armorican orocline.

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