Age and grade of metamorphism in the eastern Monts de Lacaune – implications for the collisional accretion in Variscan externides (French Massif Central)

Doublier, Michael P.; Potel, Sébastien; Wemmer, Klaus

doi:10.3166/ga.19.391-407

Cited by 10 publications

(21 citation statements)

References 25 publications

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“…In the first case, they would have been positioned still south of the Schistes X9/15. The age of deformation and metamorphism in the Eastern Monts de Lacaune and further to the NW is known from earlier investigations and new data presented in Doublier et al (2006). On the southern flank of the Zone Axiale, the age of deformation and metamorphism in the flysch-bearing nappes and their foreland is constrained by the biostratigraphic age of the youngest flysch sediments.…”

Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 91%

“…NE of the Zone Axiale (Eastern Monts de Lacaune), Cambrian and Ordovician sediments are repeated by SEfacing folds and thrusts. This association is of very low grade and is juxtaposed against the high-grade rocks of the gneissic core (Doublier et al 2006). On both the north and the south flank of the Zone Axiale, the lower grade sedimentary rocks abut against the gneissic core along ENE-trending, dextral transtensional faults (e.g., Echtler 1990).…”

Section: Geological Settingmentioning

confidence: 96%

“…In the eastern Espinouse dome, Krause et al (2004), dated a ''Roc Noir Granite'' at c. 308 Ma and contact metamorphic events in orthogneiss (U-Pb TIMS on monazite, 316-313 Ma and c. 308 Ma) (samples Kr 1 and Kr 2 in Fig. 2, location after Doublier et al 2006). The Folat Granite NE of the Zone Axiale has been dated at 314 ± 8 Ma (U-Pb TIMS on zircon fractions, Lévêque 1986).…”

Section: Geological Settingmentioning

confidence: 96%

“…Unstacking follows the basic principle, which transforms the vertical tectonic sequence into a lateral sequence in the same order. This brings the highest units-Eastern Monts de Lacaune (including the St. Gervais) and the Albigeois Units (see Guérangé-Lozes and Burg 1990;Doublier et al 2006)-into the northernmost position. The former nappes of the southern flank of the Zone Axiale follow towards the South.…”

Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 98%

“…Detailed mapping of large parts of the southeastern Montagne Noire and the findings of Doublier et al (2006) from the northern flank permit a reappraisal of the largescale structural relationships acquired during D 1 . The Palaeozoic sediments on the southern flank of the Montagne Noire, have been subdivided into several tectonic units, which evolved from recumbent fold nappes (Arthaud 1970), but are separated, today, by extensional or transtensional faults (see below).…”

Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 99%

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Hot metamorphic core complex in a cold foreland

Franke

Doublier

Klama

et al. 2010

Int J Earth Sci (Geol Rundsch)

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The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Viséan/Namurian boundary (B320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D 1 ), all structural levels shared the same tectonic evolution: D 2 (extension and exhumation), D 3 (refolding) and post-D 3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M 2 ). M 2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D 2 effects defines a bulk prolate strain in a ''pinched pull-apart'' setting. Ductile D 2 deformation during M 2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D 1 is only represented by the inverted position of bedding (overturned limbs of recumbent D 1 folds) and by refolded D 1 folds. U-Pb monazite and zircon ages and K-Ar muscovite ages are in accord with Ar-Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at C330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shearzone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward

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Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 91%

Section: Geological Settingmentioning

confidence: 96%

Section: Geological Settingmentioning

confidence: 96%

Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 98%

Section: Reconstruction Of the Accretionary Wedge (D 1 )mentioning

confidence: 99%

See 3 more Smart Citations

Hot metamorphic core complex in a cold foreland

Franke

Doublier

Klama

et al. 2010

Int J Earth Sci (Geol Rundsch)

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Very low-grade metamorphism of Rheno-Hercynian allochthons (Variscides, Germany): facts and tectonic consequences

Doublier

Potel

Franke

et al. 2011

Int J Earth Sci (Geol Rundsch)

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The Hörre Belt in the SE part of the Rhenish Massif has been regarded as autochthonous because of the apparent lack of metamorphic inversion. We have performed a K-white mica study using powder X-ray diffraction derived Kübler Index (KI) and K-white mica b cell dimension, and the spectrally derived wavelength of the 2,200 nm absorption feature (''2200wvl'') of supposedly allochthonous units and the underlying par-autochthon. KI reveals a slight, but significant inversion of metamorphic grade, which shows that the Hörre Belt and the Hessische Schieferserie adjacent to the South were emplaced by thrusting after they had acquired peak metamorphism (M 1 ) in deeper parts of the orogenic wedge. Only in the westernmost part of the study area, the par-autochthon and the allochthon show the same grade of anchizonal metamorphism. This observation, together with K-white mica b cell dimensions\9.000 Å and 2200wvl\2,204 nm, suggests a second, low-pressure metamorphism M 2 subsequent to the emplacement of the allochthon. This is supported by an anomaly of published conodont alteration index data. Our results expand the original width of the Rheno-Hercynian passive margin to at least 580, more probably to 670 or even 750 km. Palinspastic restoration also reveals that the Givetian to Early Carboniferous volcanic belt of the Lahn/Dill area is not only younger than the Emsian MORB-type lavas of the Rheno-Hercynian ocean, but was also separated, from the latter, by a largely nonvolcanic belt of at least c. 220 km width. Apparently, the Givetian to Early Carboniferous volcanic episodes were unrelated to the Rheno-Hercynian drift stage.

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