Geochemistry of eclogites and associated rocks of the southeastern area of the French Massif Central: Origin of the protoliths

Piboule, Michel; Briand, Bernard

doi:10.1016/0009-2541(85)90120-2

Cited by 30 publications

(7 citation statements)

References 27 publications

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“…By combining the results of whole-rock based major and trace element geotectonic diagrams with the zircon trace element provenance diagrams, all results suggest a crustal contamination (Fig. 2) and are in agreement with previous studies that suggested emplacement in a thinned continental crust in a spreading environment (e.g., Briand & Piboule, 1985;Briand et al, 1988;Bouchardon et al, 1989).…”

Section: Eclogite Protoliths: Origin and Emplacement Settingsupporting

confidence: 89%

“…In the Variscan belt, the timing of the HP metamorphism and the subsequent exhumation of the subducted rocks is controversial. The convergence between two megacontinents, Laurussia to the north and Gondwana to the south, with the contribution of microplates, such as Armorica or Avalonia (e.g., Murphy et al, 2009;Kroner & Romer, http://www.petrology.oupjournals.org/ 6 Briand (1985) interpreted the LAC as a tholeiitic suite from a shallow crustal origin in a spreading environment; (iv) Pin & Piboule (1988) proposed a collision between two independent magmatic suites to account for the composite character of the LAC; (v) for Briand et al (1988), the magmatic signature of the LAC corresponds to an emplacement in a back-arc basin environment; and (vi) Lardeaux (2014) finally reinterpreted the chemical composition of all the LAC occurrences from the FMC, including the LAC from the Lévézou, as reflecting a hyper-extended continental margin. However, all these authors agree on the fact that the LAC does not correspond to a metamorphosed mature oceanic crust.…”

Section: Introductionmentioning

confidence: 99%

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From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

Lotout

Poujol

Pitra

et al. 2020

Journal of Petrology

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Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge due to subsequent retrograde overprinting. A fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U–Pb in zircon, rutile and apatite, Lu–Hf and Sm–Nd in garnet) in a petrological context tightly constrained by petrographic observations, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18–23 kbar and 680–800°C, whereas the retrogressed eclogite later equilibrated at 8–9·5 kbar and c.600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U–Pb date of 472·3 ± 1·7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace element record and whole-rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, a few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5·7 Ma, interpreted as a prograde pre-eclogitic age. Lu–Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358·0 ± 1·5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350·4 ± 7·7 Ma and 352 ± 20 Ma dates for Sm–Nd garnet dating, and 367·8 ± 9·1 Ma and 354·9 ± 9·5 Ma for U–Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351·8 ± 2·8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U–Pb, rutile, apatite; Lu–Hf, garnet; Sm–Nd, garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15–8·5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6·3 mm/yr.

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Section: Eclogite Protoliths: Origin and Emplacement Settingsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

Lotout

Poujol

Pitra

et al. 2020

Journal of Petrology

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“…This view has been profoundly modified first by the results of absolute dating demonstrating ubiquitous Variscan reworking (Gebauer et al, 1981;Pin and Lancelot, 1982;Rolin et al, 1982;Duthou et al, 1994). Another turning point was to interpret the felsic/mafic gneisses with a tholeiitic signature, defined as the Leptynite-Amphibolite Complex (LAC), as remnants of ophiolites (Forestier, 1961;Briand and Piboule, 1979;Cabanis et al, 1983;Maillet et al, 1984;Mercier et al, 1985;Piboule and Briand, 1985;Briand et al, 1988;Dubuisson et al, 1989;Pin, 1990). In the Armorican Massif, ophiolites define the Medio-European suture also referred to as the Galicia-South Brittany suture, or the Eo-Variscan suture (Bernard-Griffiths and Cornichet, 1985;Hanmer, 1977;Faure et al, 1997;Ballèvre et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

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

Vanderhaeghe

Laurent

Gardien

et al. 2020

BSGF - Earth Sci. Bull.

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We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and 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. These features reflect the horizontal growth of the Variscan belt and the 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 Paleothethys 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 correspond to post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

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“…thesis, Univ. Lyon, 1979;Piboule & Briand, 1985;Gardien, 1993) and felsic gneiss lenses closely associated with the metabasites.…”

Section: Geological Settingmentioning

confidence: 95%

Geochemistry of bimodal amphibolitic—felsic gneiss complexes from eastern Massif Central, France

et al. 1995

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High-grade basic and acidic meta-igneous rocks are widespread in the bimodal amphibolitic-felsic gneiss complexes, which are characteristic formations of the 'Middle Allochthonous Unit' from eastern and southern French Massif Central. The metabasites from the Lyonnais and Doux complexes are chemically diverse and range from N-MORB type tholeiitic to transitional types. The two populations are not related by fractional crystallization or crustal contamination processes and their chemical characteristics reflect differences in their mantle sources. An ensialic setting is supported by the crustally-derived character of some of the associated felsic rocks, but the presence of N-MORB-type metabasites argues for an extensional environment. This bimodal association compares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic crust formation during the CambroOrdovician spreading event.

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Geochemistry of eclogites and associated rocks of the southeastern area of the French Massif Central: Origin of the protoliths

Cited by 30 publications

References 27 publications

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

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

Geochemistry of bimodal amphibolitic—felsic gneiss complexes from eastern Massif Central, France

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