EGU2020-7913
https://doi.org/10.5194/egusphere-egu2020-7913
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?

Jean-baptiste Jacob1, Stéphane Guillot1, Daniela Rubatto2, Emilie Janots1, Jérémie Melleton3, and Michel Faure4
Jean-baptiste Jacob et al.
  • 1Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France
  • 2University of Bern, Institute of Geological Sciences, Baltzerstrasse 1-3, CH-3012 Bern, Switzerland
  • 3Bureau de Recherches Géologiques et Minières, 3 avenue Claude-Guillemin, BP 36009, 45060 Orléans Cedex 2, France
  • 4Institut des Sciences de la Terre d'Orléans, Université d'Orléans, CNRS, 45071 Orléans Cedex 2, France

The Paleozoic basement exposed in the External Crystalline Massifs of the Western Alps (ECM) contains numerous relics of Variscan eclogites and high pressure granulites preserved in high grade migmatitic gneisses. These relics are taken to indicate that the ECM underwent an early HP metamorphic stage during the Variscan Orogeny. However, due to the scarcity of recent thermobarometric and geochronological data, the geodynamic significance of this high pressure metamorphism remains unclear. Based on petrological similarities with other eclogite-bearing formations in the European Variscides (especially the “leptyno-amphibolic compex” in the French Variscides), it has been suggested that the high pressure rocks from the ECM mark a mid-Devonian subduction cycle, preceding the main Carboniferous Variscan collisional stage (Fréville et al., 2018; Guillot and Ménot, 2009). This interpretation mostly relies on one mid-Devonian U-Pb zircon age (395±2 Ma) obtained in eclogites from the massif of Belledonne (Paquette et al., 1989), which has been interpreted as the age of eclogitization. However, dating of high pressure granulites in the Argentera Massif (Rubatto et al., 2010) yielded a Carboniferous age (ca. 340 Ma) for the high pressure stage, questioning the previous geodynamical interpretation. We present here the results of a detailed petrological and geochronological investigation of the high grade formation of the Lacs de la Tempête in NE Belledonne, where some of the eclogites dated by Paquette et al. (1989) were sampled. This area exposes mostly high-grade migmatitic metasediments with intercalated lenses of orthogneiss and garnet-bearing amphibolites, preserving locally eclogitic assemblages. Thermobarometric estimations coupling forward pseudosection modelling, Zr in rutile thermometry and garnet growth modelling constrain the minimal P conditions during the high pressure stage at ca. 1.4-1.6 GPa and 700 °C. The early HP assemblage was then strongly overprinted by granulite facies metamorphism at ca. 1.0-1.2 GPa and 750 °C, also recorded in the surrounding metasediments. U-Pb dating of zircon reveals that the eclogites derived from Ordovician protoliths. Zircon overgrowth in the eclogites and the surrounding metasediments constrain the age of HP metamorphism between ca. 350-305 Ma, with no evidence for a Devonian event. Rutile dating in the eclogites supports the late Carboniferous age of metamorphism. The middle-late Carboniferous corresponds to the main period of Variscan nappe stacking in the ECM, following a period of arc magmatism during late Devonian-Tournaisian (ca. 360-350 Ma, Fréville et al., 2018). We therefore suggest that the 350-305 Ma ages recorded in the HP units of the ECM do not correspond to a Devonian subduction, but rather represent the equilibration of orogenic lower crust at HP-MT conditions during the Variscan nappe stacking events, followed by re-equilibration at lower P during late Carboniferous. This evolution presents striking similarities with the high pressure units of the Moldanubian zone in the Bohemian massif (Schulmann et al., 2009). However, deciphering the exact meaning of U-Pb ages in retrogressed eclogites remains a challenge, and further field and petrological investigation is required to produce a consistent history of the Variscan collision in the ECM.

How to cite: Jacob, J., Guillot, S., Rubatto, D., Janots, E., Melleton, J., and Faure, M.: Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7913, https://doi.org/10.5194/egusphere-egu2020-7913, 2020

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Presentation version 1 – uploaded on 15 Apr 2020
  • CC1: Comment on EGU2020-7913, Jana Kotkova, 04 May 2020

    Hello Jean-Baptiste, I had more questions for you, however, I left space to the other ones.

    You got to one point I had in your last comment, in fact. When reading your display, I wondered, if there is not rather comparison with eclogites from the French Massif Central, not to go as far as the Bohemian Massif. What about their P-T conditions and age? Please note that the Moldanubian eclogites from the BM record higher temperatures: the strong overprint on your eclogites could have modified the composition of major phases. As for rutile, if it is enclosed in the core, couldn´t it record rather prograde evolution? And what about Ti in zircon?

    Then about dating. The temperatures of the peak and exhumation of the retrogressed eclogites are well below the TC of U-Pb in zircon, unlike the eclogites from the Moldanubian Zone of the Bohemian Massif. How do you explain that zircon, which crystallized in equilibrium with garnet i.e. at the peak, records the age of the exhumation? What is the character of this zircon domain imaged using CL/BSE?

    All the best

    Jana

    • AC1: Reply to CC1, Jean-baptiste Jacob, 04 May 2020

      Hello Jana,
      that's a lot of interesting questions, I will try to do my best to respond.

      The eclogites from the western Alps are indeed lower T than the Bohemian eclogites. I used the figure from the paper of Maierova et al. (2016) mostly because it makes an interesting comparison between Variscan and Himalayan eclogites: in these two orogenic systems, there are two stages of eclogitic metamorphism, an early stage with subduction-related low T eclogites, and a latter stage with higher T eclogites and HP granulites, which indicates a longer time of residence in the lower crust before exhumation. My point was here to show that the samples presented fit better with this second group and are not really consistent with an early Devonian subduction. As you suggest, the Variscan eclogites from the Alps compare better with the eclogites from the Massif Central in terms of P and T (you can find a good  review presenting a comparison the HP rocks in these two domains in Regorda et al. (2019), Geoscience Frontiers). Regarding the age, I would however be careful because many of the ages reported for the Massif Central eclogites were obtained a long time ago by multigrain ID-TIMS U-Pb zircon and are probably a mix between the protolith age and the metamorphic recrystallization. More recent data (e.g. Lotout et al. 2018) support a late Devonian age for the HP metamorphism (370-380 Ma) in the Massif Central, but I think that the Alpine eclogites compare better with those from the Montagne Noire dome (e.g. Whitney et al. 2020, JMG), which yield younger ages. 

      The rutile may have been enclosed in garnet during the prograde evolution, but the temperature estimated for these inclusions is very close to that estimated for the rutile in the matrix, which probably records conditions closer to the peak T. I doubt that these eclogites have experienced temperatures greater than 750°C. Ti in zircon temperatures are lower than the rutile temperatures (~650°C), and I still don't have an explanation to this.

      Finally, regarding zircon dating, the metamorphic domains dated correspond to thin (10-20µm) CL bright rims around the zircon cores.
      Please note that the zircon U-Pb dates are quite dispersed (~335-315 Ma) and REE composition presents large variations, from flat HREE patterns and no Eu anomaly, which suggests HP growth in presence of garnet, to enrichment in HREE and negative Eu anomaly which suggest a growth during garnet breakdown reactions in presence of plagioclase. They therefore probably record a protracted evolution starting at HP in the eclogite facies and then continuing at lower P in the Pl stability field.

      regards,
      Jean-Baptiste