The Chamrousse Ophiolite : A Double Witness of Ancient Oceans





In the Belledonne Massif above Grenoble, the Chamrousse ophiolite was long considered a rare remnant of Cambro-Ordovician oceanic crust. However, new U–Pb zircon dating reveals a much more complex history.

The Chamrousse ophiolite (A) Geological map of the southwestern Belledonne Massif. The line indicates the position of the cross-section shown in Figure 3. The stars show the sample position, with colour code depending on the new igneous ages. Inset : Location of the Belledonne ECM in the Variscan Belt (Jacob et al. 2022). (B) The inverted ophiolitic sequence of Chamrousse (modified after Ménot et al. 1988), with sample position and new U–Pb zircon age.
Chamrousse
Chamrousse


At the base, the amphibolite unit (460–520 Ma) represents magmatism associated with continental rifting of the northern Gondwana margin, with inherited Proterozoic zircons reflecting Pan-African influence. Higher up, the gabbros, ultramafic rocks, and basaltic dikes (345–360 Ma) record Devono-Carboniferous oceanic crust formed in a short-lived back-arc basin at the onset of the Variscan collision.

Chamrousse thus records two tectonic episodes separated by 150 million years : the onset of Gondwana rifting, followed by the opening of a back-arc ocean. As a true geological time capsule, this complex sheds light on Europe’s Paleozoic evolution and the formation of the Variscan mountain belt.


The full study is published in TERRA NOVA

References

Arenas and Martínez, 2015
R. Arenas, S. S. Martínez
Variscan Ophiolites in NW Iberia : Tracking Lost Palaeozoic Oceans and the Assembly of Pangea
Episodes Journal of International Geoscience, 38 (2015), pp. 315–333

Ballèvre and al., 2014
M. Ballèvre, J. R. Catalán, A. López-Carmona, et al.
Correlation of the Nappe Stack in the Ibero-Armorican Arc Across the Bay of Biscay : A Joint French–Spanish Project
In The Variscan Orogeny : Extent, Timescale and the Formation of the European Crust, edited by K. Schulmann, J. R. Martínez Catalán, J.-M. Lardeaux, V. Janoušek, and G. Oggiano, Geological Society, London, Special Publication, vol. 405 (2014), pp. 77–113

Chelle-Michou and al., 2017
C. Chelle-Michou, O. Laurent, J.-F. Moyen, et al.
Pre-Cadomian to Late-Variscan Odyssey of the Eastern Massif Central, France : Formation of the West European Crust in a Nutshell
Gondwana Research, 46 (2017), pp. 170–190

Scientific Contacts :

  • Maureen Gunia – Researcher, ISTerre – University Grenoble Alpes, France
  • Carole Cordier – Researcher, ISTerre – University Grenoble Alpes, France
  • Emilie Janots – Researcher, ISTerre – University Grenoble Alpes, France
  • Adrien Vezinet – Researcher, ISTerre – University Grenoble Alpes, France
  • Valentine Milloud – Researcher, ISTerre – University Grenoble Alpes, France
  • Jean-Baptiste Jacob – Researcher, ISTerre – University Grenoble Alpes, France
  • Stéphane Guillot – Researcher, ISTerre – University Grenoble Alpes, France