Two-stage oceanic opening with its continental margins, revealed by elemental zircon U–Pb/Hf/TE signatures: the case of the Chamrousse ophiolite in the Western Alps.

The Chamrousse ophiolite, located in the French External Alps, has long been considered as the suture of an ocean opened during the Cambro-Ordovician (496 ± 6 Ma, Ménot et al., 1988). This has been recently challenged by new U–Pb zircon results revealing a dual geological history, occurring in the Cambro-Ordovician (520-460 Ma) and in the Devono-Carboniferous (360-345 Ma), see Gunia et al., 2025. Futhermore, this complex is bounded by North-Gondwana flysch units (Fréville et al., 2018). Additional information is needed to define geodynamic interpretations. Here, we present combined zircon U–Pb/Hf and trace element signatures obtained on these two igneous populations via in-situ approaches.

Zircon showing Cambro-Ordovician U–Pb dates, identified in meta-trondhjemites and amphibolites, feature elemental geochemistry of U and Yb contents indicative of continental affinities. ε¹⁷⁶Hf_initial of ca. +5 point to a mantle source intermediate between depleted and enriched end-members. We interpret these results as indicating zircon crystallisation during the continental rifting of the North of Gondwana.

Zircon that crystallized during the Devono-Carboniferous event, identified in mafic-ultramafic units, show trace-element characteristics and an ε¹⁷⁶Hf_initial of ca. +12 suggestive of an oceanic zircon origin from a chemically depleted mantle source. This Devono-Carboniferous event is interpreted as reflecting the actual oceanic opening, facilitated by pre-thinned Cambro-Ordovician continental crust.

In addition, new structural data suggest the opening occurs through the development of an asymmetric detachment that enabled exhumation of mantle-derived rocks with limited magma production, followed rapidly by its closure during the Variscan collision (330-300 Ma, Jacob et al., 2023). The new results presented here highlight the two-stage tectonic evolution, making this short-lived oceanic basin an exceptional case study before the final assembly of Pangea.

Reference list:

Fréville, K., Trap, P., Faure, M., Melleton, J., Li, X. H., Lin, W., ... & Poujol, M. (2018). Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France). Tectonophysics, 726, 14-42.

Gunia, M., Cordier, C., Janots, E., Vezinet, A., Milloud, V., Jacob, J. B., & Guillot, S. (2025). The Chamrousse Ophiolite (Western Alps, France): Relict of a Devono‐Carboniferous Ocean. Terra Nova.

Jacob, J. B., Janots, E., Cordier, C., & Guillot, S. (2023). Discovery of Variscan orogenic peridotites in the Pelvoux massif (western Alps, France). BSGF-Earth Sciences Bulletin, 194(1), 2.

Ménot, R. P., Peucat, J. J., Scarenzi, D., & Piboule, M. (1988). 496 My age of plagiogranites in the Chamrousse ophiolite complex (external crystalline massifs in the French Alps): evidence of a Lower Paleozoic oceanization. Earth and Planetary Science Letters, 88(1-2), 82-92.