A remnant of the Palaeotethys break-up: The Variscan Chamrousse ophiolite (Western Alps)

Understanding the origin and evolution of Variscan ophiolitic sequences is crucial for reconstructing the palaeo-geography of the Variscan belt. The Chamrousse ophiolite (External Crystalline Massif, Western Alps) dated by U Pb zircon presents a Late Devonian-Early Mississippian age (355–345 Ma), which coincides with the beginning of the Palaeotethysian Cycle. By characterising the melt source of this ophiolite through new zircon chemical investigations, we propose a geodynamic opening model in a pull-apart setting during the Palaeotethysian break-up. Through low U/Yb, Nb/Yb and Th/Yb ratios coupled with highly radiogenic initial Hf isotopes (ca. +12), the Chamrousse ophiolite demonstrates an oceanic origin from a chemically depleted mantle source. Its architecture illustrates the structure of magma-poor oceanic ridges and rifted margins suggesting that the Chamrousse ocean opened in an intra-cratonic position, likely in a transtensional pull-apart setting. This oceanic basin is preserved between its two conjugate margins. To the west, the bimodal Séchilienne unit reflects the Cambro-Ordovician continental rifting of the North-Gondwana margin. To the east, the Late Devonian-Early Mississippian Rioupéroux-Livet-Gavet formation is made of a bimodal magmatic and volcano-sedimentary association. The Rioupéroux meta-trondhjemite (ca. 367 Ma) could record the earliest stage of rifting, that may have led to the continental break-up and the Chamrousse opening. This geodynamic scenario can be integrated into the broader triangular Palaeotethysian break-up, where the Chamrousse basin and other extensional volcano-sedimentary continental basins located in the Moroccan Meseta and Bohemian Massif, are triggered by the westward propagation of the Palaeotethys Ocean, prior to the final assembly of Pangea.