Variscan eclogites from the Argentera–Mercantour Massif (External Crystalline Massifs, SW Alps): a dismembered cryptic suture zone

Abstract

We document structural, geochemical, petrological and 40 Ar/ 39 Ar geochronological data performed on Variscan eclogites from the Argentera-Mercantour Massif, southwestern Alps. Based on the high-resolution field mapping, we present new eclogite occurrences and discuss the relationships between eclogites and surrounding migmatites. We recognized for the first time preserved eclogite facies assemblages. Trace elements and REE patterns establish that eclogites protoliths are MORBs contaminated by continental crust. Standard thermobarometry and thermodynamic modeling are consistent with P-T values of 640-740 °C for 1.5 ± 0.25 GPa, coherent with paleo-geotherms predicted for warm subduction of oceanic crust. We interpret these eclogites as a dismembered cryptic suture zone. 40 Ar/ 39 Ar dating on amphiboles yields an age of 339.7 ± 12 Ma for eclogite retrogression under amphibolite facies conditions. All these data are combined to link the Argentera-Mercantour Massif in the tectonic framework of both pre-Mesozoic Alpine basement and European southern Variscides.

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…For highest convergence velocities, metamorphic imprints characterized by low P/T ratios can diffusely coexist with metamorphic imprints distinguished by high P/T ratios and lying along gradients traditionally associated to cold and warm subduction complexes, as described by Cloos (1993) (see dashed‐dotted and continuous arrows in Figure 13). Such observation well fits, for example, with the occurrence in the Variscan subduction complex of blueschist‐ to granulite‐facies conditions (e.g., Ballèvre et al., 2009; Faure et al., 2005; Jouffray et al., 2020; Lardeaux, 2014b), in accordance with higher velocities generally proposed for the Variscan convergence (Regorda et al., 2020, and refs. therein).…”

“…P-T conditions predicted by the models are also consistent with the contrasted P-T metamorphic conditions and gradients that have been described in the internal zones of some collisional belts. Significantly contrasted P-T metamorphic conditions have been recorded during the Variscan subduction, from blueschist, to eclogite and HP-granulite facies (e.g., Ballèvre et al, 2009;Faure et al, 2005;Jouffray et al, 2020;Lardeaux, 2014b;Regorda et al, 2020), as observed in all models (Figure 5).…”

Carboniferous high‐P metamorphism and deformation in the Belledonne Massif (Western Alps)

“…They all derive from normal to enriched MORB‐type tholeiites emplaced during the Ordovician between ca. 480 and 450 Ma (Jouffray et al., 2020; J. L. Paquette et al., 1989; Rubatto et al., 2010; Von Raumer & Bussy, 2004), presumably in a thinned continental crust during the opening of the Rheic and Saxo‐Thuringian oceans. Peak pressure is constrained between 1.3–1.8 GPa and 650–750°C (Ferrando et al., 2008; Liégeois & Duchesne, 1981; Vanardois et al., 2020; this work), and is followed by near‐isothermal decompression to ca.…”

“…They all derive from normal to enriched MORB-type tholeiites emplaced during the Ordovician between ca. 480 and 450 Ma (Jouffray et al, 2020;J. L. Paquette et al, 1989;Rubatto et al, 2010;Von Raumer & Bussy, 2004), presumably in a thinned continental crust during the opening of the Rheic and Saxo-Thuringian oceans.…”

Reconstructing the Variscan Terranes in the Alpine Basement: Facts and Arguments for an Alpidic Orocline

“…In the Helvetic zone, the Variscan basement is exposed in the External Crystalline massifs of Argentera, Pelvoux, Grandes Rousses, Belledonne, M t -Blanc, Aiguilles Rouges, and Aar-Gothard (Figures 5 and 6). Due to the weakness of the Alpine overprint, the pre-Permian geology of the External Crystalline massifs has been investigated since a long time (e.g., [4,9,13,36,38,[53][54][55][56][57][58][59]). However, the widespread crustal melting erased the early structures.…”