A high-temperature tectonic mélange marking the Variscan suture in the Argentera-Mercantour Massif of Western Alps

In the Alps, numerous mafic, ultramafic, and sedimentary protoliths have been affected by Variscan metamorphism due to processes including subduction, collision, and late-orogenic extension, persisting until Early Permian times. Variscan eclogites derived from oceanic lithosphere are extensively documented, particularly in the External Crystalline Massifs (EMC) of the Western Alps (Helvetic-Dauphinois-Provençal domain). These eclogites are typically found as lenses and boudins enclosed within migmatitic gneiss and offer crucial insights into the location of a Variscan suture zone in the southern segment of the Variscan belt.

The Variscan basement of this massif consists predominantly of migmatitic metaintrusives and paragneisses, interspersed with mafic and ultramafic rocks that have been transposed into the migmatitic foliation. The metabasites are composed of amphibolites, eclogites, and granulites, while the ultramafic rocks occur as variably serpentinized peridotites and pyroxenites. These lithologies are often associated with calc-silicate lenses (clinopyroxene-, epidote-, and garnet-bearing) and marble layers.

This study presents new data from different rocks forming this high-temperature tectonic mélange within the Argentera-Mercantour Massif of the EMC, with a focus on lithostratigraphy, protolith origin, and metamorphic conditions. New lithostratigraphic, structural, geochemical, and petrological data are integrated with LA-ICP-MS U-Pb zircon dating, which reveals REE profiles indicative of crystallization under igneous to high-pressure metamorphic conditions. The eclogite protoliths display distinct geochemical affinities—sometimes oceanic in origin—and emplacement ages ranging from the late Cambrian to Silurian. In some samples, the prograde metamorphic evolution, progressing from prehnite-pumpellyite to eclogite facies, is evidenced by low-grade mineral inclusions within the cores of eclogitic garnet. However, omphacite is only locally preserved.

 Petrological modeling and zircon-rutile Ti-Zr thermometry consistently indicate peak eclogite-facies conditions. The prograde path, transitioning from very low- to high-pressure conditions with a temperature-depth ratio of ≤10°C/km, suggests that these rocks were deformed and metamorphosed during oceanic subduction and subsequent continental collision. The occurrence of such a HT-tectonic mélange in the core of the Argentera-Mercantour Massif represents a portion of fragmented Variscan suture zone within the pre-Alpine crystalline basement of the Alps.