Hidden pre-Alpine records preserved within the garnets of the Briançonnais nappes of the Central Alps

Studying the poly-metamorphic and polycyclic evolution of metasedimentary rocks often proves to be challenging, mainly because of the tendency of their mineral associations to easily re-equilibrate during subsequent metamorphism. On the other hand, garnets, which are often important constituents of these rocks, can be regarded as time-capsules due to their generally high resistance during retrograde and/or polycyclic metamorphism (e.g., Baxter et al., 2017and references therein). The analysis of the inclusions and of the textures preserved within these minerals thus became a well-known tool for deciphering the metamorphic evolution of such rocks, especially in high-grade terrains (e.g., Tacchetto et al., 2019). Recent geological mapping campaigns in the Tambo and Suretta nappes (Central Alps) already highlighted the presence of a widespread HT signature despite the general Alpine overprint, interpreted as pre-Alpine in age. Two samples from the southernmost portion of the Tambo in particular show relic garnets with peculiar cloudy textures, due to the high density of inclusions mainly composed by rutile-ilmenite needles, carbonates, graphitic material and micrometer-sized multiphase inclusions. Similar textures are usually reported for garnets found in high-grade metamorphic contexts (e.g., Ague & Eckert, 2012), conditions frequently observed within the rocks of the Variscan remnants across Europe. Despite the necessity for additional focused analyses, both meso- and micro-scale investigations point toward the conclusion that the Tambo Nappe still records long-lasting evidence related to the Variscan Orogeny and subsequent evolution. These new observations suggest the possible presence of a high-grade metamorphic event not yet characterized within this nappe. HP remnants of Variscan age have been already identified within the structurally higher Suretta Nappe (Nussbaum et al., 1998), and, considering the similarities observed between the two units, it is not unlikely to hypothesize a common pre-Alpine metamorphic evolution for the entire Briançonnais terrain of the Central Alps. If proved, the recognition of a pre-Alpine high-grade imprint within these units would possibly open new interpretations regarding the geodynamic significance of the easternmost portion of the Briançonnais microcontinent during and after the Variscan Orogeny.