Magmatic and metamorphic history of mafic dykes from the Argentera-Mercantour Massif, southwestern Alps: implications for Variscan and Alpine tectonics

In the Argentera-Mercantour Massif (External Crystalline Massifs of the Alps, ECMs), Variscan migmatites and calc-alkaline peraluminous granites are crosscut by several mafic dykes never exceeding 10 meters of thickness. This intrusive dyke swarm has been mapped in Valscura and Val du Boréon, which are located respectively north and south of the Fremamorta-Colle del Sabbione Shear Zone (FSZ), a regional early-Miocene E-W striking fault that runs through migmatites and granites in the central part of the massif. Sub-vertical and NE-SW striking andesitic to dioritic dykes are always characterised by well-developed chilled margins and by the occurrence of degassing bubbles; soft contacts and magmatic breccias in the dykes, as well as their cross-cutting relationships, highlight a complex short-living sequence made of discrete intrusions. Whole-rock analyses indicate magnesian, alkali-calcic/calc-alkalic, and peraluminous signatures for this dyke swarm, without any clear relationships between chemical variations and the inferred relative chronology. Nevertheless, an alkaline affinity is suggested for the last impulses, whose products crystallise into the latest magmatic veins. Dykes are heterogeneously deformed and metamorphosed. To distinct microstructural domains correspond variations in the metamorphic assemblages, the former of which (M1) characterises only the dykes and their nearest host rocks, while the following (M2, M3, M4, M5) mark the regional structures in medium to highly deformed domains. On the other hand, the latter assemblages exhibit coronitic texture where dykes are poorly deformed. This dyke swarm broadens the magmatism in the Argentera-Mercantour Massif at ages younger than the Carboniferous-Permian boundary, corresponding to the age of the hosting granites. Magnesian magmatic suits post-dating the ferriferous granitoids are not yet known in the other ECMs. Syn-intrusive structures suggest a shallow emplacement, which matches with the sub-aerial condition under which the basement of the Argentera-Mercantour Massif was, at least locally, from late Carboniferous to Lower Triassic. The structural level of intrusion is confirmed by thermobarometric estimates on late magmatic assemblage (P < 0.1 GPa), as well as on M1, which is interpreted as related to a post-magmatic hydrothermal system (T ~ 300°C; P < 0.1 GPa). In both the investigated areas, thermobarometric constrains on deformation-related metamorphic assemblages and pseudosections highlight a prograde P-T path, from sub-superficial conditions to greenschist facies (T ~ 400°C; P > 0.2 GPa), before the final exhumation. The inferred P-T evolution is coherent with the Alpine evolution described in the literature for the Argentera-Mercanotur Massif, except for the prograde path, here described for the first time. These results on the metamorphic history suggest that the basement rocks in Valscura and in Val du Boréon belong to the same Alpine tectono-metamorphic unit, despite the interposition of the FSZ.