From Variscan to Alpine tectonics: magmatic and metamorphic evolution of the Argentera-Mercantour dioritic dyke swarm

In the Argentera-Mercantour Massif (External Crystalline Massifs, ECMs) Variscan migmatites and late Carboniferous granites are crosscut by a dioritic dyke swarm. More than 60 years after the first report (Faure Muret, 1955), these dykes have been mapped in detail at Valscura and Val du Boréon, located, respectively, N and S of the main post-collisional granitic pluton, the “Central Granite”. NE-SW trending and steeply dipping dioritic dykes show chilled margins and degassing bubbles, while, in the innermost parts, soft contacts and magmatic breccias mark the boundary between different igneous types, testifying a complex intrusive history. As inferred by whole rock and EMPA analyses, these dykes are magnesian, calc-alkalic to alkali-calcic, and peraluminous; an alkaline affinity only for the latest magmatic pulse is otherwise suggested by amphibole composition. Intrusive structures and textures indicate a shallow emplacement level, consistently with the sub-aerial condition locally experienced by the Argentera-Mercantour basement from late Carboniferous to Lower Triassic. Thermobarometric estimates on the first metamorphic assemblage developed within these dykes (T~300°C, P<0.1 GPa), and interpreted as related to a post-magmatic hydrothermal system not affecting the host rocks, confirm the shallow intrusion. At a later time, dykes were intersected by the mylonitic shear zones that reworked the massif during the Alpine cycle: syn-kinematic mineral assemblages within dykes confirm the metamorphic peak conditions already inferred for this tectonic event (Sanchez et al., 2011). The occurrence of this dioritic dyke swarm implies an extension of the magmatic activity in the Argentera-Mercantour Massif beyond the Permo-Carboniferous boundary: intrusives similar in field relationships and compositions are not yet known in the other ECMs (Debon &Lemmet, 1999), but are commonly reported in other parts of the southern European Variscan belt (e.g. Bonin et al., 1998). Moreover, thermobarometric constrains on metamorphic assemblages have allowed for the first time to define a P-T-d-t path relating to the Alpine evolution of Argentera-Mercantour Massif that include both prograde and retrograde stages.