Structure of lamprophyres: a discriminant marker for Variscan and Alpine tectonics in the Argentera-Mercantour Massif, Maritime Alps

Abstract

Structural and microstructural analyses are carried out in two sites of the Argentera-Mercantour Massif, Valscura and Val du Haut Boréon, where swarms of lamprophyres intruded into Variscan migmatites and early Permian granitoids. Efforts aim at defining the structural relationships between lamprophyres and country rocks, and at constraining the structural and metamorphic evolution the dykes record. Mesoscale structural data are synthesised in geologic maps originally surveyed at 1/10 000 scale, supported by form-surface maps at 1/100 scale. The lamprophyres are magnesian, calc-alkalic to alkali-calcic, and metaluminous; they emplaced at very shallow crustal levels intersecting three generations of ductile structures in the host migmatites (D1, D2, D3). Epidote- and actinolite-bearing mineral assemblages result from late-intrusive hydrothermal circulation that has not affected the host rocks. Mylonitic shear zones of Alpine age (D4) are continuous through migmatites, granites, and lamprophyres: in these latter, they are supported by albite, actinolite, biotite, chlorite, epidote, phengite, and titanite. This detailed multi-scale structural analysis, coupled with major and trace elements geochemistry, highlights two main results: i) the lamprophyres, which post-date both the late- to post-collisional “high-Mg” and the “low-Mg” granitoids, reflect the last magmatic event in the Argentera-Mercantour Massif related to the Permian-Triassic lithospheric thinning; ii) the metamorphic assemblages that support the Alpine shear zones in the lamprophyres are consistent with the transition between the greenschist and amphibolite facies conditions.