Eocene-Miocene igneous activity in Provence (SE France). 40Ar/39Ar data, geochemical-petrological constraints and geodynamic implications

Provence (SE France) was affected by two main phases of sporadic igneous activity during the Cenozoic. New40Ar/39Ar laser step-heating data constrain the beginning of the oldest phase to late Eocene (40.82 ± 0.73 Ma), with activity present until early Miocene (~ 20 Ma). The products are mainly andesites, microdiorites, dacites and basaltic andesites mostly emplaced in the Agay-Estérel area. Major- and trace-element constraints, together with Sr[sbnd]Nd[sbnd]Pb isotopic ratios suggest derivation from a sub-continental lithosphere mantle source variably modified by subduction-related metasomatic processes. The compositions of these rocks overlap those of nearly coeval (emplaced ~ 38–15 Ma) late Eocene-middle Miocene magmatism of Sardinia. The genesis of dacitic rocks cannot be accounted for by simple fractional crystallization alone, and may require interaction of evolved melts with lower crustal lithologies. The youngest phase of igneous activity comprises basaltic volcanic rocks with mildly sodic alkaline affinity emplaced in the Toulon area ~ 10 Myr after the end of the previous subduction-related phase. These rocks show geochemical and isotopic characteristics akin to magmas emplaced in intraplate tectonic settings, indicating a sub-lithospheric HiMu + EM-II mantle source for the magmas, melting approximately in the spinel/garnet-lherzolite transition zone. New40Ar/39Ar laser step-heating ages place the beginning of the volcanic activity in the late Miocene–Pliocene (5.57 ± 0.09 Ma). The emplacement of “anorogenic” igneous rocks a few Myr after rocks of orogenic character is a common feature in the Cenozoic districts of the Central-Western Mediterranean area. The origin of such “anorogenic” rocks can be explained with the activation of different mantle sources not directly modified by subduction-related metasomatic processes, possibly located in the sub-lithospheric mantle, and thus unrelated to the shallower lithospheric mantle source of the “orogenic” magmatism.