The refertilized nature of the southern lithospheric mantle domain beneath the Massif Central (France): An example of xenoliths from Mt. Briançon

Mantle xenoliths from the Cenozoic Mt. Briançon cinder cone in the Massif Central (France) were studied in order to test previously proposed hypotheses on the structure and chemical evolution of the lithospheric mantle in the region. Mineral major and trace element compositions coupled with characterization of crystal preferred orientation (CPO) of grains allow to recognize the compositional evolution and deformation history of peridotites and show which minerals constitute the primary vs. later fabric of the rock. The Mt. Briançon xenolith suite (n = 21) consists of spinel lherzolites, except one spinel harzburgite and one olivine clinopyroxenite. Lherzolites (olivine Fo 88.5–90.4) form three major groups. Those of Group I (51–65 vol% olivine and 10–16 vol% clinopyroxene) contain pyroxenes of variable Al content (orthopyroxene 0.142–0.217 apfu; clinopyroxene 0.244–0.316 apfu) and aluminous spinel (Cr# 0.09–0.12). Clinopyroxene is LREE-depleted and exhibits CPO indicating that it is a newly crystallized/recrystallized phase added to the rock framework consisting of olivine and orthopyroxene. Group II lherzolites (67–83 vol% olivine, 5–9 vol% clinopyroxene) contain Al-poor clinopyroxene (0.208–0.245 apfu) and spinel of Cr# 0.16–0.28. Clinopyroxene is LREE-enriched and its CPO is coherent with the fabric defined by olivine and orthopyroxene, indicating a joint deformation history. The lherzolites of Group Ia (46–66 vol% olivine, 11–26 vol% clinopyroxene) are transitional between those of Group I and II, and the majority contain pyroxenes with Al contents similar to those of Group I, and spinel of Cr# 0.11–0.15. Clinopyroxene REE patterns are spoon-shaped. Three lherzolites cannot be classified into the mentioned groups because they exhibit REE contents in clinopyroxene that change gradually from LREE-depleted to LREE-enriched on the scale of a petrographic section. In one of them, this change is observed at the grain scale, with spoon-shaped REE patterns in the cores and LREE-enriched ones at the margins. The harzburgite is rich in Mg (olivine Fo 91.1, pyroxenes Mg# 0.92) and impoverished in Al (spinel Cr# 0.43). Clinopyroxene is enriched in LREE, the contents of which are significantly lower than those of lherzolitic clinopyroxene. It forms nearly undeformed grains in an olivine-orthopyroxene framework. The lherzolites of Group I were formed by reactive percolation of MORB-like melts under suprasolidus conditions, which enabled replacement and/or addition of clinopyroxene to the solid olivine-orthopyroxene framework. Zonal REE distribution in clinopyroxene from one of the lherzolites documents alkaline melt percolation overprinting the Group I, which ultimately formed the Group II lherzolites. The lherzolites of Group Ia document transition between those of Group I and Group II. Hypothetical melt in equilibrium with clinopyroxene in olivine clinopyroxenite has a composition similar to that of the host lava. This rock likely represents a cumulate from the same melt that infiltrated the lithosphere during the Cenozoic volcanic activity of the Massif Central.