Relationship between deformation, equilibration temperatures, REE and radiogenic isotopes in mantle xenoliths (Ray Pic, Massif Central, France): An example of plume-lithosphere interaction?

Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chemistry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high Nd-143/Nd-144, low Sr-87/Sr-86 and unradiogenic Pb isotope ratios; (ii) lherzolites, harzbugites and a wehrlite with LREE-enriched patterns, low Nd-143/Nd-144, high Sr-87/Sr-86 and radiogenic Pb isotope ratios. The former closely resemble depleted MORE-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the ''low velocity component'' observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 degrees C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 degrees C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions.