Basanites and nephelinites from Bas-Langnedoc: mantellic heterogeneities and crustal contamination

Plio-Quaternary basanites, nephelinites and leucito-nephelinites from Bas-L anguedoc (South France) show Sr and Nd isotopic compositions similar for al l samples and quite analogous to the Massif Central volcanics. Nevertheless , the Pb isotope data display large variations. Two major processes are inv oked: contamination by sedimentary levels (1-2%), and hybridation of the ma ntle source between asthenospheric and lithospheric components probably by metasomatism of the lithospheric source by asthenospheric fluids. These lav as are subdivided in two groups according to their major and trace elements contents: the first one is potassic (Na2O/K2O around 1), the second one, m ore sodic (Na2O/K2O between 2 and 5). The first group may result from a par tial melting (between 1 and 3%) of a garnet phlogopite mantellic source enr iched in LILE (1.6 times primitive mantle composition). The second group ma y be issued from the previous one by fractionation (15%) of magmatic phlogo pite.