Abstract

Plio-Quaternary basanites, nephelinites and leucito-nephelinites from Bas-Languedoc (South France) show Sr and Nd isotopic compositions similar for all samples and quite analogous to the Massif Central volcanics. Nevertheless, the Pb isotope data display large variations. Two major processes are invoked: contamination by sedimentary levels (1-2%), and hybridation of the mantle source between asthenospheric and lithospheric components probably by metasomatism of the lithospheric source by asthenospheric fluids. These lavas are subdivided in two groups according to their major and trace elements contents: the first one is potassic (Na 2 O/K 2 O around 1), the second one, more sodic (Na 2 O/K 2 O between 2 and 5). The first group may result from a partial melting (between 1 and 3%) of a garnet phlogopite mantellic source enriched in LILE (1.6 times primitive mantle composition). The second group may be issued from the previous one by fractionation (15%) of magmatic phlogopite.

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