New geochronological and isotopic data on the Cenozoic volcanism of Ahaggar (southern Algeria): evidence for a mantle plume.

The Taharaq district is located exactly at the centre of the gravity anomal y associated with the Ahaggar swell and comprises Upper Eocene (34 Ma) thol eiitic traps. By comparison, the other volcanic districts of Ahaggar are yo unger (Mio-Plio-Quaternary), of alkali affinity, and are located on the swe ll flanks. The isotopic signature of tholeiites shows that they derive from a heterogeneous mantle dominated by a lithospheric component near the EM1 endmember. The subsequent alkali lavas are characterized by isotopic enrich ment in radiogenic Nd and Pb, suggesting participation of a near HIMU compo nent of possible plume origin. (C) 2000 Academic des sciences / Editions sc ientifiques et medicales Elsevier SAS.