TRACE-ELEMENT AND ISOTOPIC GEOCHEMISTRY OF CENOZOIC ALKALI BASALTIC LAVAS FROM ATAKOR (CENTRAL SAHARA)

The Late Miocene to Recent Atakor volcanic complex of the Hoggar (sout hern Algeria, central Sahara) is composed of alkali basalts and relate d rocks (mainly phonolites and trachytes). Most lavas have a restricte d spread of their Sr and Nd isotopic ratios. Sr-87/Sr-86 ratios range from 0.70318 to 0.70422 and Nd-143/Nd-144 ratios range from 0.512776 t o 0.512937. An exception is peralkaline trachyte which has the highest Sr (0.71271) and the lowest Nd (0.512593) isotope values and was form ed by an assimilation-fractional crystallization process from basaltic magma. The basaltic rocks have trace element and isotopic composition s similar to ocean island basalts. It is suggested that the parent bas altic magmas were derived from a mantle plume which was modified durin g its ascent by interaction involving infiltration of a subcontinental lithospheric peridotite by plume-derived melts.