THE GEOTECTONIC DISCRIMINATION OF BASIC M AGMATIC ROCKS FROM TRACE-ELEMENTS - REAPPRAISAL FROM A DATA-BASE AND APPLICATION TO THE PAN-AFRICAN BELT OF TOGO

We improve, by the mean of a geochemical data base including a very gr eat number of presentday and recent magmatic rocks classified accordin g to precise geotectonic criteria, the efficiency of trace elements fo r the geotectonic discrimination of lavas and plutons containing less than 55% SiO2. We propose new limits for the discriminant fields on th e Th-Tb-Ta diagram, and show that they do not locate at random with re spect to the Primordial Mantle: each setting being characterized by it s own mode of fractionation of the Th-Ta and Tb-Ta systems and therefo re, by a given source (or given sources). These results are used to in fer the paleotectonic setting of fifteen metabasites (amphibolites and eclogites) sampled from different lithotectonic units of the Panafric an belt in Togo. We show that these rocks show geochemical characteris tics which are closely similar to those of transitional basalts (Afar/ Red Sea system) or continental tholeiites, but clearly distinct from t hose of present N-type MORB or subduction-related basalts. These resul ts gain no support for the consumption of a large oceanic domain in th is part of the Panafrican belt.