NEW PETROGRAPHIC, GEOCHEMICAL AND GEOCHRO NOLOGICAL DATA ON GRANITOIDS FROM THE PALEOPROTEROZOIC OF THE DIALE-DALEMA SUPERGROUP (EASTERN SENEGAL) - PETROGENETIC AND GEODYNAMIC SIGNIFICANCE

The granitoids intruding in the Birimian Diale-Dalema sedimentary and volcanosedimentary formations are potassic in composition, of calc-aka line affinity and are similar to collision type granites. They are cha racterised by low values of Yb-N (1,82-9,4) and high ratios of La-N/Yb -N up to 137. New isotopic and geochronological data of these granitoi ds indicate an Eburnean age (2008 +/- 16 Ma) and low Sr initial ratios related to a mantle source. The Diale-Dalema granitoids seem to be yo unger than the trondhjemitic Kakadian granitoids dated at 2190 Ma (Dia , 1988) and intrusive into the tholeiitic rocks of the Mako Supergroup . They could have been generated by the melting of a young and hydrate d oceanic crust, favoured by a low subduction slope. (C) 1997 Elsevier Science Limited.