ISOTOPIC (RB-SR, U-PB AND SM-ND) AND TRACE-ELEMENT GEOCHEMISTRY OF ECLOGITES FROM THE PAN-AFRICAN BELT - A CASE-STUDY OF REE FRACTIONATION DURING HIGH-GRADE METAMORPHISM

Major and trace element analyses [including rare earth elements (REE)], as well as Rb-Sr (whole rocks and micas), U-Pb (zircons) and Sm-Nd (whole rocks) isotopic data for eclogites and basic granulites from the pan-African Belt suggest the existence of two distinct tectonic settings. An ocurrence of high grade rocks, located along the pan-African suture zone in Togo, involves basic protoliths derived from depleted mantle source(s) with epsilon-Nd(T) generally close to +7, or +2 in cumulate rock-types. The ages of their protoliths are not precisely determined but are probably Proterozoic (younger than 1.15 Ga). They are thought to have been involved into subduction-obduction processes (640-580 Ma ago) which affected pan-African (s.l.) oceanic and continental crust. A second occurrence of high-grade rocks located in the Aleksod area of the central Hoggar (Algeria) has also been investigated. It corresponds to older (congruent-to 2.5 Ga) basic protoliths metamorphosed during the pan-African orogeny. These rocks are considered to belong to an intracontinental context as proposed for this part of the belt. A model is proposed to account for the "step-shaped" REE patterns sometimes observed in certain eclogite whole-rock samples. In the case of eclogites from the Mount Lato area, these REE patterns are not representative of magmatic protoliths. Some LREE-loss occurred during high-grade events particularly evidenced in syn-eclogite blastomylonites.