ISOTOPIC CONSTRAINTS ON THE AGE AND FORMATION OF A PALAEOPROTEROZOIC VOLCANIC ARC COMPLEX IN THE KEDOUGOU INLIER, EASTERN SENEGAL, WEST-AFRICA

The Palaeoproterozoic Kedougou basement in the Mako area in eastern Se negal consists of four major rock complexes: 1) high-grade amphiboliti c and gneissic rocks occuring as large xenoliths (the Sandikounda amph ibolite-gneiss complex); 2) voluminous bimodal volcanic rocks (the Mak o volcanic-plutonic complex); 3) the Sandikounda layered plutonic comp lex; and 4) trondhjemitic to calc-alkaline intrusive granitoids (the L aminia-Kaourou plutonic complex). Zircons from xenoliths of the Sandik ounda amphibolite-gneiss complex yielded ages of similar to 2200 Ma, w hich are slightly older than those of the Mako Supergroup volcanic roc ks. These xenoliths also have epsilon Nd(t) values ranging from +2.4 t o +4.1. The volcanic sequence represents an important bimodal volcanic event with Mg-tholeiites and subordinate felsic calc-alkaline lavas h aving epsilon Nd(t) values ranging from +2.6 to +3.8. The plutonic int rusions yield ages between 2158 and 2079 Ma with positive epsilon Nd(t ) values of +2.6 to +4.7. Isotopic evidence indicates that the Mako Pa laeoproterozoic crust is a juvenile, are-related terrain derived from a depleted mantle source, emplaced between 2300 and 2079 Ma and subseq uently accreted to the growing West African Craton. An early magmatic event, represented by the protolith of the amphibolite and the gneiss, is inferred to represent the initial stage of this Palaeoproterozoic crustal growth and may represent the root of the Mako volcanic-plutoni c complex. Thus, the Birimian-Eburnian development of the western part of the Kedougou Inlier is constrained to the interval 2300-2079 Ma; t his is slightly older than other parts of the Birimian-Eburnian domain s elsewhere in western Africa. (C) 1997 Elsevier Science Limited.