EVOLUTION OF AN EARLY BIRIMIAN OCEANIC IS LAND-ARC IN LIPTAKO (SIRBA REGION OF NIGERIA) - GEOLOGY, GEOCHRONOLOGY AND GEOCHEMISTRY

The Sirba region (Nigerian Liptako) is made up of volcanics, sedimenta ry sequences and granitoid batholiths belonging to the Birimian domain of the West African craton (stabilized at ca 1.9 Ga). Field relations hips have distinguished three main magmatic groups. The first group is made up of basic magmatic rocks located below the sediments. Two subg roups have been set up: la (northeast Sirba), pillowed basalts (thickn ess up to 1500 m) and Ib (southwest Sirba), various basalts, dolerites and gabbros. Group 2 comprises basaltic dykes cutting Group 1 (Subgro up 2a) and gabbroic to dioritic sills intruding Group 1 and the sedime nts (Subgroup 2b). Even if rather limited in volume, this group is ubi quitous. Group 3 represents large granitoid batholiths and frequent ac id dykes, which intrude all other types of rocks. A granodiorite from Group 3 has been dated at 2170+/-9 Ma (zircon U-Pb) giving a minimum a ge for all the studied groups, which are not much older. Subgroup la c orresponds to a typical oceanic tholeiitic island are with depleted LR EE (La-N/Yb-N: 0.3-0.9); Subgroup Ib is richer in LREE (La-N/Yb-N: 1.1 -2.7) and LIL elements pointing to an island are low-K tholeiites sign ature with a calc-alkaline affinity. Group 2 rocks are low-K calc-alka line and are enriched in LREE (La-N/Yb-N: 2.3-7.8) and LIL elements. T hat indicates a transitional island are environment (transition to a b ack-are setting?). Group 3 granitoids are more differentiated and cons titute a medium-K ti calc-alkaline sequence with higher LREE (La-N/Yb- N: 11.1-25.6) and LIL elements contents but low abundances in Y and Yb , typical of volcanic are continental margins. This group is contempor aneous to large horizontal strike-slip movements and to greenschist to lower amphibolite facies metamorphism. All groups have Sr and Nd init ial ratios within the mantle range (Sr-i: 0.7015+/-0.0005; epsilon(Nd) =+2.8+/-0.8) indicating no Archaean crustal participation in their gen esis (mean of T-DM model ages: 2207+/-56 Ma). The Sirba area displays a continuous evolution from a primitive (Sgr 1a) to a more mature (Sgr Ib) tholeiitic oceanic island are and to an evolved low-K calc-alkali ne island are (Gr 2) intruded during its accretion at 2.17 Ga by large syn-kinematic and syn-metamorphic medium-K calc-alkaline granitoid ba tholiths (Gr 3). This preserved Early Birimian evolution, the far east ern position of the Sirba belt and the weak intensity oi both tectonic s and metamorphism suggest an early oblique intra-oceanic collage of K iptchak type (Sengor rf al., 1993), The proposed model of intra-oceani c tectonics could be applied to a large part of the Birimian domain.