U-PB DATING AND ISOTOPIC SIGNATURE OF THE ALKALINE RING COMPLEXES OF BOU NAGA (MAURITANIA) - ITS BEARING ON LATE PROTEROZOIC PLATE-TECTONICS AROUND THE WEST AFRICAN CRATON

In die West African fold belt of Mauritania, high-grade metamorphic series, similar to those of Amsaga (Reguibat shield-West African Craton), are exposed in a window. At Bou Naga-Mauritania (19-degrees-N, 13-degrees-15'W) in the South of this window, an alkaline ring complex has intruded the metamorphic country rocks. This complex consists of two geological formations: the Eastern formation is mainly composed of red rhyolite sills, whereas die Western formation is made up of several kinds of alkaline rocks both saturated and undersaturated which cross cut the earlier saturated units. Three U-Pb zircon age measurements have been made on die alkaline complex, and one on an orthogneiss from the metamorphic country rocks. The syenite and die alkaline granite of the Western block are 676 +/- 8 and 687 +/- 5 Ma old. The orthogneiss is Archaean with an age of 2709 +/- 136 Ma, but the lower intercept of discordia on concordia, shows an age of 756 +/- 25 Ma linked with the genesis of the alkaline complex. A major crustal contribution is recorded by Nd and O isotopes in die SiO2-satured rocks. These results provide evidence for the correlation of the metamorphic country rocks with the Reguibat Archaean basement and for an early Pan-African continental rifting phase in this area before the tectonometamorphic events in the Mauritanide belt. Furthermore, with regards with previous geodynamic works of the West African Craton, our results lead us to suggest a significant diachronism between late Proterozoic crustal evolution to the West and to the East of the West African Craton. This is a further evidence for modern-type plate tectonics at this time.