EVOLUTION OF THE PRECAMBRIAN IN-OUZZAL BLOCK (CENTRAL SAHARA, ALGERIA)

New structural and geochronological data are used to constrain the evo lution of the Precambrian In-Ouzzal block situated on the eastern side of the West African Craton. The results of this study can be summariz ed as follows: (i) The block can be subdivided into two series; one is mainly composed of Archaean granitic gneisses (3.2-2.5 Ga old), while the other contains metasedimentary and basic to ultrabasic rocks whic h can be compared with those described in Archaean supracrustal terrai ns. (ii) The entire block is affected by very high temperature granuli te-facies metamorphism. (iii) The metamorphism is associated with two main deformational phases, the first of which is responsible for a reg ional foliation refolded during the second phase. (iv) The major tecto no-metamorphic event in the area is related to a 2.0 Ga-old (Eburnian) orogeny which ended with carbonatite emplacement. Nearly all evidence of the earliest crustal evolution events (i.e. the initial emplacemen t and deformation of the granitic and supracrustal materials) has been erased by the Eburnian event.