A REVIEW OF THE IN OUZZAL GRANULITIC TERRANE (TUAREG SHIELD, ALGERIA)- ITS SIGNIFICANCE WITHIN THE PAN-AFRICAN TRANS-SAHARAN BELT

The In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an e longated Palaeoproterozoic block within the Neoproterozoic Pan-African belt of north-west Africa. The granulites derive from Archaean protol iths that include a large volume of metasediments which were deposited on a 3.2-Ga gneissic basement. Near-peak granulite facies conditions between 2.17 and 2 Ga were estimated at P = 10 kbar and T rising from 800 to 1000 degrees C. Premetamorphic orthogneisses were intruded at 2 .5 Ga, and followed by the emplacement of syn- to late-kinematic charn ockites, syenites and carbonatites at around 2 Ga. Cooling of the gran ulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, t he IOT was buried beneath late Palaeoproterozoic and Neoproterozoic co ver sequences, and then behaved as a rigid block. Both margins ale lit hospheric faults, as evidenced by the occurrence of shear-zone-related mafic and felsic plutons. Pan-African tectonothermal events were negl igible in the north, but granulites in the south were significantly re worked under lower greenschist facies conditions during the northern m otion of the block with respect to both the western and the eastern Pa n-African terranes. The Cambrian molasse, associated with widespread a lkaline volcanism and subvolcanic granites, is horizontal in the north . The IOT, which was part of a larger continental mass including its c ounterpart in northern Mall, is interpreted as an exotic terrane which may have docked during Pan-African plate convergence and lateral coll ision. The unchanged pediplain since c. 1.7 Ga in the north suggests t hat the IOT is underlain by thick Palaeoproterozoic lithospheric mantl e, whereas its southern part is probably allochthonous and overlies Pa n-African structural units.