EARLY AND LATE PAN-AFRICAN OROGENIES IN THE AIR ASSEMBLY OF TERRANES (TUAREG SHIELD, NIGER)

Our study of the (60,000 km2) Air massif, which comprises the southeas tern extension of the Tuareg shield in the Niger Republic, has identif ied three major displaced terranes: Aouzegueur, Barghot and Assode. Th ese terranes were all affected differentially by the Pan-African oroge ny during an early major phase (750-660 Ma) and a late ''follow-up'' p hase (650-580 Ma). The greenschist facies Aouzegueur terrane comprises an ophiolite and is intruded by a late-kinematic medium-K calc-alkali ne tonalite-trondhjemite-granodiorite (TTG) suite (approximately 730 M a). The amphibolite facies Barghot terrane, made up of a basement and a supracrustal sequence, is intruded by late- to post-kinematic high-K calc-alkaline granitoids (715-665 Ma). Both terranes form a NNE to E- verging belt thrust (680-670 Ma) upon an unknown eastern rigid contine nt and are covered unconformably by an early Pan-African molasse. In c ontrast, the amphibolite facies Assode terrane, composed of a basement and several supracrustal sequences, collided with the eastern rigid b lock without being thrust upon it. Tibetan-type crustal thickening occ urred and HT-LP metamorphism proceeded to regional anatexis enhanced b y widespread emplacement of anatectic granites of lower crustal origin (approximately 670 Ma). This thermal event is attributed to the delam ination of the continental lithospheric mantle and consequent rise of the asthenosphere. The late Pan-African phase affecting mainly the Ass ode terrane is marked by the intrusion of high-K calc-alkaline N-S elo ngated batholiths (645-580 Ma) and by right-lateral displacement along the N-S Raghane mega-shear zone, located on the edge of the eastern r igid continent. This movement along the northern extent of the rigid c ontinent in Eastern Hoggar (8-degrees-30'E) created the Tiririne intra continental linear fold belt made up of early Pan-African molasse. In the model proposed, we estimate that the Assode terrane, driven by an oblique spreading ridge, moved to the north over a distance of approxi mately 1000 km during 60 Ma along the lithospheric Raghane shear zone. The latter was the subsequent locus of the Silurian-Devonian alkaline ring-complexes. The calc-alkaline granitoids are regarded as subducti on-related mantle products contaminated by lower and medium-level crus t. The model and mechanisms proposed can be applied to the entire Tuar eg shield.