Early and late Pan-African orogenies in the Aïr assembly of terranes (Tuareg shield, Niger)

Abstract Our study of the (60,000 km 2 ) Air massif, which comprises the southeastern extension of the Tuareg shield in the Niger Republic, has identified three major displaced terranes: Aouzegueur, Barghot and Assode. These terranes were all affected differentially by the Pan-African orogeny during an early major phase (750–660 Ma) and a late “follow-up” phase (650–580 Ma). The greenschist facies Aouzegueur terrane comprises an ophiolite and is intruded by a late-kinematic medium-K calc-alkaline tonalite-trondhjemite-granodiorite (TTG) suite (∼730 Ma). 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 continent and are covered unconformably by an early Pan-African molasse. In contrast, the amphibolite facies Assode terrane, composed of a basement and several supracrustal sequences, collided with the eastern rigid block without being thrust upon it. Tibetan-type crustal thickening occurred and HT-LP metamorphism proceeded to regional anatexis enhanced by widespread emplacement of anatectic granites of lower crustal origin (∼6770 Ma). This thermal event is attributed to the delamination of the continental lithospheric mantle and consequent rise of the asthenosphere. The late Pan-African phase affecting mainly the Assode terrane is marked by the intrusion of high-K calc-alkaline N-S elongated batholiths (645–580 Ma)) and by right-lateral displacement along the N-S Raghane mega-shear zone, located on the edge of the eastern rigid continent. This movement along the northern extent of the rigid continent in Eastern Hoggar (8°30′E) created the Tiririne intracontinental 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 ∼1000 km druing 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 subduction-related mantle products contaminated by lower and medium-level crust. The model and mechanisms proposed can be applied to the entire Tuareg shield.