high-K Calc-alkaline Granitoids Research Articles

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

Our study of the (60,000 km 2) Aïr massif, which comprises the southeastern extension of the Tuareg shield in the Niger Republic, has identified three major displaced terranes: Aouzegueur, Barghot and Assodé. 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 Assodé 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 Assodé 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 Assodé 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.

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony

Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.