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Jean-Paul Liégeois
Earth Sciences
Geodynamics and mineral resources
Geodynamics and mineral resources
Publication details
Acef, K, Liégeois, J.P., Ouabadi, A & Latouche, L. 2003. ‘The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization’. Journal of African Earth Sciences 37: 295-311. Amsterdam : Elsevier. I.F. 1.219.
Article in a scientific Journal / Article in a Journal
Abstract
The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km2) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (c. 2 Ga) high-grade basement belonging to the LATEA metacraton at c. 608 Ma (recalculated from the U-Pb dating of Bertrand et al., 1986) and cooled at c. 4 kbar, with a temperature of about 750°C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks.
The Anfeg batholith defines a Nd-Sr isotopic initial ratios trend (Nd/(87Sr/86Sr)i from -2.8/0.7068 to -11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (Nd/87Sr/86Sr)i of +6.2/0.7028 for the depleted mantle, -22/0.708 for the old lower crust.
The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature.