U-Pb dating of the end of the Pan-African orogeny in the Tuareg shield: the post-collisional syn-shear Tioueine pluton (Western Hoggar, Algeria)

The Tioueine pluton intrudes the Neoproterozoic series of the Iskel terrane , located in the Tuareg shield, western Hoggar. The consistency of the inte rnal structures as well as the nature and organization of the associated mi crostructures demonstrate that the Tioueine pluton was emplaced syn-kinemat ically while N-S strike-slip shear zones were active. The syn-tectonic empl acement of the Tioueine massif implies that this pluton, although belatedly crystallized, entirely belongs to the concept of post-collisional magmatis m. In order to date precisely the late Pan-African tectono-metamorphic even t in the studied area, an U-Pb age of 523 +/- 1 Ma was obtained from abrade d zircons of a late quartz-syenite from the Tioueine pluton. This early Cam brian age is younger than the other plutons of the Tuareg shield, which wer e mainly emplaced between 630 Ma and 580 Ma. This dating also shows that th e Tuareg shield was not a single coherent block at 525 Ma, but rather an am algam of active terranes moving each other along major shear zones. Finally , the Tioueine massif represents probably the final welding of the Tuareg s hield assembly of terranes and consequently the end of the post-collisional orogenic episode in the whole Pan-African belt. (C) 1998 Elsevier Science B.V. All rights reserved.