Répertoire du personnel
Jean-Paul Liégeois
Sciences de la Terre
Géodynamique et ressources minérales
Géodynamique et ressources minérales
Détails
Navez , J., Liégeois, J.P., Latouche, L. & Black, R. 1999. ‘The Palaeoproterozoic Tchilit exotic terrane (Aïr, Niger) within the Pan-African collage of the Tuareg shield’. Journal of the Geological Society of London 156: 247-259. I.F. 2.797.
Article dans une revue scientifique / Article dans un périodique
The Tchilit terrane in Aïr (Niger) is one of the most exotic terranes among those constituting the Tuareg shield. It is composed of low-K calc-alkaline metabasalts and meta-andesites (amphibolites), alkaline metarhyolites, detrital metasediments and of a syenogranite. Volcanic protoliths are Palaeoproterozoic in age (within the 1600-2200 Ma time span; Sm-Nd, Rb-Sr and Ar-Ar systematics) and underwent two metamorphisms accompanied by mylonitization. The first metamorphism occurred in the amphibolite-facies (690 ± 40 °C) during the Palaeoproterozoic whereas the second is a greenschist retrogression, late Pan-African in age (646 ± 6 Ma, 2, Ar-Ar age). The metasediments have only been affected by the Pan-African event and are probably Neoproterozoic. The syenogranite (619 ± 39 Ma, 2, Rb-Sr) is posterior to both metamorphisms: it is the only Pan-African material in Tchilit. The Pan-African tectonics, a N-S transpression (dextral shear) induced the overthrusting of the neighbouring Assodé terrane onto Tchilit. Major, trace elements and Sr-Nd isotopes give to the protoliths of amphibolites a continental active margin signature (low-K calc-alkaline) and to those of the metarhyolites an alkaline one, more typical of a post-collisional or intraplate setting. Both were significantly contaminated by an early Archaean crust (Sm-Nd TDM model ages >3 Ga). Whether these two magmatisms were generated in the same environment or not is unresolved. In any case, the Tchilit terrane is distinctive from the 2.1 Ga Birimian volcano-sedimentary series of the West African craton that are entirely juvenile. Tchilit is the first Palaeoproterozoic non-granulitic terrane found in the Pan-African assembly of the Tuareg shield which raises questions about its provenance.