The Palaeoproterozoic Tchilit exotic terrane (Air, Niger) within the Pan-African collage of the Tuareg shield

The Tchilit terrane in Air (Niger) is one of the most exotic terranes among those constituting the Tuareg shield. It is composed of low-E; calc-alkali ne metabasalts and meta-andesites (amphibolites), alkaline metarhyolites, d etrital metasediments and of a syenogranite. Volcanic protoliths are Palaeo proterozoic in age (within the 1600-1200 Ma time span: Sm-Nd, Rb-Sr and Ar- Ar systematics) and underwent two metamorphisms accompanied by mylonitizati on, The first, amphibolite-facies (690 +/- 30 degrees C) metamorphism occur red during the Palaeoproterozoic whereas the second is a greenschist retrog ression, late Pan-African in age (646 +/- 6 Ma, Za, Ar-Ar age). The metased iments have been affected only by the Pan-African event and are probably Ne oproterozoic. The syenogranite (619 +/- 39 Ma, 2 sigma, Rb Sr) is later tha n both metamorphisms: it is the only Pan-African material in Tchilit. The P an-African tectonics, a N-S transpression (dextral shear) induced the overt hrusting of the neighbouring Assode terrane onto Tchilit. Major, trace elem ents and Sr-Nd isotopes give to the protoliths of amphibolites a continenta l active margin signature (low-K calc-alkaline) and to those of the metarhy olites an alkaline one, more typical of a post-collisional or intraplate se tting. Both were significantly contaminated by an early Archaean crust (Sm- Nd T-DM model ages >3Ga). Whether these two magmatisms were generated in th e same environment or not is unresolved. In any case, the Tchilit terrane i s distinctive from the 2.1 Ga Birimian volcanosedimentary series of the Wes t African craton that are entirely juvenile. Tchilit is the first Palaeopro terozoic non-granulitic terrane found in the Pan-African assembly of the Tu areg shield which raises questions about its provenance.