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

Abstract

The Tchilit terrane in A¿r (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, amphibolite-facies (690 ± 40°C) metamorphism occurred during the Palaeoproterozoic whereas the second is a greenschist retrogression, late Pan-African in age (646 ± 6 Ma, 2σ, Ar-Ar age). The metasediments have been affected only by the Pan-African event and are probably Neoproterozoic. The syenogranite (619 ± 39 Ma, 2 σ , Rb-Sr) is later than 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 Assode 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 T DM 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 volcanosedimentary 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.