Mineralogical and geochemical characteristics of the N-S trending Libiri doleritic dyke swarm of the Paleoproterozoic Liptako basement (West Niger): Genesis and magmatic evolution

The NNW-NNE Libiri doleritic dyke swarm of the Liptako region of West Niger is one of multiple mafic dyke swarms that crosscut the Birimian formations of the Léo-Man shield in the West African Craton. This approximately N-S trending swarm consists of two different groups based on mineralogy, texture and geochemical composition. Both groups have a matching assembly of minerals (ferro-edenite, pyroxene, plagioclase, biotite, apatite, chlorite, grains with granophyric texture and titanomagnetite-ilmenite). In detail Group I dykes have a distinct intergranular texture and some particular minerals (bytownite, pigeonite, clinoenstatite and sanidine) that differ from those in Group II dykes which have a modified ophitic texture and less calcic plagioclases (albite to labradorite). Variation in Mg# and differentiation index (DI) indicate that Group I (Mg# = 55-65, DI = 16.26-20.19) is more primitive in composition than Group II (Mg# = 30-35, DI = 22.82-50.00). REE and other incompatible element values are elevated in Group II compared to Group I, and the REE slopes are parallel one to each other, which suggests that the two groups are co-genetic and that Group II units are more evolved magmas from Group I. The presence of negative Nb and Ta values normalized to primitive mantle in both groups and the Lan/Ybn (4.73-6.21) ratios suggests that the magma was formed by partial melting of hydrated garnet lherzolite from the lithospheric mantle, modified by interaction with previously metasomatized lithosphere in subduction setting. Alteration minerals such as ferro-edenite and chlorite on augite crystal rims indicate later deuteric alteration or medium to high grade late magmatic metamorphism. Petrography and chemistry of a sample SD (Samira Dyke, a sample from the Libiri pit in the Samira gold mine, which was previously dated at 1791 ± 3 Ma) suggests the entire swarm of NNW-SSE to NNE-SSW trending dykes belong to the Libiri swarm of a plume-generated Large Igneous Province that extends into formerly adjacent Sarmatia (Baltica) and Amazonia.