REE patterns, Nd-Sm and U-Pb ages of the metamorphic rocks of the Diagorou-Darbani greenstone belt (Liptako, SW Niger): implication for Birimian (Palaeoproterozoic) crustal genesis

Abstract

Abstract The Palaeoproterozoic Diagorou–Darbani greenstone belt in Liptako (Niger) is made up of micaschists, various amphibolites, metaconglomerates, and metabasalts intruded by granodioritic plutons. One of these plutons, the Dargol granodiorite, is dated at 2174±4 Ma, this age is comparable with those previously reported by many researchers. The micaschists (Type 1 sediments) and intercalated amphibolites have REE patterns variously enriched in light REE (LREE), suggesting oceanic arc-related rocks. The protolith of these micaschists have calc-alkaline affinities, with crystallization ages around 2273–2278 Ma, and T DM close to 2.3 Ga. This age is suggestive of an early Palaeoproterozoic magmatic event in crustal growth. The metaconglomerates (Type 2 sediments) exhibit REE patterns depleted in heavy REE (HREE) typical of tonalite–trondhjemite–granodiorite (TTG), the protolith of which may have been crystallized at 2187±55 Ma. These results, together with the earlier ones, led to a Palaeoproterozoic geodynamic model in which the crustal genesis was completely related to subduction zones, with an early Palaeoproterozoic magmatic event. Partial melting of a mantle slab generated the granitoid rocks of calc-alkaline affinities, whereas those with TTG characters could have been produced by direct partial melting of subducted oceanic crust. The crustal growth may have been the result of a continuous input of crustal materials in the interval time of 2.3–2.15 Ga, corresponding to ages recorded by various detrital zircon grains of micaschists and conglomerates.