Deciphering vertical zonation of the Archean crust in the Tasiast Terrane (Reguibat Shield, Mauritania): Microstructural and geochemical evidence

The Tasiast terrane of the Reguibat Shield in Mauritania preserves a complex Archean crustal history recorded in tonalite–trondhjemite–granodiorite (TTG) grey gneisses and younger leucocratic gneisses. This study uses field observations, petrography, microstructural analysis, and geochemistry to reconstruct the thermomechanical evolution of this crustal segment. The TTG grey gneisses reveal evidence of high-temperature ductile deformation (>650°C), including mylonitization, elongated quartz, subgrain rotation (SGR), and grain-boundary migration (GBM). These features are consistent with deep crustal conditions and ages ranging from 3.07 to 2.91 Ga. These rocks formed through the partial melting of thickened mafic crust within the garnet-amphibolite to rutile-eclogite stability fields. In contrast, leucocratic gneisses, which formed at shallower depths, exhibit deformation at lower temperatures (300–450°C), featuring moderately deformed quartz, static feldspar recrystallization, and partial brittle overprinting. Their geochemistry indicates localized partial melting or injection of felsic melts derived from older TTG protoliths and is associated with a late thermal event between 2.56 and 2.48 Ga. Comparative analysis with other Archean cratons worldwide reveals a two-stage model of crustal evolution: early deep TTG formation and high-temperature, ductile deformation, followed by late-stage partial melting and the emplacement of leucocratic units. These results highlight the interplay between deformation, thermal evolution, and chemical differentiation during the formation and reworking of the Archean continental crust in Tasiast, establishing it as a key example of vertical crustal zonation in Archean terranes.