Petrostructural and Microstructural Characterization of Granitoids from the Ziniare Region (Central Burkina Faso, West Africa)

Abstract

The Ziniare region is mainly made up of volcano-sedimentary and granitoid formations: the highly altered volcano-sedimentary formations are shales corresponding to the relics of the Birimian furrows, whose general orientation varies from north-south (N-S) to north-east-south-west (NE-SW). This structure is the result of regional shortening from east-west (E-W) to north-west-south-east (NW-SE), associated with the first phase of deformation (D1), which was ductile in nature. The granitoids present in the region can be divided into two groups: first-generation granitoids, belonging to the TTG series (tonalite-trondhjemite-granodiorite) and second-generation granitoids, represented in particular by the Ziniaré granitic pluton. The first-generation granitoids are oriented and affected by a second phase of deformation (D2), with a mean direction of N40. This deformation, marked by sinister shearing, is part of a tectonic continuum ranging from low-temperature to high-temperature conditions. It led to the development of characteristic tectonic structures such as sigmoidal cracks and pressure shadows, reflecting the intensity of the stresses exerted. The microstructures observed in these granitoids reflect a post-magmatic evolution acquired during this D2 phase. The D2 phase, which is predominantly east-west (NE-SW), may or may not have been taken over locally by a third phase of deformation (D3), marked by dextral shearing. In some areas, this phase affected the Ziniaré granitic pluton. However, on the whole, the microstructures observed in this pluton are mainly of magmatic origin, reflecting emplacement in a weakly deformed regime.