Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications
Résumé
The Fada N?Gourma area in Burkina Faso is underlain by Paleoproterozoic rocks that make the northeastern West-African Craton. This region is composed of NE-trending volcano-sedimentary belts and foliated tonalites, affected by several shear zones. A generation of younger, ?2100 Ma-old, non-foliated biotite-bearing granites intrudes the former rock units. We have investigated the younger granite pluton of Kouare that was previously considered as forming a single body with the pluton of Satenga to the west, a pluton which likely belongs to the ?20 Ma more recent Tenkodogo-Yamba batholith. Magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data. The granite encloses angular enclaves of the host tonalites. Magmatic microstructures are preserved inside the pluton and solid-state, high-temperature deformation features are ubiquitous at its periphery. The presence of steeply plunging lineations in the pluton of Kouare and its adjacent host-rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact. Around Kouare, the foliation in the host tonalites conforms with a map-scale, Z-shaped fold in between NNE-trending shear zones, implying a bulk clockwise rotation of the material contained in-between the shear zones, including the emplacing pluton. Regionally, the Fada N?Gourma area is concluded to result from NW-shortening associated with transcurrent shearing and vertical transfer of granitic magmas. This study concludes that the ?2200 Myears old juvenile crust of Burkina Faso was brittle before the intrusion of the biotite-granites, became softened close to them and that gravity-driven and regional scale wrench tectonics were active together.