Microstructures and temperature variability during the Eburnean deformations in the Daléma area, Eastern Senegal

Abstract

In the Dalema area, the eastern part of the Kedougou-Kenieba inlier, the petrographic study of feldspar and quartz microstructures revealed the variability of temperatures related to partitioned D2 Eburnean deformation and retrograde metamorphism from the superimposed D3 Eburnean deformation. Two major thermal events, Q1 and Q2, related to D2 and D3 Eburnean deformation phases, were identified in intracrystalline microstructures in the Dalema area. The first identified thermal event called Q1, which would have created the largest heat flow, was recorded especially in the shear zones. The microstructure features of this thermal event are syntectonic to the emplacement of granitoids around 2,100 Ma, during the D2 Eburnean deformation phase. This D2 Eburnean deformation phase is characterized by the partitioning of sinistral transpressive deformation between pure shear to simple shear-dominated domains. The thermal gradient associated with this event is very important in the inner shear zones (≥750 °C) and decreases towards their borders, where it would be estimated at ∼450 °C. Outside the shear zones, the metamorphic temperature peaks are less important with thermal gradients lower than 400 °C. They would be very low (≤250 °C) in undeformed rocks with well preserved primary beddings. The intensity of the second thermal event is weaker than in the first one, with temperatures not exceeding 500 °C. The intracrystalline microstructures which characterize this second thermal event are closely connected to those of the D3 Eburnean deformation phase, and cross-cut or are superimposed on those of the first thermal event related to the D2 Eburnean deformation phase. This second thermal event is then subsequent to the late D3 Eburnean deformation phase. The intracrystalline microstructures related to the D1 Eburnean deformation phase were not identified, and their absence is interpreted in various ways.