Structural evolution and tectonic setting of Pan-African igneous rocks around Boki, Bamenda Massif, southeastern Nigeria.
The Pan-African syntectonic felsic and mafic rocks of Boki region in southeastern Nigeria were comprehensively studied with focus on their chemical compositions and structural characteristics, the goal was to ascertain their tectonic setting and the impact of Pan-African thermotectonic events on the rocks. The intricate tectonic history of the area is reflected in the structural and mineralogical characteristics of these granitic bodies, which were created during the Pan-African orogeny. Extensive field investigations and structural examinations reveal that the granitoids have experienced notable deformation, manifesting as fractures and shear zones. High tectonic stress regimes that fostered the crystallization and evolution of mineral veins inside the rocks are indicated by these deformation characteristics. The attitude of more than 180 mode-I fractures and over 120 mineral veins were measured and documented. Rose diagrams and stereographic projections of the various structures were plotted and analyzed to show the impact of tectonic stress on the preferred orientation of the joints and veins. The granitic bodies include garnetiferous granite and granodiorites, which consist of quartz, biotite, k-feldspars, and subordinate muscovite, and tourmaline. They exhibit two major trends of fractures, which include the E –W trending mode-I fracture and the N – S trending fractures and veins. The E – W fractures (ac extensional fractures) and veins are oriented in the direction of the maximum principal stress (σ1) while the N – S fractures (bc tensile fractures) are oriented in the direction of minimum principal stress (σ3). The mode-I fractures are pervasive in the granitoids since they require less stress for their initiation and propagation relative to mode-II and mode-III factures. The widest veins are those oriented in the direction of the σ1. The mineral veins are very narrow and thins out easily in the N –S direction of σ3. The orientations of these veins is a reflection of the pattern of fluid pressure build-up during the Pan-African orogeny, which initiated the crystallization of the minerals in the veins. This study explains the configurations of the principal stresses and indicates E – W direction for σ1 and N – S direction for σ3 during the most part of the Pan-African orogeny.
Keywords: Stress configuration, Mode I fractures, Conjugate fractures, Mineral vein, Crack and seal mechanism.
