Kinematic analysis of the Dschang granitic pluton (West-Cameroon): Implications to the Pan–African deformation of the Central African fold belt in Cameroon during the post-collisional history of western Gondwana

Abstract

The Pan-African (578–563 Ma, U-Pb zircon age) Dschang granitic pluton belongs to the edge of the central domain of the Central African Fold Belt in Cameroon. It is made up of biotite granites that intruded pre-Pan-African biotite-hornblende orthogneiss basement. A new detailed structural study testifies that this area recorded four deformation phases (D1-D4). The D1-related structures described in the basement rocks are folded. They were progressively transposed into the main S2 foliation during D2 deformation which is synchronous with the emplacement of E-W granitic dykes. The D2 strikes NNW-SSE to NW-SE. It is characterized by main S2 metamorphic foliation (mean pole of the whole data at 269°/66°), NW–SE low to moderate plunging lineations (mean lines of the whole data at 159°/15°), F2 folds with NW plunging axes, C2 sinistral E-W ductile shear indicators (marked by injection of granite dykes into the biotite-hornblende orthogneiss). The D3 phase is marked in the biotite-hornblende orthogneiss by C3 dextral shear planes, followed by dextral rotated boudins (ß3). In the granites, it is characterized by S3 magmatic foliation (mean pole of the whole data at 323°/61°) due to the reorientation of platy megacrystals of K-feldspar, F3 folds profile with moderate plunging axes (mean lines of the whole data at 145°/28°) and sinistral vergence. The D4 phase is usually ductile. It is characterized by minor faults with C4 sinistral shear planes which crosscut the S3 magmatic foliation. The D2 phase is synchronous with the early emplacement of the Dschang granites at 578 Ma whereas D3 marks its end at 563 Ma. Mesoscopic (phenocrysts alignment, elongated enclaves, schlierens) and microscopic features (quartz, plagioclase and alkali feldspar deformation microstructures) are consistent with a continuum of deformation from magmatic to submagmatic states in the Dschang granites, the later state being more common. We conclude that the history of the Dschang area began with the emplacement of biotite-hornblende orthogneiss before 578 Ma; it was followed by the emplacement of granite that started towards the end of the D2 phase at 578 Ma (probably aided by the E-W sinistral shear fractures) and ended during the D3 phase at 563 Ma. This is consistent with the emplacement of the Dschang granitic pluton during the transition period between the sinistral left lateral wrench movement and the dextral right lateral wrench movement of the history of western Gondwana. However, the scarcity of typical structures of a synkinematic intrusion or a post-magmatic ductile shearing (mylonites, S/C-structures) in the study granite suggests its emplacement into a low strain crustal domain within an anastomosing pattern of shear zones.