Boula-Ibi granitic pluton syn-shearing emplacement during sinistral Neoproterozoic deformation: insights from microstructures and magnetic anisotropy

Abstract

Microstructure determinations and magnetic parameter measurements are conducted on the Neoproterozoic Boula-Ibi granitic pluton (BIGP) in the western domain of the Neoproterozoic Central African Fold Belt (NCAFB) in Cameroon. We deciphered the deformation and emplacement of the BIGP that contributed to highlight the magmatic sinistral kinematic context of its emplacement in the belt. It is emplaced in a basement made of monzonite, syenite, and gneiss. The magnitude of the magnetic susceptibility, dominantly ferromagnetic, is controlled by magmatic magnetite of different size ranging from single- to multi-domains as confirmed by thermomagnetic and hysteresis curves. The degree of magnetic anisotropy value is < 1.2 and AMS ellipsoids, of both oblate and prolate shapes, imply that flattening and shearing components have both influenced the granitic pluton emplacement. Microstructure data reveal that the pluton emplaced as syn- to late-tectonic magma that registered magmatic to solid state deformations with the drop in temperature in a progressive deformation. The magnetic fabrics of the BIGP strike NW–SE and NE-SW, with low plunges of lineation and moderate dips of foliation and display a sigmoidal S-type pattern, inferring a magmatic sinistral shearing context of the emplacement during transpressive tectonic. This tectonic is correlated to the transpressive D₂ deformation event of regional tectonic. Microstructures, kinematic markers, S-shape of the magnetic fabrics, and the emplacement age (596–566 Ma) of the BIGP indicate that it was emplaced during the transition period between the sinistral D₂ and the dextral D₃ deformation events in the NCAFB history in Cameroon.