Field observations, magnetic fabrics and microstructures evidences of syn-kinematic emplacement of the Numba granitic pluton (western Cameroon domain)

Abstract

The N–S elongated Pan-African Numba granitic pluton (NGP), hosted in banded biotite hornblende gneiss and amphibolite country-rocks, belongs to the Western Domain of the Pan-African Belt of Central African in Cameroon. It consists of biotite hornblende granite and fine-grained biotite granite crosscut by gabbroic dykes. The western border of the granite is mylonitized. Structures and microstructures recorded in granite reveal magmatic, submagmatic and high- to moderate- temperature solid-state deformations. Magnetic characterisation of the NGP reveals its ferromagnetic behaviour due to the presence of multi-domain automorphic or xenomorphic pure magnetite grains. Field and AMS measurements testify that the study area recorded three deformation events: (1) a D1 event of ENE-WSW to E-W flattening deformation; (2) a D2 event of sinistral top-towards south sense of shear movement characterised by low to moderate dips (i) NE-SW strike foliation in country-rocks; (ii) NNE-SSW to NE-SW strike of magmatic foliation marked by the preferred orientation of K-feldspar megacrysts in granite and (iii) low plunges NE-SW mineral stretching lineation in biotite hornblende gneiss parallel to the axes of SW vergence of "S" shape F2 folds and (3) a D3 event of dextral top-towards east sense of shear movement that developed narrow NNE-SSW strike mylonite corridor. The NGP emplaced in a fracture within an amphibo-gneissic country-rock and has recorded opposite sinistral top-towards south (D2 Pan-African event) to dextral top-towards east senses of shear movements (D3 Pan-African event). It can be inferred that the emplacement and the deformation of the NGP was initiated during regional D2 deformation (dated at 613-585 Ma) when its magnetic fabric was acquired, and it was completely achieved during regional D3 deformation (dated ca. 585-540 Ma). Tj vs. Pj diagram associated with Flinn diagram suggests an emplacement in dominant flattening strain regime with a considerable shear component. The different data recorded in the NGP imply that its emplacement mechanism points to a ballooning. Indeed, the eastern domain of NGP with elliptical shape represents the inner part of the architecture while its western domain represents the outer part. The moderate to high dips observed in the inner part of the elliptical domain suggest the probable location of a feeder zone, and the lineation pointing away from the centre of the intrusion in the outer part may be developed related to the magmatic flow. This concentricity is easily associated with internal microstructures that show moderate-temperature solid-state to submagmatic and magmatic microstructures from the outside of the pluton towards the centre. The magma was then emplaced by ballooning in a gneissic country-rock that was subjected to sinistral shearing. Afterwards, the deformation characterised by a dextral shearing contributed to a generation of a NNE-SSW mylonitic band at the western border of the pluton.