Emplacement of the Dschang granitic pluton (west-Cameroon): Constraints from microstructures and magnetic fabrics

Abstract

Microstructures and Anisotropy of Magnetic Susceptibility (AMS) data were investigated in the Pan-African Dschang granitic pluton and its basement rocks in order to decipher its emplacement and time relationship within the Pan-African evolution of the Central Cameroon Shear Zone (CCSZ). The Dschang granitic pluton, emplaced in biotite-hornblende gneiss country rocks, is made of biotite megacrystic granite and fine-grained biotite granite. The pluton has mostly low susceptibility magnitude (Km ≤ 500 μSI) implying that AMS susceptibility is mostly controlled by biotite. The foliation of the pluton is underlined by platy alkali-feldspar megacrystals that strike ENE-WSW to E-W or WNW-ESE with moderate to high dips, associated with biotite schlieren and spindle-shaped xenoliths. The magnetic foliations also strike dominantly NE-SW to E-W or NNW-SSE to NNE-SSW directions with moderate to steep dips. Magnetic lineations have low to moderate plunges and show radiating pattern in the Schmidt projection diagram. The microstructures within the pluton point to continuous deformation from magmatic to sub-magmatic state. The Jelinek shape parameter Tj shows abundant triaxial and oblate shape AMS ellipsoids and uncommon negative values (42% of the stations). Low to very low values of Pj parameter associated to (i) mostly triaxial and oblate shape AMS ellipsoids and (ii) dominantly magmatic to submagmatic state microstructures suggest a syn-to post-kinematic emplacement of a high viscosity granitic magma by flow mechanism. Magnetic fabrics infer that the Dschang granitic pluton was emplaced in a ENE-WSW to E-W oriented fracture initiated during the transcurrent strike-slip CCSZ.