Time relationship between emplacement, fabric development and regional deformation of the Manchi granitic pluton (western - Cameroon domain)-an integrated AMS, CPO and microstructural investigation
Résumé
In this study, we integrate anisotropy of magnetic susceptibility (AMS) and microstructural data including quartz crystallographic preferred orientation (CPO) from SEM-EBSD from the Manchi Pan-African granitic pluton with the objective of deciphering the time relationship between fabric development, emplacement and regional tectonics. The Manchi pluton, located in the western Cameroon domain of the Pan-African Fold Belt of Central Africa in Cameroon, is made of hornblende-biotite granite (HBtG), biotite granite (BtG) and protomylonites of HBtG. The pluton intruded the high grade country rocks made of banded gneiss and amphibolites. AMS of the pluton is controlled dominantly by paramagnetic phases such as, Fe-bearing silicates (biotite and hornblende). This is inferred from the low Km values (<500 × 10-6 SI units) as well as from hysteresis data. The AMS ellipsoid shape is mostly oblate, implying that flattening component of the strain was important during granite emplacement. Microstructural observations reveal presence of magmatic to sub-magmatic and solid-state deformation textures developed at moderate- to low-temperatures. This is consistent with the presence of rhomb and basal slip systems established from quartz c-axis data through EBSD analysis. The pluton recorded top-toward SW to top-toward south sense of shear as established by the kinematic markers. This indicates a component of simple shear during tectonic evolution of the pluton. We infer that the magnetic fabrics recorded in the Manchi granitic pluton is a manifestation of the D2 regional deformation. The dominance of flattening strain and the concentric pattern of magnetic data are indicative of ballooning being an important mechanism under transpressive regime.