GEOCHEMICAL CHARACTERIZATION OF ORTHO-DERIVED GNEISS AND GRANITOIDS OF THE PAN-AFRICAN PROVINCE OF DAMAGARAM (SOUTH-EAST NIGER)

The Pan-African province of Damagaram, object of this study, corresponds to the north-eastern extension of the Benin-Nigerian Shield. The main geological formations that outcrop in this region consist of migmatitic gneiss, banded gneiss, mylonitic gneiss, porphyritic granites, two-mica granites, biotite granites and alkaline granites. The petrogenesis of ortho-derived gneisses and granitoids of Damagaram province was studied using the methods of geochemistry on total rock (major elements, trace elements and rare earth elements). The chemical composition (major elements and trace elements) shows that the precursor magmas come from the subalkaline series. The analyzed samples are characterized by a high SiO2 content (≥ 65%) and show a calc-alkaline potassium to shoshonitic affinity typical of the subduction zone. The high alkaline content (Na2O + K2O) and the A/CNK and A/NK parameters indicate that these rocks are aluminous to hyperalumino. Enrichment in certain large lithophilic elements (LILEs) (Rb, Ba, Th and Sr) and depletion in certain high field intensity elements (HFS) such as Nb, P and Ti are favorable criteria for the involvement of the crust in the genesis of these rocks. The light rare earth enrichment (LREE) and heavy rare earth depletion (HREE), as well as the negative europium (Eu) anomaly, suggest that these rocks as a whole would come from either fractional crystallization or partial melting of a pre-existing continental crust. The geochemical characteristics indicate that the geodynamic domain of these formations corresponds to the syn-collisional and intraplate volcanic arc geotectonic environments.