TECTONIC STRUCTURES AND METAMORPHIC FACIES CHARACTERIZATION OF THE PAN-AFRICAN SOUTHERN OUADDAI MASSIF (EASTERN CHAD): CONSTRAINTS FROM FIELD OBSERVATIONS, MICROSTRUCTURES AND MINERAL CHEMISTRY

The Pan-African Southern Ouaddai Massif (eastern Chad) records a polyphase tectono-metamorphic evolution involving three successive deformation stages. An initial regional foliation (D1) is expressed by composite S0/S1 surfaces oriented NE-SW to NW-SE with shallow to moderate dips, reflecting NW-SE shortening. The most pervasive phase (D2) is defined by S2 schistosity transposing earlier fabrics into S0/S1-2, associated with isoclinal folds (F1, F2), gently plunging lineations (L1, L2), and dextral mylonitic shear zones. Syn-D2 emplacement of muscovite-bearing leucogranites within shear zones highlights the contemporaneity of magmatism and deformation. A late-stage event (D3) corresponds to weakly developed axial-plane schistosity (S3), trending ENE-NE with steep dips, and is accompanied by brittle deformation expressed by NE-SW fracture networks locally filled with aplitic and pegmatitic veins.Metamorphic assemblages reveal a retrograde evolution from upper amphibolite facies (almandine garnet, biotite) to greenschist facies (quartz, muscovite, biotite, albite), with low-grade chlorite-sericite-actinolite-epidote associations preserved in the NW sector.