Re ´sume Ĺesbasaltes transitionnels e ´oce `nes occupent plus de la moitie ´ouest des affleurements laviques du plateau Bamoun. Les basaltes de ce type restent exceptionnels dans la ligne volcanique du Cameroun (LVC). Ils posse `dent des caracte ´ristiques pe ´trographiques (basaltes sans olivine, BSO ; a ˆge de 51,8 AE 1,2 Ma ; le plus vieux de la LVC) et ge ´ochimiques, diffe ´rentes de celles des basaltes alcalins de ce me ˆme plateau (basaltes a `olivine, BAO ; a ˆge de 46,7 AE 1,1 Ma) et de tous les autres basaltes alcalins, classiques, de la LVC. Inversement, ils apparaissent plus proches des roches analogues des rifts est-africains et des ı ˆles Kerguelen. Ils se caracte ´risent par la rarete ´de l'olivine modale et par la valeur relativement e ´leve ´e du rapport Y/Nb (e ´gal a `1), mais faible de La N /Yb N (10) et de Ce N /Yb N (7). Les valeurs des rapports isotopiques initiaux 87 Sr/ 86 Sr (0,7044) et 143 Nd/ 144 Nd (0,5126) des BSO sont respectivement plus e ´leve ´es et plus faibles que celles relatives aux BAO (0,7034 et 0,5128). Ces re ´sultats mettent en e ´vidence l'existence d'un re ´servoir mantellique enrichi par un po ˆle de type EM lithosphe ´rique a `l'origine des basaltes transitionnels, diffe ´rent du re ´servoir plus proche du po ˆle HIMU, a `l'origine des basaltes alcalins.
The Magba Shear Zone is made up of granites, migmatites, orthogneiss, metagabbro, mafic dyke and mylonites with coarse grained texture, porphyroblastic, granoblastic, cataclastic and mylonitic texture respectively. Structural features and kinematic indicators testify the syntectonic emplacement of Magba granitoids and also provide detailed information on the relative timing of deformation as follows: (1) D1 of tangential movement immediately followed by (2) the D2 phase which is heterogeneous simple shear in dextral transpressive context with a NW-SE direction (3) D3 tectonic phase is marked by sinistral transpressive tectonic and superposed folding with a NE-SW kinematic direction. Combined ductile NE-SW shear movements and NW-SE compressional movements defined a transpressional tectonic regime during the D3 deformation (4) A brittle stage D4 is controlled by transcurrent tectonics and responsible for the emplacement of faults, and joints. The Magba granites would have intruded along sub-vertical mid-crustal feeder channels and were emplaced as a sheet or sheets along the shear zone during the early stage of the C3 shearing, followed by gabbro and mafic dyke at the late stage. Strike-slip dilatancy pumping under transpressive tectonic is suggested as a possible mechanism for the emplacement of the Magba granites.
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