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Abstrait

Granitoids of the older granite suites in Southeastern Nigeria

Bassey Edem Ephraim

The Older Granite suite is represented in southeastern Nigeria as in other parts of the country. In northern Obudu area, they are exposed as oval or dome-shaped isolated bodies that are sporadically distributed within the Precambrian basement complex. The granitoids, comprising mainly gneisses of monzogranitic composition, constitute the basement for the emplacements of both gabbroic and doleritic intrusions. The pervasive structural trend of the basements rocks in the N – S to NE – SW possibly point to the fact that the rocks were affected by the Pan – African orogeny. Field and petrographical evidences abound to support magmatic origin for the parent rocks of the granite gneiss. The occurrence of ferromagnesian phase(s) as isolate flakes or as oriented clots that probably followed magmatic flux direction, which is similar to the regional N –S to NE – SW foliation of the area suggest syntectonic emplacements of the granitoids. The granite gneiss is extensively dissected by a network of intersecting or crosscutting quartzofeldspathic stringers or veins/veinlets of various widths and orientations which also indicate the relevance of post-tectonic deformation and deuteric alteration in the evolutionary history of the rocks. Moreover, the granite gneiss is charged with numerous xenolithic lenses, rafts and blocks of the country rocks suggesting that one of the mechanisms of emplacement of the parent granitic rocks may have been the process of piecemeal stoping. The parent rocks were possibly emplaced into high-grade metamorphic rocks as parts of the Pan- African (600 ± 150 Ma) remobilization of the Nigerian basement. The N - S trending shear zones and other structures in the basement provided zones of weakness for the ascent of magmas, thereby causing the rocks to conform to the N - S to NE – SW structural trend of the country rocks.