NEOPROTEROZOIC ALKALINE META-IGNEOUS ROCKS FROM THE PAN-AFRICAN NORTHEQUATORIAL FOLD BELT (YAOUNDE, CAMEROON) - BIOTITITES AND MAGNETITE RICH PYROXENITES

The southern part of the Cameroon Pan-African North Equatorial Fold Be lt displays unusual alkaline rocks, i.e. biotitites and magnetite rich pyroxenites. Chemical and petrographical studies show that: (1)the bi otitites are mafic alkaline rocks with a high accessory phase content, (2) the magnetite rich pyroxenites have exceptionally high P contents , and an ultramafic composition similar to alkaline ultramafic rocks f ound in intracontinental domains. Field relationships indicate that th ese unusual rocks were emplaced during the main D-2, compressional eve nt. Moreover, the Yaounde Series, corresponding to shallow-water near- shore metasediments, also contains rocks of alkaline affinity (volcani clastic metagreywackes). All these data suggest that alkaline magmatic activity was initiated during an extensional stage, but ended during a collisional stage. This implies the persistence of mantle activity a long the northern edge of the Congo Craton over quite a long period of time from crustal thinning to collision, and emphasises the role of e xtensional processes on the geodynamic evolution of the Pan-African No rth Equatorial Fold Belt in the Yaounde area. (C) 1998 Elsevier Scienc e Limited.