Neoproterozoic alkaline meta-igneous rocks from the Pan-African North Equatorial Fold Bel (Yaounde, Cameroon): biotitites and magnetite rich pyroxenites

Abstract

The southern part of the Cameroon Pan-African North Equatorial Fold Belt displays unusual alkaline rocks, i.e. biotitites and magnetite rich pyroxenites. Chemical and petrographical studies show that: (1) the biotitites 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 found in intracontinental domains. Field relationships indicate that these unusual rocks were emplaced during the main D 2 compressional event. Moreover, the Yaounde Series, corresponding to shallow-water near-shore metasediments, also contains rocks of alkaline affinity (volcaniclastic 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 along the northern edge of the Congo Craton over quite a long period of time from crustal thinning to collision, and emphasises the role of extensional processes on the geodynamic evolution of the Pan-African North Equatorial Fold Belt in the Yaounde area.