PROTEROZOIC AND CENOZOIC INTERMEDIATE MAGMATISM IN CAMEROON: CONSTRAINTS ON THE GEODYNAMIC ENVIRONMENT

One of the main features of igneous activity in Cameroon is a 400 m.y. gap in magmatism between Proterozoic and Cenozoic times. In spite of abundant studies on these rocks, little has been done concerning comparative studies between the Proterozoic and the Cenozoic magmatism. This paper deals with a comparative study of the magmatism of both epochs, with particular attention to the intermediate plutonic rocks, from which the following remarks can be drawn: a) similarities in textural and petrographic characters b) (52<SiO2<65) with two analogous evolutionary trends, and c) mildly alkaline, high K to shoshonitic (K2O/Na2O>2) character for Proterozoic plutons and alkaline (K2O<Na2O) character for Cenozoic plutons. From our observations the following conclusions can be drawn: Chemical characteristics of Proterozoic intermediate magmatism are consistent with production in a subduction environment; this is supported by the abundance of associated abundant juvenile, and more voluminous, felsic plutons of calc-alkaline affinity during the magmatic climax of Pan-African orogenesis (600 Ma). Furthermore, the late Neoproterozoic plutons range from calkalkaline through mildly alkaline to alkaline. The increasing alkalinity is explained by a geodynamic model where calc-alkaline magmas are formed during plate convergence and subduction which ends with collision (assembly of West Gondwanaland). The collisional phase formed major lithospheric fracture zones, and mantle-derived alkaline magmas could therefore be formed at the end of Neoproterozoic. After 400 m.y. of inactivity, magmatism began again through reworking of these fractures, forming alkaline and hyperalkaline magmatism during the Cenozoic. In this framework, the nature and origin of plutons in the Cenozoic Cameroon Volcanic Line are interpreted, not in terms of mantle heterogeneities beneath the Line, but as a consequence of a lithospheric environment that was formed during the Proterozoic.