NEW DATA ON THE CENOZOIC VOLCANISM OF THE GAROUA VALLEY (UPPER BENUE TROUGH, NORTHERN CAMEROON)

Thirty-one plugs of Cenozoic age (37 Ma) outcrop in the Garoua rift (n orthern Cameron). Basaltic lavas are similar to others from the Camero on Line. Phonolites contain Fe-salite and aenigmatite phenocrysts. Tra chytes are biotite-, biotite and richterite-bearing. ore peralkaline ( with richterite, arfvedsonite and Ti-aenigmatite). Rhyolites contain F -Mn-arfvedsonite and Ti-aegirine. Major and trace element distribution s suggest a co-magmatic relationship between the basaltic and the fels ic lavas, except the rhyolites. Late biotite fractionation governs the differentiation process of felsic lavas and fluid phases played a maj or role during its late stages. High initial Sr-87/Sr-86 suggest a pro gressive crustal contamination.