PETROLOGY OF ULTRAMAFIC XENOLITHS AND HOST LAVAS OF NGAOUNDÉRÉ (ADAMAWA PLATEAU, CAMEROON VOLCANIC LINE)

The Cameroon Volcanic Line is a NE-SW intraplate alkaline volcanic alignment extended for more than 1,700 km. It lies, for about 700 km, on oceanic lithosphere of the Gulf of Guinea (ca. 80-120 Ma), and for about 1,000 km on the continental African plate extending to the Lake Chad. The present study focused on the Adamawa Plateau, an asymmetric N70°E horst located in the area at North of the Ngaoundéré Plateau and bordered at North and South by the Adamawa and Djérem-Mbéré fault systems, respectively. Peridotitic xenoliths revealed hints on the nature of subcontinental African lithospheric mantle and the origin of their host lavas. Lherzolite and olivine-websterite are found among the peridotite xenoliths. Lherzolites are divided into two distinct groups equilibrated within different P-T conditions: a first group equilibrated in the ranges of 1.2-1.3 GPa and 913-979°C, whilst the other one equilibrated at deeper and hotter conditions (of 1.6-1.8 GPa; 996-1079°C). Olivine-websterite equilibrated within P-T conditions intermediate between the two lherzolitic groups, estimated at 1.4 GPa and 997°C. Chemical data on minerals from the peridotite xenoliths suggested the possible occurrence of at least one mantle metasomatism event overprinting a highly heterogeneous sub-continental lithospheric upper mantle. Geochemical and petrological data on host alkaline basalts suggested a clear OIB signature showing consistently high contents in Nb and Ta (46-89 ppm, 2.8-5.3 ppm, respectively) with a strong primitive character that suffered no crustal contamination during ascent to the surface. On the basis of incompatible trace element ratios we propose a magma genesis of the host lavas from low degree of partial melting (<2 vol.%) of an asthenospheric upper mantle, equilibrated within the garnet stability field (8-9 vol.%) and characterised by the presence of an EM I component.

Keywords: Cameroon Volcanic Line, petrology, Sub-Continental Lithospheric Mantle, peridotitic xenoliths, intraplate magmatism.