Petrology of the Mio-Pliocene volcanism to the North and East of Ngaoundéré (Adamawa, Cameroon)

Basaltic flows and thirty-five plugs of felsic alkaline volcanic rocks (phonolites and trachytes) of Mio-Pliocene age are exposed to the north and east of Ngaoundéré (Adamawa, Cameroon). The series is composed of alkaline basalts, hawaiites, mugearites, benmoreites, phonolites, and trachytes. Despite the gap between basaltic and felsic lavas, major- and trace-element distributions are in favour of a co-magmatic origin for the whole series. The basalts are similar in their chemical and isotopic compositions (0.7031 < 87Sr/86Sr)i < 0.7041; +2.2 < ε N d i < +5.0) to basalts from the continental and oceanic sectors of the ‘Cameroon Hot Line’. Peralkaline trachytes have a more radiogenic initial Sr isotopic composition (0.7064); they have probably been contaminated by crustal material. The Ngaoundéré basalts may derive from 1 to 2% partial melting of an infra-lithospheric source of the FOZO type (79% primitive mantle +20% altered MORB +1% pelagic sediments) at ∼80-km depth in the garnet stability field.