Clinopyroxene of spinel-peridotite mantle xenoliths from Lake Nji (Cameroon Volcanic Line, W Africa): crystal chemistry and petrological implications

The crystal chemistry of six clinopyroxenes enclosed in protogranular spine l-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studie d by means of single-crystal X-ray diffraction and electron microprobe. The se spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-perid otite xenoliths of the same region (19-11% and 15-8% respectively) and by h igh clinopyroxene/orthopyroxene ratios: 0.7-2.7 (present study) and about 0 .4 (Lee et al, 1996), The clinopyroxene crystal chemistry indicates that th e xenoliths show the compositional features of a mantle source contaminated by small-volume melts, responsible for the high clinopyroxene content, The protogranular texture of the Cameroon xenoliths (upper portion of lithosph eric mantle) testifies that the spinel peridotite and the associated small- volume melts completely re-equilibrated at the spinel-peridotite facies. Th is is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde-Biu Plateau spinel-peridotite xenoliths, which underw ent Late Proterozoic-Early Paleozoic enrichment in incompatible trace eleme nts.