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

摘要

The crystal chemistry of six clinopyroxenes enclosed in protogranular spinel-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studied by means of single-crystal X-ray diffraction and electron microprobe. These spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-peridotite xenoliths of the same region (19 11percent and 15-8 percent respectively) and by high clinopyroxene orthopyroxene ratios: 0.7-2.7 (present study) and about 0.4 (Lee et al. 1996). The clinopyroxene crystal chemistry indicates that the 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 lithospheric mantle) testifies that the spinel peridotite and the associated small-volume melts completely re-equilibrated at the spinel-peridotite facies. This is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde-Biu Plateau spinel-peridotite xenoliths, which underwent Late Proterozoic Early Paleozoic enrichment in incompatible trace elements.