A comparison between the sub-continental lithospheric mantle of Libya, Morocco and Cameroon: Evidences from structural data and trace element of mantle xenolith Cr-diopsides

Abstract

We analysed the crystallography and the chemistry (major and trace elements) of 16 Cr-diopsides belonging to mantle xenoliths from northern (Waw-En-Namus, Libya; Bou-Ibalrhatene and Tafraoute, Morocco) and central Africa (Nyos and Barombi maars, Cameroon). These mantle xenoliths were extracted from mantle domains underlying a metacraton (Libya), a pericraton (Morocco), and a Pan-African mobile belt (Cameroon). The clinopyroxenes from pericratonic and mobile belt occurrences show similar geochemical features. They usually have high R3+ content and low Ca. When normalized to chondrite abundances, their incompatible trace elements show a wide spectrum of patterns, from slightly depleted to slightly enriched and U-shaped patterns. Such features may indicate interaction with carbonatitic and silicic melts. Their Vcell suggest different equilibrature pressures that are compatible with an ascending diapir and/or entrapment by magmas at different depths within the lithospheric mantle. It is important to stress that variable cell and tetrahedron volumes suggest that Libyan clinopyroxenes formed at variable depths. Those arising from shallower depth are less depleted and may display enriched and U-shaped patterns, while clinopyroxenes deriving from deeper zones display depleted and flat incompatible element patterns, possibly suggesting a melt extraction preceding the delamination of the SCLM.