Xenoliths of dunites, wehrlites and clinopyroxenites in the basanites from Batoke volcanic cone (Mount Cameroon, Central Africa): petrogenetic implications

摘要

The lavas of the Mount Cameroon, a Plio-Quatemary stratovolcano and the most important volcano along the Cameroon Volcanic Line (CVL), constitute a weakly differentiated alkaline series: mainly comprising basanites as well as alkaline basalts, hawaiites and mugearites. Ultramafic xenoliths (1-5 X 0.5-4 cm) of dunites, wehrlites and clinopyroxenites have been discovered in the basanites of a strombolian cone, located near Batoke on the South flank of the massif at an elevation of 500 m. K-Ar whole rock dating of the basanitic host rock has yielded an age of 0.73 + - 0.08 Ma. This result falls within the range of the seven new K-Ar age determinations of mafic lavas, between 2.83 Ma and the Present. These are the first K-Ar data on this massif. The ~(87)Sr/~(86)Sr ratios of basic lavas are low (0.703198-0.703344), and ~(143)Nd/~(144)Nd ratios are intermediate (0.512851-0.512773). These ratios are typical of a mantle origin. The main characteristics of the xenoliths are: (a) total FeO contents are 15.1 to 19.1 wt. percent in olivines (chrysolite, Mg# ranging from 79 to 84) of xenoliths, and 4.7 to 6.9 wt. percent in diopsides of xenoliths, (b) diopsides of the clinopyroxenites have up to 7.2 wt. percent Al_2O_3 and 2.3 wt. percent TiO_2, (c) spinels occur as interstitial grains between chrysolite and diopside grains, i.e. Cr_2O_3-rich magnetites (19 to 21 wt. percent Cr_2O_3) in the dunites as well as (22 to 25 wt. percent Cr_2O_3) in the wehrlites and titanomagnetites (14 to 15 wt. percent TiO_2) in the clinopyroxenites. Mineralogical analyses show an important re-equilibration between the chrysolite xenocrysts and the host basanitic magma. We observed a decrease in Mg and Ni towards the rim, and an enrichment in all others cations like Fe, Mn, Ca, Si. The changes of Fe~(2+)/ Mg~(2+) are the most important. The xenoliths are interpreted as cumulates: clinopyroxenite xenoliths have probably crystallized and fractionated at an early stage from the mafic (host basanitic) magma, while dunite and wehrlite xenoliths seem to have crystallized from a previous more primitive batch of magma. These alkaline liquids could have been derived from partial melting of a garnet- rich lherzolite in the upper mantle beneath the Cameroon Volcanic Line. The Al~(IV)/Ar~(VI) ratios remain high (1.2 to 4.9) in the clinopyroxenes of the xenoliths. This suggests crystallization under a lower pressure than that of equilibration of the clinopyroxenes (ratios 0.6 to 0.8) found in typical mantle xenoliths from the CVL.