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

Abstract

International audienceThe lavas of the Mount Cameroon, a Plio-Quaternary 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 × 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 87Sr/86Sr ratios of basic lavas are low (0.703198–0.703344), and 143Nd/144Nd 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.% in olivines (chrysolite, Mg# ranging from 79 to 84) of xenoliths, and 4.7 to 6.9 wt.% in diopsides of xenoliths, (b) diopsides of the clinopyroxenites have up to 7.2 wt.% Al2O3 and 2.3 wt.% TiO2, (c) spinels occur as interstitial grains between chrysolite and diopside grains, i.e. Cr2O3-rich magnetites (19 to 21 wt.% Cr2O3) in the dunites as well as (22 to 25 wt.% Cr2O3) in the wehrlites and titanomagnetites (14 to 15 wt.% TiO2) 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 Fe2+ / Mg2+ 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 AlIV/AlVI 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

Morphology and structure of the 1999 lava flows at Mount Cameroon Volcano (West Africa) and their bearing on the emplacement dynamics of volume-limited flows

“…The Mount Cameroon edifice surface is dominantly covered by basanite and hawaiite lavas and subsidiary fall-out tephra deposits. Recent K-Ar dates on seven mafic lava samples yielded ages between 2.83 ± 0.11 and 0.00 ± 0.09 Ma (Wandji et al 2009). Though weak Strombolian activity is observed, effusive eruption of lavas is the dominant phenomenon.…”

Geo-Electrical and Borehole Investigation of Groundwater in Some Basalts on the South-Eastern Flank of Mount Cameroon, West Africa

“…The mountain consists of more than a hundred cones which originate from moderate explosive activity [11]. It is the only active volcano [12] of a 1600 km long series of volcanoes known as the Cameroon Volcanic Line (CVL).…”

Physical Volcanology of Pyroclastic Tephra Deposit at Batoke Mt. Cameroon, West Africa: An Over View.

“…The eruptive style of this volcano has ranged from dominant effusive (Suh et al, 2003) to moderately explosive resulting in the production of pyroclastic deposits and numerous Strombolian cones. There has also been prehistoric phreatomagmatic activity at the volcano (Wandji et al, 2008, Ngwa et al, 2010. With an annual population growth rate of 2.7% and much infrastructural development at the foot and flanks of this volcano, there is a need for constant monitoring of the edifice to allow for hazard mitigation and management campaigns.…”