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Xenoliths of dunites, wehrlites and clinopyroxenites in the basanites from Batoke volcanic cone (Mount Cameroon, Central Africa): petrogenetic implications |
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| Authors: | P Wandji J P F Tsafack J M Bardintzeff D G Nkouathio A Kagou Dongmo H Bellon H Guillou |
| Institution: | 1. Laboratoire de Géologie, école Normale Supérieure, Université de Yaoundé I, BP 47, Yaoundé, Cameroun 2. Département des Sciences de la Terre, Faculté des Sciences, Université de Yaoundé I, BP 812, Yaoundé, Cameroun 3. Laboratoire de Pétrographie-Volcanologie, bat. 504, Université Paris-Sud, 91405, Orsay Cédex, France 4. IUFM, Université de Cergy-Pontoise, 95000, Cergy-Pontoise, France 5. Département des Sciences de la Terre, Faculté des Sciences, Université de Dschang BP 67, Dschang, Cameroun 6. UMR CNRS 6538, IUEM, Laboratoire de Géochronologie, Université de Bretagne Occidentale, 6 av. Le Gorgeu, CS 93637, 29238 Brest Cédex 3, France 7. CNRS, LSCE, Av de la Terrasse, 91198, Gif sur Yvette, France |
| Abstract: | The 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. |
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