Les basaltes éocènes à affinité transitionnelle du plateau Bamoun, témoins d’un réservoir mantellique enrichi sous la ligne volcanique du Cameroun

Abstract

Re ´sume Ĺesbasaltes transitionnels e ´oce `nes occupent plus de la moitie ´ouest des affleurements laviques du plateau Bamoun. Les basaltes de ce type restent exceptionnels dans la ligne volcanique du Cameroun (LVC). Ils posse `dent des caracte ´ristiques pe ´trographiques (basaltes sans olivine, BSO ; a ˆge de 51,8 AE 1,2 Ma ; le plus vieux de la LVC) et ge ´ochimiques, diffe ´rentes de celles des basaltes alcalins de ce me ˆme plateau (basaltes a `olivine, BAO ; a ˆge de 46,7 AE 1,1 Ma) et de tous les autres basaltes alcalins, classiques, de la LVC. Inversement, ils apparaissent plus proches des roches analogues des rifts est-africains et des ı ˆles Kerguelen. Ils se caracte ´risent par la rarete ´de l'olivine modale et par la valeur relativement e ´leve ´e du rapport Y/Nb (e ´gal a `1), mais faible de La N /Yb N (10) et de Ce N /Yb N (7). Les valeurs des rapports isotopiques initiaux 87 Sr/ 86 Sr (0,7044) et 143 Nd/ 144 Nd (0,5126) des BSO sont respectivement plus e ´leve ´es et plus faibles que celles relatives aux BAO (0,7034 et 0,5128). Ces re ´sultats mettent en e ´vidence l'existence d'un re ´servoir mantellique enrichi par un po ˆle de type EM lithosphe ´rique a `l'origine des basaltes transitionnels, diffe ´rent du re ´servoir plus proche du po ˆle HIMU, a `l'origine des basaltes alcalins.

Plates Amalgamation and Plate Destruction, the Western Gondwana History

“…This conclusion is corroborated by the ages recently obtained for the Kokoumi complex southwest of Mboutou (39 Ma) and for the Hossere Nigo west of Adamawa plateau (65 Ma) (Ngounouno et al, 2001;Kamdem et al, 2002;Montigny et al, 2004). New K/Ar and Ar/Ar data on volcanic rocks of the CL and reference therein; Moundi et al, 2007;Wandji et al, 2008) show that volcanic activity ranges from Upper Eocene (47 Ma) to the Present and not from Oligocene (30 Ma) as established for the distribution of volcano-capped swells along the line (Burke, 2001). Lee et al (1994) noted a SW younging of volcanism in the oceanic sector of the CL from Principé (31 Ma) to Pagalu (5 Ma).…”

“…Cameroon and Etinde) increasing to mantle (MORB-like) values in its oceanic (towards Annobon) and continental (towards Ngaoundéré) terminals. Even though a review of the CL volcanism by Déruelle et al (1991) suggests that it is entirely alkaline in nature, some transitional affinities have recently been described in the Mbam, Bangou, Bambouto and Oku volcanic centers in the west and northwest Cameroon (Marzoli et al, 2000;Fosso et al, 2005;Moundi et al, 2007). Some mafic lavas of the CL are rich in ultramafic xenoliths (Lee et al, 1996;Aka et al, 2004).…”

“…Lee et al (1994) noted a SW younging of volcanism in the oceanic sector of the CL from Principé (31 Ma) to Pagalu (5 Ma). The oldest volcanic ages (K-Ar) so far obtained (46.7 ± 1.1 Ma and 45.5±1.1 Ma) are respectively in an olivine basalt from the Bamoun plateau (Moundi et al, 2007) and the Mbépit rhyolites in the Noun plain (Wandji et al, 2008), next to the Bamoun plateau. The transitional lavas from the Bamoun plateau give a K-Ar age of 51.8±1.2; Ma (Moundi et al, 2007) and those of Mount Bangou, a K-Ar age of 44.5±1 Ma (Fosso et al, 2005).…”

“…The oldest volcanic ages (K-Ar) so far obtained (46.7 ± 1.1 Ma and 45.5±1.1 Ma) are respectively in an olivine basalt from the Bamoun plateau (Moundi et al, 2007) and the Mbépit rhyolites in the Noun plain (Wandji et al, 2008), next to the Bamoun plateau. The transitional lavas from the Bamoun plateau give a K-Ar age of 51.8±1.2; Ma (Moundi et al, 2007) and those of Mount Bangou, a K-Ar age of 44.5±1 Ma (Fosso et al, 2005). Therefore, as for anorogenic complexes, it is difficult to consider volcanic activity along the whole CL in terms of a steady time-space migration, symptomatic of a mantle plume reference frame as in the Hawaiian Islands.…”

Hazards from Pyroclastic Density Currents at Mount Bamenda (West-Cameroon, Cameroon Volcanic Line): Mapping and Assessment by AMS Method

“…1). Volcanism along this line began in the Eocene by the formation of Bamoun plateau between 51.8 and 46.7 Ma (Moundi et al, 2007) and Mount Bangou between 44.7 and 43.1 Ma (Fosso et al, 2005) and continues until the present day on Mount Cameroon (Suh et al, 2003). The products of the CVL volcanism are mainly rhyolites, phonolite, trachyte and basalt.…”