Petrology,Geochemistry And Petrogenesis Of The Volcanic Rocks From The Bangangté Area,west Cameroon,Central Africa

The basaltic lavas of Bangangté area belong to Bameleke Plateau,located within the West Cameroon highlands(Bambouto,Bamenda and Oku Mountains)in the central part of the Cameroon Volcanic Line.The studied area is geographically bounded by the latitudes N 4°00'00" and N 5°35'00",and longitudes E 10°19'00" and E 10°49'00",and is average 1400 meters at its highest point.It lies between the Bambouto,Bamenda,and Oku mountains in the North,the Manengouba mountain in the South-West,the Bana and the Mbo plain in the West.New major,trace and rare earth elements geochemistry,and geochronological data of representative samples help to distinguish two magmatic series in the Bangangté area:alkaline and transitional series.The alkaline series forms the main volcanic rocks on the outcrop and compressing basanite,basalt,hawaiite,and mugearite,whereas the subalkaline(or exactly transitional)series is limited on the outcrop and are exclusively composed of basalt.These lavas have alkaline and transitional affinities and were emplaced during the Cenozoic between 36.67 and 8.89 Ma,with two distinct episode of volcanism:the first one it is the emplacement of basalt at?37 Ma representing the transitional series,the second one corresponding the alkaline series is characterised by the emplacement of basanite,basalt,hawaiite,and mugearite between?15 and 9 Ma.The composition of major ant traces elements show that these lavas evolved from basic to more differentiated rocks involving fractionation of olivine,clinopyroxene and Fe-Ti oxides.The isotopic compositions show minor effect of crustal contamination processes for transitional lavas,whereas alkaline lavas were not affected by the crustal contamination during their ascent in the crust.The Sr-Nd-Pb isotopic ratios of the transitional lavas(87Sr/86Sri=0.70380-0.70384,143Nd/144Ndi=0.51268-0.51277,206Pb/204Pb=18.06-18.65,207Pb/204Pb=15.52-15.62,and 208Pb/204Pb=38.15-39.07)are different to those of the alkaline lavas(87Sr/86Sri=0.70300-0,70330,143Nd/144Ndi=0.51287-0.51298,206Pb/204Pb=19.67-19.89,207Pb/204Pb=15.63-15.66,and 208Pb/204Pb=39.35-39.59).Chondrite-normalized rare earth element(REE)and primitive-mantle-normalized trace element patterns of the lavas from the study area are similar to those of oceanic islands basalts(OIB).The transitional lavas have Nb/Y ratios of 0.67-1.18,(La/Yb)N ratios of 4.96-5.84,and Mg#=29-39,and the alkaline lavas have Nb/Y ratios of 1.65-3.55,(La/Yb)N ratios of 7.76-20.13,and Mg#=35-58.Both the transitional and alkaline lavas are enriched in Nb,Ta,Ti,and Ba,and depleted in K and Pb on primitive-mantle-normalized incompatible element diagrams.The general features of trace elements and REE suggest that garnet and amphibole perform an important part during the generation of lavas from the study area.The batch partial melting modelling result suggests that transitional lavas were formed by relatively high degrees(3%-5%)as compared to alkaline lavas(0.6%-2.0%)of partial melting of amphibole-bearing garnet lherzolite.The geochemical and geochronological information imply that transitional and alkaline volcanism in the study area is related to exchange between the upwelling asthenosphere and the lithospheric mantle,which might have triggered a significant lithospheric extension.