Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa)

摘要

Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An_(67.97-15.84)Ab_(69.19-30.43)Or_(20.59-1.51)) and anorthoclases (Ab_(68.11-61.20)Or_(33.87-20.91)An_(10.98-4.93)). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An_(67.97-59.3)_(0)Ab_(38.74-30.43)Or_(2.75-1.60)) and sanidine (An_(45.44-31.82)Ab_(62.66-51.79)Or_(5.52-2.77)), whereas in Low Magnesian basalt (LMg-B) there are labrador (An_(67.4.75-51.96)Ab_(44.98-33.72)Or_(3.06-1.51)), andesine (An_(45.44-31.82)Ab_(62.66-51.79)Or_(5.52-2.77)), oligoclase (An_(26.65-15.84)Ab_(69.19-63.57)Or_(20.59-8.55)) and anarthoclase (Ab_(68.11-61.20)Or_(33.87-20.91)An_(10.98-4.93)). Olivines are magnesian (Fo_(86.7-50.1)) and ferriferous (Fo_(48.8-37.8)). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo_(51.94-45.02)En_(44.41-33.16)Fs_(16.42-10.70)) and augites (Wo_(44.88-43.64)En_(41.03-37.04)Fs_(18.25-14.43)). Oxides are magnetites represented by ulvospinel (Usp_(90-75)Sp_(2-7)Mt_(5-23)). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.