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

Nicaise Blaise Tchuimegnié Ngongang, Merlin Patrick Njombie Wagsong, F. Owono, I. Badriyo, P. Essomba, Nathalie Love Ngongang Tchikankou, Dieudonné Youmen, Pierre Kamgang, G. Chazot
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引用次数: 1

Abstract

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 (An67.97-15.84Ab69.19-30.43Or20.59-1.51) and anorthoclases (Ab68.11-61.20Or33.87-20.91An10.98-4.93). 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 (An67.97-59.30Ab38.74-30.43Or2.75-1.60) and sanidine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), whereas in Low Magnesian basalt (LMg-B) there are labrador (An67.4.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55) and anarthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Olivines are magnesian (Fo86.7-50.1) and ferriferous (Fo48.8-37.8). 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 (Wo51.94-45.02En44.41-33.16Fs16.42-10.70) and augites (Wo44.88-43.64En41.03-37.04Fs18.25-14.43). Oxides are magnetites represented by ulvospinel (Usp90-75Sp2-7Mt5-23). 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.
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西非巴方及其周边(喀麦隆火山岩线)新生代板内碱性火山岩的矿物学及岩浆作用
八坊及其周边碱性玄武岩由长石、橄榄石、辉石和氧化物组成,它们以斑晶、微斑晶和微晶的形式出现。长石是斜长石(An67.97-15.84Ab69.19-30.43Or20.59-1.51)和斜长石(Ab68.11-61.20Or33.87-20.91An0.98-4.93)。斜长石是这些长石中含量最丰富的。斜长石仅出现在所研究的碱性玄武岩中分化程度最高的木格岩(BAF3和BAF37)中。在高镁质玄武岩中,(HMg-B)斜长石为拉布拉多岩(An67.97-59.30Ab38.74-30.43Or2.75-1.60)和萨尼丁岩(An45.44-31.82Ab62.66-51.79Or5.52-2.77),而在低镁质玄武岩(LMg-B)中,有拉布拉多岩,寡长石(An26.65-15.84Ab69.19-63.57Or20.59-8.55)和反长石(Ab68.11-61.20Or33.87-20.91An0.98-4.93)。橄榄石为镁质(Fo86.7-50.1)和含铁质(Fo48.8-37.8)。在HMg-B中,橄榄石仅为镁质。这些橄榄石是温绿石和透明橄榄岩。在LMg-B中,橄榄石为镁质和含铁质,以含铁为主。它们是温绿石、透明石岩和短石岩。辉石为Ca、Mg和Fe斜辉石。透辉石(Wo51.94-45.02En44.41-33.16Fs16.42-10.70)和辉石(Wo44.88-43.64En41.03-37.04Fs18.25-14.43)。氧化物是以ulvospinel为代表的磁铁矿(Usp90-75Sp2-7Mt5-23)。铁磁体矿物(不透明氧化物、橄榄石和辉石)的分馏是主要的分化过程。分离结晶的两个阶段可以区分:第一个阶段是碧玄岩,第二个阶段是夏威夷岩到木格岩。所研究的玄武岩熔岩中的现象晶体的化学成分记录了喷发前新岩浆脉动侵入现有岩浆库的岩浆补给特征。
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