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大岛火山是地球化学变异的一个端员,这种变异的特征是横穿东北日本弧向其弧后一侧FeO含量降低。玄武岩分析资料显示出具有原始岩浆的特征。化学成分的变化趋势表明,从苦橄岩到分异玄武岩,开始时是橄榄石结晶,然后是橄榄石+单斜辉石结晶。越富镁的玄武岩具有越富镁的橄榄石斑晶。所有岩石样品中最富镁的(MgO15%)样品都含有作为液相线相的橄榄石斑晶(Fo93.7)。由此认为这是幔源岩浆的产物。就大岛岩浆而言,其幔源岩浆的FeO和MgO含量的大致范围是可以确定的。此外,东北日本弧弧后的一目澙二辉橄榄岩包体不可能和富FeO的地幔源是一样的。本文认为,弧后一侧之下的上地幔成分是分带的,其上部是富铁地幔(一目澙二辉橄榄岩),下部为贫铁地幔(即弧后火山的地幔源)。
The Oshima volcano is a member of the geochemical variation that is characterized by a decrease in FeO content across the northeastern Japan arc toward its arc. Basalt analysis data show the characteristics of the original magma. The trend in chemical composition shows that from picrites to divisible basalts, olivine crystallites are the first to be followed by olivine + clinopyroxenes. The more magnesium-rich basalts have the magnesium-rich olivine phenocrysts. The most magnesium-rich (MgO15%) sample in all rock samples contains olivine plaque (Fo93.7) as the liquidus phase. This is considered as the product of mantle magma. In the case of the Oeda magma, the approximate range of FeO and MgO content of the mantle source magma can be determined. In addition, the monoclinic peridotite inclusions after arc in Northeastern Japan can not be the same as the mantle sources that are rich in FeO. This paper argues that the upper mantle beneath the arc-back is zonally subdivided, with the top of the iron-rich mantle (monocular lherzolites) and the lower iron-depleted mantle (the mantle source of back arc volcanoes).