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从日本弧东北部的弧后一侧的第三纪火山岩中测定了 Nd 和 Sr 的同位素组份。在整个时期内 Sr 同位素由富集状态特征~(87)Sr/~(86)Sr=0.705437)到富集贫化状态~(87)Sr/~(86)Sr=0.70270)的线性趋势。Nd 同位素以互相消长的形式,从低值(∈Nd=-0.80)开始然后随年代的减少而渐增。Nd 和 Sr 从富集到贫化的同位素变化与日本海在15百万年前的张开是同时期的。这种相同现象表明:日本海张开是由软流圈贯入引起的。在张开期前,地幔楔是由双层结构组成的:即陆下岩石圈和下伏的软流圈。这一阶段的火山岩的特征是富集同位素的状态,此火山岩系来自具有较大比例陆下岩石圈的物源。弧后一侧的陆下岩石圈因同位素贫化的软流圈贯入而变薄,它加速了在弧后一侧地幔楔内 MORB(大洋中脊玄武岩)物源的长大,并形成了张开后期 Nd 和 Sr 同位素呈贫化状态的岩浆的生成。
The isotopic compositions of Nd and Sr were determined from Tertiary volcanic rocks on the back arc side of northeastern Arc in Japan. The linear trend of the Sr isotopes from the enriched state (87) Sr / ~ (86) Sr = 0.705437 to the enriched / depleted state ~ (87) Sr / ~ (86) Sr = 0.70270 over the entire period. The Nd isotopes begin with a low value (∈Nd = -0.80) in the form of extinction with each other and then increase with decreasing age. The isotopic variation of Nd and Sr from enrichment to depletion is contemporaneous with the opening of the Sea of Japan 15 million years ago. This same phenomenon shows that the opening of the sea of Japan is caused by the influx of asthenospheric waves. Before the opening period, the mantle wedge was composed of double-layered structure: the submarine lithosphere and the underlying asthenosphere. Volcanic rocks at this stage are characterized by isotope-rich isotopes originating from a source with a large proportion of subterranean lithosphere. The subtropic lithosphere on the back-arc side is thinner due to the infiltration of isotope-depleted asthenosphere, which accelerates the growth of MORB (mid-oceanic basalts) provenance within the mantle wedge on the back-arc side Formation of magma depleted in Nd and Sr isotopes during late open.