Middle Miocene bimodal volcanism by asthenospheric upwelling: Sr and Nd isotopic evidence from the back-arc region of the Northeast Japan arc.
Middle Miocene bimodal volcanism by asthenospheric upwelling: Sr and Nd isotopic evidence from the back-arc region of the Northeast Japan arc.
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DOI：
10.2343/geochemj.28.473
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发表时间：
1994-12
影响因子：
0.8
通讯作者：
J. Ohki;K. Shuto;H. Kagami
中科院分区：
文献类型：
--
作者：
J. Ohki;K. Shuto;H. Kagami
根据第三纪至第四纪火山岩的Sr、Nd同位素资料，讨论了日本东北弧下上地幔的同位素特征和中中新世双峰火山作用的成因。弧后侧的玄武岩喷发时间为35 Ma至今，根据初始Sr同位素比值（Sri）、初始eNd值和年龄可分为两组。与另一组（~ 0.7029 ~ ~ 0.7040和~ +3 ~ ~ +8）相比，其中一组具有较高的Sri比率（~ 0.7040 ~ ~ 0.7058）和较低的eNd值（~ 0 ~ ~ +3）。高Sri组的玄武岩活动在30 ~ 10 Ma连续发生，而其他组的活动主要发生在两个时期；即在34-35 Ma左右和-15 Ma之后。在过渡带和海沟一侧的玄武岩中也观察到类似的模式，其中高Sri比的玄武岩活动在约15 - 0 Ma期间连续发生，而低Sri的玄武岩喷发发生在约15 Ma和约3 Ma之后。对这些特征的一种解释是，日本东北弧下的次大陆地幔可能是分层的；包括上部同位素未耗尽（Sri = ~ 0.7040 ~ ~ 0.7058, eNd = ~ 0 ~ ~ +3）和下部同位素耗尽（Sri = ~ 0.7029 ~ ~ 0.7040, eNd = ~ +3 ~ ~ +8）部分。具有未耗尽同位素特征的玄武岩岩浆被认为是由上层形成的，而具有耗尽同位素特征的玄武岩岩浆则来自下层。在-15 Ma之后，弧后侧具有贫同位素特征的玄武岩活动急剧增加，可能是由于日本海打开期间，热贫层从地幔深处（软流层）上涌至东北日弧弧后侧上方的未贫层。软流圈上升流也可能导致下地壳融化，在弧后侧和过渡带形成中中新世酸性火山岩。富岛和弧后侧Tsugawa-Tadami地区流纹岩的Sri比值（0.7055 ~ 0.7072）和eNd值（~ 0 ~ ~ -5）明显高于伴生玄武岩，表明这些流纹岩的地壳起源较低。另一方面，过渡带Kakudate-Rokugo地区的白云岩和流纹岩的eNd值（~ +4）相似，表明它们来自一个共同的地幔源，其同位素特征介于上部未衰竭层和下部衰竭层之间。
On the basis of Sr and Nd isotopic data for Tertiary to Quaternary volcanic rocks, the isotopic characteristics of the upper mantle beneath the NE Japan arc and genesis of Middle Miocene bimodal volcanism are discussed. Basaltic rocks from the back-arc side, erupted from 35 Ma to present, can be divided into two different groups based on initial Sr isotopic ratios (Sri), initial eNd values, and ages. One group has higher Sri ratios (∼0.7040–∼0.7058) and lower eNd values (∼0–∼+3) than the other (∼0.7029–∼0.7040 and ∼+3–∼+8). Basaltic activity of the high Sri group took place continually from 30 to 10 Ma, whereas activity of the other group occurred mainly in two periods; i.e., at around 34–35 Ma and after –15 Ma. A similar pattern is also observed in basaltic rocks from the transitional zone and the trench side, where basaltic activity characterized by higher Sri ratios took place continuously during ca. 15–0 Ma, whereas eruption of lower Sri basalts occurred at around 15 Ma and after about 3 Ma. One interpretation of these features is that the sub-continental mantle beneath the NE Japan arc may be layered; comprising upper isotopically undepleted (Sri = ∼0.7040–∼0.7058, eNd = ∼0–∼+3) and lower depleted (Sri = ∼0.7029–∼0.7040, eNd = ∼+3–∼+8) parts. Basaltic magmas with an undepleted isotopic signature are considered to have formed from the upper layer, while those with a depleted isotopic signature are from the lower layer. The sharp increase in basaltic activity with a depleted isotopic character after –15 Ma in the back-arc side may be due to the upwelling of hot depleted layer from a deeper region in the mantle (asthenosphere) to the upper undepleted layer beneath the back-arc side of the NE Japan arc during the opening of the Japan Sea. This asthenospheric upwelling might have also caused melting of the lower crust to produce the Middle Miocene acidic volcanic rocks in the back-arc side and the transitional zone. Rhyolitic rocks from Tobishima Island and the Tsugawa-Tadami area in the back-arc side show significantly higher Sri ratios (0.7055–0.7072) and lower eNd values (∼0–∼-5) than the associated basaltic rocks, indicating a lower crustal origin for these rhyolitic rocks. On the other hand, the similarity in eNd values (∼+4) of dolerites and rhyolites from the Kakudate-Rokugo area of the transitional zone suggests derivation from a common mantle source with an isotopic signature between the upper undepleted and lower depleted layers.