Across-arc Variations in Geochemistry of Oligocene to Quaternary Basalts from the NE Japan Arc: Constraints on Source Composition, Mantle Melting and Slab Input Composition
Across-arc Variations in Geochemistry of Oligocene to Quaternary Basalts from the NE Japan Arc: Constraints on Source Composition, Mantle Melting and Slab Input Composition
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DOI：
10.1093/petrology/egv073
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发表时间：
2015-11
影响因子：
3.9
通讯作者：
K. Shuto;Rikako Nohara-Imanaka;M. Sato;Toshiro Takahashi;E. Takazawa;H. Kawabata;K. Takanashi
中科院分区：
文献类型：
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作者：
K. Shuto;Rikako Nohara-Imanaka;M. Sato;Toshiro Takahashi;E. Takazawa;H. Kawabata;K. Takanashi
为了研究地幔楔源岩浆中跨弧地球化学变化的性质和起源，我们对日本东北弧的玄武岩进行了地球化学研究，年龄范围从35 Ma到现在。24-18 Ma、10-8 Ma、6-3 Ma和2.5 ~ 0 Ma喷发的弧后玄武岩，无论在哪个时期，其高场强元素（HFSE）和稀土元素（REE）（特别是轻稀土（LREE）和中稀土（MREE））的含量都比弧前玄武岩高，微量元素的不相容比值也比弧前玄武岩高。Nb/Yb与Nb的地球化学模拟表明，弧前和弧后成分的差异与俯冲改造无关，在许多情况下，可以用不同程度的熔融来解释（弧前岩浆熔融程度较高，弧后岩浆熔融程度较低）的一个几乎均匀的亏损洋中脊玄武岩（MORB）地幔（DMM）样源，尽管有几个例外。其中包括一些可能起源于比DMM稍亏损的源区的上新世弧前玄武岩，几个35-32 Ma和24-18 Ma的弧后玄武岩起源于HFSE比DMM富集的岩石圈地幔源区，一种罕见的16-12 Ma的玄武岩，在弧后喷发，但与弧前玄武岩的熔融程度相当。35-0马东北日本玄武岩的流体流动和非流动元素和熔体流动和非流动元素的比例的变化表明，这些玄武岩岩浆的产生之前，添加到源地幔的主要俯冲成分是沉积物衍生的熔体。太平洋MORB玄武岩、日本东北部玄武岩和俯冲沉积物的Sr、Nd同位素组成对比表明，大多数16 Ma后亏损程度更高的弧后玄武岩的同位素组成可解释为加入了<2%的块状沉积物;最富集的同位素组成的次大陆岩石圈衍生岩浆可以解释为增加了最大5-7%的日本海沟沉积物（JTS），如果岩石圈原始Sr和Nd成分接近DMM的话。前缘弧玄武岩的Sr、Nd同位素组成可通过加入1- 5%JTS来解释。软流圈幔源岩浆与上覆岩石圈地幔相互作用的亏损软流圈地幔（DMM）上涌模型可以解释35-0 Ma日本东北部玄武岩的地球化学特征。弧前岩浆一般是由软流圈地幔较浅部分的较高程度熔融产生的，而弧后岩浆则是由软流圈地幔较深部分的较低程度熔融产生的。后者的岩浆与岩石圈地幔的相互作用，导致更丰富的Sr和Nd同位素签名的前18 Ma弧后玄武岩和后22 Ma弧前玄武岩，但较少的相互作用，导致更多的亏损Sr和Nd同位素签名，为大多数弧后玄武岩的年龄小于16 Ma。
To investigate the nature and origin of across-arc geochemical variations over time in mantle wedge derived magmas, we have carried out a geochemical study of basalts in the NE Japan arc spanning an age range from 35 Ma to the present. Back-arc basalts erupted at 24–18 Ma, 10–8 Ma, 6–3 Ma and 2·5–0 Ma have higher concentrations of both high field strength elements (HFSE) and rare earth elements (REE) [particularly light REE (LREE) and middle REE (MREE)], and higher incompatible trace element ratios compared with frontal-arc basalts at any given time. Geochemical modeling of Nb/Yb versus Nb shows that the frontal-arc and back-arc compositional differences are independent of subduction modification and can, in many cases, be explained by different degrees of melting (higher degrees of melting for frontal-arc magmas and lower degrees of melting for back-arc magmas) of a nearly homogeneous depleted mid-ocean ridge basalt (MORB) mantle (DMM)-like source, although there are several exceptions. These include some Pliocene frontal-arc basalts that may originate from a source that is slightly more depleted than DMM, several 35–32 Ma and 24–18 Ma back-arc basalts derived from a lithospheric mantle source that is enriched in HFSE compared with DMM, and a rare 16–12 Ma basalt that was erupted in the back-arc but was produced by a similar degree of melting to frontal-arc basalts erupted at the same time. Variations in ratios of fluid-mobile and -immobile elements and those of melt-mobile and -immobile elements for the 35–0 Ma NE Japan basalts indicate that the principal subduction component added to the source mantle prior to generation of these basalt magmas is a sediment-derived melt. Comparison of Sr and Nd isotopic compositions for Pacific Ocean MORB, the NE Japan basalts and subducting sediments suggests that the isotopic compositions of most post-16 Ma more depleted back-arc basalts can be explained by the addition of <2% bulk sediment; the most enriched isotope compositions of the subcontinental lithosphere-derived magmas can be accounted for by addition of a maximum 5–7% Japan Trench Sediment (JTS), if the original Sr and Nd compositions of the lithosphere approximated that of DMM. The Sr and Nd isotope composition of the frontal-arc basalts can be accounted for by the addition of 1–5% JTS. A depleted asthenospheric mantle (DMM-like) upwelling model with interaction between asthenospheric mantle-derived magmas and overlying lithospheric mantle can account for the geochemical characteristics of the 35–0 Ma NE Japan basalts. The frontal-arc magmas were generally generated by higher degrees of melting of the shallower part of the asthenospheric mantle, whereas the back-arc magmas resulted from lower degrees of melting of the deeper part of asthenospheric mantle. These latter magmas underwent interaction with the lithospheric mantle, resulting in more enriched Sr and Nd isotopic signatures for the pre-18 Ma back-arc basalts and post-22 Ma frontal-arc basalts, but less interaction, resulting in more depleted Sr and Nd isotopic signatures, for most of the back-arc basalts younger than 16 Ma.