Ordovician arc ophiolite, the Hayachine and Miyamori complexes, Kitakami Mountains, Northeast Japan: isotopic ages and geochemistry
Ordovician arc ophiolite, the Hayachine and Miyamori complexes, Kitakami Mountains, Northeast Japan: isotopic ages and geochemistry
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日本东北部北上山脉奥陶纪弧蛇绿岩、Hayachine 和 Miyamori 杂岩：同位素年龄和地球化学
DOI：
10.2343/geochemj.26.85
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发表时间：
1992
影响因子：
0.8
通讯作者：
K. Ozawa
中科院分区：
文献类型：
--
作者：
K. Shibata;K. Ozawa
对日本北上山早宫弧蛇绿岩进行了Sm-Nd、Rb-Sr和K-Ar的年代学和同位素分析。4个全岩样品的Sm-Nd等时线年龄为510±70 Ma，初始143Nd/144Nd比值为0.51229±0.00010 （eNd = +6.4±2.0）。单个岩石的初始eNd值在+6.3 ~ +7.0之间，低于MORB。510 Ma的年龄被解释为Hayachine-Miyamori蛇绿岩的结晶年龄。8个角闪石的K-Ar年龄在244 ~ 473 Ma之间，其中小于400 Ma的年龄被认为是受白垩纪花岗岩侵入的影响。K-Ar角闪石年龄可以指示构造侵位向浅层的时代。8个完整岩石样品的初始87Sr/86Sr比值变化较大，eSr值在-4.0 ~ +31.9之间。近恒定的eNd和变化的eSr表明这些样品存在不同程度的海水蚀变。原岩可能为挤压岩或浅侵入岩的角闪岩的eSr高于深成岩，这与变质过程中海水的循环作用相一致。侵入寄主橄榄岩的粗粒辉长岩的eSr仍高于MORB。这些辉长岩的低eNd和高eSr特征表明，这些同位素特征记录在弧状上地幔内，并具有与弧相关的地球化学特征。这种地幔特征可以解释为俯冲板块的放射性成因流体或熔体的参与，这些板块在洋中脊的海水作用下发生了变化。
Sm-Nd, Rb-Sr and K-Ar geochronological and isotopic analyses were made on the Hayachine-Miyamori arc ophiolite in the Kitakami Mountains, Northeast Japan. The Sm-Nd isochron for four whole-rock samples gives an age of 510 ± 70 Ma and an initial 143Nd/144Nd ratio of 0.51229 ± 0.00010 (eNd = +6.4 ± 2.0). The initial eNd for individual rocks ranges from +6.3 to +7.0, and is lower than that of the MORB. The age of 510 Ma is interpreted to represent the crystallization age of the Hayachine-Miyamori ophiolite. K-Ar ages of eight hornblendes range from 244 to 473 Ma, within which ages younger than about 400 Ma are thought to be affected by Cretaceous granitic intrusions. The K-Ar hornblende ages may indicate the times of tectonic emplacement to shallower levels. The initial 87Sr/86Sr ratios of eight whole rock samples from the Hayachine-Miyamori complexes show a wide variation: ranging from –4.0 to +31.9 as eSr. The nearly constant eNd and variable eSr suggest various extent of seawater alteration for these samples. Amphibolites whose protolith may be extrusive or shallow intrusive rocks give higher eSr than plutonic rocks, which is consistent with circulation of sea water during the metamorphism. However, coarse-grained gabbro intruded into host peridotites still have higher eSr than the MORB. The lower eNd and higher eSr for these gabbroic rocks, which occur in the host peridotites with arc-related geochemical characteristics, suggest that these isotopic signatures were recorded within an arc upper mantle. This mantle signature may be explained by an involvement of radiogenic fluid or melt derived from subducting slab, which had been altered by sea water at a mid-ocean ridge.