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首页> 外文期刊>Contributions to Mineralogy and Petrology >Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge in the Philippine Sea, and growth of Izu-Ogasawara (Bonin)-Mariana arc crust
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Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge in the Philippine Sea, and growth of Izu-Ogasawara (Bonin)-Mariana arc crust

机译：菲律宾海九州-帕劳岭北部的小松-戴尼海山的玄武岩浆形成的斜长石，以及伊豆-小gas原（波宁）-玛丽安娜弧壳的生长

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Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite-hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37-38 Ma, indicating that felsic plutonic activity occurred during the early stage of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These Tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO_2, except for an increasingly strong negative u anomaly at higher SiO_2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the dame arc from the Tanzawa Complex (10-5Ma), CENTRAL Japan, considered to represent the lower-middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO_2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex.
机译：从九州-帕劳山脊北部的Komahashi-Daini海山挖出的菱形岩被分类为黑云母-角闪闪长晶岩和角闪闪长晶岩。这些岩石具有37-38 Ma的辐射年龄，表明在伊豆-小ara原（波宁）-马里亚纳（IBM）弧形火山爆发的早期发生了长短岩浆活动。因此，这种方钠石复合体对于理解岛弧和大陆壳形成的初始过程非常重要。这些Tonalitic岩石表现出以下岩石学和地球化学特征：（1）在整个组成范围内斜长石型隐晶岩中常见的层状孪晶和振荡带状分布; （2）角闪石斜长石显示出平行的REE模式，并且随着SiO_2的增加，总REE含量也增加，但在更高的SiO_2含量下负u异常越来越强; （3）同位素组成在较大的二氧化硅变化范围内保持恒定。我们将这种托纳石与日本中部Tanzawa Complex（10-5Ma）的年轻弧形贵妇弧相比较，认为代表了IBM弧的中下壳，并发现以下差异：（1）累积的纹理在小桥戴尼海山的样品中未发现淡泽斑岩。 （2）与Tanzawa的那些不同，Komahashi-Daini Seamount的Torites表现出Zr和REEs与SiO_2图的线性变化。这些数据和其他观察结果支持了这样的解释，即科马桥-戴尼海山中的托纳石是由玄武岩浆的晶体分离产生的。我们认为，部分结晶作用是在大洋洲弧形成的早期阶段进行的，从而产生了天然的方钠石，而后期的色调则主要由玄武岩下地壳的部分融化形成，如Tanzawa Complex中的色调钠石所代表的那样。

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