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首页> 外文期刊>Contributions to Mineralogy and Petrology >Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus
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Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

机译：日本的侏罗纪羽状蛇绿岩：海洋平台的增生碎片

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摘要

The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geo-chemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to F093.5, and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T_p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as theMikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T_p mantle. Evidence for the high T_v mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high PIT meta-morphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.
机译：日本的Mikabu和Sorachi-Yezo带由侏罗纪的蛇绿岩复合体组成，那里大量的玄武岩到苦荞岩形成熔岩和破硅质岩块。在已研究的Mikabu地带的Hamamatsu北部和Dodaira北部地区，这些岩石被分为两种地球化学类型，即贫化（D-）和富集（E-）类型。另外，文献中其他领域也报道了高度富集（HE-）型。 D型微晶岩含有高至F093.5的高镁质遗迹橄榄石表晶，并且它们的Fo-NiO趋势表明从高MgO初级岩浆中有部分结晶。计算出的MgO含量高达25 wt％，表明地幔在高达1650°C的异常高的潜在温度（T_p）下熔化。相对于D型岩石，E型岩石代表Fe和LREE的富集以及Mg，Al和HREE的贫化。这些化学特性与石榴石辉石熔炼的熔体的化学特性非常吻合。 Sorachi-Yezo带中的火山可以分为与Mikabu带相同的类型，带有镁质橄榄石的D型微晶岩也显示出从高T_p地幔生产的证据。与来自海洋和大陆大火成岩省（LIPs）的高Mg苦味粉质和科马提岩具有很高的T_v地幔和地球化学相似性的证据表明，Mikabu和Sorachi-Yezo带是由晚晚期热的大地幔柱形成的增生的海洋LIP。侏罗纪太平洋。 E型和D型岩石分别是在LIP岩浆作用初期由石榴石辉绿岩融化而在后期由贫化橄榄岩融化产生的岩浆。 Mikabu带典型地具有丰富的超镁铁质堆积体，这可能是由富铁橄榄岩地幔源产生的初级岩浆的晶体堆积形成的，而HE型岩浆是由石榴石辉石体源的低度部分熔融产生的。它们的形成时间应晚于E型和D型岩石。 Mikabu和Sorachi高原形成于侏罗纪晚期太平洋的低纬度地区，可能在俯冲带附近，在俯冲过程中部分经历了高PIT变质作用，然后伴随着（或没有，就Mikabu而言）升高超俯冲带蛇绿岩。 Mikabu和Sorachi高原可能是侏罗纪晚期的海洋LIP，可能与Shatsky Rise建立了兄弟情谊。

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