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首页> 外文期刊>Journal of Asian earth sciences >K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene-Oligocene thermo-tectonic reactivation
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K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene-Oligocene thermo-tectonic reactivation

机译：日本西南部古近纪大锅团的K-Ar年龄和地球化学：始新世-渐新世热构造再活化的意义

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摘要

Systematic K-Ar dating and geochemical analyses of Paleogene cauldrons in the Sanin Belt of SW Japan have been made to explore the relationship between the timing of their formation and the Paleogene sub-duction history of SW Japan documented in the Shimanto accretionary complex. We also examine the magma sources and tectonics beneath the backarc region of SW Japan at the eastern plate boundary of Eurasia. Fifty-eight new K-Ar ages and 19 previously reported radiometric age data show that the cauldrons formed during Middle Eocene to Early Oligocene time (43-30 Ma), following a period of magmatic hiatus from 52 to 43 Ma. The hiatus coincides with absence of an accretionary prism in the Shimanto Belt. Resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench, as a common tectono-thermal event within a subduc-tion system. The cauldrons are composed of medium-K calc-alkaline basalts to rhyolites and their plutonic equivalents. These rocks are characterized by lower concentrations of large ion lithophile elements (LILE) including K_2O, Ba, Rb, Th, U and Li, lower (La/Yb)_n ratios, lower initial Sr isotopic ratios (0.7037-0.7052) and higher ε_(Nd)(T) values (-0.5 to +3.5) relative to Late Cretaceous to Early Paleogene equivalents. There are clear trends from enriched to depleted signatures with decreasing age, from the Late Cretaceous to the Paleogene. The same isotopic shift is also confirmed in lower crust-derived xenoliths, and is interpreted as mobilization of pre-existing enriched lithospheric mantle by upwelling depleted astheno-sphere. Relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the back-arc and forearc of the SW Japan arc-trench system during the Eocene to Oligocene. Newly identified Late Eocene low silica adakites and high-Mg andesites in the Sanin Belt and Early Eocene A-type granites in the SW Korea Peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate. The backarc region was an extensional tectonic setting, and some Paleogene rift basins and Sanin Belt cauldrons occur in linear arrays. The Eocene-Oligocene Sanin-SE Korea continental arc lies on the NE extension of the East China Sea Basin, the initial stage of which probably formed by continental arc rifting. This rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab, in response to decreased convergence rate between the Pacific and Eurasian plates.
机译：进行了系统的K-Ar测年和对日本西南山地古近纪大锅的地球化学分析，以探索它们的形成时间与Shimanto增生复合体中记载的日本西南古俯冲历史之间的关系。我们还研究了欧亚大陆东部板块边界的日本西南部后弧区下方的岩浆源和构造。 58个新的K-Ar年龄和先前报道的19个辐射年龄数据表明，在中新世至渐新世早期（43-30 Ma）期间形成了大锅，随后发生了52-43 Ma的岩浆裂隙。该裂口与Shimanto带中没有增生棱镜相吻合。作为俯冲系统中常见的构造热事件，在后弧中恢复形成大锅团的岩浆作用与陆源碎屑大量涌入海沟同时发生。该大锅由中等K钙碱性玄武岩和流纹岩及其深成岩等价物组成。这些岩石的特征是：较大的离子亲石元素（LILE）浓度较低，包括K_2O，Ba，Rb，Th，U和Li，较低的（La / Yb）_n比，较低的初始Sr同位素比（0.7037-0.7052）和较高的ε_。 （Nd）（T）值（-0.5至+3.5）相对于白垩纪晚期至古近纪早期等价物。从白垩纪晚期到古近纪，随着年龄的增长，从富集特征到耗尽特征有明显的趋势。在下地壳派生的异岩中也证实了相同的同位素位移，并且被解释为通过贫化软流层上升流动员了先前存在的富集岩石圈地幔。据推测，在始新世至渐新世期间，西南日本弧-沟系统的弧后和前弧的较宽区域普遍存在较高的地热梯度。新发现的山南地带的晚始新世低硅质Adakites和高镁安山岩以及西南朝鲜半岛的始新世A型花岗岩可能是由于热流软流圈的上升和年轻板块的俯冲而形成的。弧后地区是一个伸展的构造环境，一些古近纪裂谷盆地和萨宁带大锅以线性阵列出现。始新世-渐新世山南—东南韩国大陆弧位于东中国海盆地的东北延伸，其初始阶段可能是由大陆弧裂谷形成的。响应太平洋和欧亚板块之间收敛速度的降低，这种裂谷可能是由于热流软流圈上升到俯冲板块回滚形成的楔形空间中引起的。

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  • 来源
    《Journal of Asian earth sciences》 |2011年第2期|p.509-533|共25页
  • 作者

    T. Imaoka; K. Kiminami; K. Nishida; M. Takemoto; T. Ikawa; T. Itaya; H. Kagami; S. Iizumi;

  • 作者单位

    Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan;

    Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan;

    Fukuoka Prefectural Police Department, Criminal Affairs Division, Fukuoka 818-0041, Japan;

    Nippon Expressway Research Institute Co., Ltd., Machida, 194-8508, Japan;

    Yachiyo Engineering Co. Ltd., Shiromi 1-4-70, Chuo-ku, Osaka 540-0001, Japan;

    Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan;

    Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan;

    Department of Geoscience, Shimane University, Matsue 690-8504, Japan;

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  • 正文语种 eng
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  • 关键词

    k-ar ages; sr-nd isotopes; paleogene; cauldron; rifting; high-mg andesite; adakite;

    机译：钾时代;锶同位素;古近纪;大锅;裂谷;高镁安山岩;赤铁矿;

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