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首页> 外文期刊>Precambrian Research >A systematic rare-earth elements and yttrium study of Archean cherts at the Mount Goldsworthy greenstone belt in the Pilbara Craton: Implications for the origin of microfossil-bearing black cherts
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A systematic rare-earth elements and yttrium study of Archean cherts at the Mount Goldsworthy greenstone belt in the Pilbara Craton: Implications for the origin of microfossil-bearing black cherts

机译：在皮尔巴拉克拉通山戈德斯沃西山绿岩带上的太古代石的系统化稀土元素和钇研究：含微化石的黑色石起源的启示

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摘要

A systematic REE + Y study of Archean (ca. 3.0 Ga) cherts from the Mount Goldsworthy greenstone belt in the northeastern Pilbara Craton, Western Australia was performed in order to understand their origin and depositional environment. Analyzed samples include microfossil-bearing black cherts from the Farrel Quartzite and a black vein chert from the underlying Warrawoona Group, and laminated to banded chert including carbonaceous chert, jaspilite and banded iron-formation from the overlying, deepening-upward Cleaverville Formation. Laminated to banded cherts from the Cleaverville Formation show a clear stratigraphic trend upsection of increasing Y/Ho and positive Eu-anomalies, with HREE-enrichment and positive La-anomalies. The data comprise a mixing array between two end-member components on a newly proposed Y/Ho-Eu-anomaly diagram. One end-member is interpreted to be Archean seawater, with a super-chondritic Y/Ho ratio (～100) and a weak positive Eu-anomaly (～3). The other end-member with a chondritic Y/Ho ratio and a negligible Eu-anomaly is assumed to be non-marine water such as continental run-off, ground water, or geothermal water. Black cherts containing microfossils in the Farrel Quartzite are characterized by a positive La-anomaly, HREE-enrichment, negligible to a slight positive Eu-anomaly, and a chondritic to slightly super-chondritic Y/Ho ratio. They are distinct from vein cherts with a distinct MREE-enrichment and contemporaneous hydrothermal cherts with pronounced Eu-anomaly, and plot close to the supposed non-marine end-member component on the Y/Ho-Eu-anomaly diagram. The black cherts were thus precipitated from a water mass influenced significantly by for example continental run-off, ground water and/or geothermal water, but not from high-T hydrothermal solution, increasing the credibility of microfossils contained in them.
机译：为了了解其起源和沉积环境，对来自澳大利亚西部Pilbara Craton东北部的Goldsworthy绿岩带的太古代（约3.0 Ga）硅质石进行了系统的REE + Y研究。分析的样品包括来自Farrel石英岩的微化石黑色石和来自潜在的Warrawoona群的黑色脉石石，并层压到带状的t石中，包括碳质石，碧玉和带状铁质，来自上覆，向上加深的Cleaverville地层。层压至切沃维尔组的带状石显示出明显的地层趋势向上变化，其Y / Ho和正Eu异常增加，HREE富集和La正异常。数据包括新提出的Y / Ho-Eu异常图上两个端部成员之间的混合数组。一个末端成员被认为是太古代海水，具有超软骨质的Y / Ho比（〜100）和弱的正Eu异常（〜3）。另一个具有软骨Y / Ho比和可忽略的Eu异常的末端成员被认为是非海洋水，例如大陆径流，地下水或地热水。在Farrel石英岩中含有微化石的黑ts石的特征是正La异常，HREE富集，略微正Eu异常可忽略不计，以及软骨至略高于超级软骨的Y / Ho比。它们不同于具有明显的MREE富集的脉石和具有明显Eu异常的同时期热液thermal石，并且在Y / Ho-Eu异常图上与假定的非海洋末段成分接近。因此，黑硅质ts石是从受大陆径流，地下水和/或地热水等明显影响的水团中沉淀出来的，而不是从高T水热溶液中沉淀出来的，从而增加了其中所含微化石的可信度。

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