首页> 外文期刊>Science in China. Series D, Earth sciences >Origin of the deep fluids in the paleosubduction zones in western Tianshan: Evidence from Pb- and Sr-isotope compositions of high-pressure veins and host rocks
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Origin of the deep fluids in the paleosubduction zones in western Tianshan: Evidence from Pb- and Sr-isotope compositions of high-pressure veins and host rocks

机译:天山西部古俯冲带深层流体成因:高压脉和宿主岩中Pb和Sr同位素组成的证据

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Fluids in the deep subduction zones play an important role in crust-mantle exchange related to the subduction process. Identification of fluids origin, internal or external, can help us to evaluate the contribution of crust materials to mantle during this process. High-pressure veins, which developed in the western Tianshan HP-metamorphic belt extensively, are the direct products of the fluids in subduction zones. In western Tianshan, high-pressure veins and host rocks have overall Pb- and Sr-isotope compositions. At t = 340 Ma, the high-pressure metamorphic time, the ratios of ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, ~(208)Pb/~(204)Pb and ~(87)Sr/~(86)Sr of high pressure veins are 17.122-18.431, 15.477-15.611, 37.432-38.689 and 0.70529-0.70705, and are 17.605-17.834, 15.508-15.564, 37.080-38.145 and 0.70522-0.70685 for host rocks. However, high-pressure veins show a much larger variation in Pb-isotope compositions than host rocks. Plots of samples are distributed roughly along a line parallel to and far away from the Northern Hemisphere reference line (NHRL) on the plot of ~(206)Pb/~(204)Pb vs ~(207)pb/~(204)pb. On the plot of ~(206)pb/~(204)pb vs ~(208)pb/~(204)pb, plots of data are distributed nearby mid-ocean-ridge basalts (MORB) area or along a line parallel to and not far away from NHRL, which shows that the data plots are farther away from MORB and ocean island basalts (OIB) areas on the plot of ~(206)Pb/~(204)Pb vs ~(207)Pb/~(204)Pb than on the plot of ~(206)Pb/~(204)Pb vs ~(208)Pb/~(204)Pb. The compositions of Pb- and Sr-isotopes of these high-pressure metamorphic rocks are between that of enriched mantle 1(EMI) and of sediments or upper crust. A notable characteristic can be seen from plots of Pb-isotope ratio vs Pb-isotope ratio and ~(206)Pb/~(204)Pb vs ~(87)Sr/~(86)Sr that array of plots shows an evident tendency of mixture of two end members. Ratios of Rb/Ba, Ce/Pb, Nb/U and Ta/U of high-pressure veins and host rocks are between that of OIB or MORB and that of continental crust, which indicates that continental crust materials probably have been incorporated into the rocks. The fluids in the paleosubduction zones in western Tianshan are mixtures of two sources, one originating from the devolatilzation of the host rocks with protolithes similar to ocean basalts from enriched mantle 1 (EMI), the other from the dehydration of subducted sediments.
机译:深层俯冲带中的流体在与俯冲过程有关的地幔幔交换中起着重要作用。识别内部或外部流体的来源可以帮助我们评估在此过程中地壳材料对地幔的贡献。在天山西部高压变质带中广泛发育的高压脉是俯冲带流体的直接产物。在天山西部,高压脉和宿主岩总体上具有铅和锶的同位素组成。在t = 340 Ma时,即高压变质时间,〜(206)Pb /〜(204)Pb,〜(207)Pb /〜(204)Pb,〜(208)Pb /〜(204)之比高压静脉的Pb和〜(87)Sr /〜(86)Sr为17.122-18.431、15.477-15.611、37.432-38.689和0.70529-0.70705,分别为17.605-17.834、15.508-15.564、37.080-38.145和0.70522-主体岩石为0.70685。但是,高压脉脉中的Pb同位素组成的变化远大于基质岩。在〜(206)Pb /〜(204)Pb与〜(207)pb /〜(204)pb的图上,样本图大致沿平行于北半球参考线(NHRL)且与之远离的线分布。在〜(206)pb /〜(204)pb与〜(208)pb /〜(204)pb的图上,数据图分布在中海脊玄武岩(MORB)区域附近或沿平行于并且离NHRL不远,这表明数据图更远离〜(206)Pb /〜(204)Pb与〜(207)Pb /〜(图上的MORB和海洋岛玄武岩(OIB)区域。 204)Pb大于〜(206)Pb /〜(204)Pb与〜(208)Pb /〜(204)Pb的关系图。这些高压变质岩的Pb和Sr同位素组成在富集地幔1(EMI)与沉积物或上地壳之间。从Pb同位素比对Pb同位素比和〜(206)Pb /〜(204)Pb对〜(87)Sr /〜(86)Sr的图可以看出一个显着的特征,该图阵列显示出明显的趋势两个末端成员的混合物。高压矿脉和主岩的Rb / Ba,Ce / Pb,Nb / U和Ta / U之比介于OIB或MORB与大陆壳之间,这表明大陆壳物质可能已被纳入岩石。天山西部古俯冲带的流体是两种来源的混合物,一种来自母岩的脱挥发分,其原生质类似于富集地幔1(EMI)的海玄武岩,另一种来自俯冲沉积物的脱水。

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