Variational Characteristics of Sm/Nd Ratio in Cambrian-Triassic Paleoseawater from Three Gorges Area Conodont Evidence
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摘要: 牙形石作为划分地层的标准化石, 具有确切的地层层位和较高的REE含量, 并不容易受成岩作用的影响, 因此作为研究古海水演化的指示剂引起人们的广泛关注.利用同位素稀释法对扬子克拉通三峡地区寒武纪—三叠纪海相碳酸盐岩中代表古海水化学组成牙形石的Sm/Nd比值进行测定, Sm/Nd比值在0.108-0.348之间, 有较大的变化范围.建立Sm/Nd比值随时间的演化曲线, 并结合已有沉积地层学和沉积地球化学的成果进行研究, 结果表明, 古海水Sm/Nd比值变化与海平面升降、古陆块的隆起有明显对应关系, 同时, 区域性构造运动, 如海底地壳拉张产生的幔源组分和地幔柱活动也会影响古海水的Sm/Nd比值.因此, 古海水的Sm/Nd比值对海平面的升降、古大陆的风化作用和区域构造事件有指示意义.Abstract: Conodonts have received extensive attention in terms of their potential as geochemical tracers, due to their precise stratigraphic age and high REE concentration as well as their relative resistance to diagenetic alternation. The ratios of Sm/Nd of paleoseawater from the Cambrian to the Triassic in the northern of the Yangtze craton were determined using the Isotope Dilution Mass Spectrometry method. The samples exhibit a large variation in Sm/Nd ratio (0. 108-0. 348). It shows that the temporal variability in Sm/Nd ratios of the seawater from the Cambrian to the Triassic is characterized by strongly correlated eustatic changes of sea level and submergence history of old continental in the Three Gorges area, corresponding to the previously published sedimentological and geochemical studies on these areas. In the other hand, the preliminary conclusion revealed that the submarine hydrothermal system (mantle flux) and the Emeishan mantle plume with higher Sm/Nd ratios, which were existing in the subarea adjacent to the Three Gorges area during the Phanerozoic, may likely influence the Sm/Nd values of contemporaneous seawater. It allows us to speculate that the Srn/Nd curve of paleoseawater constructed by using conodonts has a potentially wide range of applications in studies relating to the paleoenvironmental and regional tectonic event.
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Key words:
- Sm/Nd ratio /
- conodont /
- paleoenvironment /
- Yangtze craton.
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图 1 三峡地区地质略图(汪啸风等(1987))
1.黄陵花岗岩崆岭群片麻岩；2.震旦纪；3.取样位置
Fig. 1. Schematic geologic map of the Three Gorges area
图 2 三峡地区显生宙牙形石Sm/Nd比值随时间演化
Fig. 2. Sm/Nd ratio distribution of conodonts in response to the age changes during the Phaneroaoie in the Three Gorges area
表 1 三峡地区寒武纪一三叠纪不同时期牙形石Sm/Nd比值
Table 1. Sm/Nd ratios of conodont from Cambrian to Triassic in the Three Gorges area
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