Evidences of hydrothermal fluids recorded in microfacies of the Ediacaran cap dolostone: Geochemical implications in South China
Evidences of hydrothermal fluids recorded in microfacies of the Ediacaran cap dolostone: Geochemical implications in South China
复制标题
DOI：
10.1016/j.precamres.2017.12.028
复制
发表时间：
2018-03
影响因子：
3.8
通讯作者：
Yan-Yan Zhao-Yan;Mingyu Zhao;San-zhong Li
中科院分区：
文献类型：
--
作者：
Yan-Yan Zhao-Yan;Mingyu Zhao;San-zhong Li
中国南部陡山沱组海相南坨英云母上覆的埃迪卡拉期盖层白云岩中含有丰富的神秘构造。在贵州省乌河的这些构造中可以观察到两代碳酸盐胶结物(分别命名为C1和C2)。C1的特征是白云石胶结物排列在结构的墙壁上，C2的特征是分布在中心的赤方解石晶石。C_1的稀土元素 + -Y模式、δ~(13)C和δ_(18)O值范围与泥晶基质相似，表明C_1与泥晶基质形成于同一流体中。C_1的稀土元素 + Y配分模式具有轻度富稀土、正Ce和Eu异常、Y/Ho比值变化大的特点。这些模式反映了正常的海相成因，在早期成岩作用中，孔隙水中释放了铁-锰氧化物。正Ce异常和基质的Th/U比值反映了缺氧的沉积条件。C2具有富MREE、Eu正异常、高Y/Ho比值和不明显Ce异常的特征，不同于C_1和基质。Eu异常表明，C_1和C_2的降水流体可能参与了高温热液流体的不同贡献。根据C_1和C_2的 ~(18)O变化，C_2的析出温度为10~5 °C~120δ°C。基质、C_1、C_2的δ~(18)O与δ~(13)C值呈正相关，可能与热液作用有关。而C2的δ~(13)C与δ~(18)O值之间的负相关可能是有机质氧化和甲烷渗漏的结果。不同微相碳酸盐岩的地球化学特征反映了全球马里诺冰期后神秘的流体化学特征。
The Ediacaran cap dolostones overlying the Marinoan Nantuo diamictite of the Doushantuo Formation in South China contain abundant enigmatic structures. Two generations of carbonate cement (named C1 and C2, respectively) can be observed in these structures at Wuhe of the Guizhou Province. C1 is characterized by dolomite cements that line the wall of the structures, and C2 by equant calcite spars that distribute in the center. The REE + Y patterns and the range of δ13C and δ18O values of C1 are similar to those of the micritic matrix, indicating that C1 precipitated from the same fluid as the matrix. The REE + Y patterns of C1 are characterized by minor MREE-enrichment, positive Ce and Eu anomalies, large variations in Y/Ho ratios. These patterns reflect normal marine origin with the release of Fe-Mn oxides in pore water during early diagenesis. Positive Ce anomalies and Th/U ratios of matrix suggest the anoxic depositional conditions. C2 is characterized by MREE-enrichment, positive Eu anomalies, high Y/Ho ratios and unobvious Ce anomalies, which is different from C1 and matrix. The Eu anomalies indicate that the precipitation fluids of C1 and C2 may have involved in different contribution of high-temperature hydrothermal fluids. The precipitating temperatures of C2 would be from 105 °C to 120 °C based on the δ18O variations of C1 and C2. The positive correlation between δ18O and δ13C values of matrix, C1 and C2 may attribute to the alteration by hydrothermal fluids. However, the negative correlation between δ13C and δ18O values of C2 may result from the oxidation of organic matter and methane seeps. The geochemical features of different microfacies of the cap carbonates reflect enigmatic fluid chemistry after the global Marinoan glaciation.