A study of element geochemistry of mudstones of upper Ordovician Wufeng Formation and lower Silurian Longmaxi Formation in southern Daba Mountain
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摘要: 元素地球化学在沉积环境判别中发挥重要的作用，尤其是沉积物形成时的氧化-还原环境和底层水体状况。本文对南大巴山上奥陶统五峰组-下志留统龙马溪组泥岩开展了元素地球化学分析，结果表明：临湘组Al2O3含量高于五峰组，五峰组-龙马溪组向上逐渐增加，TiO2和Al2O3具有相似的特征，二者与TOC呈负相关；氧化-还原敏感元素U、V和Mo较为富集，表明沉积时处于缺氧环境，不同剖面不同层位存在两次强烈富集异常，指示其为硫化环境；稀土元素均表现为轻稀土富集、重稀土亏损，略微右倾的稀土元素配分模式，具有明显Eu负异常，其中五峰组-龙马溪组Ce负异常，缺氧环境，栗子坪少数Eu正异常，可能存在热液活动；氧化-还原敏感元素比值、Mo/TOC及MoEF-UEF的协变模式揭示了南大巴山奥陶纪-志留纪之交主体上处于弱滞留贫氧-中等滞留缺氧的开阔海环境，主要受控于海平面升降变化和沉积时古海底地形，局部受北侧秦岭洋盆内洋流影响。Abstract: Element geochemistry plays an important role in judging sedimentary environments such as redox environment and bottom water condition during deposition process of sediments. A detailed element geochemistry study of mudstones from the upper Ordovician Wufeng Formation to lower Silurian Longmaxi Formation in southern Dabashan mountain is carried out in this paper.The results show that Linxiang Formation has a higher Al2O3 content than Wufeng Formation,and from bottom Wufeng Formation to upper Longmaxi Formation, the contents of Al2O3increase gradually.Both contents of Al2O3 and TiO2 have a negative correlation with TOC. Redox-sensitive elements such as U, V and Mo are enriched, indicating an anoxic environment during the deposition of the sediments.Two dramatically enriched anomalies of U, V and Mo in different strata from different profiles show a sulfidic condition. The distributions of NASC-normalized REEs display slightly right-tilted patterns, which are enriched in LREEs and depleted in HREE sand have an obvious Eu negative anomaly.The Ce negative abnormalies of Wufeng Formation and Longmaxi Formation signify an anoxic environment in bottom water and the minor Eu positive abnormalies from Liziping profile probably imply hydrothermal activities during precipitating. Covariation of ratios of redox-sensitive elements such as Mo/TOC and MoEF-UEF reveal that southern Daba Mountain during Ordovician-Silurian transition is generally in an open oceanic environment with weak stagnant oxygen depletion or medium stagnant anoxia. The sedimentary environment of study area at that time is mainly controlled by fluctuations of sea level and paleo-seafloor topography,locally influenced by oceanic hydrothermal currents from Qinling Ocean in the north.
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