西沙石岛礁相白云岩稀土元素地球化学特征及成岩环境分析

摘要

Xisha Islands,characterized by development and evolution of typical reef systems,are the key area for reef oil and gas exploration in the South China Sea.By determining the REE contents of 107 dolostone samples selected from Well Xike1 in Shi Island,Xisha Islands,an analysis of REE geochemical characteristics of borehole reef dolostone was conducted for the diagenetic environment and interrelation between paleo-climatic event and dolostone stratum.Seven layers of dolostone in Well Xike-1 were similar in REE geochemical characteristics such as abundance,PAAS-normalized pattern and element anomaly,indicating a parallel diagenetic environment.Burial compaction and corresponding diagenesis caused no significant change in REE geochemical characteristics of dolostone,which could basically reflect the nature of diagenetic environment and dolomitization fluid.Several geochemical indices showed that the emergence of dolomitization in Xisha Islands could be interpreted by a seepage-reflux model under a low-temperature and meta-reducing environment with the action of highsalinity seawater.The high-salinity brine,formed in relatively closed reef-shoal lagoon and infiltrating along the rock pore,resulted in the metasomatic alteration of dolomite for calcite.The distribution of dolostone strata,corresponding to cooling events since the Miocene,was controlled by the eustatic fluctuation caused by the climate change.%西沙群岛是南海生物礁体系发育的典型地区,也是我国生物礁油气资源勘探与开发的重点区域.对采自西沙石岛“西科1井”岩心7层共107个白云岩样品进行了稀土元素(REE)含量测试分析,旨在通过分析白云岩层的REE地球化学特征,探讨西沙岛礁白云岩的成岩环境及其与古气候事件的相互关系.研究结果表明:西沙石岛礁相碳酸盐岩中7层白云岩具有相似的REE地球化学特征(含量、配分模式和元素异常等),表明其形成于相近的成岩环境;埋藏压实及其所导致的成岩作用未造成白云岩层中REE主要特征的显著变化,白云岩的REE地球化学特征基本反映了白云岩化作用的成岩环境及成岩流体的性质;多种地球化学指标(REE、Sr含量和碳、氧同位素特征)表明,西沙岛礁白云岩主要形成于高盐海水作用条件下的低温偏氧化环境,潟湖环境下高盐卤水的渗透回流作用是导致白云岩层形成的主要形式;白云岩的形成在时间上与中新世以来的古气候变冷事件相对应,气候变化导致的海平面升降控制了岛礁白云岩层的分布;由于气候变冷在岛礁周围形成礁滩潟湖,相对封闭的潟湖和蒸发量大于降水量的气候环境形成高盐度卤水,这种比重较大的高盐卤水沿孔隙下渗,交代蚀变早先形成的方解石类碳酸盐,从而形成了西沙岛礁中的白云岩层.