西沙岛礁白云岩化特征与成因模式分析

The characteristics and genetic model of the dolomitization in Xisha Reef Islands

关于碳酸盐岩的白云岩化作用至今仍是一个争论颇大的科学问题。在2012-2013年期间,中海石油（中国）有限公司湛江分公司在西沙石岛钻取了长达1 268.02m的＂西科1井＂岩心,在0-1 257.52m主要由碳酸盐岩组成的岩心中存在有7个白云岩层。本文基于西科1井岩心样品的矿物组成、常量、微量及稀土元素含量等地球化学特征分析,探讨了西沙岛礁特有的白云岩化作用过程。结果表明：西科1井7层白云岩在成因上与埋深和压实成岩作用无关,其分布与海平面下降的地质事件有良好的对应关系,白云岩层元素地球化学特征表明白云岩成岩流体为浓缩的高盐度海水。根据白云岩层的矿物学和地球化学特征,提出了礁滩潟湖环境下高盐卤水渗透回流作用的白云岩化模式：伴随海平面的升降,礁体处于淹没与暴露的交替之中,海平面下降导致了礁滩潟湖的形成,由于西沙海区蒸发量大于降水量而使澙湖中海水盐度增高,最终形成富Mg卤水;澙湖中高盐度富Mg卤水在礁体内下渗和侧向扩散,Mg逐渐替代早期CaCO3晶格中Ca而形成白云石;海平面的升降变化和礁体淹没与暴露的交替导致了钻井岩心多个白云岩层的形成。海平面升降直接控制了岛礁碳酸盐岩的发育和白云岩化作用,导致海平面升降的古气候变化在岛礁的形成发育中起到主导作用。

The dolomitization of carbonate rocks is a controversial scientific problem.During the 2012-2013,the Zhanjiang branch of CNOOC drilled the Well Xike-1.The core of well is 1 268.02 mlong and 0-1 257.52 m carbonate rocks interval has 7dolomite tiers.This paper discusses the dolomitization of reefs in the Xisha Islands based on mineral composition,major elements,trace and rare earth elements of sample of well Xike 1.It turned out that the cause of formation of 7dolomite tiers is not relevant to buried depth and compaction diagenesis and the distribution of dolomite tiers has a close relationship with sea level falling.According to characteristics of element geochemistry of dolomite tiers,the dolomite diagenetic fluid is concentrated high salinity seawater.The paper proposes the hypersaline brine seepage backflow dolomitization pattern in reefs lagoon environment.With the rise and fall of sea level,the reefs are in the loop of submersion and exposure.The descending of sea leval leads to the formation of the lagoon.Because the evaporation capacity is more than precipitation in the Xisha sea area,the salinity of seawater in lagoon is increased and Mg-rich brine is formed.With the infiltration of the Mg-rich brine in mushy reefs,the Mg replaces the Ca formed in CaCO3 in early stage and the dolomite is formed.The cycles of submersion and exposure of reef cause several dolomite tiers formed.The sea level fluctuation controls the development of reefs carbonatite and dolomitization.The changes of paleoclimate leading to the sea level fluctuation play an important role in the formation and development process of reef islands.