四川盆地东部下寒武统龙王庙组白云岩类型及其成因

通过对四川盆地东部板凳沟剖面龙王庙组露头进行了密集采样（1块/m），并根据X-衍射得到的白云石含量对样品进行筛选后，结合主微量元素、稀土元素及碳、氧同位素测试结果，系统分析了该地区白云岩的类型及其成因。分析结果表明，研究区发育两大类不同成因的白云岩。一类发育有序度低的泥-粉晶白云石，具有正相关的MgO和CaO含量，Mg/Ca值总体较高，且低Sr含量，高Fe，Mn含量，ΣREE+Y含量高，δEu和δCe负异常明显，δ¹³C与δ¹⁸O同位素值与同期海水形成的灰岩特征一致，发育于蒸发环境，为准同生的萨布哈白云石化作用和回流渗透白云石化作用的结果，且受到陆源碎屑混入的影响。而另一类发育粉-细晶白云石，MgO和CaO含量呈负相关，具有低Mg/Ca，高Sr，Fe，Mn含量，低ΣREE+Y含量，也同样具δEu与δCe负异常以及同期正常海水形成的灰岩的δ¹³C同位素值，但δ¹⁸O同位素值略低于同期海水形成的灰岩，这类白云石是浅埋藏白云石化作用形成的。结合白云岩的岩石学特征，认为研究区寒武系龙王庙组灰岩在沉积时，经历了准同生白云石化作用（包括萨布哈白云石化作用和回流渗透白云石化作用），晚期叠加了浅埋藏白云石化作用，因此表现出的碳-氧同位素数据具有分区性。

Typological analysis and genetic mechanism of dolomite in the lower Cambrian Longwangmiao Formation,eastern Sichuan Basin

The dense sampling of the dolomite from the Bandenggou outcrop of Longwangmiao Formation in the eastern Sichuan Basin was carried out with the samples selected through X-ray diffraction.The classification and genesis of the dolomite were demonstrated based on test results such as major and trace elements,rare earth elements (REEs) and carbon and oxygen isotopes.The results show that the dolomitization of the study area could be divided into two types in terms of origins.One is the micritic dolomite with low order degree,which developed in evaporating environment and was influenced by para-syngenetic Sabkha dolomitization,seepage-reflux dolomitization,and terrigenous clastic minerals.For this kind of dolomite,MgO and CaO contents are positively correlated,and Mg/Ca ratio is high in general,with low content of Sr,high content of Fe,Mn and REE+Y,and significant δEu & δCe negative anomalies.Additionally,the δ¹³C & δ¹⁸O values are consistent with those of the limestone deposited in coeval sea water.The other is the powder fine crystal dolomite,formed by shallow burial dolomitization,with negatively correlated MgO and CaO content,low Mg/Ca value,high content of Sr,Fe and Mn,low content of REE+Y,δEu & δCe negative anomalies,and the same δ¹³C value of the limestone deposited in coeval sea water.However,the value of δ¹⁸O is a little bit lower than that of the limestone in coeval sea water.According to the petrographic characteristics of the dolomite,we believe that the rock has undergone the syndepositional dolomitization (including Sabkha effect and reflux infiltration)superimposed by shallow burial dolomitization at later time.As a result,the values of carbon and oxygen isotopes became different through the processes.