四川盆地震旦系灯影组白云岩成因

基于岩芯观察、薄片鉴定、地球化学分析,结合区域地质背景,系统研究了四川盆地震旦系灯影组白云岩成因。岩石学特征分析发现,灯影组白云岩可分为泥粉晶云岩、粒屑云岩、微生物云岩、岩溶角砾云岩4类,其结构类型一般为泥晶—粉晶,说明白云岩化时间较早,为准同生期。地球化学分析表明,灯影组白云岩δ^13C平均值与震旦纪原始海水较为接近,δ^18O值呈现中—高负偏的特征,有序度和Fe、Mn含量相对偏低,Na含量相对较高,稀土元素配分模式则与海水配分曲线类似,表明白云岩化主要受到海源流体的影响。盆地周缘的古陆和水下隆起使盆内海水极易封隔浓缩,盆内发育的富Ca^2+菌藻类可间接提高海水的Mg^2+/Ca^2+比值,加之灯影期干旱炎热的古气候让盆内蒸发作用强烈,可伴生石膏等蒸发性矿物,促进白云石化。综合上述特征,四川盆地灯影组白云岩成因为微生物参与的蒸发海水回流渗透白云石化。

Genesis of Dolomite in the Sinian Dengying Formation in the Sichuan Basin

Based on core observations, thin section identification, geochemical analysis, and regional geological background research, this paper systematically studied the genesis of dolomite in the Sinian Dengying Formation in the Sichuan Basin. The petrological characteristics show that Dengying dolomite can be divided into four types, including micritic to powder crystal dolomite, grain dolomite, microbial dolomite, and carneule. Its structural type is micritic crystal to powder crystal, indicating that dolomitization occurred earlier, during the penecontemporaneous stage. Geochemical analysis showed that the average δ13C of Dengying dolomite is close to that of the Sinian original seawater, and the δ18O value is characterized by a medium-high negative bias. The order degree and the Fe and Mn contents are relatively low, but the Na content is relatively high. The rare earth element partition curve of the dolomite is similar to that of the seawater, indicating that dolomization was mainly affected by sea-source fluids. Further analysis shows that the ancient lands and submarine uplifts around the basin were able to easily trap the seawater in the basin and keep it concentrated. The Ca2+-rich algae in the basin could increase the Mg2+/Ca2+ ratio of the seawater indirectly. Meanwhile, the evaporation in the basin was strong, caused by the dry-hot palaeoclimate during the Dengying period, and the dolomite was often accompanied with evaporite minerals such as gypsum, which could promote dolomitization. Based on the above characteristics, this paper considers that the genesis of dolomite from the Dengying Formation in the Sichuan Basin was evaporative seawater seepage-reflux dolomitization under the participation of microorganisms.