塔里木盆地玉北地区蓬莱坝组鞍形白云石地球化学特征

根据岩相特征将玉北地区蓬莱坝组白云岩分为粉细晶基质白云岩、粗晶基质白云岩和鞍形白云石充填物。鞍形白云石充填物各类地化特征都与粗晶基质白云岩相似,是粗晶基质白云岩自身溶解后又在缝洞内形成的一种再沉淀,应该属于白云石胶结物,而与交代灰岩作用无关。高温、高盐度的地层卤水应该是鞍形白云石充填物形成的主要成岩流体。微量元素方面,粉细晶基质白云岩的Sr含量明显高于粗晶基质白云岩和鞍形白云石充填物,而鞍形白云石充填物的Ba含量远远高于基质白云岩和粗晶基质白云岩;Mg/Ca比方面,三者差别极小;鞍形白云石充填物与基质白云岩稀土含量都很低且配分曲线特征相似,稀土分异程度相近;包裹体测温、测盐显示鞍形白云石充填物与基质粗晶白云岩在流体性质上存在重叠,表明其在流体性质上存在对基岩的继承性。

Geochemical features of saddle dolomites in the Penglaiba Formation,Yubei area,Tarim Basin

Dolomites of the Penglaiba Formation in Yubei area were divided into fine-crystalline matrix dolomites, coarse-crystalline matrix dolomites and saddle dolomite fillings according to the petrographic features. The saddle dolomite fillings have geochemical features similar to the coarse-crystalline matrix dolomites. They appear to be dolomite cements, and are not associated with limestone replacement. Diagenetic fluids for saddle dolomite fillings are inferred to be high-temperature and high-salinity formation brines. The average Sr content of fine-crystalline matrix dolomites is higher than that of coarse-crystalline matrix dolomites and saddle dolomite fillings, while the average Ba content of saddle dolomite fillings is much higher than that of matrix dolomites and coarse-crystalline matrix dolomites. The Mg/Ca ratios of fine-crystalline matrix dolomites, coarse-crystalline matrix dolomites and saddle dolomite fillings are similar. The REE distribution curves of saddle dolomite fillings and matrix dolomites are similar. The inclusion analyses of coarse-crystalline matrix dolomites and saddle dolomite fillings show overlapping fluid properties, which indicates that saddle dolomite fillings are inherited from matrix dolomites.