塔里木盆地北部下奥陶统碳酸盐岩孔洞充填物特征及其成岩环境分析

为了正确分析和评价碳酸盐岩溶蚀孔洞在埋藏成岩环境中的发育和保存条件,对塔里木盆地北部露头剖面中下奥陶统白云岩及其溶蚀孔洞的充填物进行了详细的矿物学和地球化学研究,试图对这类孔洞充填物沉淀时的成岩环境进行恢复。大型洞穴充填物的矿物学特征和微孔洞充填物扫描电镜的分析表明,此类孔洞的环带状充填物具有明显的世代性,由基岩向孔洞中心,至少可识别出4期充填物,即第1期的乳白色白云石,第2期的铁质浸染的杂色高镁方解石,第3期的无色透明白云石和第4期的自形石英。碳、氧同位素分析结果表明,孔洞充填物形成于高温条件,结合微量元素、稀土元素所示的成岩信息可进一步推断其形成于高温埋藏环境,而非现代表生暴露期的充填物;第1期和第3期白云石充填物形成在相对还原且高盐度的流体性质条件下,而第2期高镁方解石充填物沉淀时相对偏氧化,盐度也偏低。据下奥陶统此类4期充填物的特征分析可知,第2期高镁方解石充填物的发育可能与构造抬升导致成岩环境的改变有关。

The characteristics of cave fillings of carbonates in the Lower Ordovician and their diagenetic environments in northern Tarim basin

In order to rationally analyze and evaluate the developing and preserving condition of carbonate caves in burial diagenetic environment,the mineralogy and geochemistry of the host Lower Ordovician dolomites and their cave fillings in the northern Tarim Basin have been studied in details,and the diagenetic environment of the cave fillings has been recovered.It has been discovered that the cave fillings show obvious generational features.There are at least four generations of fillings from the bedrock to the center of a cave.Ordinarily,they are ivory-white dolomite,iron-dipped high magnesium calcite,achromatic transparent dolomite and euhedral quartz.The analytical data of carbon and oxygen stable isotope,trace elements and REE compositions show that the cave fillings of carbonates were formed in a high-temperature and burial environment,instead of an exposure environment;that the dolomite fillings of the first and the third generations may have deposited in a relatively reductive and high saline condition;that the calcite fillings of the second generation may have formed under a relatively oxygenic and lower saline environmental conditions,which is corresponding to the tectonic uplift in the Cretaceous.