川西地区中二叠统栖霞组中—粗晶白云岩成因

Genesis of Medium-Coarse Crystalline Dolomite of Middle Permian Qixia Formation, Western Sichuan

通过野外剖面、岩心、薄片的观察及地球化学分析等手段,对川西地区栖霞组中—粗晶白云岩成因进行了研究。结果表明,中—粗晶白云石晶形呈自形、半自形及他形,可见残余生物(碎屑)幻影,裂缝发育,白云石被缝合线切割,储集空间以溶蚀孔洞和晶间孔为主,孔洞内充填有自形白云石及鞍状白云石。中—粗晶白云岩的碳同位素值处于二叠系海相碳酸盐岩的范围;川西的南部地区中—粗晶白云岩氧同位素相对于川西的北部地区更偏负;川西的北部中—粗晶白云岩锶同位素比值处于二叠系海相碳酸盐岩的范围,而川西的南部则高于此范围;稀土元素配分模式显示白云岩基本继承了石灰岩特点;白云岩Ce显示正异常,Eu无明显正异常,川西的南部地区具有较明显的La负异常。综合研究得出:颗粒滩相是白云石化作用发生的有利相带,浅埋藏阶段的海水白云石化作用是中—粗晶白云岩的主要成因,后期局部遭受热液改造作用,孔洞内充填的自形白云石也为浅埋藏阶段的海水白云石化作用所形成,而鞍状白云石为后期热液成因。

The genesis of medium-coarse crystalline dolomites of Middle Permian Qixia Formation in western Sichuan Basin is researched based on the geochemical methods and observation of outcrops, cores and thin-sections. The results showed that crystalline dolomites can be divided into three types： idiomorphic, hypidiomorphic, and xenomorphic. The residual bioclastic structure can be seen in moderately to coarsely crystalline dolomites. Fractures are well developed,and some crystalline dolomites are cut by stylolites. The pore space mainly consists of intercrystalline solution pores,solution vugs and intercrystalline pores. The solution pores are usually filled by idiomorphic dolostones and saddle dolostones. The δ13C values of the medium-coarse crystalline dolomites are in a range of marine carbonate rocks of the Permian strata, and the δ18O values in the north of Western Sichuan are higher than those in the south.87Sr/86 Sr values of dolomites in the north of Western Sichuan are also in a range of marine carbonate rocks of the Permian strata, and those in the south are higher than this range. The REE pattern model shows that the dolomites inherit characteristics of limestones. Ce of the dolomites shows positive anomalies, but Eu shows not obviously. La of the dolomites in the south of Western Sichuan shows relatively obvious negative anomalies. The study concluded that the grain beach face is positive for dolomitization, and the seawater dolomitization is the main genesis of medium-coarse crystalline dolomites during shallow burial stage, and at the late diagenetic stage they were suffering from hydrothermal modification in local areas. Seawater dolomitization in shallow burial stage is also the genesis of idiomorphic dolostones filled in solution vugs, but saddle dolostones are the products of later hydrotherm.