羌塘盆地南坳陷古油藏带中侏罗统布曲组白云岩元素地球化学特征及意义

Trace element geochemical signatures and significance of the Buqu Formation dolostones in southern Qiangtang depression

关键词:  元素地球化学  白云岩  布曲组  南羌塘坳陷

Key words:  element geochemistry  dolomite  Buqu Formation  southern Qiangtang depression

摘要:

在前期详细岩石学分类的基础上,研究南羌塘坳陷布曲组碳酸盐岩微量元素地球化学特征及意义。结果表明:(1)布曲组碳酸盐岩具有较低的微量元素含量,与班公湖-怒江洋盆海水快速入侵及炎热、半潮湿-半干旱条件下缺少地表径流补偿陆源物质有关;(2) Na元素和K元素指示布曲组保留先驱灰岩原始组构的白云岩和不保留先驱灰岩原始组构的晶粒白云岩的初始流体为同期蒸发海水,岩石中Sr元素的含量受方解石和白云石的相对含量控制;(3)经历不彻底白云石化作用的过渡性岩类(RD5-1)是在中-深埋藏阶段中-晚期不彻底交代先驱灰岩形成,细-中晶、自形-半自形白云石充填物由微-粉晶白云岩在浅埋藏阶段末期重结晶形成;(4)鞍形白云石的形成有外来高温流体参与,流体成因具大气淡水参与、流经碎屑岩地层深循环流体特征。新生代(78~64Ma)构造热事件是形成高温流体的热源,部分基质白云岩受到构造热事件的高温改造。

Abstract:

Based on the previous detailed petrological classification, the present paper focuses on the geochemical signatures and significance of trace elements in the carbonate rocks from the Buqu Formation in the southern Qiangtang depression. The results of research in this study are outlined in the following. (1) The carbonate rocks in the Buqu Formation have lower trace element contents, which are related to the rapid invasion of sea water in the Bangong Lake-Nujiang Ocean Basin and the lack of surface runoff compensation for terrigenous materials under hot, semi-humid and semi-arid conditions; (2) Na and K contents indicate that both of the primitive fluids resulting the formation of the dolostones retaining primordial compositions and structures of pioneer limestones and the primitive fluids resulting the formation of the crystal grain dolostones not retaining pioneer limestone primordial composition in the Buqu Formation are the seawater evaporated in the same periods, and the contents of Sr are generally controlled by the relative contents of calcite and dolomite in the rocks; (3) The transitional rocks experiencing incomplete dolomitization (RD5-1) were formed by incomplete replacement of pioneer limestones in the middle to late phases of medium to deep burial stages. Fine-to medium-sized crystalline, euhedral-subhedral dolomite fillings were formed by the recrystallization of micro-powder dolomite at the end of shallow burial stage; (4) The formation of saddle dolomite involves the participation of exotic high-temperature fluids, and genetic fluids have the characteristics of atmospheric fresh water and deeply-cyclic fluids flowing through clastic formations. The high temperatures are provided by the Cenozoic (78-64 Ma) tectonic thermal events, and the partial matrix dolostones are transformed by the high temperatures caused by tectonic thermal events.