热液活动对川东中二叠统茅口组的影响及与峨眉山大火成岩省的关系

The dolomite of the Middle Permian Maokou Formation in the eastern Sichuan Basin is often associated with chert. Previous studies have shown that the formation of dolomite and chert in the Maokou Formation is related to hydrothermal fluids. It is suspected that hydrothermal fluids and the Emeishan igneous province (LIP) have a close relationship but no hard evidence is given. The project is guided by theories of sedimentology, mineralogy, petrology, geochemistry and tectonics, and takes the Maokou Formation in the eastern of Sichuan Basin as the research object, making full use of outcrop sections, drilling, seismic data, and research results of previous generations. A variety of testing methods were used to systematically study the origins of dolomite and chert in the Maokou Formation, analyze the effects of hydrothermal activity on the formation of dolomite and chert, and clarify the main controlling factors and distributions of the development of dolomite and chert. The Mg isotope analysis of calcite which filled in the geode, dissolved pores or cracks of hydrothermal dolomite and chert layers, and the exact age data achieved by U-Pb dating of calcite with LA-ICP-MS. Combined with regional geological evolution, the period of hydrothermal activity affecting the origins of dolomite and chert and their affiliation with the Emeishan basalt were analyzed and to discuss the geodynamic mechanism during deposit of the Maokou Formation and its response to the Emeishan large igneous province (LIP).

四川盆地东部中二叠统茅口组白云岩常与硅质岩相伴生，前人的研究表明茅口组的白云岩和硅质岩的成因均与热液有关，猜测热液与峨眉山大火成岩省有密切的关系，但没有给出确凿的证据。本项目以沉积学、矿物学、岩石学、地球化学及构造地质学等理论为指导，以川东茅口组为研究对象，充分利用露头剖面、钻井、地震和前人研究成果等资料，运用多种测试分析手段，系统研究茅口组白云岩与硅质岩的成因问题，分析热液活动对白云岩和硅质岩形成的影响，阐明白云岩和硅质岩发育的主控因素和分布范围。对热液白云岩和硅质岩层内晶洞、溶孔或裂缝内充填的方解石进行Mg同位素分析和LA-ICP-MS的U-Pb测年分析，得出具体的年龄数据，在与峨眉山玄武岩喷发时间对比的基础上，结合区域构造演化，分析影响白云岩和硅质岩形成的热液活动的期次及与峨眉山玄武岩的亲缘性，探讨中二叠统茅口组形成时的地球动力学机制及其对峨眉山大火成岩省的响应。

四川盆地东部中二叠统茅口组白云岩常与硅质岩相伴生，前人的研究表明茅口组的白云岩和硅质岩的成因均与热液有关，猜测热液与峨眉山大火成岩省有密切的关系。本项目以沉积学、矿物学、岩石学、地球化学及构造地质学等理论为指导，以川东茅口组及川西中二叠统为研究对象，充分利用露头剖面、钻井岩心和前人研究成果等资料，运用多种测试分析手段，系统研究茅口组白云岩与硅质岩的成因问题，分析热液活动对白云岩和硅质岩形成的影响，探讨白云岩和硅质岩发育的主控因素。研究表明川东中二叠统茅口组白云岩和川西中二叠统栖霞组白云岩的成因均与浅埋藏有关，而且均受到高温的影响；川东茅口组硅质岩既有层状的也有非层状的，层状的硅质岩既有生物的成因也有高温流体（如热水）的成因，非层状的硅质岩一般是成岩作用的产物。对川东茅口组进行碳酸盐U-Pb定年分析，发现茅口组至少经历了两期白云石化流体和两期富钙流体的成岩改造：第一期的白云石化流体的年龄是261.70 ± 2.90 Ma或261.76 ± 8.47 Ma，第二期的白云化流体的年龄是256.20 ± 4.80 Ma或253.70 ± 4.20 Ma，均与峨眉山大火成岩省有关系；第一期的富钙流体的年龄是220.80 ± 4.10 Ma，与燕山期构造运动有关；第二期的富钙流体的年龄是59.62 ± 0.98 Ma，与喜山期构造运动有关。研究成果对于四川盆地东部乃至整个四川盆地中二叠统的油气勘探具有重要的意义，同时对探讨中二叠世四川盆地的演化和研究峨眉山大火成岩省提供新的证据和思路。