四川盆地南江栖霞组白云岩岩石学、地球化学特征及成因

四川盆地南江桥亭剖面中二叠统栖霞组二段灰岩局部白云岩化,基质白云岩主要由非平直晶面他形晶白云石构成,孔洞缝中充填鞍形白云石和结晶方解石,溶解作用主要发生在白云石中。在岩石学研究基础上,综合运用碳氧同位素、元素、包裹体均一化温度、阴极发光测试手段,结合埋藏史、热史以及峨眉山玄武岩喷发非正常地热增温事件,研究了基质白云石和鞍形白云石的特征和成因。研究表明:基质白云石、鞍形白云石和结晶方解石的包裹体均一化温度分别为100~110,130~230,130~230℃;富含基质白云石、鞍形白云石和结晶方解石样品的δ¹⁸O_PDB值分别为-4.27‰~-6.28‰,-4.87‰~-5.80‰,-5.16‰~-6.50‰,δ¹³C_PDB值分别为3.97‰~5.22‰,4.88‰~5.35‰,2.73‰~4.29‰;与基质白云岩相比较,鞍形白云石和结晶方解石的δ¹⁸O值略负,并显示三者碳源为同时期海水。反演的基质白云岩形成流体的δ¹⁸O_SMOW值分布在3‰~6‰之间,鞍形白云石δ¹⁸O_SMOW主要分布在7‰~14‰之间,盐度均显著高于同期海水,因而两者都是在高温和高盐度的流体中形成的。三者都受到了峨眉山玄武岩喷发活动所引起的热事件影响,高温和高盐度克服了白云石沉淀的动力学屏障,形成交代灰岩的基质白云岩和孔洞缝中充填的鞍形白云石,流体的持续高温和Mg²⁺的消耗,导致了高温方解石的沉淀,显示热液对碳酸盐岩的改造作用。

Petrologic and geochemical characteristics and the origin of dolomites in the Qixia Formation in Nanjiang county of the Sichuan Basin

Limestones are locally dolomitized in the 2nd member of the Middle Permian Qixia Formation of Qiaoting section, located in Nanjiang county of the Sichuan Basin. Matrix dolomites primarily consist of non-planar xenotopic dolomite crystals, and vugs and fractures are filled with saddle dolomites and crystalline calcites. Dissolution occurs mainly in dolomites. Characteristics and origin of matrix dolomites and saddle dolomites have been determined based on petrographic examinations, integration of carbon and oxygen stable isotopes, trace element content, fluid inclusion data, cathodoluminescence images, burial and thermal maturation history and abnormal thermal events associated with the Emeishan basalt eruption. The research showed that the fluid inclusion homogenization temperatures of matrix dolomites, saddle dolomites and crystalline calcites range 100~110℃, 130~230℃ and 130~230℃, respectively. The δ18O values of the samples which mainly consist of matrix dolomites, saddle dolomites and crystalline calcites range -4.27‰~-6.28‰, -4.87‰~-5.80‰ and -5.16‰~-6.50‰, respectively. Saddle dolomites and crystalline calcites have the characteristics of more negative δ18O values than matrix dolomites. The δ13C values of the samples which mainly consist of matrix dolomites, saddle dolomites and crystalline calcites range 3.97‰~5.22‰, 4.88‰~5.35‰ and 2.73‰~4.29‰, respectively, indicating that the carbon source is generally from the coeval seawater. The δ18O values of dolomitizing fluids for matrix dolomite and saddle dolomite range +3‰~+6‰ (SMOW) and +7‰~+14‰ (SMOW), respectively. This is consistent with the formation of matrix and saddle dolomites in fluids with elevated salinity and temperature. Matrix dolomites, saddle dolomites and crystalline calcites were influenced by thermal events caused by Emeishan basalt eruption activity. The kinetic barrier of dolomite precipitation was overcome by high salinity and temperature, and it allowed the transformation of limestones into replacive matrix dolomites and precipitate saddle dolomites in vugs and fractures. Due to lasting high temperature and Mg2+ loss, calcites with high temperature precipitated, which showed that hydrothermal fluids altered the carbonate rocks.