四川盆地西部中二叠统栖霞组晶洞充填物特征与热液活动记录  

Vug fillings and records of hydrothermal activity in the Middle Permian Qixia Formation,western Sichuan Basin

作　　者：黄思静[1,2] 兰叶芳[1,2] 黄可可[1,2] 吕杰[1,2] 

机构地区：[1]油气藏地质及开发工程国家重点实验室,成都理工大学,成都610059 [2]成都理工大学沉积地质研究院,成都610059

出　　处：《岩石学报》2014年第3期687-698,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41172099;41272130)资助

摘　　要：热液环境在碳酸盐岩成岩作用中十分重要,MVT铅锌矿床和热液白云岩储层都与之有关。四川盆地西部中二叠统栖霞组普遍发育厘米级大小的晶洞,其充填物主要为晶体粗大的非平直晶面鞍形白云石,这些鞍形白云石经历了广泛的溶解作用,次生方解石充填于鞍形白云石溶解空间及其晶间孔隙中。本文在碳酸盐岩岩石学特征研究的基础上,测试了晶洞充填物的碳氧同位素组成、元素构成和包裹体均一化温度,结合川西中二叠统埋藏历史和二叠纪以来的非地热增温热事件,研究了晶洞充填物中鞍形白云石的沉淀与溶解流体,以及充填于鞍形白云石溶解空间和晶间孔隙中次生方解石的沉淀流体。研究表明:在作为晶洞充填物的碳酸盐矿物中,鞍形白云石和沉淀于白云石晶间、晶内的次生方解石具有显著不同的氧同位素组成和包裹体均一化温度,前者δ18O值-5.94‰～-4.35‰,包裹体均一化温度主要为110～210℃,后者δ18O值-10.34‰～-8.75‰,包裹体均一化温度主要为70～110℃,据此反演的鞍形白云石沉淀流体的δ18O值为+4‰～+14‰(SMOW),方解石相应值为-4‰～+5‰(SMOW),显示白云石是在高盐度和高温流体中沉淀的,方解石是在相对低盐度和低温流体中沉淀的;晶洞充填物的碳同位素分析表明,鞍形白云石和沉淀于白云石晶间和晶内的次生方解石的δ13C值大致分布在0.7‰～2.6‰的范围内,显著低于同期海水的δ13C值,两种碳酸盐矿物中的碳具有同期海水和深部CO2混合碳源的特征;中二叠世末东吴运动期间峨眉山玄武岩喷发时岩浆活动的热效应导致了当时处于浅埋藏环境的栖霞组地层中鞍形白云石的沉淀,而热事件后流体温度和盐度的同时降低则使得鞍形白云石溶解,同时伴随方解石沉淀在鞍形白云石溶解后的孔隙和晶间孔隙中。The hydrothermal environment is very important in the study of carbonate diagenesis. Both MVT lead-zinc deposits and some dolomite reservoirs are associated with the hydrothermal environment. Centimeter-sized vugs are very common in the Middle Permian Qixia Formation of the western Sichuan basin. Vug fillings consist mainly of coarse-grained saddle dolomite and calcite, as well as a small amount of authigenic non-carbonate minerals, such as fluorite, fluorapatite, barite, quartz, and illite. Saddle dolomites with non-planer and anhedral crystals show typical curved crystal faces and undulatory extinction. Intensive dissolution is well developed in saddle dolomite, and even some of the dissolution spaces have been completely replaced by calcites. The calcite, occupying the dolomite dissolution spaces and inter-granular pores, occurs after the formation of the saddle dolomite. The analyses of petrography, chemical composition, carbon and oxygen isotopes, cathodoluminescence images, and fluid-inclusion homogenization temperature of the vug fillings suggest that ： （ 1 ） Saddle dolomite and calcite have significantly different oxygen isotope compositions and fluid-inclusion homogenization temperatures. The former with δ^18O ranging from -5.94‰ -4. 35‰ and homogenization temperature ranges of 110 -210℃. The latter with δ^18O ranging from - 10. 34‰-8.75‰ and homogenization temperature ranges of 70 110℃. The inversion of the oxygen isotope composition and homogenization temperature shows that the 81SO composition of the precipitating fluids of the saddle dolomite and calcite ranges respectively from ＋ 4‰ - ＋ 14‰ （SMOW） and - 4‰ - ＋ 5‰ （SMOW）. These values indicate that the fluids for the precipitation of saddle dolomite have a relatively high salinity and temperature, while the calcite that forms after the saddle dolomite may precipitate from fluids with a relatively low salinity and temperature. （2） Saddle dolomite and calcite have similar carbon isotope compositions with δ

关 键 词：四川盆地西部 中二叠统栖霞组 碳 氧同位素 鞍形白云石 热液环境成岩流体 

分 类 号：P588.245[天文地球—岩石学]