摘要
川东飞仙关组白云岩包括微晶白云岩、具原始结构的粒屑白云岩和结晶白云岩等3种主要的结构类型，其中结晶白云岩是重要的天然气储集岩。根据白云岩的锶含量和锶同位素组成，结合相关岩石学和地球化学资料，本文研究了白云石化过程中锶的迁移和相应的同位素变化。三种类型白云岩具有类似的锶含量，但其^87Sr/^86Sr比值均不同程度地高于同期海水值，锶在白云石和方解石（或文石）中分配系数的差别是影响白云石化过程中白云石锶含量最为重要的因素。不同流体中锶的混合方程计算表明：如果白云石化流体是大气淡水和海水的混合流体，则结晶白云岩的白云石化流体相当于94％的大气淡水和6％的海水混合，具原始结构的粒屑白云岩的白云石化流体相当于81％的大气淡水和19％的海水混合，微晶白云岩的白云石化流体相当于92％的大气淡水和8％的海水混合。但极低的锰含量、相对较高的锶含量和包裹体均一化温度等资料均不支持有94％的大气淡水参与了结晶白云岩的白云石化作用，其白云石化流体应是埋藏成岩过程中囚禁的近同期、但是非同期的海源流体，其来源可能与埋藏成岩过程中该时间段蒸发盐的溶解有关。锰含量、锶含量、^87Sr/^86Sr比值都表明具原始结构的粒屑白云岩和微晶白云岩的白云石化过程都存在大气淡水的介入，微晶白云岩的白云石化可能与潮坪环境的蒸发泵机理有关；具原始结构的粒屑白云岩的白云石化可能与混合水白云石化作用有关，这类白云岩分布于向上浅滩化旋回滩体的顶部，这是埋藏前或浅埋藏阶段大气淡水与海水混合白云石化作用最容易发生地带。
Three major dolomite types including micrite dolomite, grainy dolomite （with original texture） and crystalline dolomite, occur in the Feixianguan Formation of Triassic, Eastern Sichuan of China, in which the crystalline dolomite is the most important gas reservoir. Based on the strontium concentration, isotope composition of the dolomites, the dolomitization fluids were accordingly inferred here. The three types of dolomite have similar strontium content with higher ^87Sr/^86Sr ratios to some extent than that of the coeval seawater. The difference of distribution coefficient of Sr in calcite （or aragonite） and dolomite is the most important control factor on the strontium content in dolomites. Calculation by strontium mixing equation of seawater with meteoric water shows that the dolomitization fluid for crystalline dolomite is mixed by 94% meteoric water and 6% seawater; the fluid for grainy dolomite （with original texture） is mixed by 81% meteoric water and 19% seawater, and that for micrite dolomite is mixed by 92% meteoric water and 8% seawater. However, very low Mn contents （average 19. Sppm）, relatively high Sr contents （average 137ppm） and the homogenization temperatures measured in fluid inclusions of crystalline dolomite do not support the hypothesis that meteoric water as high as 94% involved the dolomitization. The dolomitization fluid for this kind of dolomite was probably the trapped coeval seawater （with the age of 242 to 245Ma）, which may have been released into the dolomite horizon through unidentified pathways. The Mn and Sr contents, as well as ^87Sr/^S6Sr ratios of the micrite dolomite and grainy dolomite （with original texture） suggest the existence of meteoric water during the dolomitization. The mechanism of dolomitization for micrite dolomite could be explained by evaporative pumping, and that for grainy dolomite （with original texture） explained by mixing-zone model.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第8期2123-2132,共10页
Acta Petrologica Sinica
基金
国家自然科学基金(40472068)
教育部博士点基金(20050616005)资助课题
关键词
四川盆地东部
三叠系飞仙关组
白云石化作用
锶含量与锶同位素
成岩流体
Eastern Sichuan, Feixianguan Formation of Triassic, Dolomitization, Strontium concentration and isotope, Diagenesis fluids
