四川盆地东北部晚二叠世-早三叠世白云岩与同期海水锶同位素组成的对比研究

在四川盆地东北部14条野外剖面和地下钻井的二叠系长兴组、三叠系飞仙关组和嘉陵江组石灰岩和白云岩岩石学研究的基础上,对其中189个不同类型的碳酸盐样品(包括代表海水的石灰岩样品和在不同成岩阶段形成的各种白云岩样品)进行了锶同位素组成和相应的MgO、CaO和Mn、Sr元素分析,获得了系统的晚二叠-早三叠世海水的锶同位素组成数据并建立了相应演化曲线.在此基础上,对不同地层组/段和不同类型白云岩的锶同位素组成与同期海水锶同位素组成进行了对照研究,取得了如下主要认识:(1)川东北晚二叠-早三叠世白云岩的锶同位素组成与同期海水具有类似的演化趋势,结合白云岩的低锰、高锶特征,说明白云化流体与海水存在显著的亲缘关系,与铝硅酸盐地层无关；(2)白云岩的锶同位素组成与同期海水存在差别,各地层组/段白云岩的87Sr/86Sr比值都不同程度地高于同期海水,但从下往上,即从长兴组、飞2+3段、嘉2段到嘉4段,这种差值逐渐缩小,嘉4段白云岩的锶同位素组成已和同期海水基本一致；(3)川东北长兴组、飞2+3段白云岩形成的时间显著晚于同层石灰岩,白云化流体为时间上更晚的海源流体,但嘉2、嘉4段白云岩的形成时间仅略晚于同层石灰岩,白云化流体来源于非常近同期的蒸发浓缩的高Mg/Ca比值海水,一些嘉4段的白云岩的白云化流体就是同期海水,因而这些白云岩是同生或准同生的；(4)如果把白云化的时间看作白云岩的形成时间,则违背地层叠置原理是川东北长兴组和飞仙关组结晶白云岩的主要特征之一,其形成机制可用非同期海源流体的隐伏回流-对流模式来解释,嘉陵江组白云岩形成机制可用活跃回流-萨布哈模式来解释.白云岩和代表同期海水的石灰岩锶同位素组成的对比为解决白云化流体与海水之间的时间关系提供一种新的研究途径.

A comparative study on strontium isotope composition of dolomites and their coeval seawater in the Late Permian-Early Triassic, NE Sichuan basin

Based on the research of petrography of limestones and dolomites in Changxing Formation of Permian, Feixianguan Formation and Jialingjiang Fromation of Triassic from 14 outcrop sections and wells, NE Sichuan basin, 189 carbonate samples (including limestones, which contain the information of the coeval seawater, and dolomites formed in different diagenetic stages) were selected and their isotope composition and contents of MgO, CaO, Mn and Sr were detected. Systematic data of the strontium isotope composition of the Late Permian-Early Triassic seawater were obtained, and then corresponding evolution curve was established. Accordingly, comparative studies of strontium isotopic composition between the different dolomites and their coeval seawater are carried out. And the following main points are obtained from the research: (1) The strontium isotope composition of dolomites of the Late Permian-Early Triassic in NE Sichuan basin has a similar evolutionary trend with that of the coeval seawater, suggesting that dolomitization fluids have notably close relationship with the coeval seawater; (2) There are differences between the strontium isotope composition of dolomites and that of the coeval seawater. In other words, the ⁸⁷Sr/⁸⁶Sr ratios of dolomites of each formation/member are higher than that of the coeval seawater in different degrees. However, from Changxing Formation to the second and third members of Feixianguan Formation, and to the second Member of Jialingjiang Formation, and to the fourth Member of Jialingjiang Formation, the difference narrows gradually. Furthermore, the strontium isotope composition of dolomites of the fourth Member of Jialingjiang Formation is nearly consistent with that of the coeval seawater. And also, because of the low Mn and high Sr content of dolomites, it can be concluded that relative enrichment of radiogenic Sr in dolomites has nothing to do with the siliciclastics; (3) the forming time of the dolomites of Changxing Formation and the second and third members of Feixianguan Formation in NE Sichuan basin is significantly later than that of the limestone in the same strata and the dolomitization fluids with higher Mg/Ca ratios should derive from the concentrated seawater in the overlying strata. But it is just slightly later that the forming time of dolomites in the second and fourth members of Jialingjiang Formation is than that of the limestone in the same strata and their dolomitization fluids should be very close to the coeval seawater. And even, the dolomitization fluids of dolomites of the fourth Member of Jialingjiang Formation could be the coeval seawater; (4) Violating the principle of stratigraphic superposition is one of the main features of crystalline dolomite of Changxing Formation and Feixianguan Formation in NE Sichuan basin, and their formation mechanisms could be explained using the latent reflux-convection model by non-coeval seawater dolomitization fluids. The ⁸⁷Sr/⁸⁶Sr ratio of dolomite of Jialingjiang Formation is very close to that of the coeval seawater, so its formation mechanism could be explained by active reflux and sabkha model. The comparative study on the strontium isotope composition of dolomites and limestones, which contain information of the coeval seawater, provides a new choice to research the time relationship between dolomitization fluids and the coeval seawater.