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摘要
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 (87)Sr/(86)Sr 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 Chanting 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 (87)Sr/(86)Sr 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.
- 出版日期2011-12
- 单位成都理工大学
