Strontium isotopic compositions of Cambrian (Upper Miaolingian-Furongian Series) dolomites from south-eastern Sichuan Basin, China: Significance of sources of dolomitizing fluids and timing of dolomitization

摘要

Tracing the nature of dolomitizing fluids and constraining the dolomitization time are of great significance for understanding dolomite genesis and reservoir formation, especially for ancient dolomites. Strontium isotopes can be used as very useful tools to study this aspect. Thick dolomite-bodies developed in the Loushanguan Group of the Upper Miaolingian-Furongian Series (Cambrian), in the south-eastern margin of the Sichuan Basin, China. These dolomites are probably good hydrocarbon reservoirs. Little research has been carried out on the Sr isotopes in these ancient dolomites. Three different textures of dolomites have been identified in the study area: dolomicrite, fabric-retentive oolitic dolomite, and fabric-obliterative dolomite. The dolomicrite displays a higher Sr isotopic value (avg. 0.71052) compared with the contemporaneous seawater, as well as a high Sr and low Mn content. These chemical characteristics indicate that the dolomitizing fluids of the dolomicrite were mainly derived from modified seawater mixed with a little continental input. The Sr-87/Sr-86 ratios (avg. 0.70936) of the fabric-retentive oolitic dolomites are close to those of normal seawater, indicating that the dolomitizing fluids were derived from the contemporaneous seawater and that they possibly formed soon after sedimentation as a result of an early replacement during shallow burial. The low Sr isotopic values (avg. 0.70854) of the fabric-obliterative dolomites indicate that these dolomitizing fluids are related to Late Cambrian seawater, and that the dolomites are the result of recrystallization of the fabric-retentive dolomites in a relatively closed diagenetic system during burial, with combined characteristics of relatively high Sr and low Mn contents. The recrystallization began approximately 490 Ma, and was completed not earlier than 485 Ma. This study is not only beneficial to understand the characteristics and multistage process of the dolomitization of ancient Cambrian dolomites but provides a reference for sourcing dolomitizing fluids and constraining the dolomitization time using Sr isotopes.