Dolomitization process of oolites in the Lower Triassic Feixianguan Formation, northeastern Sichuan Basin: evidence from magnesium isotopes

Oolitic dolostones are widely distributed in the Triassic Feixianguan Formation (FXG Fm) in northeastern (NE) Sichuan Basin, South China. Although multiple dolomitization processes have been proposed, including meteoric-mixing zone, seepage-reflux, and burial dolomitization, their origin remains controversial. While traditional petrological and geochemical analyses provide valuable insights into the formation mechanism of the oolitic dolostones, the source of Mg²⁺ and formation processes cannot be exclusive constrained. In this study, we investigated three representative sedimentary intervals within the FXG Fm: a non-oolitic shoal interval in the platform interior (S1), an oolitic shoal interval at the platform margin (S2), and an oolitic shoal interval in the platform interior (S3). These intervals, characterized by multiple shallowing-upward cycles, collectively record the deposition–diagenesis process of oolitic facies in the FXG Fm. By integrating sedimentary, petrographic, and geochemical data, including major and trace elements, carbon and oxygen isotopes and Mg isotopes, the dolomitization process of the oolitic dolomite in the FXG Fm can be constrained. The stratigraphic covariation of δ²⁶Mg_dol values with sedimentary cycles suggests periodic dolomitization controlled by sea-level fluctuations. During the relatively high sea-level stage, oolitic limestone precipitates in the grain shoals. When sea-level falls, the carbonate platform is prone to experience penecontemporaneous sabkha dolomitization characterized by a downward decreasing δ²⁶Mg_dol trend in intertidal flat facies, and seepage-reflux dolomitization marked by downward increasing or unchanging δ²⁶Mg_dol trend in shoal facies. Thus, this study provides a testable approach to investigate the origin of oolitic dolostone formed in grain shoals.