Dolomitization controlled by geological events during the critical tectonic transition of the Tethyan domain: A case study from the Permian Maokou Formation in eastern Sichuan Basin, SW China

Abstract

Focusing on the dolomites within the Permian Maokou Formation in eastern Sichuan Basin, this study integrates petrographic observation, geochemical analysis and in situ U-Pb dating to constrain the timing of dolomitization and trace the sources of dolomitizing fluids, analyze the intrinsic links among geological events during the tectonic transition of the Paleo-Tethys to Neo-Tethys oceans, strike-slip faulting and dolomitization, so as to reveal the dolomitization mechanism of the Maokou Formation. Three types of matrix dolomites occur in the Maokou Formation in eastern Sichuan Basin, with U-Pb ages indicating three dolomitization phases at (260.6 ± 6.8)–(265.1 ± 2.4), (244.0 ± 11.0)–(247.7 ± 6.0), and (220.6 ± 8.5)–(221.4 ± 7.8) Ma, respectively. Geochemical data indicate distinct fluid origins for each phase of dolomitization. Three geological events and the resulting three episodes of faulting during the tectonic transition from Paleo- to Neo-Tethys Ocean are key controlling factors of three phases of dolomitization. Specifically, the Middle Permian Emeishan magmatism activated the Houba-Peng’an-Fengdu strike-slip fault zone and induced thermal anomalies, promoting thermal convection between contemporaneous seawater and the Lower Silurian siltstone aquifer, and initiating the first phase of dolomitization. During the Middle Triassic, oblique closure of the Mianlüe Ocean induced transtensional faulting, and density-driven downward migration of residual evaporitic seawater and brines from evaporates in the Lower–Middle Triassic facilitated the second phase of dolomitization. The Late Triassic continental collision between the South China Block and North China Block induced transpressional faulting, driving the upward migration of brines within the Lower Silurian to mix with residual evaporitic seawater in the Lower–Middle Triassic, thus supplying the magnesium source for the third phase of dolomitization. A strike-slip fault-controlled dolomitization model is established, providing new insights into the formation mechanisms of dolomite reservoirs in the Tethyan domain.