Middle Permian dolomites of the SW Sichuan Basin and the role of the Emeishan Large Igneous Province in their origin

Abstract

Dolomitization of the Middle Permian carbonate platform in South China occurred in successive phases, partially influenced by the emplacement of the Emeishan Large Igneous Province. The main replacement dolomites are: (1) finely crystalline (˂50 μm) Md I, (2) mesocrystalline (50–250 μm), non-planar-a to non-planar-s Md II; and (3) coarse crystalline (250–500 μm), non-planar-s to non-planar-a Md III. Dolomite cement (Cd) is characterized by pores and/or fractures filled with saddle crystals within the Md III dolomites. The Md I dolomites share similar rare earth element patterns and carbon and strontium isotopes to host limestones and δ18O values that are in equilibrium with the limestone. These characteristics indicate that Md I dolomite was formed via the seepage/reflux dolomitization of evaporated seawater during the penecontemporaneous phase. The Md II dolomites share δ13C values and 87Sr/86Sr ratios similar to those of typical Permian carbonates, characterized by a range of low δ18O values. Analyses of fluid inclusions in these Md II dolomites showed homogenization temperatures of 90–130 °C and salinities of 4–9 wt% NaCl equivalent. These results suggest that Md II represents typical replacive dolomitization occurring within convective half-cells under geothermal conditions. They are particularly pervasive when the platform is heated by magmatic underplating and/or igneous emplacement to form thermal convection. The coarser Md III dolomites are characterized by high porosity and subsequently partially filled with saddle dolomite cement (Cd). These Md III and Cd dolomites are rich in radiogenic strontium, have low δ13C values, very low δ18O values, positive Eu anomalies, and contain fluid inclusions indicating high homogenization temperatures and salinities. These characteristics suggest that the Md III and Cd dolomites precipitated from high-temperature hydrothermal fluids, potentially associated with the emplacement of the Emeishan Large Igneous Province (LIP). Regional heating from the Emeishan LIP induced basin uplift and faulting. These were important in channeling the upwelling deep brine fluids into the overlying platform carbonates, leading to the localized intense dolomitization. This suite of Middle Permian dolomites is an excellent example of the successive phases of different dolomitization processes, including that associated with hydrothermal and thermal convection during the emplacement of a LIP.