Magma-driven thermal convection dolomitization of the Lower-Middle Ordovician strata caused by Mg sourced from underlying Cambrian dolomites in the Tarim Basin, China

Thermal convection arises due to a density gradient between geothermally heated formation water within the underlying strata and the water in the overlying strata. The thermal convection model has long been accepted as a stable and long-lived flow mechanism for dolomitization, which can affect large volumes of rock. This study investigates the dolomitization of the Lower-Middle Ordovician strata in the Tarim Basin using petrological evidence, geochemical data, and the Calculation of Mg2+ flux. The Ordovician dolomites occurred in several types, including 1) individual dolomite crystals; 2) patchy dolomites; 3) layered-massive dolomites; 4) cement dolomites in pores and fractures. The dolomite content decreases vertically with decreasing burial depth. Geochemically, the dolomitizing fluids of the Ordovician carbonates are suggested as deep-formation water slightly influenced by hydrothermal fluids, with an obvious genetic connection with the underlying Cambrian dolomites. In addition, mass balance calculations were conducted to help identify the Mg2+ sources, supporting that the Mg-rich dolomitizing fluids were derived largely from the underlying strata. Large volumes of Mg-rich fluids were released when the underlying contemporaneous Cambrian dolomites were physically and chemically compacted. The released fluids were driven by heat from a high-temperature Early Ordovician magma chamber and migrated upward through fractures and/or permeable strata due to hydrothermal convection. These processes caused thermal convection dolomitization of the overlying Ordovician limestones from bottom to top. This study explains how these widespread dolomites developed in the Ordovician strata of the Tarim Basin, which can help identify the locations of the oil-gas reservoir rocks. In addition, this revised thermal convection model highlights the contribution of Mg-rich fluids derived from the underlying dolomites, and the circulation of fluids may have been driven by the heat from a magmatic source. This model is believed to be of significance to areas affected by abnormal geothermal activities.