Dolomitization Fluid and Genesis of Dolomite in the Devonian Guanwushan Formation in Upper Yangtze

Abstract

Thick-block dolomite, including microcrystalline dolomite (Type 1), fine crystalline dolomite (Type 2) and medium crystalline dolomite (Type 3), is extensively developed in the Guanwushan Formation of Devonian in the upper Yangtze region. Petrological, mineralogical and geochemical analyses indicate that the source of dolomitization fluid is contemporaneous seawater. The Type 2 and Type 3 dolomites have resulted from further dolomitization of the Type 1 dolomite. The order degree of the three types of dolomites increased in sequence, and they all show dull cathodoluminescence. The three types of dolomites have higher Sr²⁺ and lower Mn²⁺ concentrations than those of limestone. The total amount of rare earth elements of dolomites is lower than that of limestone. However, the dolomites are enriched with LREE and lack of HREE, with a consistent REE distribution pattern of the limestone. The weak negative anomalies of Ce and Eu indicate that the dolomites formed in a weak oxidizing and reducing environment with low temperature, and the dolomitization fluids inherited from the original seawater. The δ¹⁸O_PDB values of Type 1 and Type 2 dolomites are higher than those of the limestone, indicating that the dolomitization fluid is influenced by seawater evaporation at the penecontemporaneous stage. The δ¹⁸O_PDB value of Type 3 dolomite is lower than that of limestone, and it also shows anhedral crystal, which indicates that the dolomite has experienced more intense dolomitization stage at greater burial depth and higher temperature.