Characteristics and genesis of dolomites in the Upper Devonian Shawozi formation of the northwestern Sichuan basin, southwestern China

Devonian thick–layer dolostone is widely developed in the Shawozi Formation (SWZF) of the Sichuan Basin, however, the properties of the dolomitisation fluid and its genesis require clarification. This study adopted field observations, microscopic identification, and geochemical data analysis methods that major and trace elements, rare–earth element (REE) analyses, carbon and oxygen isotopes based on 31 samples. The results showed that dolomites were divided into crystalline dolomites and calcareous dolomites. Crystalline dolomite, which is the most widely distributed, can be further divided into fine crystalline dolomite (FCD), fine–medium crystalline dolomite (FMCD), medium crystalline dolomite (MCD), and coarse crystalline dolomites (CCD). Calcareous dolomite, fine crystalline dolomite (FCD), and fine–medium crystalline dolomite (FMCD) have higher Fe, higher Mn, and lower Sr, whereas medium crystalline dolomite (MCD) and coarse crystalline dolomite (CCD) have the opposite characteristics. The REE content was lower and weak negative anomalies of Ce and Eu were apparent, indicating that dolomite was formed in a fragile redox environment and at a relatively low temperature. The δ13CPDB value had a small variation range, with an average value of 0.04‰. Although the variation range of δ18OPDB value was more extensive, most samples were concentrated in the range of -6.53‰~-2.1‰, indicating that the overall effect of diagenetic alteration was more negligible. Two dolomitisation models were developed: reflux seepage and burial dolomitisation. The reflux seepage model mainly developed calcareous dolomite, whereas the burial model mainly developed crystalline dolomite. The degree of dolomitisation increased with increasing burial depth and formation pressure.