Trace element geochemical signatures and significance of the Buqu Formation dolostones in southern Qiangtang depression

Based on the previous detailed petrological classification, the present paper focuses on the geochemical signatures and significance of trace elements in the carbonate rocks from the Buqu Formation in the southern Qiangtang depression. The results of research in this study are outlined in the following. (1) The carbonate rocks in the Buqu Formation have lower trace element contents, which are related to the rapid invasion of sea water in the Bangong Lake-Nujiang Ocean Basin and the lack of surface runoff compensation for terrigenous materials under hot, semi-humid and semi-arid conditions; (2) Na and K contents indicate that both of the primitive fluids resulting the formation of the dolostones retaining primordial compositions and structures of pioneer limestones and the primitive fluids resulting the formation of the crystal grain dolostones not retaining pioneer limestone primordial composition in the Buqu Formation are the seawater evaporated in the same periods, and the contents of Sr are generally controlled by the relative contents of calcite and dolomite in the rocks; (3) The transitional rocks experiencing incomplete dolomitization (RD5-1) were formed by incomplete replacement of pioneer limestones in the middle to late phases of medium to deep burial stages. Fine-to medium-sized crystalline, euhedral-subhedral dolomite fillings were formed by the recrystallization of micro-powder dolomite at the end of shallow burial stage; (4) The formation of saddle dolomite involves the participation of exotic high-temperature fluids, and genetic fluids have the characteristics of atmospheric fresh water and deeply-cyclic fluids flowing through clastic formations. The high temperatures are provided by the Cenozoic (78-64 Ma) tectonic thermal events, and the partial matrix dolostones are transformed by the high temperatures caused by tectonic thermal events.