| Abstract:The continental red bed is widely distributed around the world and considered to be the product of the special environment, but the research on the aspect of coloration of continental red bed in the Songnen Plain is relatively weak. In this study, we take the red bed of the Luojiawopeng Formation on the southeastern edge of the Songnen Plain as the research object, analyze the geochemistry, petrology and mineralogy of the collected samples, explore the coloration mechanism of it and the environmental background. Through the geochemical analysis of these samples, the result shows that the Fe2O3 content of the Luojiawopeng formation red bed is relatively high (6.76%); Whole rock X-ray diffraction analysis, thin section identification and diffuse reflection spectrum (DRS) analysis indicate that the content of hematite is extremely high, which indicate that hematite is a red bed coloring mineral. The incomplete filling of hematite between grains in thin section analysis and the absence of local red bed provenance indicate that hematite is authigenic origin. High Resolution Transmission Electron Microscope（HRTEM）analysis shows that hematite adheres to the surface of clay mineral kaolinite, and kaolinite can serve as a transportation medium for iron oxides during migration. Therefore, it is believed that some hematite is transformed from other clay minerals, the iron in the crystal lattice is leached out, adsorbed and oxidized on the surface of kaolinite during the formation of kaolinite. The Fe2O3 content of the sample is higher than other red beds, while the iron in the clay mineral lattice is limited. Combining CIA, A-CN-K ternary diagram, and UCC standardization, we concluded that the Luojiawopeng red bed is in the final stage of chemical weathering, while besides clay mineral source another part of iron is gradually accumulated as an immobile element. Luojiawopeng continental red bed of coloration mineral is authigenic hematite, and the iron comes from the gradual accumulation of secondary clay mineral transformation leaching and chemical weathering. Based on the climatic indicative significance of hematite and kaolinite, MAT, MAP and strong leaching of major elements, we infer that the Luojiawopeng red bed tends to be the product of alternating dry and wet weather under the warm and wet background of the East Asian summer monsoon. |
