科研之友
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摘要
The existence of a 0.3 Ga"great unconformity" between the Mesoproterozoic "Undefined System" Xiamaling Formation and the "Neoproterozoic" Longshan Formation has long been controversial. In this study, stratigraphy, sedimentology, detrital zircon dating, lithium isotope, and major and trace elements were applied to systematically analyze their relationship. Results demonstrate that coarse to fine sandstone-mudstone normal cycles with different grades and thicknesses exist from the Xiamaling to the Longshan formations. Sedimentary succession formed from a restricted platform with lagoon-shallow marine shelf-tidal flat-beach and tidal shoal developing in ascending order. The pebbly sandstone at the bottom of the Longshan Formation does not have the properties of basal conglomerate with tectonic significance. The youngest detrital zircon is older than 1.6 Ga for the Longshan Formation and overlying Jing'eryu Formation sandstones. In addition, no (zircon) provenance has been found from the large igneous province of 1.32-1.2 Ga in Xiamaling Formation. The delta Li-7 isotope values exhibit a gradually increasing trend. The Li content and CIA (Chemical index of alteration) gradually decreases from the top of the Xiamaling Formation to the lower part of the Longshan Formation. Simultaneously, as K2O/Al2O3 and FeO/Fe2O3 decreases, Na2O/Al2O3 increases, and Li/Al, V/Cr, and V/(V+Ni) fluctuates slightly. At the boundary, the Ce/Ce* and Eu/Eu* show positive and negative anomalies, respectively. Further, the Rb/Sr and Al/Si values decrease, and U/Th values increase. This indicates that the geochemistry of the weathering crust at the interface of the Xiamaling Formation-Longshan Formation is not characteristic. However, it indicates a weakening of continental weathering intensity from the bottom to the top of the interface. These results do not support the existence of a large unconformity between the Xiamaling and the Longshan formations. Therefore, it is suggested that the Longshan Formation-Jing'eryu Formation should be moved to the lower level in the stratigraphic chart, where it should form part of the Mesoproterozoic "Undefined System" along with the Xiamaling Formation. On this basis, the paleogeography of the Xiamaling Formation-Longshan Formation-Jing'eryu Formation sedimentary period in the Yanliao area of North China has been reclassified.
