Zircon U-Pb Dating of 'Mung Bean Rock' in the Tongliang Area, Chongqing and Its Geological Significance

'Mung Bean Rock' or 'Green Bean Rock' is a type of clay-like rock formed by Paleomatization and diagenesis. These rocks are distributed mainly between early and middle Triassic strata in Southwestern China and are good regionally isochronal mark beds. Studying the diagenetic age of 'Mung Bean Rock' is of great significance for further understanding of its diagenetic mechanism and age background. The debris zircon of 'Mung Bean Rock' in Chongqing Tongliang was used for U-Pb dating by LA-ICP-MS in this study. The zircon crystals have almost complete shape and zoning textures with high U and Th content, typical of magma zircon. Weighted average ages of the zircon ²⁰⁶Pb/²³⁸U peak at 245.86±0.98 Ma and 277.6±3.8 Ma. 'Mung Bean Rock' is located in the base of the Leikoupo Formation, the age of 245.86 Ma represents the timing of the zircons entering the sedimentary basin with volcanic ash at the base of the Leikoupo Formation in Tongliang, Chongqing. The age is similar to that of the tuff in Guizhou and Guangxi, indicating that regional volcanic eruption occurred at around 246 Ma. The older zircon was brought into the sedimentary basin during weathering of early igneous rocks and deposited with 'Mung Bean Rock'. A peak age at 277 Ma may imply that there was magmatic activity in the middle Permian period.