Zircon U-Pb ages of the cetaceous sedimentary rocks in the Laiyang Basin, eastern China and their tectonic implications

Abstract

The Laiyang Basin is located in the northern margin of the Sulu high-pressure and ultrahigh-pressure (HP-UHP) metamorphic belt. The sediment source and depositional time of the Laiyang Basin can provide important insights into the exhumation mechanism of the Sulu Orogen and the formation of the Tancheng-Lujiang (Tan-Lu) Fault, but have not been well constrained. In this study, seven samples were collected from typical sections of the Laiyang and Qingshan groups in the southern margin of the Laiyang Basin in the Sulu Orogen. LA-ICPMS U-Pb dating of detrital zircon from these samples yielded ages from 3145 Ma to 115 Ma and can be divided into four groups: (1) 3145–1728 Ma, which indicate a provenance from the Precambrian basement rocks of the North China Craton; (2) 908–522 Ma, which are consistent with the ages of the protolith of the Sulu HP-UHP rocks; (3) 240–220 Ma, which are identical to the ages of the HP-UHP metamorphism. This suggests that the HP-UHP rocks of the Sulu Orogen had been exposed to the surface and contributed to the deposition of the Laiyang Group by the Early Cretaceous; (4) 148–115 Ma (weighted mean age: 125 ± 0.5 Ma; MSWD = 1.2), which, as the main group, suggests that the major source of the sediments in the Laiyang Group was from Early Cretaceous igneous rocks. These young ages, combined with the ages of 119 ± 1 (MSWD = 0.7) Ma determined from the overlain Qingshan Group, confine the depositional age of the Laiyang Group to 119–125 Ma. Further comparison of detrital zircon U-Pb age spectrum between the Laiyang Basin and the Hefei Basin indicates that these two basins were developed independently, rather than developed from single basin and then truncated by the Tan-Lu Fault as suggested by previous studies. Based on the formation age of the Laiyang Basin and other age constraints, we further suggest that the Tan-Lu Fault was formed during the Late Triassic to Early Jurassic. The Mesozoic geological settings in the Sulu Orogen, including the sediments of the Laiyang Basin, Early Cretaceous magmatic intrusive and detachment activities of the Wulian Fault, imply that the Cretaceous exhumation of the Sulu HP-UHP belt was consistent with a metamorphic core complex model.