U-Pb Ages and Lu-Hf Isotopes of Detrital Zircons from Sedimentary Units across the Mid-Neoproterozoic Unconformity in the Western Jiangnan Orogen of South China and Their Tectonic Implications

摘要

The Jiangnan Orogen, located at the southeastern margin of the Yangtze Block, South China, records the complete history of assembly and evolution between the Yangtze and Cathaysia blocks. Neoproterozoic sedimentary sequences are widespread in the Jiangnan Orogen, with a regional angular unconformity separating the Sibao and Danzhou groups. U-Pb geochronology and Lu-Hf isotope analyses were carried out on detrital zircons across this unconformity and also the associated granites in the western Jiangnan Orogen in this study. Our new results, combined with previous data, indicate that the Sibao and Danzhou groups were deposited at 860–832 and 803–764 Ma, respectively. Thus, this unconformity was constrained to be 832–803 Ma. Detrital zircons from the Sibao and Danzhou groups have generally similar age populations, with three major peaks at 2000–1550, 1000–880, and 850–830 (760) Ma and one minor at 2600–2400 Ma. The most important generation of juvenile crust appears to have occurred at 2600–2400 Ma. Both recycling of ancient crustal materials and addition of juvenile mantle components took place in the time intervals of 2000–1550 and 1000–880 Ma. Detrital zircons in the age population of 850–830 (760) Ma were probably derived from proximal magmatic rocks in the western Jiangnan Orogen, reflecting fast erosion of newly formed igneous rocks. The Sibao and equivalents were deformed and intruded by granites posterior to the deposition of the Sibao and equivalent sequences after 832 Ma but before deposition of the Danzhou and equivalent units before 803 Ma. The pre-Danzhou deformation marked a collision along the southeast margin of the Yangtze Craton. The regional angular unconformity sealed the Jiangnan Orogeny, and thus the collision between the Yangtze and Cathaysia blocks was during 832–803 Ma. The Danzhou and equivalent units unconformably overlying the Sibao Group record the onset of rifting along the southeastern Yangtze margin beginning around 803 Ma.