Paleomagnetism and U-Pb geochronology of the Lower Cretaceous Qingshan Group in the Jiaodong Peninsula, China and implications for tectonics

The Jiaodong Peninsula of Shandong Province is located in east of the Tan-Lu fault, the largest fault system in the eastern margin of Asia. Although accumulating evidences suggest that the Tan Lu fault had a certain strike slip displacement in Mesozoic, there are still controversies regarding the time of its evolution. Paleomagnetism and U-Pb geochronology study was conducted on Cretaceous volcanic rock samples of the Qingshan Group in the Jiaodong Peninsula. LA-ICP-MS zircon U-Pb dating was carried out for five samples of Qingshan Group. It yields concordant ages were obtained, which are 121.9±2.8 Ma, 120.4±3.0 Ma, 120.4±2.8 Ma, 121.0±2.3 Ma and 119.0±1.9 Ma, from the bottom to the top, respectively. Stepwise thermal demagnetization conducted on 297 samples from two sections (39 sites) yielded stable characteristic remanent magnetization (ChRM) directions. Except two sites from the bottom of section, 37 sites have reversal polarity, which were potentially formed in a relatively short period of time, likely just before the Cretaceous Normal polarity superchron. The average declination/ inclination directions of two sections are 205.6°/-49.6°(α₉₅=5.5º) with a corresponding average virtual geomagnetic pole (VGP) at 76.7ºN and 162.6ºE (A₉₅ = 6.2º). Of note, the paleolatitude of sampling sites were lower than expected. This will provide constraints on the regional tectonic movement during Late Mesozoic and useful clues for the paleo-Pacific subduction process.