Zircon U-Pb dating and Hf isotopes of K-bentonites from the Tieling Formation in a new exposure of the Jixian Section, Tianjin, North China Craton

The first age constraints for the Jixian System at the well-known, carbonate-dominated Mesoproterozoic Jixian Section, North China Craton (NCC), were derived from U-Pb age date in 2014. One of these dating results was SHRIMP zircon U-Pb ages from a K-bentonite bed in the basal part of the second member of the Tieling Formation at the Dayushan Section, northwestern Jixian County, Tianjin. Although it has been accepted as one of the critical "anchor points" for the chronostratigraphic calibration and correlation of the Meso-and Neoproterozoic standard section for the NCC, new highway construction around the Jixian urban area has destroyed almost all of the Tieling Formation at the Dayushan Section, meaning that most of the section is not accessible to those who need it. Recently, on the northern slope of Mount Fujunshan, northeastern Jixian County, a new continuous outcrop section of the Tieling Formation has been found at the Dawujian limestone quarry (mining-banned now), and several K-bentonite beds occur in basal parts of the second member of the formation. LA-ICPMS zircon U-Pb ages(1445±12Ma and 1442±10Ma)have been obtained from two of the K-bentonite beds, while another LA-ICPMS zircon U-Pb age of 1439±11Ma was also obtained from the same K-bentonite bed in the Tieling Formation at the former Dayushan Section. Within the range of error, not only the LA-ICPMS zircon U-Pb ages of all three new samples are consistent with each other, but they are also consistent with the two earlier reported SHRIMP zircon U-Pb ages(1437±21Ma and 1439±14Ma)of K-bentonites at the same horizon of the Tieling Formation, which are from the Liujiagou Section(Pingquan, Hebei, NCC)and the former Dayushan Section, respectively. In addition, a series of Lu-Hf isotope of the dated zircons of the three new K-bentonite samples was analyzed. They yielded¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.281703 and 0.281880, withϵ_Hf(t)=-6.7~-0.2,t_DM2=2200~2603Ma and 2360Ma Gaussian distribution peak age. These values indicate that the parent magmas of the current K-bentonites from the Tieling Formation were largely derived from crustal materials of Neoarchean to Early Paleoproterozoic. In summary, these new data provide additional and reliable age constraints for the Tieling Formation, both at the Jixian Section and the entire Yanshan Mountains in the northern NCC, with possible information about the parent magmas of the K-bentonite beds. In addition, the section in the Dawujian quarry is in a rural area far from the planned development zone of the Jixian urban area, in which mining has been officially prohibited, meaning that it will be available to preserve longer. Consequently, this research has laid a more solid foundation for the chronostratigraphy of the Jixian Section as the conventional Standard Section for the Meso-and Neoproterozoic strata in the NCC.