中-新元古界标准剖面蓟县系首获高精度年龄制约———蓟县剖面雾迷山组和铁岭组斑脱岩锆石ＳＨＲＩＭＰＵ-Ｐｂ同位素定年研究

Although known as the Meso- to Neoproterozoic Standard Section of China for more than half a century, the age of the famous Jixian Section (Jixian County, Tianjin City) has been rather poorly constrained, mainly by Rb-Sr and K-Ar ages that have proven unreliable and inaccurate in most cases. In recent years, significant progress has been made in studying the Meso- to Neoproterozoic geoehronology of the North China Craton. To date, however, only one accurate U-Pb zircon age of 1625 ± 6Ma from volcanic rocks of the Dahongyu Formation, and two duplicate zircon U-Pb ages of about 1635Ma from mafic sills crosseutting the Chuanlinggou Formation of the Changehengian System in the lower part of the Jixian Section, has been reported. In addition, no one until now has obtained this kind of age dates directly from carbonate-dominated parts of the succession at the Jixian Section. Fortunately, bentonite beds were recently discovered for the first time in the Tiding and Wumishan formations at the Jixian Section, and ages of ~ 1440Ma (1439 ± 14Ma) and ~ 1485Ma ( 1483 ± 13Ma and 1487 ± 16Ma) for the bentonite beds were obtained by means of SHRIMP zircon U-Pb dating. This achievement means that the shallow-marine, carbonate-dominated Tieling and Wumishan formations, the upper two stratigraphic units of the conventional Jixian System at the Jixian Standard Section, are now temporally constrained by direct zircon U-Pb dating. Furthermore, it is the first time that the Wumishan Formation from the entirety of the North China Craton has been precisely dated. Moreover, the current SHRIMP zircon age of about 1440Ma for the bentonite in the Tieling Formation at the Jixian Standard Section is the same as the age reported earlier in Pingquan County, Hebei Province, in the east Yanshan Mountains on the northern margin of the NCC. Consequently, this progress in age dating of the bentonite beds in the carbonate-dominated succession provides new, accurate, benchmark ages for further clarification of the ehronostratigraphic framework of the well-known Jixian Meso- to Neoproterozoic Standard Section and helps to consolidate its position for correct correlation with relevant strata, both regionally and internationally.