摘要
mary name="EnDivSummary">A tuff bed in the upper horizon of Gaoyuzhuang Formation has been newly discovered in Yanqing County,Beijing.Two almost identical precise zircon U-Pb ages,1 559±12 Ma(SHRIMP)and 1 560±5 Ma(LA-MC-ICPMS),have been obtained for the tuff bed,which indicates that the Gaoyuzhuang Formation in the northern North China Craton was formed in the early stage of Mesoproterozoic Calymmian Period(1 600~1 400 Ma).Combining with the available zircon U-Pb age data(1 622~1 625 Ma)for the volcanics from the upper part of Dahongyu Formaiton,the lower boundary age of Gaoyuzhuang Formation could be accurately positioned at 1600Ma.Moreover,with the newly determined SHRIMP U-Pb age(1 440 Ma)of the K-bentonite in Tieling Formation,the authors therefore propose that the basal boundary of Gaoyuzhuang Formation should be taken as the lower boundary of Mesoproterozoic Jixian System.The voluminous carbonate succession from Gaoyuzhuang Formation to Tieling Formation with time span of 1600Ma to 1 400 Ma comprises the Jixian System,almost identically comparable with the international Mesoproterozoic Calymmian System.