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北京市密云水库附近分布着一些北-东走向,个体宽度达40m以上,长度达8000m的岩墙,本文称之为密云岩墙群.从一条宽约40m的岩墙中分选出了大量斜锆石,同位素稀释法获得其207Pb/206Pb年龄为1731±4Ma(n=4;1731±1Ma,n=3).该岩墙被长城群底部常州沟组底砾岩不整合覆盖,这一年龄限定常州沟组沉积起始时代晚于1731Ma.由于长城群是长城系的标准剖面,因此,这一年龄指示长城系的起始时代应晚于1731Ma.本区斜长岩杂岩体及环斑花岗岩(1730~1680Ma)被认为与长城群所在的燕辽裂谷有成因联系.但是,密云岩墙群的成因与上述岩体并不相同,指示不同的地质事件.长城群是我国地质时代单元长城纪时代划分的重要依据之一,考虑到1731Ma前后地质事件性质可能发生了变化,建议将长城纪起始时代定为1730Ma或者稍晚.
Near the Miyun Reservoir in Beijing, there are some dikes that run north-east with individual widths of over 40m and lengths of 8000m, which are called Miyun dyke swarms in this paper. A large number of slopes are selected from a dyke wall of about 40m in width The 207Pb / 206Pb age was 1731 ± 4Ma (n = 4; 1731 ± 1Ma, n = 3) by zircon and isotopic dilution.The wall was unconformably covered by the basal conglomerate of Changzhougou Formation at the bottom of the Great Wall Group, Since the Changzhou Goup Formation depositional onset was later than 1731Ma, since the Great Wall Group is a standard section of the Great Wall, this age indicates that the initial age of the Great Wall should be later than 1731 Ma. The granite (1730 ~ 1680Ma) is thought to be genetically linked to the Yanliao rift where the Great Wall Group is located, but the genesis of the Miyun dyke group is not the same as that of the above-mentioned rock mass, indicating different geological events.The Great Wall Group is a unit of geochronology One of the important grounds for the division of the Great Wall Records is that considering the possible changes in the nature of the geological events around 1731Ma, it is proposed that the Great Wall age be set at 1730Ma or later.