The Changchengian System, with its basal boundary being a key to the delimitation between the Pa-leo- and the Meso-Proterozoic in the North China Platform, serves actually as an absorbing Proterozoic research field in Chinese geological circle. For a long time it has been difficult to precisely delimit the intra-Prot-erozoic ages of North China and their global stratigraphic correlations has been controversial because of lack of available rocks or bodies for age dating. In resent years, the Proterozoic rapakivi granites (dike) with the pa-leo-weathered mantle - clastic rocks covered directly by the sandstones of the Changzhougou Formation of Changchengian System are discovered in the Miyun County, Beijing. The detrital components embrace the in-situ weathered rocks of rapakivi granites in the lower and the rapakivi-like coarse-grained clastic rocks in the upper. Using SHRIMP and LA-ICP-MS dating methods, we have obtained the U-Pb dating ages of 1682±20 Ma and 1708±5. 6 Ma from the detrital zircons sampled in the rocks; those age data are the same with the nearby Miyun Rapakivi Granite. Preliminary analysis has revealed that the Miyun Rapakivi Granite and its syn-chronic dikes had once been uplifted and outcropped in the air after they were emplaced. The processes of weathering and denudation resulted in both the saprolites and the almost in-situ accumulated rapakivi debris. The formation of the weathered rocks was to be pre-Changzhougou time in age. The existence of this suite of weathered rocks and clastic rocks, and the analysis of the dating results, have shown that the age of the base of the Changzhougou Formation (which belongs to the Changchengian System) should be younger than 1682 Ma instead of the formerly reported 1800 Ma. According to the analysis of the dating results of the in-situ rapakivi-sourced sedimentary elastics and the correlative age data, we here propose a new age of the lower limit of the Changchengian System to be about 1665 - 1670 Ma in North China.%华北地区长城系在中国元古宙研究中占有相当重要的地位,其底界是目前划分古、中元古界的重要界线标志.长期以来,由于在靠近长城系底部缺少可定年的岩层或地质体,其底界年龄一直难以确定.近年来笔者在北京密云地区,发现了长城系常州沟组砂岩层直接覆盖在发育有古风化壳的环斑花岗岩(脉)之上.风化壳物质的组成主要为环斑花岗岩的原地风化岩石,以及向上过渡为貌似环斑花岗岩的粗碎屑岩.已获得古风化壳-环斑花岗质碎屑岩的碎屑锆石U-Pb年龄值为(1 682±20)Ma(SHRIMP)和(1 708±5.6)Ma(LA-ICP-MS)等,与相邻的密云环斑花岗岩年龄相同.初步分析表明,密云环斑花岗岩及其同期岩脉在侵位之后,曾随着区域地壳抬升暴露出地表.经过一定时期的风化和部分剥蚀,在其表面形成残积物和接近原地堆积的环斑花岗质碎屑物质.从形成时间来看,这套岩石应属于“前常州沟期(组)”.它的存在和测年结果显示,长城系常州沟组的底界年龄应小于1 682 Ma,而不是以前划定的1 800 Ma.根据环斑花岗质沉积岩碎屑锆石以及已有的上覆地层相关年龄数据的分析,我们初步提出长城系底界年龄约为1 665～1 670 Ma.
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