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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(SHRI MP)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。
The Great Wall in North China occupies a very important position in the study of the Chinese Archean. The bottom boundary of the Great Wall in North China is the important boundary mark of dividing the ancient and the Middle Proterozoic. For a long time, the bottom boundary age has been difficult to determine due to the lack of ageable rock formations or geologic bodies near the bottom of the Great Wall. In recent years, in the Miyun area of Beijing, the author discovered that the Changchenggou Formation sandstone layer of the Great Wall System is directly covered by a ring-shaped granite (vein) developed ancient weathering crust. The weathered crust material is mainly composed of in situ weathered rocks of the ringed granite and uplifted into coarse clastic rocks that look like ringed granite. The detrital zircon U-Pb ages of the ancient weathered crust-ring-shaped granitic clastic rocks have been obtained as (SH-MP) and (SH-ICP-MS ), Etc., and the adjacent Miyun ring spot granite age. Preliminary analysis shows that the Miyun ring-shaped granite and its contemporaneous dikes have exposed the surface as regional crustal uplifts after emplacement. After a certain period of weathering and partial erosion, the surface of the formation of residual sediment and close to the original accumulation of ring granite debris material. From the formation of time, this set of rocks should belong to the “former Changzhou ditch period (group) ”. Its existence and dating results show that the bottom boundary age of the Changchenggou Formation in the Great Wall should be less than 1 682 Ma instead of the previous 1,800 Ma. Based on the analysis of detrital zircons from the Ramadae granitic sedimentary rocks and the related age data of the overlying strata, we initially proposed that the bottom boundary of the Great Wall series is about 1665 to 1670 Ma.