华北克拉通北缘的中元古代多旋回复合盆地及其地质意义:来自碎屑锆石U-Pb年龄的统计学证据

华北克拉通是划分我国中元古代地层序列的标准地区,查明华北地区各中元古代盆地的发育与演化机制、寻找长城系下部1. 80～1. 65Ga的沉积盖层、完善1. 4～1. 0Ga待建系标准剖面,是我国中元古代综合地层学研究亟待解答的三个主要问题。现有资料显示,华北北部的燕辽盆地缺失长城系底部地层,南部熊耳盆地各地层小区的蓟县纪-待建纪盆地演化过程差异较大。与之相比,北缘盆地的中元古界相对完整且较连续出露,是系统开展中元古代年代地层学和盆地动力学研究的理想对象。鉴于狼山地区的"中、新元古界"已被重新厘定为新元古界狼山群,中元古代的华北北缘盆地应以渣尔泰群、白云鄂博群、腮林忽洞群和化德群为代表。由于缺乏火山岩夹层年龄的有效约束,与这些地层单元的时代划分、区域对比和盆地类型有关的认识分歧,普遍发端于对碎屑锆石年代学数据的不同理解。因此,本文对渣尔泰群和白云鄂博群的部分层位进行了碎屑锆石年代学研究,并全面收集了北缘盆地碎屑锆石年代学研究的相关数据,参考新近提出的两种统计学分析方法,对渣尔泰群、白云鄂博群(含腮林忽洞群)和化德群的碎屑锆石年龄数据进行了真实沉积时代和盆地构造属性的统计分析。通过设置单点精度高、但样本库计算总量降低的对照组进行方法可信性的对照分析,结果显示样本库总量对统计结果的影响更大,因此利用相关方法解析前寒武纪哑地层的沉积时代和盆地属性,应当遵从"合理降低单点精度限制以谋求更大样本库"的原则。结合前人建立的中元古代沉积-岩浆事件序列,本文分析结果显示中元古代的北缘盆地是在长城纪造山后伸展盆地之上、叠加了蓟县-待建纪裂谷事件而成的多旋回复合盆地。通过北缘和燕辽地区沉积、岩浆事件的综合对比,提出华北克拉通北部的中元古代盆山耦合过程,可分为:由造山后向陆内伸展转换的构造反转期阶段(1. 82～1. 60Ga)、被动裂谷早期的陆内伸展阶段(1. 6～1. 4Ga)、裂谷作用阶段(1. 4～1. 3Ga)和成因机制不明的长期沉积间断阶段(1. 3～1. 0Ga)。

A Mesoproterozoic multi-cycled composite basin in the northern margin of the North China Craton and its geological implications: Constraints from statistics of the detrital zircon U-Pb data

The North China Craton (NCC) is an exclusive region to establish the standard stratigraphic frame of the Mesoproterozoic strata in China. Recently, three issues arisen from the integrated stratigraphic researches on the Mesoproterozoic strata in the NCC have been presented, including the evolving mechanism of each Mesoproterozoic basin, the identification of the cover sequence during the Early Changcheng Period (1.80~1.65Ga) and the refinement of the Unnamed System (1.4~1.0Ga). According to the available data, the depositional succession corresponding to the Lower Changcheng System is absent in the Yanliao Basin, and the evolutionary processes of the three stratigraphic sub-regions in the Xiong''er Basin are distinct from each other during the Jixian-Unnamed periods. In contrast, the northern marginal basin (NB) in which the relatively successive and intact strata have cropped out is an ideal region to carry out the chronostratigraphic and dynamic researches on the Mesoproterozoic NCC. Since the depositional sequence in the western part of the NB, known as the Langshan Group, has been calibrated as Neoproterozoic, the Mesoproterozoic NB is represented by the Zhaertai, Bayan Obo, Sailinhudong and Huade groups. To date, the ongoing disputes on depositional timing, regional correlation and tectonic discrimination of these groups are mainly resulted from the different recognitions of the detrital zircon U-Pb data. This research has collected all these data from the Mesoproterozoic NB and complementarily analyzed several clastic layers of the Zhaertai and Bayan Obo groups. Referring to the two statistical methods which are recently proposed to analyze the detrital zircon U-Pb data, this contribution presented the tectonic discriminations and chronologic results approaching to the real initiating age for each depositional cycle. We have set a control group consisting of less analyzing points but with higher precision for each point. The control experiment indicates the statistical results are more sensitive to the sample size rather than the individual precision, suggesting that the larger sample size has the priority in the applications to the Precambrian strata. On basis of the pioneers'' sedimentary-magmatic timing sequence, this contribution proposed that the Mesoproterozoic NB was a multi-cycled composite basin which was a post-collisional basin in the Changcheng Period and was juxtaposed by a rift event during the Jixian-Unnamed periods. Integrating the sedimentary and magmatic records from the NB and Yanliao Basin, the Mesoproterozoic tectonic evolution of the northern part of the NCC could be subdivided into 4 stages:(a) Reversion (1.82~1.60Ga), post-collisional setting is gradually transited to an intra-continental extensional setting; (b) Intra-continental extension (1.6~1.4Ga), the early stage of a passive rifting event; (c) Rifting (1.4~1.3Ga), the peak stage of the passive rifting event; and (d) Interruption (1.3~1.0Ga), a long-term depositional interruption was lasted by one or several unknown mechanisms.