燕山地区团山子组层凝灰岩SHRIMP锆石U-Pb年龄及其地质意义

燕辽地区长城群年代学工作一直是华北克拉通中元古代地质学研究的重点之一。由于团山子组与下伏串岭沟组界限年龄不明确, 且此前的年龄都是来自于其中的火山熔岩锆石的年龄, 亟待新的测年工作加以验证和完善。本文作者在北京市密云区、平谷区的团山子组下部发现了层凝灰岩, 并对其开展了系统的SHRIMP锆石U-Pb测年, 获得两个高精度年龄1634±9Ma和1637±8Ma, 进一步精确限定了该组的形成年代。结合前人研究, 燕辽地区团山子组的底界、也即下伏串岭沟组顶界年龄, 可进一步限定为1638Ma。由此可对燕辽地区长城群地层格架进行更准确的年代学厘定, 并进一步确认其整体处于持续拉张的裂谷-被动陆缘背景下的快速沉积特征、以及长城群团山子组-大红峪组与华北克拉通南缘汝阳群洛峪口组等相关沉积更为精确的对比关系。结合其他资料还可推断, 在团山子组-大红峪组及洛峪口组等形成时, 华北克拉通南北两侧都开始被碳酸盐岩浅海覆盖, 且最终还可能在其中部出现了南北联通的碳酸盐岩浅海通道。这显示出华北克拉通与早先所拼合克拉通之间已产生明显的拉张性疏离及新生海洋, 应标志着华北克拉通从哥伦比亚超大陆裂解的新阶段。

SHRIMP zircon U-Pb ages of the tuffite beds in the Tuanshanzi Formation (Changcheng Group) of the Yanshan Mountains,North China Craton

The geochronological research of the Mesoproterozoic Changcheng Group, North China Craton (NCC), has been concentrated for several decades. Although lots of significant progress have been achieved in recent years, the exact age of some key horizons and the relevant units, including that of the Tuanshanzi Formation, are still somewhat ambiguous. So far, all the previous chronological studies of the Tuanshanzi Formation have been focused on the zircon U-Pb age dating of the intercalated alkalic volcanic rocks in this formation. However, due to the complicated category of the zircons from the eruptive alkalic rocks, to some extent, it resulted in dispute both about the precise depositional age of the Tuanshanzi Formation and that of the underlying Chuanlinggou Formation, especially while the porphyry dikes and the rapakivi-granite apophysis, truncated by the Changcheng Group in the Yanshan Mountains, have been dated successfully since early 2010's. Hence, further work is needed to verify the exact depositional age of the Tuanshanzi Formation. Recently, some interbedded tuffite beds in the lower part of the Tuanshanzi Formation are firstly recognized in the Miyun and the Pinggu districts, Beijing City, in the Yanshan Mountains. High-precision SHRIMP zircon U-Pb dating yields tuffite ages of 1634±9Ma and 1637±8Ma, and the depositional age of the Tuanshanzi Formation can be calibrated more precisely. Combining with the previous works, the bottom age of the Tuanshanzi Formation, the same with the top age of the underlying Chuanlinggou Formation, can be constrained more tightly as ~1638Ma, and the chronostratigraphic framework of the Changcheng Group in the Yanshan Mountains can be revised, too. This may conform with the characteristic of the rapid depositional setting in this area during the latest Changchengian Period (1650~1600Ma), which should represent a sort of successions from the constantly extending rift-basin to the passive continental margin along the northern NCC. Further, it can lead a more credible stratigraphic correlation of all the Changchengian successions at the NCC, especially between the Luoyukou Formation at the southern margin of the NCC, and the Tuanshanzi-Dahongyu formations in the Yanshan Mountains. Moreover, it may be inferred that, corresponding to the deposition of the Tuanshanzi-Dahongyu and the Luoyukou formations, around ~1638Ma, for the first time, the warm, clean, shallow carbonate sea covered both at the northern and southern margins of the NCC. Simultaneously, probably a narrow but unobstructed carbonate sea passage, roughly north-south, emerged in the central of the NCC. This indicated that, from this time on, the NCC had been not only developed in the more extensional tectonic setting, but also isolated enough with the surrounding newborn seas (juvenile oceans). All these would prevent the input of the terrigenous clasts both from the NCC and the previous converged cratons, i.e. the main body of the Supercontinent Columbia (Nuna), and be suitable for the formation of carbonate rocks. Therefore, it should mark the new stage of the break-up between the NCC and the Columbia, as well as the supercontinent itself.