滹沱超群火山沉积夹层的U-Pb测年研究：华北克拉通古元古代表部层圈演化的年代学约束

Due to the lack of direct, accurate age constraints and poor understanding of the tectonic setting of the Hutuo Supergroup and coeval successions in the Trans-North China Orogen (TNCO) on the North China Craton (NCC), some critical issues regarding the Paleoproterozoic NCC are still controversial and uncertain. For example, the final closure of the TNCO, which is causatively linked to the collision between the Eastern and Western blocks of the NCC and basement formation of the NCC is poorly constrained. Similarly, position of the NCC within the Columbia/Nuna Supercontinent is uncertain. The Lomagundi-Jatuli Event (LJE), the most prolonged Paleoproterozoic carbon isotopic shift in Earth’s history, has not been recognized on the NCC. This project will mainly focus on identification and high-resolution U-Pb dating of magmatic zircons in intercalated volcanic deposits, including K-bentonite beds and tuff layers, in the Hutuo Supergroup. In addition, U-Pb dating of microbaddeleyite from mafic volcanic layers reported earlier in this succession will be attempted. It will lead to a direct and accurate calibration of the Hutuo Supergroup and its subdivisions, and the two phases of the Lüliang Orogeny, which are indicated by two regional angular unconformities at the top of the Supergroup. In addition, based on a comprehensive study, including sedimentary facies and lithospheric-flexure analysis, carbon and oxygen isotopes of carbonates, geochemistry of intercalated volcanic deposits and Lu and Hf isotopes of the magmatic zircons, the tectonic setting of the Hutuo Supergroup and the nature of the Lüliang Orogeny will be investigated, and their relationship with the development of the TNCO will be explored. Possible depositional horizons/intervals corresponding to Earth’s major surface events during the Paleoproterozoic, specifically the LJE and its tailing Shunga Event, and the final assembly of the Columbia/Nuna Supercontinent, will be searched for and calibrated using the Hutuo Supergroup records. These efforts will provide a precise and integrated chemo- and chronostratigraphic framework for the central NCC for further leading-edge research, such as global reconstruction of the Columbia/Nuna Supercontinent, evolution of the early life, the rise of oxygen in deep-time atmosphere, and the possible coupling of the surface processes on the Earth during the Paleoproterozoic.

本项目拟通过识别、确认滹沱超群相关地层中的钾质斑脱岩、层凝灰岩等中酸性火山沉积夹层，结合该群已知基性火山岩等，系统开展其岩浆锆石（中酸性）及斜锆石（基性）的高精度U-Pb测年研究，完善对该超群各沉积阶段及“吕梁运动”直接而精确的年代学约束；在此基础上，结合沉积相-岩石圈负载效应分析、火山岩岩石地化组成及锆石Hf同位素、碳酸盐岩C(O)同位素等综合分析，揭示滹沱超群及“吕梁运动”与华北中部造山带(TNCO)演化的时空关系，标定古元古代中-晚期全球层圈演化重大事件（包括Lomagundi-Jatuli Event即LJE事件、以及哥伦比亚/努纳超大陆聚合等）在滹沱超群可能存在的沉积记录与具体层位（段），为深入开展古元古代超大陆再造、早期生命演化、深时地球大气氧含量变化及可能的表部层圈耦合等重大命题，提供由年代地层-化学地层精确约束的华北克拉通中部综合地层格架。

本项目主要研究内容有如下几方面：（1）通过识别、确认滹沱超群相关地层中的钾质斑脱岩、层凝灰岩等中酸性火山沉积夹层，结合该群已知基性火山岩等，系统开展其岩浆锆石（中酸性）及斜锆石（基性）的高精度U-Pb测年研究，完善对该超群各沉积阶段及“吕梁运动”直接而精确的年代学约束；（2）结合沉积相-岩石圈负载效应分析、火山岩岩石地化组成及锆石Hf同位素、碳酸盐岩C(O)同位素等综合分析，揭示滹沱超群及“吕梁运动”与华北中部造山带(TNCO)演化的时空关系，标定古元古代中-晚期全球层圈演化重大事件（包括Lomagundi-Jatuli Event即LJE事件、以及哥伦比亚/努纳超大陆聚合等）在滹沱超群可能存在的沉积记录与具体层位（段）；（3）在可能的情况下，在其他相关地区和层段（包括中条群、辽河群等），适当开展一些相关工作。.遗憾的是，虽然项目在上述几方面都陆续取得了许多重要进展，包括在华北克拉通第一次明确识别并获得高精度SHRIMP锆石测年（~2170Ma)）约束的Lomagund碳酸盐岩碳同位素正漂移事件（最大值>+11‰），在包括滹沱群以及中条群、辽河群等古元古代典型沉积当中，识别并获得了一批火山夹层的锆石SHRIMP测年等等，但迄今为止，这些进展都还未能及时的公开发表。其主要原因，就是由于新冠疫情的突然爆发，导致本项目原计划的许多野外工作和室内分析工作不得不推迟、甚至暂时搁置。其中包括项目计划当中，由项目主要成员、美国加州大学河滨分校Andrey Bekker副教授帮助完成的碳酸盐岩样品的碳氧同位素分析等，由于中美之间邮路长期处于停滞状态，样品不能及时寄送到对方，其学校相关实验室也一直处于关闭状态，导致相关样品的测试分析等，直到最近也还没能彻底完成；与此同时，项目原来计划的锆石SHRIMP测年以及TIMS测年等工作，也因为疫情导致的相关实验室不定期的被迫关闭，使得相关任务的完成也相对滞后。尽管如此，但是一些关键目标还是在最近得以“撞线式”完成了，项目组成员将努力在接下来的最近一段时间里，陆续完成这些数据的额分析以及相关论文撰写发表等工作。