从关键地质事件看华南的前寒武系划分

2004年的《国际地层表》与以往“国际地层表”的最大不同是对前寒武系的划分原则,即:以保存在地质记录中的地球系统巨变中的关键地质事件作为划分地层单位的界线,并据之建立“自然的”前寒武纪地质年表。在这一思想指导下,本文以川滇赣三省为重点,重新审视华南的前寒武系资料,分中太古—新太古代、跃迁期、古元古—中元古代和新元古代四大阶段阐述了关键地质事件并讨论了其在大陆岩石圈形成和演化中的构造意义。这四大阶段的主要构造面貌分别是:片麻岩基底和绿岩带盆地,古陆壳的形成与裂解,第一个超级大陆即哥伦比亚(或称努纳)超大陆的形成与离散,板块构造体制下罗迪尼亚超大陆的汇聚与离散,反映了大陆岩石圈的形成和由低级向高级的演化,因而每一阶段都有不同于前一阶段的沉积建造、火成岩类型及组合、变质作用、构造样式、构造线方向、变形方式和成矿作用。基于这些事件介绍了前寒武系的分布,列述了若干条相对连续的地质剖面上所见的这四大阶段的代表性地层序列及其对比方案。文章认为“参考方案”对前寒武系研究有推动和导向作用,并探讨了地层研究具体操作中关于事件的内容、事件与过程、现存地质记录的残破不全等问题。

DIVISION OF THE PRECAMBRIAN IN SOUTH CHINA IN THE LIGHT OF KEY GEOLOGICAL EVENTS

An obvious change of the International Stratigraphic Chart 2004 is the principles for the Precambrian stratigraphic subdivision. More specifically, the Precambrian Time Scale is recommended to be (re)defined in terms of the only objective physical standard: the extant rock record, with a goal to achieve a “natural” geological time scale for the Precambrian. In the light of this new development, I critically re-examine the Precambrian stratigraphic data of Sichuan, Yunnan, and Jiangxi provinces of South China, in order to systematically understant the key events in the Mesoarchean-Neoarchean, Transition, Paleoproterozoic-Mesoproterozoic, and Neoproterozoic.I also discuss the tectonic significances of these stratigraphic data in the creation and evolution of continental lithosphere. The main tectonic features of the four stages mentioned above can be summarized as follows: gneissic basement and greenstone belt in the Archean, formation and rifting of ancient continental crust in the Transition, creation and disposal of the first supercontinent Columbia (or Nuna) in the Paleoproterozoic-Mesoproterozoic, and the assembly and disassembly of the supercontinent Rodinea under plate tectonic framework in Neoproterozoic. The four stages indicate the creation and progressive evolution of the continental lithosphere, so each stage has its unique (different from its proceeding or succeeding stages) records of sedimentary formation, type and association of igneous rocks, metamorphism, structural pattern, tectonic strike, deformational mode and mineralization. On the basis of these events, the distribution of Precambrian strata in Sichuan, Yunnan, and Jiangxi is described, and the representative stratigraphic successions from some relatively continuous profiles and their comparison to the four stages are discussed. The proposed “Precambrian reference time scale” would promote and guide the study of Precambrian stratigraphy. The application of the Precambrian reference time scale, including the definition of events, relation between events and processes, and incomplete geological records, are also discussed.