古/中元古代界线:1.8Ga

年代地层表是我们描述地球历史演化的时间框架,也隐含着我们对地球演化过程的基本认识,承载着一系列核心科学问题。现有的国际前寒武纪地质年表存在不少问题,还没有被普遍接受的新方案。在现行国际地层年表(IUGS 1989—2004)中,20～18亿年被称为造山纪,18～16亿年被称为固结纪,16～14亿年被称为盖层纪,也即:造山运动结束到盖层发育之间的过渡时期为固结纪,而将盖层的广泛发育作为中元古代的开始。在我国,由于"吕梁运动"是华北克拉通结晶基底最终形成的标志性构造-热事件,此后发育以长城系-蓟县系-青白口系为代表的地台型沉积盖层,因此我国地质界一直以长城系的底界代表中元古代的开始,并根据"吕梁运动"结束时间,将古/中元古代的时间界线置于18亿年。2012年国际地层委员会提出了一份全新的全球前寒武纪地质年代表划分建议方案,其中2060～1780Ma阶段被称为古元古代哥伦比亚纪,主要以Columbia超大陆的聚合为特征;而之后17. 8～8. 5亿年的近10亿年间,则被定义为罗迪尼亚纪,代表了从Columbia超大陆裂解到Rodinia超大陆聚合的漫长阶段。即这一新建议方案的古/中元古代界线为17. 8亿年,与我国学者长期以来所坚持的古/中元古代分界是基本一致的。在华北克拉通,"吕梁运动"的结束时间在～18亿年,此后整体处于多期裂解的陆内伸展环境,长城系-蓟县系基本上属于连续的陆表海沉积。近年来的研究表明,燕山地区长城系的底界年龄约为17亿年,长城系与蓟县系的分界则为16亿年。如果一味按照现行国际地层划分方案,其古/中元古代的界线(16亿年)将对应于长城系-蓟县系的分界,华北克拉通这套盖层型沉积将被人为分割为两部分,这显然是很不合理的。值得注意的是,在华北克拉通中部吕梁地区发育的小两岭组火山岩和在其南部地区发育的、也是世界范围内同时期最大规模的火山活动——熊耳群火山-沉积岩系,是华北克拉通结晶基底之上最早发育的盖层沉积,其起始形成时间约为18亿年。这与新建议的古/中元古代分界非常接近。从全球地质演化来看,从18到16亿年,造山作用结束,Columbia超大陆开始裂解,岩浆作用方式和岩浆岩组合类型及其地球化学特征发生明显改变,如斜长岩、环斑花岗岩在世界主要克拉通均有发育。与此同时,稳定沉积盖层开始广泛发育,条带状铁建造(BIF)消失,代之以鲕状或粒状矿物集合体组成的浅海富铁沉积,另外海相硫化物沉积、有核原生生物等也首次出现。所有这些都标志着,在此前后,地球岩石圈、水圈和大气圈均发生了重大转折,生物圈也进入新的演化阶段,标志着地球进入"中年期"演化阶段。虽然地球演化发生重要转折的根本原因和细节还有待进一步深入探讨,但这一转变的起点或最重要的时间点,无疑就在18亿年前后,因此,本文认为,应将其作为全球古/中元古代的时间界线。

The Paleo-Mesoproterozoic boundary: 1.8Ga

The chronostratigraphic chart describes the timescale of the geological evolutions in the Earth''s history, records our understanding of the global evolution, and carries a series of key scientific questions. There are many problems in the current Precambrian Geological Time Scale, yet no new chart has been widely accepted by far. 2.0~1.8Ga, 1.8~1.6Ga and 1.6~1.4Ga are referred as the Orosirian, Statherian and Calymmian in the international chronostratigraphic chart (IUGS 1989-2004), respectively. That is, the transition between the end of orogenesis and the development of stable sedimentary covers is termed Statherian, while the widely development of stable sedimentary covers represents the start of the Mesoproterozoic. The "Lvliang Event" is usually considered as the symbolic tectono-thermal event marking the final solidification of the crystalline basement of the North China Craton (NCC), which was followed by development of extensive platform-type sedimentary covers collectively known as the Changcheng-Jixian-Qingbaikou systems. The deposition of the Changcheng System has long been considered as the beginning of the Mesoproterozoic in China. The boundary between Paleoproterozoic and Mesoproterozoic was set at 1.8Ga according to the end time of the "Lvliang Event". The International Stratigraphic Commission (ISC) launched a new proposal in 2012 for the global Geochronological Time Scale, in which the 2060~1780Ma period is called the "Columbian", followed by the "Rodinian", the long historical stage from the break-up of the Columbia Supercontinent to the assembly of the Rodinia Supercontinent (1.78~8.5Ga). That is, the boundary between Paleoproterozoic and Mesoproterozoic of this new proposal global Geochronologic Time Scale is 1.78Ga, which is largely consistent with the boundary that Chinese scholars have long insisted on. The termination of the "Lvliang Movement" in the NCC is at ca. 1.8Ga. Subsequently, the NCC has been in an intra-continental extension environment until the Late Neoproterozoic. The Changcheng and Jixian systems are basically continental epicontinental deposit. Recent studies showed that the initial age of the Changcheng System in the Yanshan area is ca. 1.7Ga and the boundary between the Changcheng System and Jixian System is 1.6Ga. It is obviously inappropriate to regard the Changcheng System and Jixian System boundary (1.6Ga) as the Paleo-Mesoproterozoic boundary considering the successive sedimentation of these two systems. It is noteworthy that the volcanic rocks of the Xiaoliangling Formation distributed in the Lvliang area in central NCC and the Xiong''er Group in the southern NCC, the largest volcanic-sedimentary rocks globally at that time, are the earliest sedimentary covers overlying the crystalline basement of the NCC dated at 1.8Ga, which is quite close to the new proposed Paleo-Mesoproterozoic boundary. The period between 1.8Ga and 1.6Ga witnesses significant geological changes globally. The orogenic process terminated, the Columbia Supercontinent began to break-up, and the magmatism model, magmatic rock assemblage types and their geochemical characteristics changed significantly, characterized by the development of anorthosite and rappakivi granite in many cratons. Meanwhile, stable sedimentary covers began to develop worldwide, the banded iron formation (BIF) disappeared, shallow-sea iron-rich deposits consisting of oolitic and granular mineral aggregates and marine sulphide deposits and eukaryotes first appeared. All these features marked that the earth''s lithosphere, hydrosphere and atmosphere have undergone major turning points, and the biosphere has entered a new evolutionary stage, indicating that the Earth entered into an evolution stage of "middle age". The major reason and detailed information for the Earth turning into the "middle age" are still unclear and further in-depth study is requested, yet the initial point or foremost turning period of this transformation is at ca. 1.8Ga, which should be regarded as the global Paleo-Mesoproterozoic boundary.