太古宙的洋-陆与古地理问题*

大陆最古老的陆壳物质是沉积岩中4.4 Ga的碎屑锆石,最古老的陆壳岩石年龄为4.1-4.0 Ga,出露面积约20 km²。3.9-3.6 Ga的古老陆核出露在不同克拉通中,而大陆的生长峰期是在2.9-2.7 Ga,全球稳定的陆壳圈层形成是在~2.5 Ga,被称为克拉通化。陆壳以英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)为代表,体积占古老陆壳的~70%以上。古陆表现为高级片麻岩区-花岗绿岩带格局(穹隆-龙骨格局),与显生宙的洋-陆格局不同,暗示构造体制的差异。火山沉积组合即是围绕高级片麻岩地体以层状向斜方式存在的绿岩带,后者相对变质很浅或未变质。早期地球演化中,先有陆还是先有洋、陆核形成和生长的机制、什么时候开始有露出海面的陆地、太古宙时期的洋-陆格局等等都还没有定论。古元古代时期,全球长期处于伸展阶段,巨厚的裂谷型沉积以及伴随的大氧化事件,可能是开启古地理研究的最早地质时期。本文还以华北克拉通为例,作了陆壳演化的简单介绍。

Ocean and Continent in Archean

The oldest continental crust material is 4.4 Ga detrital zircon from sedimentary rocks, and the oldest continental crust rocks are between 4.1 and 4.0 Ga with an outcrop area of ca.20 km². The old continental nuclei of 3.9-3.6 Ga were exposed in various cratons,whereas the growth peak of the continental crust was in 2.9-2.7 Ga,and the global stable continental crust formed at~2.5 Ga,which is termed as cratonization. The continental crust is represented by Tonalite-Trondhejmite-Granodiorite(TTG),accounting for ~70% of the ancient continental crust by volume. The ancient continents show a high-grade gneiss region-granite-greenstone belt pattern(dome-keel pattern),which is different from the Phanerozoic oceanic-continental pattern,suggesting a difference in tectonic regime. The volcanic sedimentary sequence is the greenstone belt surrounding the high-grade gneiss region in stratified syncline form,the latter being relatively low grade metamorphic or un-metamorphic. In the early evolution of the Earth,the continent or ocean came first,the mechanism of the formation and growth of the continental remain unclear,when the surface land began to emerge in sea water,and the relation state of the ocean and continent during the Archean period remain controversial. In the Paleoproterozoic period,the entire world was in the extended stage for a long time,and the thick rift deposits and the associated great oxidation events may be the earliest geological period to initiate the study of palaeogeography. Taking the North China Craton as an example,the evolution of continental crust is briefly introduced in this paper.