冥古宙的地层学属性:了解地球形成初期古地理背景和演变历史的重要线索*

在现行的前寒武纪地质年代表中,由于太古宙底界没有得到很好的定义,只是被粗略地置于大约4000,Ma,因此也造成了一个没有得到较好定义的“冥古宙”。2个重要的发现促使学者们对冥古宙的地层学属性进行修订:(1)在西澳大利亚Jack山脉太古宙砾岩中发现了真正古老的锆石晶体,其不仅可将地层时代延伸到4404,Ma,而且包含了有关地球早期环境条件较为丰富的信息; (2)在加拿大北部发现了大约4030,Ma的Acasta片麻岩。根据这些发现,并结合月球和陨石的测年数据,就产生了大量有关太阳系和地球早期历史的新知识,包括太阳系与地球的形成、初生地球时期的重要变化及其物质记录和生命的起源及早期进化,这成为修订冥古宙地层学属性的重要基础。修订后的冥古宙代表了地球演变历史的最早时段,即从太阳系和地球在T0=4567,Ma的形成,一直延续到地球上最古老岩石的出现(4030,Ma)。冥古宙还可被进一步划分为2个代:(1)“混沌代”(4567,Ma—4404±8,Ma);(2)“杰克山代”或“锆石代”(4404±8,Ma—4030,Ma)。由于没有保存相应的地层记录,因此冥古宙顶界(4030,Ma)与底界(4567,Ma)的年龄值,仅为一个计时性的年代界限。上述这个被赋予了明确地层学属性的冥古宙,不仅代表了前寒武纪地层学研究的一个重要进展,而且也为深入了解地球早期演变历史提供了许多重要的理念。

Stratigraphical natures for the Hadean:An important clue for understanding of both evolutionary history and palaeogeographical background in forming and initial periods of the Earth

Within the current Precambrian geological time scale, it is an undefined base of the Archean Eon that it is roughly indicated at about 4000,Ma,which results in a use of Hadean is informal as it has not been defined. Two of key advances led to the revision of stratigraphical nature for the Hedean,which as follows: (1)The discovery of truly ancient zircon crystal in Archean conglomerates at Jack Hills in Western Australia,which extend back to 4404,Ma and contain a wealth of information about conditions on very early Earth; (2)The discovery of ca.4030,Ma Acasta Gneiss in northern Canada. These discoveries,together with age dating of the Moon and meteorites,have yielded a great deal of knowledge about the early history of our solar system that is just an important foundation to revise the stratigraphical nature of the Hadean. The knowledge includes the formation of the solar system and the Earth, the important changes and their physical records on very early Earth, the origin of life and its early evolution. This revised Hadean Eon that has been recognized as the earliest period of Earth history,extends from the age of formation of the solar system at T0=4567,Ma,to the age of Earth??s oldest dated rock from the Acasta Gneiss Complex,at 4030,Ma,and has further been subdivided into two eras:(1)the Chaotian Era(4567,Ma-4404±8,Ma);and(2)the Jack Hillsian Era or the Zirconian Era(4404±8,Ma-4030,Ma). Both the lower(4567,Ma)and upper(4030,Ma)age limits for the Hadean Eon would necessarily be defined as chronometric boundaries,as there is no preserved stratigraphy. A Hadean Eon that has been endowed with a definitely stratigraphical nature not only delegates an important advance of studies on Precambrian stratigraphy, but also provides lots of important ideas for the further understanding of the evolutionary history of the early Earth.