前寒武纪重大地质事件

Precambrian major geological events

本文概括性地阐述我国前寒武纪冥古宙、太古宙、元古宙三大地史阶段的重大地质事件,粗略勾绘前寒武纪地球演化的轨迹,期望了解我国与全球变化的异同,进一步突出我国前寒武纪三大地史阶段中新太古代超级地质事件及元古宙时期中国大陆块体对哥伦比亚及罗迪尼亚两个超大陆形成与破裂的地质响应。冥古宙是地球最早期的地史阶段,从太阳系形成的4 567Ma至地球上最老的4 030Ma的Acasta片麻杂岩。碎屑锆石保存最好的地点是西澳的Mt.Narryer和Jack Hills。目前在中国大陆至少有7个地点发现具有罕见的约4.0Ga的碎屑锆石,这些地点并不位于克拉通区,而是赋存于造山系新元古代至古生代以碎屑岩为主的地层中。太古宙（4 030～2 420Ma）定义为从最古老的岩石出现（4 030Ma Acasta片麻岩）至冰碛层首次广泛分布的寒冷期之间的一段地史。最古老的岩石为英云闪长片麻岩,构成加拿大西北斯拉夫克拉通4.03～3.94Ga Acasta片麻岩的一部分。西南格陵兰Isua带保存全球有最老的表壳岩,形成于3 810 Ma。太古宙最重大的地质事件莫过于2 780～2 420 Ma时期的新太古代超级事件。值得指出的是华北克拉通最古老、也是中国最古老的岩石出露在中国辽宁鞍山地区,约3.80Ga英云闪长岩-奧长花岗质片麻岩和3.30Ga的表壳岩已被识别。华北克拉通太古宙有与世界各地太古宙相似的演化历史和特点,包括花岗岩-绿岩带及高级变质片麻岩带、广泛的英云闪长岩-奧长花岗岩-花岗闪长岩（TTG）片麻岩、古陆壳的出露（略老于3.8Ga）、广泛分布的BIF等。我国太古宙花岗岩-绿岩带虽然在华北克拉通分布较广,但与南非、格陵兰、加拿大、西澳等地经典的花岗岩-绿岩带相比,时代偏新,仅以新太古代为主,规模偏小,缺少大面积分布的科马提岩,且变质程度偏高,主要为角闪岩相-麻粒岩相变质。演化到元古宙（2 420～541 Ma）,则进入成熟的、较冷的、刚性程度较高的地球,以现代样式板块构造、超大陆旋回和更复杂的疑源类（eukaryotic）生命的发育为特征。这种变化大致出现在2 420 Ma左右,与哈默斯利型BIF的消失及地史中首次广泛出现的冰川沉积物年代相近。古元古代早期十分重要的＂休伦冰川事件＂、指示大氧化事件的古老红层在我国尚未被发现,与Lomagundi-Jatuli（LJE）δ13 C的同位素漂移有关的关门山组古元古代沉积地层的同位素年代学依据不足;古元古代磷矿和具有巨大石油潜力的2.01Ga Shunga事件也未能鉴别。但中国最大特色是发育了与哥伦比亚和罗迪尼亚超大陆汇聚与裂解有关的良好地质记录,特别是华北克拉通保存了古元古代与哥伦比亚超大陆汇聚有关的超高温、高压麻粒岩等变质及岩浆事件,1 780 Ma以后的中元古代又保存了与哥伦比亚超大陆裂解有关的裂谷沉积及岩浆活动;而在扬子和塔里木陆块区则保存了与新元古代早期与罗迪尼亚超大陆汇聚有关的蛇绿岩、混杂岩、洋内弧、俯冲增生杂岩及大陆边缘弧,在约800 Ma以后则发育了与罗迪尼亚超大陆裂解有关的沉积及岩浆活动的地质记录,为中国和全球地质学者研究这一时期地球系统变化和成矿作用提供了客观的野外实验室和良好的范例。

The major geological events during the three Precambrian Eons of the Earth history,i.e.,the Hadean,the Archean and the Proterozoic,are summarily described and the Precambrian evolution nature of China continent is roughly depicted.It is desirable to understand the similarity and difference between China and the global,and it is a further highlight of China continent Neoarchean super geological event,and response to the formation and breakup of the Proterozoic Colombia and Rodinia Supercontinents.The Hadean Eon for the earliest period of the Earth history extends from the age of formation of the solar system at 4567 Ma,to the age of Earth＇s oldest dated rock from the Acasta Gneiss Complex,at 4030 Ma.The Hadean detrital zircon has been well preserved from Mt.Narryer and the Jack Hills greenstone belt of Western Australia.Meanwhile at present,rare detrital zircons of Hadean age were analyzed from 7places in China,those are located at the Neoproterozoic to Paleozoic orogens,not in the cratons.The Archean Eon can be defined as the time of Precambrian history from the first appearance of preserved rocks at the Earth＇s surface（4030 Ma Acasta Gneiss）,to the first appearance of widespread glacial rocks,cooler Earth conditions.The oldest dated rock on Earth is a tonalitic gneiss that forms part of the 4.03-3.94 Ga Acasta gneiss of the northwestern Canada Slave Craton.Supracrustal rocks are locally well preserved in the Isua supracrustal belt in southwestern Greenland.It is worth to point out that the most important event of Archean is the super geological event between 2780 Ma to 2420 Ma.The oldest rocks of the North China Craton（NCC）and also of China are exposed at the Anshan region of Liaoning Province,where about 3.8Ga tonalite-trondhjemite gneiss and 3.3Ga supracrustal rocks are recognized.Archean evolution and nature of NCC are quite similar to that of cratons in the world,such as the granite-greenstone belt and high-grade gneiss,tonalite-trondhjemite-granadiritesuite（TTG）,older continent crust（a little more than 3.8 Ga）and banded iron formation（BIF）and so on.It is noticed that Archean granite-greenstone belt is widely spreaded at the NCC,but differences with the classical ones in South Africa,Greenland,Canada and western Australia.For example,the age of granite-greenstone belt in NCC is dominantly of the Neoarchean,with smaller size of the belts,less komatiites and higher metamorphism up to amphibolite-granulite facies.Proterozoic Earth is a mature,cooler planet with large,rigid,emergent continental land masses, modern-style plate tectonics,a supercontinent cycle,and more complex life（eukaryotes）.This combination of geological and biological changes took place at about 2420 Ma,near to the time of disappearance of the Hamersley-type BIF and occurrence of the first global glacial event.It is noticed that the Huronian glacial beds and the red strata of the Great Oxidation Event are not recognized in China.The age of Paleoproterozoic Guanmenshan Formation,related to the Lomagundie-Jatuli carbon isotopic excursion,is still uncertain.In addition,Paleoproterozoic phosphorites and the worldwide Shunga event dated to 2.01 Ga,being the source rocks for giant petroleum deposits,are not discovered.However,the assembly-breakup of supercontinents-related geological records are well developed in the North China,Yangtze and Tarim Cratons,respectively.The ultra-high temperature metamorphic rocks,high pressure granulite and Columbia Supercontinet assembly-related magmatic association,as well as the arc-related sedimentary and volcanic strata are exposed.And after 1780 Ma the Mesoproterozoic rifting basins and magmatic activity with disperse of Columbia Supercontinet are widespreaded in the NCC. Meanwhile the Early Neoproterozoic ophiolite,mélange,oceanic arc,subduction-related accretion complex and continental marginal arcs occurred at the Yangtze and Tarim Cratons,which reflects the Rodinia formation-related geological events.The rifting basins and a series of magmatism with Rodinia breakup took place since ca.800 Ma.All of above mentioned provide an objective field laboratory and an outstanding example in studying Precambrian Earth system and mineralization to China and world geoscientists.