最古老陆壳物质:综述

Oldest continental materials:A review

最古老陆壳物质指≥3.9 Ga锆石和≥3.8 Ga岩石,其发现是了解地球最早期陆壳形成演化的关键.全球范围内,迄今已在近20个地区发现最古老锆石,在8个地区发现最古老岩石.最古老锆石遍布各个大陆,主要存在于太古宙岩石中,但在许多地区仅有1~2颗发现.西澳杰克山-纳瑞尔山是≥3.9 Ga锆石发现最多的地区,全球最古老4.4~4.3 Ga锆石也来自这里.它们普遍显示岩浆振荡环带,结构和组成上与花岗质岩石中的岩浆锆石类似,表明陆壳岩石在地球形成之后不久就出现了,在冥古宙晚期已有了相当的规模.最古老岩石主要分布于北半球,规模通常很小,但在西南格陵兰分布范围很大,达1000 km^(2)以上.岩石以英云闪长岩-奥长花岗岩-花岗闪长岩(tonalite-trondhjemite-granodiorite,TTG)为主,一些地方存在以变质玄武岩和超基性岩为主的表壳岩,也有条带状铁矿等沉积岩存在.>3.5 Ga TTG岩石地球化学组成上存在变化,普遍具有低的Sr/Y和La/Yb比值,表明它们大多形成于相对低压的条件下.≥3.5 Ga岩浆锆石和碎屑-外来锆石Hf同位素组成存在大的变化,ε_(Hf)(t)从正值到负值都有,大多数锆石都具有负的ε_(Hf)(t)值,暗示其大多来自于富集地幔或类似于球粒陨石储库的物源区.Hf同位素组成富集的古老TTG岩石和锆石(≥3.9 Ga)在全球多个地区存在,支持了岩浆海事件形成全球性镁铁质-超镁铁质地壳的认识.在3.8 Ga,最古老陆壳的物质组成和形成条件已显示出多样性,表明陆壳在那时就已达到较高的演化程度.代表早期陆壳演化重要转折的壳源富钾花岗岩在3.65 Ga之后才开始形成,大规模形成的时代更晚.一些地区(西南格陵兰,加拿大的萨格利克-希布伦)存在大规模~3.5 Ga基性岩墙群,表明某些地区陆壳在那时规模就十分巨大,达到了相当的刚性.最古老陆壳物质形成方式存在争论,主要有陨石撞击、冰岛模式、岛弧岩浆作用、板底垫托、热管构造、深成软盖构造等不同解释.本文对最古老陆壳物质今后的研究方向作了展望.在华北克拉通、鞍山-本溪和冀东地区最有希望取得重大突破.

In this paper,the review of oldest continental material refers to≥3.8 Ga rocks and≥3.9 Ga zircons.These rare materials are the key to understanding the formation and evolution of the Earth’s earliest continental crust.Globally,the oldest zircons have been found in~20 areas,while the oldest rocks have been identified in 8 areas.The oldest zircons have been found on all continents,mainly in Archean rocks,but from most localities,only 1−2 oldest zircons are commonly found.The metasedimentary rocks of Mount Narryer-Jack Hills in Western Australia is where≥3.9 Ga detrital zircons are most abundant,and the oldest(4.4−4.3 Ga)zircon grain in the world also came from there.Eastern Hebei is the area where known≥3.9 Ga detrital zircons are the second largest in number,but their ages are mainly between 4.0−3.9 Ga.≥3.9 Ga zircons generally show oscillatory zoning,similar in structure and composition to magmatic zircons from granitoid rocks,indicating that continental rocks formed soon after the formation of the Earth and occurred at a large scale by the late Hadean.The oldest rocks are mainly distributed in the northern hemisphere and are usually small in scale,but they are most extensive in southern West Greenland,with a distribution area up to 1000 km^(2).A~4.0 Ga component in the Acasta Gneiss Complex(Canada)is the oldest rock identified in the world,but forms less than a trillionth of the modern crust.In some areas,there are≥3.8 Ga rocks with different ages.The rocks are mainly gneisses derived from TTG(tonalite-trondhjemitegranodiorite),with supracrustal rocks in some areas dominated by meta-basalt and ultramafic rocks,with some sedimentary rocks such as banded iron formation.≥3.5 Ga TTG rocks show variations in geochemical compositions,with a broad range in Sr/Y and La/Yb ratios,indicating that they formed by melting under a range of pressure conditions.≥3.5 Ga magmatic and detrital-xenocrystic zircons show large variations in Hf isotopic compositions,ε_(Hf)(t)values vary from positive to negative,with most being negative.In the Anshan area of the North China Craton,both mantle addition and crustal recycling played important roles in each magmatic episode from 3.8 to 3.1 Ga,consistent with the understanding that a high heat flux occurred periodically over a long time.≥3.9 Ga rocks and zircons of Hf isotope enrichment features have been identified in many areas,supporting the scenario that magma sea event occurred in the very early stage of the Earth evolution,resulting in the formation of mafic-ultramafic curst worldwide.Fractionation of the silicate Earth within the planet’s first 100 million years gave rise to a depleted upper mantle,which provided the source of surviving juvenile crustal components such as many Archean TTGs.The continental crust showed diversity in composition and formation conditions as early as 3.8 Ga,indicating an obvious difference in magmatism in the deep Earth at that time.Only after multi-stage addition of juvenile crustal components from the mantle,was the scale and thickness of the continental crust large enough for the formation of large-scale K-rich true granite.Therefore,the first occurrence of crustally-derived K-rich granite represented an important turning point in the early continental crust evolution;it began after 3.65 Ga.There already were large-scale mafic dyke swarms by~3.5 Ga,as preserved in southern West Greenland and the Saglek-Hebron area of Labrador(Canada),indicating that by then the continental crust in some areas was extensive in scale and had reached considerable rigidity.Still controversial is the formation mode of the oldest continental material,with suggestions covering meteorite impact,the Iceland model,island arc magmatism,underplating,heat-pipe and plutonic squishy lid tectonics.Finally,this paper prospects the further research direction of the oldest continental material.In the North China Craton,the Anshan-Benxi and eastern Hebei areas are the most promising for major breakthroughs.