Archean belongs to Precambrian, the last one is Phanerozoic and the next one is Proterozoic. Archean includes Archean, Archean, Middle Archean and New Archean.
Archean is an ancient period in geological history. From a biological point of view, this is the initial stage of primitive life and biological evolution. At that time, there were few prokaryotes, such as bacteria and lower cyanobacteria, and they left few fossil records. From the abiotic point of view, Archean was a period of thin crust, steep geothermal gradient, intense and frequent volcanic-magmatic activities, widespread deformation and metamorphism of rock strata, lack of free oxygen in the atmosphere and hydrosphere, and formation of a series of special sediments. It is also a period of formation and growth of silicon-aluminum crusts and an important metallogenic period.
The earliest geological period belongs to the early Precambrian. The strata (rocks) formed in this period are called Archaean. According to the broad concept of time, it includes from the formation of the earth to 2.5 billion years ago, lasting about 2 billion years. At present, the isotopic geological age of the oldest rocks has been determined to be 3.8 billion years or slightly older, and the age of detrital zircon in metamorphic conglomerate in Jack Hill, Western Australia can reach 4.3 billion years. The word Archaean was first put forward by American geologist J.D. Dana in 1872, which roughly represents the Precambrian in North America. 1977 The fourth meeting of the Precambrian Stratigraphic Subcommittee of the International Stratigraphic Committee limited the Archaean to 2.5 billion years and called it Archaean. 1979, the fifth meeting of this organization put forward the Archean triad theory, and its age limit was 3.5 billion years and 2.9 billion years respectively. However, at the 6th, 7th and 8th meetings of 1983, 1985 and 1988, it was considered that the basis for further division of Archean was not sufficient, and the division of Archean was scheduled to be discussed at the 9th meeting of 19 1. At the eighth meeting, the proposal for the division of Precambrian system was formally put forward, including the view that the upper limit of Archean is 2.5 billion years, which was adopted by the International Stratigraphic Committee and the Executive Committee of the International Geoscience Union in 1989 and 1990 respectively.
1979, The Stratigraphic Guide of China proposed by the Second National Stratigraphic Conference in China is also called Archaean. However, in the minutes of the meeting of the late Precambrian professional group of the National Stratigraphic Committee at the end of 1989, it was clearly stated that it should be renamed Archaean, with an upper limit of about 2.5 billion years. There are a few ancient rocks over 3 billion years old in China, among which the oldest known rock is amphibole near Cao Zhuang-Huangbaiyu in Gan 'an County, with an age of about 3.5 billion years, and even older detrital zircon has been found. There are rocks around and in other areas that are about 2.9-3 billion years old or older. At present, although Archean in China tends to be divided into three parts, according to the chronological data of Northeast China and North China, the upper part can be tentatively set at 2.9-3 billion years, and the basis for the lower part is not enough, so it is appropriate to temporarily adopt the dichotomy scheme (hereinafter referred to as Middle Archean).
The characteristics of stratigraphic distribution area are within the distribution range of some Archean rocks in the world, which can be generally divided into deep metamorphic granulite-gneiss area (also called high metamorphic area) and low metamorphic area represented by greenstone belt.
The metamorphic degree of granulite-gneiss area mostly belongs to granulite facies to high-grade amphibolite facies. The main rock is felsic gneiss, with a small amount of dark minerals such as biotite, amphibole, hypersthene or diopside, and its chemical composition is equivalent to granodiorite or greisen diorite. There are disputes about the origin of their original rock properties, such as intrusion, volcano and volcanic deposition. Some rocks contain aluminosilicate minerals, which are roughly metamorphic from argillaceous-semi-argillaceous original rocks; Other pyroxene granulites and amphiboles are mostly equivalent to basic volcanic rocks; Light felsic gneiss is mainly equivalent to acid igneous rock; Plagioclase is usually considered as a layered intrusion. The structures in these areas are generally complex, often with multi-stage structural superposition and multiple migmatization in different degrees. Its typical areas are the continental core of the North Atlantic, the Adan Shield in eastern Siberia and the Limpopo Belt in southern Africa.
Most greenstones belong to low-grade amphibolite facies to greenschist facies. Many places are mainly composed of three parts of rock strata, the lower part is composed of ultramafic-mafic lava, and some are mainly Komatite. The middle part is calc-alkaline acid volcanic rocks; The upper part consists of sedimentary turbidite, chemically deposited banded iron-bearing layer and flint. The greenstone belt is mostly syncline structure, which can be invaded by granite. Typical areas are barberton Mountain and Zimbabwe, as well as parts of canadian shield, western Australia and eastern Finland. However, the Dawa greenstone belt in southern India has been deteriorating from north to south, reaching granulite facies. Archean strata in China are mainly distributed in the north. East of east longitude 105 and between 3 1 ~ 43 north latitude, it is basically the exposed area of the basement of North China platform. Archean metamorphic rocks are mainly composed of biotite plagioclase gneiss, amphibole plagioclase gneiss, biotite granulite and amphibole. Its northern belt starts from Jilantai in Ningxia, passes through Wulashan in Inner Mongolia and Yanshan in Hebei, and extends eastward to Liaodong and southern Jilin. There are granulite facies rocks (pyroxene granulite, biotite plagioclase gneiss, etc. ). The above rocks constitute the main body of Archean rocks. In addition, there are some metamorphic sedimentary rock interlayers in varying degrees, such as magnesium marble, cassiterite gneiss, mica schist, feldspar quartzite, banded timely magnetite layer (banded magnetite is poor), graphite gneiss and graphite schist. These interlayers can reflect the differences of original sedimentary conditions and sedimentary environment in different regions respectively. Most of the above strata have experienced many migmatization and tectonic deformation superposition. In the same area, regional metamorphism often shows that the strata with high metamorphic degree are older than those with low metamorphic degree. The above situation shows that the Archean rock distribution area in the basement of North China platform is mainly equivalent to the high-grade metamorphic area, and there are small-scale low-grade metamorphic rocks in some places such as Liaodong and Luxi. There are different opinions at home and abroad on whether the strata of Archean high-grade metamorphic area can be established and divided. Combined with China, especially the above-mentioned areas with layered strata exposed, many of them can be established and divided according to detailed work, especially geological mapping. However, some areas (such as Mount Tai) are mainly composed of ancient intrusive rocks, so it is difficult to divide strata.
In northwest China, east longitude 105, the exposed ancient metamorphic rocks are distributed between 35 and 45 north latitude. Due to the low level of geological work and the lack of isotopic age data, Archean and Proterozoic rocks in many areas have not been divided. Besides biotite plagioclase gneiss and amphibole plagioclase gneiss, there are a considerable number of mica schist, amphibole schist, marble and quartzite. In many areas, it is still difficult to completely divide the above two zones.
Archean strata in the northern belt of North China platform basement are well exposed, and the possible stratigraphic boundaries of dichotomy are temporarily represented by the boundaries between Yinshan Jining Group and Wulashan Group, Yanshan Qianxi Group and Badaohe Group (or Luanxian Group), and Longgang Group and Anshan Group in Liaoji area (or lower Anshan Group and upper Anshan Group), which generally reflect two volcanic sedimentary cycles. Because Archean has not yet been built, it is still called Lower, Middle and Upper Archean. The lower and middle Archean are often represented by rocks containing granulite facies, while the upper Archean is dominated by amphibolite facies. In other areas, such as Taihang, Huaiyang and the northern slope of Qinling Mountains, granulite facies rocks are sporadically exposed, so Upper Archean and Middle Archean are not divided into granulite facies and amphibolite facies. In recent years, another opinion on the Archean upper limit in China is that it should be placed at the top boundary or middle of Wutai Group and its corresponding strata. Some recent rock age data confirm that some early Proterozoic volcanic rocks exposed in Shanxi and Hebei are about 2.5 billion years old (the error is less than 50 million years), indicating that some parts of the basement of North China platform are on the Archean top boundary, which may be slightly more than 2.5 billion years old.
According to the primary diagenesis, volcanic activity, tectonic changes and geochemical characteristics of Archean rocks in China, combined with its crustal evolution history, some characteristics of Archean rocks in China are different from those in the western northern hemisphere and the southern hemisphere, but they are similar to most of the neighboring Soviet Union.
Atmosphere, hydrosphere and life forms According to general speculation, Archaean primitive atmosphere is relatively dense, mainly composed of water vapor, carbon dioxide, hydrogen sulfide, ammonia, methane, hydrogen chloride, hydrogen fluoride and other components. These gas components may come from frequent volcanic activity. The general trend is that carbon dioxide decreases with time, because carbon dioxide is fixed in carbonate sediments when carbonate precipitates. The primitive atmosphere lacks free oxygen, and the appearance of oxygen is the result of photochemical action. According to the similarity of sedimentary rocks in different places, it is speculated that most of the earth was covered by oceans at that time. The primitive ocean may not be deep and is rich in chloride, but it lacks sulfate, because there is also a lack of free oxygen in the hydrosphere.
With the passage of time, the transparency of the atmosphere is enhanced, which provides favorable conditions for biological photosynthesis. According to the fossil records, a large number of fungi and low-grade cyanobacteria appeared in the late Archean. If 3.5 billion years is taken as the early boundary between Middle Archean and Middle Archean, there are already low-grade cyanobacteria and stromatolites in Middle Archean.
tectonic movement
At present, the research on Archean tectonic movement is not clear enough, and there may be three major tectonic movements in the world. There are few early discoveries, for example, southern Africa ended 3.4 billion years ago, while North America ended 3.3-3.5 billion years ago, and India may have ended 3.2 billion years ago. The middle stage was the Dahomey Movement in south-central Africa, which was manifested in America, the United States, Australia, India and China and ended about 2.9 billion years ago. The later period was equivalent to the obvious Kai Nuo thunder movement in canadian shield, about 2.7-2.5 billion years ago. There seems to be some differences in the development and time of Archean crustal movement in different places, and only the late tectonic movement has found evidence in many places. At present, some people think that plate tectonics first occurred in Proterozoic, especially in northern Europe and North America, while others think that plate tectonics existed in Archean.
The late Archean Fuping movement (or Tie Bao movement) in northern China is an obvious tectonic movement, which may be equivalent to Kennoley movement. In addition, in Taihang Mountain area, we can see the unconformity between Longquan Guan Group and Fuping Group, but it has only local significance.
mineral
Archean is an important metallogenic period, and the minerals formed are very rich. There are mainly iron, gold, nickel, chromium, copper, zinc, rare elements and some nonmetallic minerals. Compared with other times, many minerals are in the forefront, especially nickel, gold, copper, iron and other minerals. Such as Superior, South Africa and Western Australia, are rich in these minerals.
The large-scale iron mines in Anshan, Benxi, Jidong and Lvliang, and the gold mines in southern Jilin, western Liaoning, Jidong and Xiaoqinling all occur in Archean rocks, but so far there is no evidence that the gold mineralization period is Archean.
The mineral resources in granulite-gneiss area mainly include iron ore and nonmetal deposits, and a small amount of chromium and nickel deposits. The greenstone belt is particularly rich in minerals, in which chromium and nickel are mainly produced in ultramafic flows and intrusive rocks in the lower part, gold, silver, copper and zinc are produced between mafic and felsic volcanic rocks in the middle part, banded iron formations are produced in upper sedimentary rocks, and rare elements are produced in pegmatites associated with granitic intrusive rocks.
Most Archean deposits belong to stratabound or layered deposits, and a few are related to various intrusive rocks or pegmatites.