At present, the mainstream view holds that the oldest primitive crust belongs to the ancient platform formed by most modern continents 3.5 billion years ago, and its composition is either granodiorite and quartz diorite (tonalite) or diorite. For these rocks, a common name-gray gneiss, has been clarified by Archean greenstone belts in Taichung, North America, Greenland, South Africa, India and Australia. Archaean gneiss and granite gneiss in the basement of eastern Europe platform can be compared with this. In addition to Murmansk block, the basement of the greenstone belt in the White Sea and Karelia, the huge belt along the Dnieper River in the Ukrainian shield and the Kursk magnetic anomaly area, there is also the ancient plagioclase gneiss basement (Nurrak complex) in the eastern part of the Russian platform. The so-called gray gneiss refers to amphibolite facies metamorphic, neutral or intermediate-basic volcanic-plutonic complex (less metamorphic sedimentary rocks), which is different from magmatic rocks with similar composition in the later stage, with higher aluminum, calcium and sodium content and lower potassium content. The rocks called gray gneiss can be further classified according to their composition (from granodiorite to diorite), origin and age (from Archean to Archean). Almost at the same time that the gray gneiss was subjected to medium-pressure metamorphism, the metamorphic conditions of high-pressure and high-temperature granulite facies rocks were formed in the deep part, and the Archean or Proterozoic deep rocks were lifted to the surface.
Archean (3.5 billion ~ 2.6 billion years) basement rocks can be divided into several main types, showing the heterogeneity of upper crust structure-material composition and development stage.
The first category is plagiogranite and plagiogranite, which contain plagioclase gneiss, rare crystalline schist and plagioclase amphibolite (Mormanke block). It can be imagined that they were formed by the regional granitization of Archean gray gneiss protocrust (protocontinent).
The second type is greenstone belt, which is developed in a narrow and deep depression above the ancient gray gneiss. It consists of volcanic rocks with Archean-specific shallow metamorphic basic and ultrabasic rock components (Komate rocks), volcanic rocks with andesite and dacite components, and terrigenous and iron-bearing sediments. The development process of greenstone belt is firstly the folding deformation of sediments, then the gneiss basement is strongly granitized to form sodium granite, and finally the metamorphic granite dominated by potassium ends, and some granites break through the greenstone belt groove and deform. The greenstone belt was formed in Archean (pre-Dnieper) or Neoarchean, but the regional granitization occurred at the end of Archean (2.7 billion years ago). The greenstone belt is combined with the adjacent granite gneiss basement to form the granite greenstone belt area, which is usually close to trough. Granite greenstone areas (Karelia, White Milkov, Dnieper River and Kursk Belt) are hundreds of kilometers wide.
The third type is thick layered supracrustal rocks, mainly gneiss, followed by amphibole, quartzite, metamorphic conglomerate and iron quartzite, which are filled in a vast depression with a thickness of several hundred meters (central and southern Kola belt, southern Warren-Potoska, Kirovgrad and western coastal zone of Azov Sea). These complexes are composed of acidic and basic metamorphic volcanic rocks, interbedded with sedimentary rocks and mainly terrigenous materials. Sediments may come from adjacent areas, which can identify early medium-pressure granulite facies and late amphibolite facies retrogradation. According to the Archean structure and development of the Russian platform, the granite gneiss of the Russian platform is likely to be similar to this type, but it is different from the similar structure in the shield area in terms of the intensity of later thermal transformation.
The fourth type is thick gneiss-plagioclase amphibolite complex, which consists of volcanic rocks, mainly basic lava (Baihai Group). The accumulation of this kind of complex (the same as the third kind) basically occurred in Archean, and has undergone many metamorphic transformations and deformations since then.
The fifth type, which has not been studied so far, is characterized by many linear elongated belts with a width of 200 ~300km, which are found in the Russian platform and have narrow weak magnetic substances in the middle. It is speculated that this kind of ancient basement plagioclase gneiss complex (Nurlak complex) was stretched and fractured in Archean, forming these narrow troughs and tensile cracks, which were filled by basic-ultrabasic rock volcanic rocks, injected by peridotite, gabbro, plagioclase and plagiogranite, and the linear belt in the late Archean was squeezed, forming rock wedges, plates and rocks composed of medium-pressure and high-pressure granulite facies rocks. This process may be repeated in ancient Proterozoic. An example is the Lapland-Kolwitz granulite belt, which extends northward, that is, the Hangel-Sk and Kandara-Keke linear belts in the basement of the platform. The granulite belt may have been formed by extrusion in Paleoproterozoic.
Most Archean complexes (except greenstone belts) have the characteristics of multi-stage metamorphism, which begins with progressive metamorphism (reaching granulite facies) and is formed during its subsidence, followed by retrogression of amphibolite facies, possibly accompanied by many Archean strata deformation. At the end of Archean, regional granitization occurred, and as a result, most of the ultrametamorphic plagioclase granite bodies of migmatite complex appeared on the platform, and then plagioclase-microcline granite appeared (not everywhere). This process is related to the large amount of potassium brought to the upper crust by the first magmatic activity. Granitization increases the volume of Archean crust, which has high plasticity and uniform rheology under the conditions of strong high temperature and high hydrostatic pressure. Due to the lateral inhomogeneity of heating and the upward flow of materials, circular and elliptical diapir dome structures (according to лии salop, elliptical uplift or granite gneiss dome) with different sizes on the plane are formed, which are separated by narrow inter-dome synclines and complicated by small deformation.
One of the main factors of the deep multi-stage metamorphism of Archean rocks is the high heat condition, and the Archean heat flow is higher than its later stage. In the early Archean, the earth's surface was surrounded by a dense atmosphere. V.i. Shuldner and others think that the atmospheric temperature can reach 300 ~ 500℃. During Archean-Paleoproterozoic period, the crustal temperature gradually decreased as a whole. This is closely related to the fact that almost all rocks in the upper crust have undergone metamorphism to varying degrees. Obviously, regional granulite facies rocks only appeared widely in Archean, while amphibolite facies and even greenschist facies rocks appeared in New Archean. At the same time, amphibolite metamorphic rocks are often superimposed on rocks that have undergone granulite facies metamorphism in the early stage. However, the overall decline of thermal conditions in the deep part of the earth is often complicated by the occasional increase of heat flow. At present, it has global characteristics: granulite facies metamorphism widely developed at the end of Archean (2.6 billion ~ 2.7 billion years ago) led to large-scale granitization, accompanied by potassium being brought into the upper crust from the mantle or redistributed in the crust, and primary or superimposed metamorphism (retrogression) occurred in the area.
Another factor of Archean regional metamorphism is high pressure. If the metamorphic transformation of the rock reaches amphibolite facies, it may occur under the pressure of 2× 108 ~ 5× 108 Pa, and the depth should be 6 ~ 15 km today. The granulite facies needs a pressure of 5× 108 ~ 8× 108 Pa or even 10 × 108Pa, and the current depth is 15 ~ 30km. Due to the widespread regional granulite facies metamorphism in the upper basement of the eastern European platform (especially the Russian platform), it can be inferred that it has experienced deep subsidence and metamorphism since its formation, and then experienced high uplift. At the same time, the Archean (and some Proterozoic) strata were cut by denudation 15 ~20km. This assumption is not very reliable. Therefore, some scientists believe that the gravity of Archean Earth is greater than that of modern times, and the pressure required to metamorphic rocks into granulite facies may appear at a smaller depth. When the depth is shallower than today, the Archean has reached the temperature (750 ~ 800℃) required for granulite facies metamorphism, and the gravity value is large. We can only set the Archean earth to be smaller and the average density to increase with the same mass (Ф. п. Mitrovan, ка. Shurkin. So the diameter of the earth may decrease by 25% ~ 30%, and the gravity will increase by 1.5 ~ 2 times. Therefore, the depth required for granulite facies metamorphism is not 15 ~ 30km, but may be equivalent to 10 ~ 20km or even 7. 5 ~ 5km。 Of course, this interesting hypothesis needs to be fully verified.
Although the shape of Archean structure is largely related to its plastic state. The plastic state is determined by intense heating in the deep part of the earth. In the process of its formation, the mutual tectonic movement between different rocks in the crust plays a decisive role, including vertical (quartzite, metamorphic sandstone and metamorphic conglomerate exist in a series of sections) and horizontal. In some large-scale greenstone belts, the linear structural form and parallelism of Archean greenstone belts can explain the subsidence during deposition and the intensity of volcanic activity in greenstone belt grooves. They are subjected to transverse tension and destroyed by a series of longitudinal tensile faults, and develop into linear depressions or fissures along the faults. On the contrary, at the end of the development of greenstone belt trough, there was lateral compression, which led to the folding of volcanic sedimentary volcanic rocks filled in it. Some scientists (аФ Greyev, вссс Fedoroff) think that it is determined by the extension in front of the greenstone trough and the unloading and consolidation of materials in the process of granitization. The author thinks that the overall width of the whole granite-greenstone belt in this area is reduced due to compression.
The banded or lenticular contours and almost parallel structural interfaces of Archean main structural zones in the basement of the shield and Russian platform indicate the existence of Archean horizontal movement. Volcanic rocks and basic-ultrabasic rocks are distributed in Archean linear belt system within the platform, which is related to the horizontal extension of basement. Among them, the banded distribution of metamorphic granulite facies is due to the extrusion and upward extrusion of local deep material plates in linear zones.
Paleoproterozoic (3.6 billion ~10.65 billion years). At the end of Archean, strong granitization spread to most areas of the platform, resulting in the formation of thick mature continental crust in the original continental crust. Compared with Archean, the thermal state of Proterozoic is generally lower. Obviously, the surface temperature (close to today's) and the average heat flux have decreased, but the decrease of thermal state is complicated, which will change in a wave form due to short-term thermal enhancement, the strongest of which is the heating of the upper crust 65.438+0.9 billion ~ 65.438+0.8 billion years ago, and Archean (especially Archean) showed high mobility (high activity-л) everywhere. However, the structure of different parts of the Paleoproterozoic platform became very different. In the active zone, the deformation has obvious linear characteristics, at the same time, the linear fold is obviously enhanced, and it is often strongly squeezed (until the equal-length fold). Related to this, many reverse faults and overthrust faults have appeared under the condition of horizontal compression. In addition to the complex active zone, thick and weakly deformed in-situ platform cover complexes began to form in some areas above Proterozoic Archean basement.
The paleoproterozoic history of the eastern European platform can be divided into three stages: 2.6 billion to 2.2 billion to 2.3 billion years, 2.2 billion to 300 million to 65.438+0.8 billion to 65.438+0.9 billion years, and 65.438+0.8 billion to 65.438+0.6 billion years. In the first and second stages, extensional conditions occurred and different types of deep depressions and depressions developed. In many cases, subsidence is accompanied by strong volcanic activity; At the end of the second stage, compressive deformation (Riffen fold) occurred, and granite plutonism and regional metamorphism were widely developed. In the third stage (Gothic period), most areas of the platform are characterized by enhanced structural stability, mainly due to the overall rise of the platform and the thermal events of large-scale homogenization. Only in the west of the platform, the crust is affected by tectonic-thermal events, which show the development of a single volcanic-plutonic complex and the regeneration of basement rocks in the radioisotope era.
According to its composition, tectonics-magmatism and the characteristics of Paleoproterozoic, the zones with the strongest activity, deepest subsidence and strongest deformation in a large range can be identified. Except for local subsidence and deformation, they are very similar to Neoproterozoic geosyncline and have similar characteristics to Neoproterozoic and Phanerozoic primitive platforms. However, judging from the activity, deformability, magmatic activity intensity and metamorphism, the Proterozoic original platform greatly exceeds the Neoproterozoic platform area. In-situ trough area and its adjacent depressions are filled with Proterozoic complex deformation, and in-situ platform area basically belongs to the main uplift area of basement. Northwest and southwest of the platform are the Baltic Sea Shield, Ukrainian Shield and Voronezh Platform anticline and its slopes. The subsidence basement of Russian platform (in the middle and east of the platform) is mostly primitive platform area, and no deep and wide depression in Proterozoic was found in this area. Therefore, there are many speculations about the evolution stages and structural characteristics of the basic structural areas of the Eastern European platform, namely, its shield (and its adjacent active Rognier platform anticline) and many Proterozoic (in-situ trough and in-situ platform) of the Russian platform.
The platform basement of the Paleoproterozoic geosyncline area appears in the wide (cross-sectional distance is close to Qian Qian meters) nearly equiaxed Rifen area, the narrow (hundreds of kilometers) East Voronezh belt, and the Osny belt in the middle of the Azov Sea and the edge of the Ukrainian shield. The tectonic position and relationship between the Ukrainian shield marginal zone and the Rifen region and the East Voronezh region are still unclear, but there is no doubt that these geosynclines are surrounded by the original platform-type region, or even closed. The scope and area of Proterozoic in-situ geosyncline are much smaller than Phanerozoic geosyncline. The fillings in the in-situ trough depression are sedimentary rocks, mainly terrigenous materials, including flysch (East Voronezh belt, east of Ruifen area) and sedimentary volcanic rocks (inner belt of Ruifen area, middle belt along Azov Sea, Osny belt). At the same time, volcanic rocks appear in the upper part of its profile. Relatively speaking, they can be compared with the geosyncline belt and the superior geosyncline belt in Phanerozoic geosyncline, which correspond to the original geosyncline belt and the original superior geosyncline belt. On the profile of the original upper geosyncline zone, acidic volcanic rocks are widely developed, but ophiolite suite is missing, which is considered as a sign of the existence of oceanic crust zone. Therefore, we speculate that it and the in-situ trough developed from the continental crust, but the thickness is thin and destroyed by magmatic rocks.
The characteristics of the Proterozoic in-situ trough area and the in-situ trough area between the narrow craton are as follows: the second Proterozoic period (2.2 billion ~10.90 billion a) experienced the strongest subsidence, which ended with fold deformation and regional metamorphism of filling amphibolite facies or epidote amphibolite facies, and acid plutonic intrusions were widely developed. It is related to the reactivation of Archean granite gneiss basement under the condition of strong heating in the upper crust, accompanied by the introduction of potassium.
In the linear depression and outer belt of the original trough, the fold structure in Ruifen area extends along the strike, while the orientation of Ruifen area changes, which may be due to the overall deformation of granite dome caused by deep diapir. In a word, the strength of Proterozoic in-situ geosyncline is lower than that of Phanerozoic geosyncline, which is obviously because its horizontal compression is lower than that of Phanerozoic geosyncline.
At present, there are in-situ platforms in areas where platforms lack Proterozoic in-situ trough areas. The whole Paleoproterozoic is a long and complicated stage of cratonization in these areas, that is, the metamorphic basement gradually consolidated and transited to the characteristics of ancient platform. This main stage can be divided into three stages. The third stage is equivalent to the Gothic stage, and can be divided into two types of primitive platforms: the first type of primitive platform is characterized by narrow and deep fault depressions and graben depressions (the first and second stages), short-axis depressions (the second and third stages) and adjacent uplift zones. These platforms are distributed in the east of the Baltic Sea Shield and the middle of the Ukrainian Shield and Voroni Block, and the second type of primitive platform is characterized by the present Russian platform. It is mainly uplift, and the Paleoproterozoic subsidence zone is missing or underdeveloped. Many belts underwent tectonic thermal activation in the second stage.
The first Proterozoic primitive platform area is different from the primitive trough and its adjacent Neoproterozoic and Phanerozoic ancient platforms in structural characteristics and development history, and its characteristics are as follows:
(1) Weathered crust is widely developed, which indicates that it has been eroded for a long time, the terrain is flat and the structure is stable. Weathered crust originally (? ) appears at the bottom of Paleoproterozoic sediments, but the bottom of Yatuli Group and its similar horizons is particularly common, that is, about 2.5 billion-2.6 billion a.
(2) The Farahov Formation and the high-alumina metamorphic rock formation, which are composed of terrigenous detritus with seasonal components, are well developed, with small to medium thickness, and are presumed to be the products of discontinuous or chemical weathering crust redeposition. Chemical weathering crust is a typical platform cover. The Moromov Formation of Karpos belongs to chemical weathering crust, which is located in the middle layer of Paleoproterozoic (Yatuli, Inguletskkoskori Group, etc.). ).
(3) There have been many intense magmatic activities in some areas, mainly composed of basalt, which is essentially dark rock, and its formation includes volcanic overflow and layered penetrating intrusions produced on the surface or in shallow water. Magmatism can be divided into three stages: the first stage is Sumia-Saliori volcanic rocks, the second stage is Behenga-Imanda-Warchug Group, Yaturi, Susari volcanic rocks, and the third stage is Vipsey basalt. The second stage is the thickest and most widely distributed.
(4) In the Middle and Late Proterozoic, some flat-bottomed depressions with short axis or equiaxed plane were formed, which were similar to Phanerozoic syncline, but with high degree of deformation (for example, the depressions in Karelia belt were filled with Tu Tu Li Group and Weipuxi Group).
(5) Deep and narrow linear adjacent faults and grabens are widely distributed, similar to continental rift zones. They were formed in the Neoproterozoic and Phanerozoic paleoplatforms, some in the first stage of Proterozoic (full of Sumia-Saliori complex), some in the second stage (Behenga, Imanda-Valchug depression), and the third type is between these two stages with discontinuity (Cuono-Vigozel, Krivorog belt, Kursk magnetic anomaly depression). These adjacent fault depressions and graben depressions are often called primitive rift zones, or to emphasize their similarity with aulacogen, that is, the ancient platform has rift structures formed in the young rift period, and is called ancient aulacogen or primitive aulacogen. Similar to the Rife Trough and the younger rift zone on the platform, the Proterozoic primitive rift zone was developed under the action of crustal tensile stress perpendicular to its strike, but it was strongly squeezed laterally at the end of its formation, resulting in a strongly twisted syncline or syncline at its axis, which was complicated by longitudinal overthrust faults (on the contrary, in the trough, the extrusion deformation at its end was weak or nonexistent).
According to the nature and deformation of fillings, the original rift zone can be divided into two basic types: ① fillings are mainly sedimentary formations, with terrigenous carbonate rocks in the upper section and jasper iron rocks (Krivorog, Kursk Group) in the lower section. Their development process ends with isosceles fold deformation; ② Thick volcanic rocks are mainly basic rocks (Bechenga, Emandra-Valzug sag, Kuono-Vigezele belt, etc. ), and they are asymmetric.
It is controversial that M.B. Muratok thinks that the Krivorog-Kursk depression is a primitive rift zone (or an ancient aulacogen). Some researchers classified it as primitive geosyncline type, and thought it was a wide fold belt in Paleoproterozoic, which was left after deep erosion. On the contrary, the thickness of Paleoproterozoic sediments in the Krivorog-Kursk depression is moderate, which is relatively weak compared with the metamorphism of the typical Proterozoic in-situ trough belt, which lacks the superposition of Proterozoic metamorphism or is weak. Therefore, it can be regarded as the exposed part of the original trough erosion chassis in Proterozoic (such as Dnieper coastal zone).
The original rift zone appears near the adjacent in-situ trough area, and its strike is almost parallel to its edge. Their dynamic mechanism is the same, stretching and filling in the early stage of development, and squeezing after settlement (about 65.438+0.9 billion years ago). The simultaneity of this deformation can be easily explained as the pulsation of horizontal compression and tension of Proterozoic primitive platform and primitive trough basement. The formation of Biyu Iron Mine is a feature of the Paleoproterozoic primitive rift zone, rather than the sedimentary features of Rifei Depression, other rifts on the platform and general platform caprocks. In addition, the formation of jasper iron rocks plays an important role in Archean greenstone belts (Roxey Group, Kangsk-ViHoffner Vite Group, Mikhainovsk Group and its formation in other platform basement). Although the formation of jasper iron ore in Archean greenstone belt is mainly volcanic rocks, and the formation of jasper iron ore in Proterozoic rift belt is mainly terrigenous materials, the formation of jasper iron ore can indicate kinship. Some primitive rift zones, such as Krivorog depression and Kursk magnetic anomaly depression, are related to Archean greenstone belt in space and structure.
The materials in Proterozoic primitive platform area are similar to Neoproterozoic and Phanerozoic ancient platforms in terms of construction composition, basic volcanism and structure. They are also very different. The Li Fei-Phanerozoic platform does not have the characteristics of the original platform, but there should be the following types of deposits: jasper iron ore, semi-graphite, not only basic and ultrabasic rocks, but also acidic volcanic rocks, which are widely distributed magmatic intrusions subjected to weak or medium-intensity metamorphism and horizontal compressive deformation (from 2.2 billion to 2.3 billion years to weak-medium-intensity local deformation, about 65.438+9 billion years to strong deformation), and their compositions are diverse from ultrabasic.
In Proterozoic, there were in-situ platform depressions and depressions in the west of the new platform, which were separated by uplift zones in the middle, providing the source area for clastic materials. According to the existence of a large number of metamorphic rocks with high alumina content, it is judged that the plain terrain has existed in this area for a long time. Except for a few periods, their uplifts are characterized by weak to medium strength, and sometimes they are covered by thin carbonate strata. The characteristics of these uplifts in Proterozoic are very similar to those of the shield area in the ancient platform, but they are different in scale. Many uplifts experienced tectonic-thermal events, starting from 2.2 billion to 2.3 billion years, and the most intense event was about 65.438+0.9 billion years, which made the Archean complex forming the shield retrogress and made its isotopic age younger (the basement of the Baihai Giant Belt, Voronezh Block and the western part of the Ukrainian shield), while the Archean basement developed granite and migmatization in a large area (mainly the Ukrainian shield).
The relative subsidence basement in the eastern part of Russian platform belongs to another Proterozoic original platform area, and there is little research at present. According to the present viewpoint, it is characterized by undeveloped Paleoproterozoic depression and dominant uplift area, composed of Archean metamorphic rocks and locally containing kyanite schist. Some researchers believe that it is the product of redeposition of weathering crust of Archean basic ultrabasic rocks. According to the basement research data of the Volga-Ural region (C.B. Bogdanov), granitization occurred in the Archean block or large rock block in Mesoproterozoic (2.2 billion ~ 65.438+0.8 billion years), resulting in a widely distributed hypermetamorphic dome dominated by potash granite and granite gneiss, accompanied by annular and radial fracture systems. Obviously, granitization has little effect on the linear zone composed of Archean amphibolite facies and granulite facies metamorphic rocks that separate large rocks. However, this does not mean that the linear zone was not exposed during the thermal activation of Proterozoic tectonics. On the contrary, it can be imagined that it is periodically subjected to extrusion and tensile deformation. For example, the Lapland-Kandara granulite belt in the Baltic Sea Shield is obviously similar to the linear belt of the Russian platform basement.
The Gothic stage of Paleoproterozoic (from 65.438+0.9 billion to 65.438+0.8 billion to 65.438+0.7 billion to 65.438+0.6 billion). In all eastern European platforms, extensive uplift and denudation dominate, while linear depression and shear deformation are lacking. However, there are several gentle and symmetrical syncline depressions in the west of the platform, which are filled with terrigenous sediments, mainly composed of seasonal, acidic and basic volcanic rocks (Weipuxi, Lower Yotni and Baihai Group). Obviously, they were large at first, but they were left behind by erosion in the later period.
Some areas distributed in the Riffen zone of the Baltic Sea Shield and the northwest of the Ukrainian Shield (Belarus and the Baltic Sea coast) were affected by the new thermal transformation in the Gothic stage, resulting in the younger isotopic age of their basement, ranging from 65.438+700 million to 65.438+500 million years. There are many granites (pyroxenite porphyry granite), gabbro plagioclase (Ukrainian shield) and the earliest alkaline rock complex in this area, as well as in the Baltic shield, Ukrainian shield and Voronezh block (their emplacement in the east is a long-term, complex and gradual process, and the basement of the new platform is gradually consolidated, which is the end of cratonization.
The development of the in-situ trough in the Ruifen belt and other belts in the northwest and southwest of the platform ended in the Ruifen fold stage 65.438+0.9 billion years ago. Some researchers think that the Gothic stage is similar to the end of Phanerozoic geosyncline orogeny. However, other researchers believe that its cratonization lags behind the East Platform. At this stage, the vertically extending Gothic volcanic plutonic complex belt was formed in the western area of Rifen, which constituted the porphyry formation of Lower Yotni and its homologous magmatic granite body. Generally speaking, in the eastern European platform, the thick mature continental crust was finally formed at the end of Paleoproterozoic.