It is generally believed that 3.8 billion years ago, the earth had not formed a sufficiently stable silicon-aluminum shell tectonic environment. This understanding stems from theoretical inference and comparison with the moon. Both the earth and the moon were formed about 4.6 billion years ago or 4.5 billion years ago. They are formed by the cooling and condensation of interstellar dust. Later, due to the release of gravitational potential energy and radioactive heat energy, it experienced total or partial melting, and the original crust appeared about 4.2 billion years ago. Due to the impact of asteroids and meteorites, in the first few hundred million years of the moon's formation, the outside was completely melted by hundreds of kilometers (deep). About 4.2 billion years ago, when the shell cooled down, the impact continued to cover its surface with craters. Between 3.7 billion and 3.5 billion years, due to the decay of radioactive elements, basaltic magma melted on a large scale at a depth of 200 kilometers, overflowing upward to fill the crater, thus forming the moon sea basalt observed today. Because the impact rate of meteorites on the moon dropped rapidly after 3.9 billion years, the lunar shell formed by the above actions can be preserved to this day. It is speculated that the earth experienced a similar process in the early stage, but the geological process in the later stage destroyed the early heritage.
No matter how the earth condenses, the original crust should be a siliceous magnesium crust like the moon, and the original siliceous aluminum crust was not preserved until about 3.8 billion years ago. It is generally believed that Proterozoic (2.5 billion years) is the period of the formation of the ancient continental nucleus. Since then, the crust has shown both marginal proliferation around the ancient land core and interspersed transformation, that is, a considerable number of new land margins have been reconstructed from the ancient land core.
2. The discovery of ancient continental nuclei in Xinjiang
The formation of Xinjiang ancient land core was also 2.5 billion years ago. The Archaean orogeny in Sweden and Finland (2.5 billion years ago) is called the Svekfen Movement, and it is generally believed that Pangea I was formed after this orogeny. Roughly the same as the first Pan-continent proposed by Academician Wang Hongzhen (1990) was formed 2.5 billion years ago. The unconformity age of Tiejiashan granite under Anshan Group in Northeast China is 2.97 billion years, and the age of Donganshan granite is about 2.99 billion years. In the south of Tsinger, Kuruktag, Xinjiang, 2.85 billion years of flesh-red gneiss-like mica granite was found in Archean gray gneiss. It is considered that this 2.8 billion-year data may be the earliest record of regional metamorphism in the Archean geological history of this area (2.5 billion years ago). These tectonic-thermal events, which occurred in Neoarchean (2.8 billion to 2.5 billion years), further consolidated Archean strata and formed the oldest continental core in the crust. Later, Lu Songnian and others discovered magmatic activities about 3.6 billion years ago in Altun Mountain, which are all evidence of the formation of ancient continental nuclei in Xinjiang.
3. The proliferation of ancient land core-the formation of primitive ancient land.
Around the ancient land core, the crust continues to proliferate and expand, developing into a primitive ancient land.
Basic concepts of crustal movement. Matter is always in motion, there is no matter that does not move, and there is no matter that does not move. The same is true of the earth's crust, which has been moving, destroyed, proliferated and developed, sometimes violently (suddenly) and sometimes slowly (gradually). This movement is called crustal movement. In a broad sense, it refers to all physical and chemical movements of materials in the crust, including deformation, metamorphism and magmatic activity of the crust; In a narrow sense, it is mainly the formation of crustal uplift, depression and various structural deformations (folds, faults, etc.). ) and magmatic activity caused by the internal force of the earth. Geologists often call the violent (abrupt) period of crustal movement tectonic movement, and some scholars call this event tectonic-thermal event.
The formation of primitive ancient land in Xinjiang. From Neoarchean (2.8 billion ~ 2.5 billion years) to Paleoproterozoic (2.5 billion ~/kloc-0.80 billion years), there were many crustal mutations in China, such as Fuping movement in 2.5 billion years, Wutai movement in 2.4 billion years, Zhongtiaoshan (Lvliang) movement in/kloc-0.80 billion years, accompanied by corresponding magma intrusion. The age of blue Shi Ying gneissic plagiogranite and quartz diorite in Kuruktag area of Xinjiang is 2.487 billion years, which seems to be equivalent to Fuping and Wutai movements. When Xinjiang called Tsinger Movement, Proterozoic strata unconformity covered Archean strata, and the ancient land core further consolidated, proliferated and expanded, and gradually developed into primitive land.
Geologists have visited, explored and pursued the face of Xinjiang, which has been far away from us for a long time, again and again in extremely difficult environment. In the vast desert mountains, in the complicated and fragmented geological relics, we are looking for clues that can reflect the past face of Xinjiang. They finally made a new discovery, and initially confirmed that in the Proterozoic (65.438+0.8 billion ~ 2.5 billion years), there were two important metamorphism and crustal proliferation events in Xinjiang, one in 2.2 billion ~ 2.3 billion years. The evidence is that the Sm-Nd model age of Proterozoic granite in Kuokesu area is mostly between 2.2 billion and 2.3 billion years, indicating that crust-mantle differentiation events occurred. The other occurred between 2 billion years and 65.438+0.8 billion years, which had great regional metamorphism. The Rb-Sr isochron age of gneiss in this area is 2 billion-65.438+0.8 billion years, zircon U-Pb age is 2.076-54.38+0 billion years, and the Rb-Sr isochron age of gray gneiss is 2 billion-65.438+0.8 billion years, which can confirm this conclusion. Later, the U-Pb age of about 65.438+0.9 billion years obtained by the National 305 Project during the Ninth Five-Year Plan in Hongweizhuang Granite Gneiss invading the Bulake Complex in Tuo Gerakl also supported the above conclusion. These reflect that in the Paleoproterozoic (2.5 billion ~ 65.438+0.8 billion years), the crust continued to proliferate and developed into a primitive ancient land. Due to the long geological history and strong structural changes in Archean-Proterozoic, the original structure has basically disappeared. Based on these scattered information, we can only infer that the so-called primitive ancient land may be a part of Pangea I and an ancient basement of Xinjiang mainland.
4. Cracking of primitive ancient land
The so-called "long-term separation must be combined, long-term separation must be combined" may be a manifestation of material movement.
Many of these ancient and primitive ancient landmasses in Xinjiang have been submerged by the vast sea again and again. Geologists have found a set of Mesoproterozoic-Neoproterozoic stable deposits composed of limestone, magnesium carbonate and single continental debris containing stromatolite fossils in Yili-Sailimu block, covering the metamorphic basement of Proterozoic, which can be called an ancient land submerged by seawater. According to the analysis of stromatolite fossils and lithologic characteristics, it belongs to coastal-shallow sea environment deposition. Similar situations have been found in many places in Xinjiang, which proves that Xinjiang was Wang Yang at that time. Although they have disappeared without a trace now, the remains of geological history have left ironclad evidence for them. The other is active deposition, which reflects that the ancient land was completely torn and formed the ocean. Geologists call a special rock assemblage representing the marine environment ophiolite suite. Although these oceans, which once divided the land into Qian Qian meters or even wider, have disappeared, people can always trace their appearance through the fragments of ophiolite or ophiolite melange left by marine extinction.
In Mesoproterozoic (65.438+0.8 billion ~ 65.438+0 billion years ago), Xinjiang mainland cracked in many places, and some of them developed from rift valleys to oceans, dividing the original unified mainland into pieces. The most obvious two oceans are the Proterozoic South Tianshan Ocean and the Proterozoic Kunlun-Altun Ocean. The former is located in the northern margin of Tarim (present position). From the end of Paleoproterozoic (65.438+08 billion years ago) to the early Mesoproterozoic (65.438+08 billion years later), the mainland began to split and formed a rift valley. The rift valley further extended to form the ocean, represented by the blue schist of Aksu Group in 65.438+66.3 million years and the albite schist and ophiolite fragments of glaucophane in Changjiawuzou in 65.438+57 million years, extending eastward to Beishan area (present position), and the seawater gradually became shallow, forming a semi-closed basin. During the Mesoproterozoic-Neoproterozoic, the Proterozoic South Tianshan Ocean shrank and subducted northward, forming a continental margin active belt around the Middle Tianshan Mountains. At the end of Proterozoic, the vast Proterozoic South Tianshan Ocean disappeared and closed, which made the Yili block in the north and the Guttari wood block in the south merge again.
Proterozoic Kunlun-Al Yang Kim is located in the southern margin of Tarim (present position). Mesoproterozoic (65.438+0.8 billion ~ 65.438+0.0 billion years) inherited Proterozoic activities and crustal extension and detachment. The spilite-keratophyre formation was discovered in the early Mesoproterozoic Great Wall system, reflecting the formation of crustal extension, and the strong bimodal volcanic eruption extended eastward to Altun Mountain and early (early) Jixian County.
The Altun Mountain Movement at the end of Jixian County (65.438+0 billion ~ 65.438+0.4 billion years) was an extremely important Proterozoic convergence collision movement, which closed the Proterozoic Kunlun-Al Yang Kim, caused strong deformation of the Mesoproterozoic strata (65.438+0 billion ~ 65.438+0.8 billion years), caused greenschist facies metamorphism, and at the same time was in the orogeny. The stable sedimentary unconformity of metamorphic shallow sea-coastal shallow sea facies covers the underlying stratum. At this time, the scope of transgression in Xinjiang has not only been greatly reduced, but also the sea water has become shallow. The majestic ocean of the past has disappeared without a trace. West Kunlun, East Tianshan, North Altai, Qimantage and Southwest Tarim all rose to land and suffered denudation. This is the formation period of the global joint old land-Rodinia supercontinent (figure 1-9- 1).
5. The formation and division of Rodinia supercontinent.
The ancients said that "the blind man touches the elephant". Touching the elephant's leg makes it feel like a stick, and another one touches the elephant's ear and says that the elephant is like a fan. In fact, an elephant is neither a stick nor a fan. An elephant is an elephant. Therefore, when we explore the characteristics of Xinjiang's geotectonics and its development and evolution, we must combine with the global tectonic facies, otherwise we will become frogs at the bottom of the well, and it is impossible to understand the true face of Xinjiang's land development and evolution. After all, Xinjiang mainland is an inseparable part of the global continent. Relatively speaking, no matter how small it is, its development and evolution must be controlled by the development and evolution of global structure.
Since the 1990s, with the establishment of the global concept of Greenville orogeny, McManamin (1990) first put forward the concept of Rodinia United continent, which refers to the post-continent formed in the early Neoproterozoic, and later other scholars constantly improved it in practice. Hoffman (199 1) and Australian scholar Li Zhengxiang (1995) think that the Rhodinia United continent was formed in the late Greenville orogenic event10/100 million years ago, and Dalzell (1992) studied the Early Cambrian. However, according to paleoclimate, paleoglaciers, paleomagnetism, isotopic age and other data, most scholars believe that continental disintegration may have occurred 700 million years ago.
In 1980s, Soviet scholar юааааааааааааааааааааааааааа10 Later research continuously enriched and developed these viewpoints, and speculated that after Yi Shi's eastward movement, a vast area including Sayanling and Mongolian Plateau was formed.
As mentioned above, when the Proterozoic South Tianshan Ocean and Proterozoic Kunlun-Al-Yang Kim formed by the cracking of the primitive ancient land in Xinjiang disappeared and closed, except for some uplifted ancient land, Xinjiang was almost submerged by coastal sea or land surface sea, and the vast ocean no longer existed. This land-sea change movement occurred about 654.38+00 billion years ago, which is called Altun Mountain Movement in Xinjiang, because it is very obvious that the stable shallow-sea facies deposits are not integrated on the underlying metamorphic rock series.
In recent years, there are similar reports in Altai area (Zhang Chuanlin et al., 2003), that is, the deep metamorphic Paleoproterozoic strata are covered by Sinian unconformity, with slight metamorphism or no metamorphism. It is speculated that the unconformity may represent a strong tectonic event from 1 100 million to 800 million years, and it can be roughly compared with the Greenville movement.
According to the research of Lu Songnian et al. (1998), it is considered that there were two eclogite and granite gneiss belts in Beishan, Liuyuan and northern margin of Qaidam in Gansu at the end of Proterozoic. The single zircon U-Pb age of Liuyuan granite gneiss is 880 million years, and that of eclogite is 860 million years. The granite gneiss in the northern margin of Qaidam is 950-870 million years old. The age of pegmatite in the same tectonic period in Daquangou, Dunhuang is 965,438+03 million years. According to the analysis of the rocks formed by these collisions, it is shown that the Tarim, North China and Qaidam blocks once converged to form a unified continent in the early Neoproterozoic, including the Yangtze block. Ren Jishun is called the ancient China platform, which may be a part of the global United ancient land Rodinia.
From the point of view that the South China-Sinian moraine and the phosphate-bearing deposits at the bottom of Cambrian are widely distributed in Xinjiang, South China, North China, Central Asia and Siberia, we have to admit that at the end of Proterozoic, there was indeed a joint ancient land-Luoding Niagu land-including Central Asia and East Asia. Many scientists believe that shallow carbonate deposits, large phosphate deposits and glacial deposits from the late Neoproterozoic (about 850 million years) to the early Paleozoic (about 520 million years) were widely covered on the Precambrian craton, such as Siberia, South China, India, Kazakhstan, Xinjiang, Mongolia and so on. Similar situations were found in North America, West Africa and Australia, reflecting that they were all in low latitudes at that time, and that this was a vast passive continental margin.
From Nanhuajian (880-680 million years) to Sinian (680-540 million years), the Niagu land in Luoding began to crack. Three glacial periods (Beiyixi, Telaiken and Hangel Qiaoke) can be seen in Kuruktag area of Xinjiang, accompanied by alkaline volcanic eruption in thick moraine clastic rocks with moraine gravel layer, which is a sign that the ancient land is cracking to form a rift valley.
The noun (short for noun) EylsC.Young( 1994) holds that every time the supercontinent breaks up, there will be a large-scale glacier activity. For example, the Early Proterozoic Ice Age represented by the Lake Huron Ice Age was distributed in Canada, South Africa, Australia and other places in North America. After the formation of Rodinia supercontinent, rifting began about 800-600 million years ago, which led to three ice ages. The Studt Glacier and Malino Glacier in South Australia are equivalent to Chang 'an Glacier and Beiyi Glacier, Nantuo Glacier and Trieken Glacier in China Yangtze Block and Tarim Block, respectively, while the luoquan Glacier or Hangel Jock Glacier may be the third Glacier, which is equivalent to the conglomerate layer at the bottom of Pound quartzite in Australia. It shows that the Yangtze-Tarim block in China may be spliced with the southeast pole of Australia in Neoproterozoic and close to North America (Zhang Huimin, 1993). Hlaskova (1988) pointed out that 620-570 million years was the peak of the disintegration of the "Rodinia" ancient land, and a rift system was formed in East Gondwana. Part of this rift remained at the edge of the present Kazakhstan-North Tianshan landmass, forming a graben combination-moral rock-like and moraine-like bimodal volcanic activity, similar to Kuru, Xinjiang. It is believed that this rifting caused the Tarim, Searle River-Wulutao-North Tianshan (Kazakhstan) and Junggar-Balkhash blocks to split from the edge of East Gondwana, and then partially formed an ocean basin in the early CAMBRIAN about 530 million years ago, completely separated from East Gondwana and drifted northward. The 620 million-year-old ophiolite melange belt was discovered in Hongguleleng, northern Junggar, Xinjiang, indicating that it opened earlier. However, from the 480-500 million-year-old ophiolite discovered in Tangbale, western Junggar, the 490-million-year-old ophiolite in arman, eastern Junggar and the 420-million-year-old ophiolite in South Tianshan Mountain, it can be seen that the Ordovician (480-440 million years) was the main period of marine formation and development.