Impact metamorphism (translated as "impact metamorphism" and "meteorite metamorphism" in recent years) is a new field of understanding since the 1960s. It is distributed near the crater and deteriorated under the strong shock wave of the meteorite hitting the surface. Instantaneous (1μ s ~ 1s) conditions of high pressure (up to several hundred gigapascals) and high temperature (up to > 1500℃) are the controlling factors. Deformation and accompanying partial melting are the main metamorphic mechanisms. Judging from the metamorphic factors, impact metamorphism is a kind of metamorphism under extreme conditions, and its temperature and pressure range and its comparison with normal metamorphism are shown in Figure 24-9. It can be seen from the figure that the temperature of impact metamorphism increases with the pressure, and the rock melts or even gasifies under the influence of high temperature. The typical impact metamorphic rock is garnet, which is a kind of lava-like breccia. Instantaneous high pressure leads to the appearance of deformation lines and bands, and even the appearance of ultra-high pressure time variants such as Shikeying and Shi Si Ying. Instantaneous high temperature makes feldspar and timely melt to form glass, and biotite darkens. Metamorphic rocks are similar to dynamic metamorphism because of their metamorphic factors, and sometimes they are classified as dynamic metamorphism (for example, Raymond, 1995). But it is not the result of the earth's internal forces, and it is more developed on the surface of stars such as the moon, Mars and its satellites. For example, on the surface of the moon, there are more than 33,000 craters with a diameter greater than 1km, accounting for 7% ~ 10% of the surface area of the moon, and there are countless smaller craters. Therefore, more generally speaking, impact metamorphism is the metamorphism that an asteroid or comet impacts a planet and produces on its surface, which can be called extraterrestrial metamorphism (Mason & Sang, 2007). This section only discusses the impact metamorphism that occurred on the earth.

Figure 24-9 Range of meteorite impact metamorphism and P-T range of various meteorite impact effects (according to French, 2003, quoted from You Zhendong and Liu Rong, 2008).

(2) the distribution of craters on the earth

The crater on the earth is a special annular geological structure formed on the surface of an asteroid or comet hitting the earth. Unlike the moon, Mars and other planets, there is an atmosphere with a thickness of about 1000km on the earth's surface, which makes the falling small celestial bodies burn up due to strong friction after entering the atmosphere. This is what is usually called a meteor. Only a large individual can crash the earth's surface into a pit. In addition, in the long history of geological evolution, the earth's surface has been affected by supergene and plate movement, so craters with older geological history are often destroyed by weathering erosion and tectonic action. Many craters are buried under the sedimentary layer in the late stage of meteorite impact, which is difficult to be found. According to the data of the Earth ImpactDatabase, the number of meteorite impact craters found on the earth so far (up to July 20 10, July 2010) is 176, including 7 in Africa/kloc-0 and 29 in Asia and Russia (Figure 24-/kloc-0 Among these craters, the oldest is Suav Jarvi Crater in Russia (about 2400Ma), and the youngest is Xihaote Alin Crater in Russia, only 63 years old. The largest crater is Vredefort crater in South Africa, with a diameter of 300km, and the smallest crater is Haviland crater in Kansas, USA, with a diameter of only 0.0 15km. These craters are distributed in 32 countries in the world. Although Chinese scholars have conducted a series of investigations on craters in China since 1980s, they have found a number of ring-shaped geological structures suspected of craters, such as Baisha in Hainan, Taihu Lake in Jiangsu, Fanshan Basin in Zhuolu, Hebei, Kowloon in Hong Kong, Duolun in Inner Mongolia and luoquan in Xiuyan, Liaoning, but unfortunately, none of them can be selected into the international crater database. For a long time, the main reason for the lack of breakthrough in the study of craters in China is that there is no key evidence to confirm the cause of meteorite impact (Chen Ming, 2007). In 2009, after scientific drilling, mixed deposits of impact metamorphic rocks were found in Xiuyan meteorite impact crater lake deposit in Liaoning Province, with the thickness of 107m, including multi-phase angle conglomerate and melt, impact molten glass and timely impact foliation pdf (Chen Ming et al., 2009). Thankfully, the research results of Xiuyan crater by Chen Ming et al. (20 10) were recently published in American magazine EPSL, and Xiuyan crater became the first internationally recognized crater in China, which was a breakthrough in the research field of impact metamorphism in China. However, we still have a long way to go in the study of impact metamorphism. Experience has proved that the determination of many large-scale meteorite impact structures has gone through repeated exploration for decades. For example, Vredefort (crater) in South Africa, 1937 was suggested to be the cause of meteorite impact, which was not confirmed until 1962. In view of the present situation of meteorite impact structure research in China, starting from the known data, strengthening the comprehensive research in the areas where clues have been found, and intensifying the research on petrology, tectonics, remote sensing geology and geophysics will surely bring more discoveries in a short time (You Zhendong, Liu Rong, 2008).

Figure 24- 10 Distribution Map of Impact Structures in Asia and Russia (according to the Earth Impact Database, Asia and Russia, July 10)

(3) Identification mark of meteorite impact structure

Because after the crater is formed, it will often be eroded and structurally destroyed in the later period. To find the meteorite impact structure on the present surface requires multidisciplinary exploration. It is necessary to combine petrology, remote sensing geology and geophysical methods for effective search. Geologically speaking, there are mainly the following identification marks.

1. Morphology and structure of crater

You can see the shape of the young crater from the aerial photos. Its main feature is that the projectile sequence around the crater with annular uplift is opposite to the original stratigraphic sequence of meteorite hitting the crater; Sometimes it is accompanied by secondary pits caused by the bombardment of giant debris. According to the shape and structure of craters, craters can be roughly divided into simple type and complex type (Figure 24- 1 1).

◎ Simple crater: the depth-diameter ratio is 1/5 ~ 1/7, which is shallow bowl-shaped and has the crater edge formed by ejecta. The deposition range of ejecta is twice the diameter of impact crater, and the particle size of ejecta decreases with the distance from the edge of impact crater. A typical example is Balinger Crater in Arizona, USA (Figure 24- 12), with an impact age of 49,000 years, a diameter of1.12km and a depth of1200m ... Xiuyan Crater in China is a simple bowl-shaped crater.

◎ Complex crater: The diameter of complex crater varies according to the geological conditions of the target area. If the target area is sedimentary strata, the pit diameter is > 2 km; If the target area is a crystalline rock development area, the pit diameter is > 4 km. The aspect ratio is very small, about1/20 ~110. Structure of complex crater: There is often a central uplift at the bottom of crater, and there are collapses and faults around crater. There are two reasons for the central uplift: first, the rocks in the target area at the bottom of the pit rebound due to decompression; The second is caused by the collapse of materials after the excavation of the impact pit. Complex meteorite impact craters often induce magmatic activity, and the detached breccia layer is often covered by lava. Typical example: sudbury giant meteorite impact crater (Figure 24- 13). Its diameter is 140km, and its area is 15000km2, including the whole sudbury igneous complex (SIC) and the bottom breccia formed by the fracture of floor rocks. The impact structure of Soderberg meteorite is located at the interface between Proterozoic Huron Supergroup and Archean basement. Archean basement rocks in the north and east, Proterozoic supracrustal rocks in the south of SIC. Meteorite breccia, mudstone and composite sandstone (Baishui Formation) are all covered on SIC. The impact cone, the structural symbol of meteorite impact, can be found in the whole range of 17 km around SIC. There are meteorite impact breccia, pseudobasaltic glass and other impact metamorphic rocks around SIC, which obviously belong to the outer ring of meteorite impact structure. In many places, meteorite-struck breccia has also become the surrounding rock of copper-nickel PGE (platinum group elements) deposits.

Figure 24- 1 1 Simple crater (a) and complex crater (b) (according to Hamilton 20065438+0; Cited by Zhen Dong and Liu Rong, 2008)

Petrology (Second Edition)

2. Meteorite fragments

Meteorite fragments can often be found in young craters. Collecting sediment samples in and along the mine pit, carefully elutriating and analyzing the heavy components in them may find more accurate standards, such as iron-nickel chondrite.

3. Impact cone

The impact cone is also called the crushing cone. There is an obvious striped cone structure on the fracture surface of the rock hit by meteorites. The length of the stripes varies from less than 1cm to several meters, and the stripes spread from the top of the cone to the flank in a ponytail shape (Figure 24- 14). The appearance of shock cone indicates that the pressure of shock wave can reach 2 ~ 25 GPA. Rocks subjected to nuclear explosions also have shock cones caused by shock waves. By systematically measuring and calculating the direction of the top of the shock cone, the center of shock wave emission can be determined

4. Plane deformation characteristics (pdf)

Impact surface deformation structure, also translated as impact surface foliation. It is characterized by the appearance of many small flakes in mineral particles such as quartz and feldspar (Figure 24- 15A), and the refractive index and birefringence of minerals are generally reduced, and some of them have even changed into amorphous bodies; In the case of strong impact, the long-range ordered crystal structure is destroyed, and lattice non-parallel domains or mosaic structures appear, which also shows wave extinction under polarizer. However, unlike the general structural stress, its spectral line widens in the X-ray diffraction sheet and appears star-awning phenomenon (You Zhendong, Liu Rong, 2008).

Pdf developed in time and feldspar, the rock-forming mineral of impact crater, are symbolic criteria for judging meteorite impact crater. FDFs is a special mineral structure produced by impact, and it is a dynamic high-pressure deformation microstructure in island and shelf silicate minerals such as Yanshi and feldspar. Mineral fdf usually occurs along a specific crystal direction (e.g. {10 1N}, n = 1 ~ 4), and flaky flakes are evenly distributed and arranged in parallel in the mineral, and the thickness of a single flake is less than1μ m. The timely fdf can be composed of the following microstructures: high-density dislocation zones. Except for artificial nuclear explosion and large-scale impact in nature, any other geological processes, including volcanic eruption, tectonic movement and high temperature and high pressure in the deep earth, can not produce this special plane deformation structure in minerals, so pdf is the decisive criterion for judging craters. The characteristics of timely fdf found in Xiuyan ring structure in China are very typical (Figure 24- 15B), which are the same as those revealed in other known meteorite craters in the world, thus providing definite evidence for determining the cause of meteorite impact in this crater (Chen Ming, 2007). It should be pointed out that pdf is easily confused with "deformation line" or "micro-foliation" in metamorphic rocks. This deformation feature is called plane fracture (PFs), which is obviously different from pdf. The width between PFs is generally greater than 5 ~ 10μ m, with uneven distribution and parallel to specific crystal planes, such as {000 1} or1kloc-0/}. PFs is usually the result of long-term slow high-pressure metamorphism, and the formation pressure is much lower than pdf. Pdf is the product of instantaneous high-pressure impact. Therefore, PFs can not be used as a conclusive basis for judging the impact crater.

Fig. 24- 13 sudbury Crater (according to Reimold, 2005).

Fig. 24- 14 seismic fracture cone in fine limestone of Horton impact structure, Canada

5. Transformation of mineral phase

Under the extreme conditions of impact metamorphism, a series of ultra-high temperature and ultra-high pressure minerals can appear (see Figure 24- 16).

◎ Coesite: Coesite is commonly found in ultrahigh-pressure metamorphic rocks, but it was first discovered in nature by Zhao Jingde (Chao, 1967) in a crater in Arizona, USA. Recently, coesite was also found in the timely impact glass that was strongly impacted by the Riess crater in Germany. In impact metamorphic rocks, coesite often appears in other silica phases in the form of particles. Because the residual heat after impact metamorphism is still as high as several hundred degrees Celsius, coesite originally formed under high pressure can easily degenerate into tridymite and Shi Ying.

Fig. 24- 15 pdf of time in impact rock

◎ Timing: Its formation pressure is higher than that of coesite, about P > 10 GPA, and most of them are fine particles and coesite in impact metamorphic rocks. High aging density (4.35) and high refractive index. The characteristic X-ray diffraction peak D = 2.96, 1.53, which is a relatively simple detection method. It is also easy to deteriorate in time and difficult to preserve, so it is difficult to find.

◎ Lecatellierite: This is a kind of silicate glass, and its forming temperature is extremely high (up to 17 10℃), which is higher than the melt ejected by ordinary volcanoes. It is famous for its Libyan desert glass (LDG) in North Africa. This glass structure is similar to schlieren, in which Shi Jiao Ying and baddeleyite are the remnants of atmospheric metamorphic molten minerals. The origin of Libyan desert glass has always been controversial. Recently, two craters (BP and Oasis) were found near the border between Libya and Egypt through satellite photos, which supported the cause of the impact.

Baddeleyite: monoclinic ZrO2 _ 2 is the product of thermal decomposition of zircon (ZrSO4 _ 4 _ 4);

Petrology (Second Edition)

In the process of impact metamorphism, zircon is wrongly interpreted as an aggregate of baddeleyite and amorphous silica. In order to keep the original crystal morphology of zircon, it is necessary to cut a series of polished pieces of impact metamorphic rock samples containing zircon and identify them by using the strong reflectivity of baddeleyite.

In addition, Fe-Ni pellets, ilmenite drops, rutile and pseudobrookite can also be found in impact glass or rock, indicating that their formation temperature should be above 1500℃.

◎ Mineral melting: timely and feldspar are selectively or completely transformed into solid silicate glass or plagioclase glass, also known as molten glass or molten feldspar. However, the dark minerals associated with it are still crystalline. There is no reaction between adjacent minerals. Both Ke Shi Ying and Ying Shi, which are formed under extremely high pressure, appear as fine inclusions in the glass matrix.

6. Impact glass

Impact glass is a kind of high-density glass with the same composition as the original rock. Oxide minerals such as magnetite in the original rock are completely melted. This kind of high-density glass is strong evidence that rocks have undergone meteorite impact metamorphism (You Zhendong, Liu Rong, 2008).

It must be emphasized here that it is far from enough to distinguish craters only from geomorphological features, and it often leads to misjudgment, which is also the main reason why many reports about suspected craters in China have not been confirmed. This is a misunderstanding. Many scholars often study craters from the macro topography, because it is the most intuitive and easy to observe. In fact, the key evidence for judging craters mainly comes from microscopic rock mineralogical characteristics, such as pdf, high-pressure mineral facies, impact glass and so on. They are decisive standards, because the traces left by shock waves in the short-term impact process can be well preserved and are not easily transformed by subsequent geological processes; The macroscopic geomorphological features are secondary and play an auxiliary role in proving. There are many geological processes on the earth that can produce annular landforms, and the characteristics of annular landforms cannot be used as a decisive criterion for judging craters. The most typical example is the Richat structure in the sub-saharan desert of Mauritania, Africa (Figure 24- 16a), which is in line with the other two craters (Figure 24- 16b). Although its shape is very similar to the crater, it is confirmed by investigation that Richat structure is not an impact structure, but a special landform formed by the uplift of weathering and denudation strata on the surface. For those craters buried deep below the surface, it is determined to be complete by geophysical (earthquake, gravity) data, which needs to be further verified by deep drilling sampling.

Fig. 24- 16 richat structure (according to Mattonet et al., 2005)

(4) Rock types of impact metamorphic rocks

Impact metamorphic rocks include stony breccia, meteorite impact breccia and pseudobasaltic glass.

◎ Debris breccia: It mainly refers to the breccia broken to varying degrees under the influence of shock wave in the impact pit and below its bottom. The composition of breccia is mostly quasi-in-situ target rock, and the matrix composition is target rock debris. It is one of the impact metamorphic rocks with the lowest metamorphic degree, which is different from other genetic breccia by its occurrence.

◎ Flint: breccia cemented by impact glass. There are a wide range of projectiles, from filling the pit to the edge of the pit. The composition of breccia can be quasi-in-situ or filled in different places. In addition to debris, the matrix also contains impact glass, which can be subdivided into fused breccia, meteorite impact breccia and impact fused breccia. (Figure 24- 17).

Pseudo-basaltic glass: Pseudo-basaltic glass can also be produced in general tectonic deformation rocks. Pseudo-basaltic glass, as an impact melt, often contains the residue of impact deformed minerals, and its occurrence scale varies greatly, which can be millimeter-level, centimeter-level veinlets or irregular filling matrix. It can be seen from the impact granite slice (Figure 24- 18A) in Ries crater, Germany, that feldspar remains in the pseudo-basaltic glass matrix. Occasionally, there are dozens of meters thick pseudo-basalt glass, and its cause remains to be discussed. They are usually filled between cracks in broken rocks with textured or irregular fillers (Figure 24- 18B).

Fig. 24- 17 Impact breccia of German Ries crater (provided by R.Mason, cited by You Zhendong and Liu Rong in 2008)

Figure 24- 18 Pseudobasalt Glass Caused by Impact

(E) the research significance of the influence structure

If an iron meteorite with a diameter of 1km (assuming its density is 8.0g/cm3) hits the ground at a speed of 25km/s, its kinetic energy is e =1/2mv2 =1.31×102/kloc. This kinetic energy is equivalent to the explosion energy of 3.12×101t TNT explosive. In 2004, the energy of the Indonesia earthquake with M8.9 was only 184× 10 16J. Therefore, the impact of giant meteorites on the earth is a major catastrophic event, which will inevitably affect the geological processes, environmental changes and biological evolution inside and outside the earth. For example, the Chicxulubo collision in Mexico was the "culprit" of the extinction of dinosaurs and many species at the end of Mesozoic (Sharpton et al.,1992); The above-mentioned sudbury-induced magmatic activities in Canada formed the famous sudbury magmatic complex SIC (see Figure 24- 13). The study of impact structure and earth evolution has become a new starting point of earth science in 2 1 century, involving a series of new basic problems about the origin and evolution of the earth. For example, the impact period, the relationship between impact and dynamic action in the earth, the change of geomagnetic and earth axis in geological history, the origin of magma, the formation of continental crust, the evidence and influence of huge impact events in geological history, the impact projection modes on different planets in the solar system and the new global tectonic view of the earth expansion theory (Qin Gongjiong et al., 200 1).

What deserves special attention is the economic value of impact structures. Almost all meteorite impact structures found on the earth have certain economic value (Reimold, 2007). Vredefort Witwatersrand in South Africa and Shodeberry in Ontario, Canada are both famous metal deposit areas with a development history of over 100 years. Vredefort is famous for its copper-uranium deposits, which Sudbury has always regarded as magmatic copper-nickel deposits. Only in 196 1 year, Dietz RS wrote in his paper Vre de Fortring structure: meteorite impact scar? It is formally proposed that the Voldford dome belongs to the meteorite impact structure. The following year, he pointed out that sudbury was also a meteorite impact crater. His view was confirmed by later discoveries. In recent years, it has been found that many meteorite impact structures are related to oil and gas reservoirs. For example, Steen River (9 1 7 Ma) in Alberta, Canada is a potentially huge oil and gas reservoir. Ames, Oklahoma, USA, Madeira Mountains, Texas, USA (< 100 Ma), etc. Are trying to produce oil. It is estimated that the hydrocarbon resources of the North American meteorite impact structure can provide an annual output value of $5 billion to $6,543.8+$0.6 billion. Other meteorite impact structures are the locations of nonmetallic mineral resources, such as the meteorite impact diamond (Vishnevsky,1997) in the Popigai impact structure of the Anabar shield in Siberia, Russia; The impact crater of Nordlingenlis meteorite in southern Germany has greater economic value. It not only found meteorites, but also its meteorite impact breccia is a good building material. In addition, the crater of the Rhys meteorite has now become a tourist attraction.

Avak impact structure

According to Reimold et al. (2005), Avaka structure (Figure 24- 19) is located in the Arctic coastal plain of Alaska, which is considered as the cause of impact by Kirchner et al. (1992). He also described the impact cone and the time-dependent plane deformation structure. According to the stratigraphic sequence data, the age of this structure is100 5 Ma, and its diameter is about 12km. It is a complex impact structure with annular groove and central uplift. The central uplift was drilled by Avak well, and the strata encountered ranged from Lower Cretaceous to Ordovician. This well also shows oil, but it has no commercial value. However, near the impact structure, there are three main natural gas fields, namely Sikulik, East Barrow and South Barrow, which all appear and pass through the ring structure and are considered to be related to the impact event. According to grive & masa itis( 1994), the shovel fault at the edge of the crater cut off the early Cretaceous Barrow sandstone and juxtaposed it with the early Cretaceous Torok shale, thus forming an effective gas seal. South Barrow and East Barrow gas fields have been developed. Lantz( 198 1) preliminarily estimated that the recoverable reserves of natural gas in this structure were 370× 108ft3.

Fig. 24- 19 structural map of superimposed impact structural belt of Avaka oil and gas field in Alaska