(A) the basic characteristics of Moho and deep faults in China.
The contour map of Moho surface in China (Figure 4- 1) clearly shows the general trend of crustal thickness. The crust thickness in the eastern region is 32-36km, the Moho depth in the Qinghai-Tibet Plateau is 73km (north Tibet) and 45-68km in south Tibet. From west to east, there are deep structural variation zones such as Helan Mountain-Liupanshan Mountain-Longmen Mountain, Daxing 'anling Mountain-Taihang Mountain-Wuling Mountain, which divide the deep structures in China into three zones: Qinghai-Tibet mantle flat zone, middle mantle flat zone and east mantle flat zone. In the western mantle plateau depression area (Qinghai-Tibet mantle plateau area), it is surrounded by an annular gravity gradient belt with great gradient and a width of about 200km. Its northern edge extends from the east of Kunlun Mountain, passes through the northern edge of Qaidam Basin along Altun Mountain and Qilian Mountain, and turns south to cross Longmen Mountain. Its east branch turns to the west along the west side of Wumeng Mountain, and its west branch turns to Gaoligong Mountain after passing through Daxue Mountain (Wang Maoji et al., 2008+0088). The seismic sounding data of Menyuan-Pingliang-Weinan (Zhang Shaoquan et al., 1985) show that the Moho surface of Weinan-Menyuan gradually deepens from 35.5km to 56.6km, Liupanshan is a steep mantle belt, which is gentle in the east and steep in the west, and the western edge of Ordos is accompanied by ultra-deep faults. The eastern mantle steep slope belt starts from Daxing 'anling in the north, passes through Wuling Mountain in Taihang Mountain in the south, and enters Vietnam through Baise, Guangxi, with a width of 80 ~ 100 km and a gravity gradient of 1mGal/km, forming a series of horst grabens, which control the sedimentary formation of the upper crust and plutonic magmatism. There are still sporadic basic-ultrabasic rocks such as kimberlite and picrite exposed along this structural belt, which can reach the mantle density.
Figure 4- 1 Relationship between Moho surface morphology and skarn ore belt in China
On the basis of the above three Moho surfaces, Wang Maoji and others (198 1) further divided nine districts according to the internal structural characteristics of each district.
On the basis of the above mantle morphology, giant fault zones or composite fault zones are often developed at the edge of each tectonic zone or at the intersection of two tectonic units, which belong to the ancient Asian fault system, the Pacific Rim fault system and the Tethys Himalayan fault system respectively. Skarn deposits and their ore-bearing magmatic formation are mainly controlled by the faults in the northern margin of China-Korea parapet, Polohoro-Middle Tianshan fault system and Kunlun-Qinling geosyncline, such as the northern Qilian-North Huaiyang fault system, the northern margin of Qaidam-Nanshan-North Qinling-North Huaiyang fault system and the eastern Qinling-South Qinling fault system, which control the development history of the geosyncline system in northern China, western Chyi Chin and the temporal and spatial distribution of ore-bearing magmatic formation. Among the eastern coastal Pacific fault systems, the Tancheng-Lujiang fault system, Daxing 'anling-Taihang-Wuling deep fault system and the southeast coastal fault system are particularly important. Yulong-Longmen fault system and Kangdian axis fault system in southwest China play an important role in controlling skarn deposits and porphyry-skarn composite deposits. The deep tectonic framework of China depends not only on the interaction among Pacific plate, Indian plate and Chinese mainland, but also on the gravity balance of the crust and mantle and the characteristics of the Eurasian plate creeping to the southeast since Mesozoic.
(2) structural types of magmatic rocks
According to the structural position, deep structural characteristics, crustal structure, basement type and magmatic rock combination of ore-bearing magmatic rocks, the magmatic rocks related to skarn deposits in China are preliminarily divided into the following structural types:
1. magmatic formation in the stable platform.
Magmatic formation, a quasi-platform of China and North Korea, is its representative. The Moho surface in this area is generally 30-40 kilometers deep, with an average of 35 kilometers, showing a platform shape. The crust thickens to the west and north and thins to the east and south. Moho isobath extends NNE-NE except the part adjacent to Yanliaotai fold belt. On the basis of the above general model, the undulating Moho surface formed local mantle uplift zone and mantle depression zone. The wave-shaped structural unit is 50 ~ 100km wide and about100 km long. The discontinuous points of Moho surface are mostly located at the inflection point of ups and downs, and Mesozoic magmatic rocks and deep fault zones are mostly located in the maximum gradient zone of crustal thickness, that is, the connecting transition zone between mantle uplift and depression. The above-mentioned deep structural characteristics of the Sino-Korean paraplatform determine that the main rock belts in this area are NNE, especially the Tanlu rock belt in the east and the Taihang rock belt in the central and western regions. The alkalinity of the crust in this area is 0.44 ~ 0.56. The basement is composed of metamorphic complexes with different crystallinity. Taiguyu-Yuanguyu is exposed in Jiaodong Group in eastern Shandong (12000~26000m ~ 26000m), Taishan Group in western Shandong (3700 ~ 2 1000 m), Fuping Group, Wutai Group and Hutuo Group in Taihang area, with a thickness of over 260000. From deep metamorphism to medium metamorphism, it is mainly composed of granulite, gneiss, amphibolite and migmatite, with schist and phyllite in the upper part. Primitive rocks are mostly basic-intermediate basic volcanic rocks and clay semi-clayey rocks, which mainly belong to the basement of intermediate basic volcanic rocks, but Jiaodong Group and Fenzishan Group are highly acidic.
The heat flow values in North China are relatively high, and the heat flow values in Mesozoic and Cenozoic subsidence areas are all above 1.5 hfu. Basic and ultrabasic rocks and eclogites are sporadically exposed along the Daxing 'anling-Taihang-Wuling deep structural variation zone and Yishu deep fault, and kimberlite cones (Mengyin) are found, indicating that geothermal activity and magmatic activity in this area are related to deep mantle activity and deep fault.
Magmatic rocks related to skarn deposits were mainly formed in Mesozoic, when the China-Korea paraplatform had entered the development stage of the continental margin active zone. There are Yishu rock belt, Taihang rock belt and Taiyuan-Linfen rock belt from east to west in this area. Magmatic rocks in these rock belts have low δ 18O values (7.7 ~ 9.8) and initial values of 87Sr/86Sr (0.7058 in Xi 'anli, Shaanxi). In the magmatic structure-magmatic assemblage map of Delaroche (Figure 6- 14), the trend lines of plutonic rocks in Yishu rock belt and Taihang rock belt are almost parallel to the trend of the source region, indicating that the composition changes of magmatic rocks in different intrusion stages in this area are the result of the orderly and gradual development of geotectonics, and the diagenetic materials mainly come from the lower crust and mantle and have weak assimilation to the upper acidic crustal materials.
2. Stabilize the magmatic formation in the depression zone at the edge of the platform.
Distributed in the northern and southern margins of the Sino-Korean paraplatform, including the Yanliaotai fold belt and Caledonian fold belt in the northern Qinling Mountains, it is the main molybdenum producing area in China. The Moho depth of Yanliaotai fold belt is 37 ~ 42 km. It is a mantle depression that decreases from southeast to northwest, and the Moho surface fluctuates, forming a secondary northeast long depression zone. On the national Moho isobath map, we can see that it is located in the deep structural variation zone of Daxing 'anling-Taihang-Wuling, and the deep structural line is distributed in an arc shape in this zone, from north to south, from north to south, and turns sharply to northeast or northeast. Most of the known ore-bearing magmatic rocks are distributed in the transition zone between uplift and depression or on one side of mantle depression. The results of seismic exploration in Shijiazhuang-Harqin Banner show that the crustal structure of Liao Yan subsidence zone is different from that of China-DPRK paraplatform. The thickness of the middle crust in Xinglong and Chengde areas can reach 16km, the low-velocity layer is about 5.5km/s, and Baoding and Shijiazhuang areas are about 12km. The development of middle and low velocity layers in the crust is often accompanied by high geothermal field and geothermal flow value, which can be regarded as an important feature of the active area of the crust.
The Qianxi Group and Dantazi Group of Middle and Lower Precambrian are widely exposed in the Yanliaotai fold belt. The lower protolith is mainly basic-neutral volcanic rocks, and the upper is clay rocks, which constitute the basement of basic-neutral volcanic rocks. However, a large area of mixed granite is developed in western Liaoning, with high molybdenum abundance. Basic-neutral volcanic rocks are relatively few. The Caledonian fold belt in North Qinling is an eastward extension of the Qilian Caledonian fold belt. The Moho surface in this area is 35 ~ 46 kilometers deep. Luanchuan and Lushi are mantle depressions that deepen to the west, and the axial direction is nearly east-west, with small mantle uplift and mantle depression. The Menyuan-Weinan seismic sounding profile in the west of this area shows that with the increase of crustal thickness from east to west, the Moho surface in Jinduicheng-Weinan-Pingliang area fluctuates greatly and faults develop. Jinduicheng, Weinan and Pingliang show relative mantle uplift, and the crust thickness is 45 ~ 50 km. And to Yongdeng, Menyuan area is more than 55 kilometers deep. According to the research data of Northwest Metallurgical Geological Exploration Company, the main molybdenum-bearing rock bodies and their deposits in East Qinling are located in the mantle depression area with the crust thickness less than 44 kilometers. The crust alkalinity in Jinduicheng, Weinan and Pingliang areas is low, which belongs to silicon-aluminum crust and increases westward. Taihua Group is the basement of this area, consisting of gneiss, migmatite, marble and amphibole, with a thickness of 3700 ~ 5000 m and an average molybdenum content of about 4. 1× 10-6. (Huang Jianjun et al., 1983). There are east-west crust-cutting faults in this area; Such as Chongningzhen-Jianshan fault, Luoyuan-Machaoying fault, Shangloucun-Miao Zi fault, especially Heigou-Luanchuan fault, largely controlled the directional distribution of Mesozoic ore-bearing magmatic formation. Ore-bearing magmatic rocks are rich in silicon and potassium and poor in calcium, magnesium, iron and sodium. The whole rock oxygen isotope δ 18O is 7.2 ~ 9.6, and the initial value of 87Sr/86Sr is 0.7034 ~ 0.708. In the structural-magmatic assemblage diagram of De La Roche (see Figure 6- 14), the curve is located in Zone 4, perpendicular to the source trend, indicating that the limited and continuous separation of basic components and the addition of acidic crust-derived substances occurred during magma crystallization. According to the analysis of Mesozoic magmatic formation in western Liaoning, we think that it originated from andesite magma and added more acidic crustal materials (Lin Wenwei, 1987). Qiao Huai-dong (1984) calculated the initial value of 87Sr/86Sr of the ore-bearing rock mass in western Henan with the mixed parameter formula of C.J.Alligre, and considered that the magma was composed of 57% ~ 76% upper mantle material and 24% ~ 43% crust material.
The above analysis shows that the Mesozoic magmatic formation, located in the northern and southern margin of the Sino-Korean paraplatform, has similar geotectonic and deep tectonic background, with high crustal acidity and high molybdenum content in Archean basement. Mesozoic magmatic rocks belong to the mixed source type of mantle and crust, and more crust-derived materials are added, which constitutes the metallogenic series of intermediate-acid magmatic rocks characterized by molybdenum.
3. magmatic formation is located in the depression zone between stable platforms.
This belt, represented by the fault depression belt in the middle and lower reaches of the Yangtze River, is an arc fault depression belt, connecting the North China Platform in the north and the South China Mantle Depression in the south. The Moho surface in this zone rises from west to east. The crust in Changshu-Qidong area is 28km thick, the first line from Changshu to Maanshan is about 32.5km, and the eastern part of Hubei is 30-30~33km. From Wuhan to Nanjing, a mantle arc uplift belt is formed, which is called the middle and lower Yangtze mantle uplift belt, and Mufu Mountain is on the north and south sides respectively. According to the data of Cao Luohua and Ge Zongxia, the apparent alkalinity of the crust in Wuning mantle uplift zone (JS) is >: 0.5 (Cao Luohua,1986; Cao Luohua, Ge Zongxia, 1987) belongs to the Si-Al-Fe-Mg crust, while Mufu Mountain and Dabie Mountain belong to JS.
From the late Indosinian cycle to the early Yanshan cycle, the Huaiyang block slipped southward, forming a structural pattern dominated by NE in the east and NW and EW in the west, which controlled the spatial distribution of Mesozoic magmatic rocks in this area. Most magmatic rocks in this area have δ 18O < 10, and the initial values of 87Sr/86Sr are 0.703 ~ 0.707.
In the structure-magma assemblage map of De La Roche (see Figure 6- 14), the evolution trend lines of Wushan, Chengmenshan and Tieshan are perpendicular to the strike of the source area, and it is speculated that they are derived from high-alumina basaltic or andesite magma. The aluminum enrichment of Wushan rock mass in the late stage may be related to the addition of K2O, Al2O3 and other components.
The rocks in the middle and lower reaches of the Yangtze River belong to light rare earth enrichment type, except for late intrusion, most of them have no europium anomaly or only weak negative europium anomaly, δ EU = 0.84 ~1.11,and some samples have slight cerium loss (δ Ce: 0.66 ~ 0.83). The REE distribution curve inclines to the right, which is similar to that of andesite.
4. magmatic formation in the acidic crustal fold belt.
The shape of Moho in South China and North China is quite different. On the national Moho isobath map, the Moho isobath in the north is mainly distributed in the northeast, while the Moho in the south outside the Nanyang-Hefei-Suzhou line is scattered and its linear characteristics tend to disappear. The Moho surface in the south of China is 28-38km deep and rises from north to south and from northwest to southeast coast. According to Yongping 1978 industrial blasting data, the average depth of Moho surface in South China is about 33km, and the depth of Kangshi surface is 2 1.46km. This area is divided into two sub-regions, the Moho surface in the north is complex, the mantle uplift zone is NE-NNE, and the crust is obviously thickened and thinned. The Moho isobath in the south area is east-west, with gentle uplift and depression and little fluctuation. The crust in the coastal zone of Guangdong is 27-30 km deep, with a maximum thickness difference of 5km, and the Kang's surface may be 13 ~. According to the morphological characteristics and spatial distribution of Moho surface, South China can be divided into the following deep structural units (Zhang Hongliang et al., 1985). From east to west, there are Pucheng-Yong 'an Mantle Depression, Poyang-Ganzhou Mantle Uplift, Xiushui-Guidong Mantle Depression, Dongting-Hengyang Mantle Uplift, Zangjiang-Yuanling Mantle Uplift, Xuefeng Mountain Mantle Depression and Nanling East-West Complex Structural Area.
The deep structural variation zone of Daxing 'anling-Taihang-Wuling passes through Hechi and Baise in the west of this area and enters Vietnam. Lujiang-Nanchang-Ji 'an-Guangzhou in the east is the boundary of gravity balance between the east and the west, and its abnormal balance is positive in the east and negative in the west. Wuchuan-Sihui fault passes through the southern end of this line. According to this, it may be the south extension of the Tan-Lu fault or one of its branches (Wang Maoji, as well as linear structures such as the southeast coastal mantle depression belt, Shangrao-Chongren mantle slope belt and Dean-Yongxing mantle slope belt, which controlled the formation of Mesozoic magma in South China to some extent.
The crust of South China is multi-layered and lateral inhomogeneity, and the low gravity area is south of Nanyang-Hefei-Suzhou line. Various genetic types of granite are developed in this area, and the distribution range of granite gradually increases from northwest to southeast, reflecting that South China is characterized by low-density granite crust. The lateral inhomogeneity between the upper mantle and the crust shows that the low-density crust thickens from west to east, the crust density under the basin increases, and there is a low-density upper mantle in the coastal volcanic area. This lateral variation of the crustal density of the upper mantle makes the Moho surface shallow in the southern and eastern coastal areas, accompanied by high geothermal field and heat flux. These deep structural features have a suitable relationship with the spatial distribution of Mesozoic granites, which proves that the Mesozoic magmatic formation in South China is controlled by the upper mantle morphology and crustal composition.
Deep faults are developed in the northeast of this area. From the southeast coast to the northwest, there are Changle-Nan 'ao fault, Lishui-Haifeng fault, Shaowu-Heyuan fault, Li Anping-Guangzhou-Siping fault, Sihui-Wuchuan fault and Guangxi Lingshan fault. There are Shaoxing-Jiangshan-Xinjiang fault and Yichun-Yongle fault in the northeast. Most of these deep faults are located in the steep mantle zone, such as Changle-Nan 'ao. Shaoxing-Jiangshan-Xinjiang fault zone is located in Shangrao-Chongren mantle slope zone. These steep mantle slopes and deep fault zones control the spatial distribution of granite in this area to a great extent, and form a continuous evolution series of in-situ granite-intrusive granite-high granite with the evolution of diagenetic age.
The fold geosyncline in South China is mainly composed of Sinian-Silurian, which is flysch and contains a small amount of carbonate rocks and volcanic rocks. In Yuanguyu, there are mainly shallow-sea clastic rocks of Bosi Group mixed with spilite porphyry, and shallow-sea flysch clastic rocks of Banxi Group mixed with volcanic rocks, which are mainly distributed in eastern Guizhou, southeastern Guizhou, north-southwest Guangxi and western Fujian, forming a silty-argillaceous basement. Gneiss, schists, amphibole schists, quartzites and granulites were found in Jian 'ou area of northern Fujian. There are many intermediate-basic volcanic rocks in the original rocks, which constitute the basement of intermediate-basic volcanic rocks. In a word, there are great differences in mantle morphology, crustal composition and basement types between South China and North China. South China is a silicon-aluminum crust dominated by granite, and the pre-Sinian system is dominated by silty mud, while the coastal areas of Fujian and Zhejiang become medium-basic volcanic basement. This deep tectonic background and the spatial changes of the crust and upper sedimentary metamorphic formations determine the spatial zoning of granite in South China. According to the deep structural characteristics of each rock belt, the following rock belts can be divided.
(1) NE tectonic magmatic belt
1) Zhejiang-Fujian-Guangdong coastal rock belt; It is distributed in the uplift area of the upper mantle east of Lishui-Haifeng fault. Gravity data show that there is an abnormally low density upper mantle in this area, and andesite-dacite-rhyolite volcanic rocks are distributed. The Mesozoic volcanic rocks in the coastal land have a width of 180 ~ 260km and an extension of 1200km, which is dominated by fault eruption, accompanied by granodiorite-biotite granite-homologous intrusion. There are relatively few minerals related to Mesozoic magmatic formation in this area, mainly Cu, Pb, Zn, Mo, Fe, W and Sn, and there is a changing trend from Cu, Pb, Zn and Mo to W and Sn from north to south, among which there are few skarn deposits, which may be related to the undeveloped carbonate rocks in this area.
2) Intracontinental NE-trending magmatic belt: this belt is located in the west of Lishui-Haifeng fault zone, which is a widely distributed area of Mesozoic magmatic rocks in South China. Among them, the granite between Mufushan-Yunkaishan and Wuyishan accounts for about 50% of the total area in this area, including Caledonian granite, Hercynian granite and Yanshanian granite. The Yanshanian granite is especially developed, including migmatized metasomatic granite, remelted granite of continental crust, mixed eutectic granite of crust and mantle and a small amount of mantle-derived magmatic rocks. Remelted granite is the main body. Intrusive rocks are mainly distributed in the NE-trending linear structural belt. From north to southeast, it is the granite belt at the junction of Hunan and Guangxi, the granite belt from Wandashan to Darongshan, the migmatite granite belt from Yunkaishan to Wuyishan, the Xinxing-Du Yu granite belt and the Baoan-Sanming granite belt. From northwest to southeast, the formation age of rock mass has new signs of change, and its scale is getting bigger and bigger, which is often a compound rock mass that has been invaded many times.
(2) East-west magmatic rocks in the acidic crust.
Located between 24 and 26 north latitude, it is the transitional part between the northeast mantle tectonic belt and the east-west structure of Guangdong and Guangxi in this area. Due to the joint action of NE-trending and EW-trending structures, the end of the northeast mantle tectonic belt on the northern edge of the belt was broken into smaller mantle uplift and mantle depression, which extends to the west of Hechi in the west and to Anyuan Shanghang area in the east. Mesozoic magmatic rocks are extremely developed in this area, accompanied by extremely rich rare, rare earth, non-ferrous and other polymetallic minerals. From north to south, there are Tashan-Gushan rock belt, Daoxian (Jiuling)-Huichang-Xianyou rock belt, Dadongshan-Jiuyishan-Quanzhou rock belt, Huashan-Gushan-Fogang-Xiamen rock belt. Generally speaking, from west to east, the age of rock mass becomes new and its scale increases. According to geological and geophysical data, remelted granites and migmatized granites in the continental crust of South China are dominated by batholiths and have the characteristics of multi-stage diagenesis. In-situ-semi-in-situ granite and high-grade intrusive granite have a transitional relationship in a series of geological and geochemical characteristics, and are controlled by deep fault zones. They are characterized by aluminum supersaturation, rich in silicon, rich in alkali and poor in basic components. Mineralization is mainly rare earth, colored (W, Sn) and U, and the oxygen isotope δ 18O of magmatic rocks is 9.5 ~ 13.5, 87sr/86sr >: 0.71.
According to the inversion results of more than 50 gravity anomalies, it is considered that the variation range of gravity anomalies of granite in South China is 30mGal, the thickness of rock mass is 5 ~ 15 km, and a few of them reach 20km. For example, the thickness of Jiuling bedrock is 7km, Yaogang fairy rock mass is1~12 km, and Shizhuyuan rock mass is/kloc.
The mineralization in this area is complex and there are many types of mineralization. Chen Yuchuan et al. (1985) divided Nanling area into five metallogenic series. Skarn-type deposits mainly belong to the metallogenic series of rare earth nonferrous and polymetallic deposits related to Yanshanian intermediate hypabyssal acidic granite, such as Shizhuyuan, Xianghualing, Huangshaping, Dachang and Yaogangxian, which are closely related to crust remelting granite series and Yanshanian hypabyssal-ultrahypabyssal intermediate acidic granite, such as copper, lead, zinc, tungsten, molybdenum and silver. The gold and uranium deposit series are mostly magmatic, pegmatite, porphyry and hydrothermal metal mineralization, and the ore-forming mother rocks are mainly granodiorite porphyry, syenite porphyry, dacite porphyry and rhyolite. The initial value of 87Sr/86Sr in the rock mass is less than 0.7 10, which belongs to mixed source granite of crust and mantle. Its representative skarn deposits are Shuikoushan and Tongshankou. Skarn deposits are commonly found in iron-tin-molybdenum (lead, zinc) metallogenic series related to acid magma intrusion, such as Makeng, Pantian, Luoyang and Datian in Fujian, Dading, Tieshangzhang and Jianshan in Guangdong. The magmatic rocks and their deposits in the latter two areas are mainly concentrated in the steep mantle slope zone or the transition zone between mantle uplift and depression, and spatially distributed along the Shangrao-Chongren mantle slope zone (Shangrao-Qianshan fault zone), Wuchuan-Sihui fault zone and Lishui-Haifeng fault zone.
The crust of South China is mainly characterized by the enrichment of rare, rare earth and radioactive elements, and the abundance of U and th is high, which is much higher than the average content of U and Th in crustal granite. Most intrusive rocks rich in U and th are multi-stage complex rocks with large exposed area, which indicates that the South China crust may have a high background content of U, Th and K, which may be one of the important factors of the high geothermal field in the South China crust.
5. magmatic formation in geosyncline fold belt.
This kind of magmatic formation belongs to synorogenic and post-orogenic granitoids, which were formed in geosynclinal fold systems, including Tianshan fold system, Inner Mongolia fold system, Jihei fold system and Kunlun-Qilian fold system.
Northeast China is a large-scale mantle uplift zone. Songliao and Lower Liaohe Plain are the axes of mantle uplift. Moho surface inclines to the east and west, showing a trend of steep east and slow west. In the west, the Moho surface of Daxinganling falls from 34 km to 39 km from east to west, forming a mantle slope belt extending from northeast to east. A series of positive and negative structures developed in the eastern part of Songliao Plain, forming a secondary uplift and depression zone. There are two main deep structural change zones in the area, one is distributed along Suihua, Shenyang-Changchun, which is the northern extension of the Tan-Lu fault. The Yulanhaote variation zone in Butha Banner is actually a part of Daxing 'anling-Taihang-Wulingshan structural zone, and there are also a series of NE-trending faults. They control the distribution of magmatic rocks in this area. According to engineering blasting exploration in southern Liaoning, the crust in southern Liaoning is thick 13 ~ 16 km, and the apparent alkalinity of the crust is 0.40 ~ 0.43. There is a lack of corresponding information in the north. The Precambrian in the west of this area is mainly schist containing staurolite, sillimanite and garnet, plagioclase schist with gneiss angle and marble. It constitutes the basement of intermediate-basic volcanic rocks and clay rocks. The eastern part is mainly carbonate rocks and medium-deep metamorphic rocks, with exposed Mashan Group, Heilongjiang Group and Chenming Group. It belongs to carbonate-sandy argillaceous basement (Jiang et al., 1982).
The granite of Xingkai and Caledonian periods, especially Hercynian granite, before exposure in this area is closely related to iron, copper, lead and zinc. In the early Hercynian period, the intensity of magmatic activity was weak. In the middle and late Hercynian period, granites were distributed on the ridge and both sides of Daxing 'anling Mountains, and the neogranite belt moved to the southeast with time, belonging to magmatic formation with geosyncline fold belt. There are late Hercynian granites in Xiaoxing 'anling, Zhangguangcailing and Taipingling, which are produced in excellent geosynclinal environment. There are biotite granite, plagiogranite, granodiorite, Baigangshi granite and granite porphyry.
At the end of Paleozoic, the development stage of geosyncline ended and entered the development stage of coastal Pacific tectonic domain dominated by fault block movement. Yanshanian magmatic rocks were controlled by Yilan-Yitong fault zone and Mishan-Dunhua-Fushun fault zone respectively, and shallow rock mass was formed in the southern margin of Nenjiang fault.
Tianshan fold system and Kunlun-Qinling fold system are controlled by Tianshan mantle trough, Altun-Qilian mantle slope and northern margin of Kunlun Mountain respectively. A series of deep and large faults developed along the above-mentioned crustal thickness variation zone, such as Altun-Beishan fault, northern and southern margin faults of the Middle Tianshan Mountains, Bayankala fault and northern margin faults of Jishishan, etc., which are mainly compressive and shear, with widespread hot springs, frequent earthquakes and active neotectonic movement. According to the Menyuan-Weinan seismic sounding profile, the alkalinity of the crust is estimated to be about 0.43. The Precambrian in Tianshan-Subei is mainly composed of medium-shallow metamorphic carbonate rocks and clastic rocks, with a small amount of neutral volcanic rocks, and the thickness exceeds15000 m m. The upper Cambrian in Qilian Mountain has a huge sedimentary thickness, consisting of metamorphic clastic rocks and carbonate rocks, and there are many intermediate-basic volcanic rocks. They all developed into geosynclines on the basis of China ancient platform, starting from Xingkaixuan cycle and gradually transformed into fold areas through Caledonian and Hercynian cycles, during which geosynclinal sedimentary formations were formed, accompanied by large-scale basic-acid volcanic activities and plutonic magma intrusion, and most of them were formed in the best geosyncline environment. Magmatic rocks related to skarn deposits include gabbro, diorite, albite granite porphyry and granite. Hercynian granite in Tianshan fold system is the most developed, concentrated in the central uplift belt and Beishan fold belt, belonging to calc-alkaline rock series, reflecting the evolution law from basic to acidic to alkaline from early to late, and the rocks are relatively rich in Na2O (Regional Geological Survey Brigade of Xinjiang Geological Bureau, 1985). Caledonian granites of the East Kunlun-Qilian Fold System are mainly distributed in the Middle Qilian and Hexi Corridor, and the intrusion time is divided into three stages, namely granodiorite, plagiogranite, quartz diorite and granite. There were many biotite granites in the late Caledonian, and there were potash feldspar granite, quartz diorite, diabase and pegmatite in the Hercynian. Indosinian intrusive rocks are concentrated in Nanshan and Dulan, Qinghai, and the lithology is biotite granite and monzogranite. Skarn deposits are related to Indosinian-Yanshanian intrusive rocks and belong to post-orogenic intrusive rocks.
In a word, the magmatic formation in the geosyncline fold belt was mainly formed in the geosyncline development stage, mostly magmatic rocks in the same orogenic period, but also intrusive rocks in the post-orogenic period and fault block activity period. Most of them occur in the deep structural variation zone, and intrusive rocks are accompanied by corresponding extrusive rocks, which are mostly formed in the compressive and shear tectonic environment. The small rock mass in the later stage may be the product of extensional conditions.
(3) Preliminary understanding of the relationship between deep structure and magmatic activity and mineralization.
1) Due to the interaction between Indian plate, Pacific plate and Eurasian plate and the gravity balance within the continent (including lateral balance and basin-mountain action), the shape of the upper mantle in China is higher in the east and lower in the west. The subduction of the Pacific plate and the northward reduction of the Tethys oceanic crust led to the warping of the upper mantle in the southeast coast and southern Guangdong. The northward differential drift of the Jurassic-Cretaceous Pacific plate (Zhang Yongxia, 1983) is the main reason for the development of NE-trending structures in eastern China.
2) Two important deep structural variation zones in China divide the deep structure in China into three regions. Magmatic activity and endogenetic metal mineralization are mostly distributed in the eastern and western regions, and intermediate-acid magmatic rocks are widely distributed in the eastern region (upper mantle uplift region), most of which are Yanshanian products. In the coastal upper mantle uplift area, Mesozoic and Cenozoic volcanic activities of basic-neutral-alkaline rocks were extensive and intense, with high geothermal field and geothermal flow, and mantle-derived inclusions were found in many places. Metal mineralization is mainly iron, copper, lead, zinc, tungsten, tin, molybdenum, uranium, thorium, rare and rare earth. Magmatic activities in the western region are produced in geosyncline fold belt, which is the product of synorogeny and post-orogeny, and are mostly distributed in the upper geosyncline. The spatial distribution of the rock belt is consistent with the regional tectonic line, and the mineralization is mainly in Caledonian and Hercynian, which is mainly related to the mineralization of iron, copper, nickel, lead and zinc.
3) The main magmatic rocks are distributed along the steep mantle zone, and the magmatic rocks in the eastern part of China are generally distributed in the thin crust zone, while the secondary mantle uplift zone controls the spatial distribution of the rock zone or rock mass, mostly on the side of the transition zone between uplift and depression. The intersection of two deep structural belts in different directions (such as Nanling) is particularly rich in minerals, and the steep upper mantle belt is often consistent with regional deep faults.
4) The magmatic formation and metallogenetic assemblage are controlled by the morphological characteristics of the upper mantle, crustal structure and composition (alkalinity and basement type) to some extent. Generally, the higher the alkalinity of the crust and basement, the magmatic rocks and related minerals are mostly the element combination of partial basic magma field, and the rock mass is obviously controlled by deep faults. Acidic-super acidic rocks and related minerals are mostly developed in acidic crust and sandy argillaceous basement, and remelted granite is widely distributed. South China and North China are very different in these aspects, so they have different metallogenic characteristics and metallogenic combinations.
5) The lateral inhomogeneity of crust and mantle components is one of the important reasons for the development of crustal movement, which leads to the compensation of gravity balance and the relative movement of crust and mantle materials. Compared with the platform area, the thickness and vertical and horizontal changes of the crust in the active zone are great, and there are low-speed and high-conductivity layers in the crust, which are developed in relatively stable areas, such as Yanliaotai Fold Belt, Maanshan-Changshu-Qidong, Menyuan-Weinan and Qinghai-Tibet Plateau.
6) Granites in South China are rich in uranium and thorium, and rocks are rich in potassium. Most of them belong to crust-derived remelted granite. Therefore, it can be considered that the granite layer in the crust of South China has high radioactive elements such as U, Th and 40K, and this radioactive energy is an important factor in the formation of geothermal field in South China. In addition, mantle thermodynamics can form extensive molten magma, leading to remelting granite and its mineralization combination.
7) crust-mantle interaction and regional geochemical field are important factors that restrict the formation of regional magma and its metallogenic assemblage. Diagenesis and mineralization are the result of crust-mantle interaction, material and energy migration and transformation, and also profoundly affect the characteristics of regional geochemical field. The interaction between crust and mantle is the relative migration of crust and mantle materials under the action of gravity, including the upwelling of mantle materials along the structural weak zone, invading the upper crust and loading it, resulting in the readjustment of gravity and the corresponding change of Moho surface morphology; Gravity compensation of lateral crustal materials (basin-mountain action); The lower crust enters the upper mantle along the deep fault, forming a mantle-crust mixed zone, and the corresponding physical and chemical changes may occur in the mantle at high temperature. When the fault continues to move, it may lead to the selective remelting of crust and mantle materials, forming a mixed magma rich in silicon and alkali. Therefore, the crust-mantle mixed magma may come from the mixing and melting of crust-mantle materials, the mixing of mantle materials with acidic materials in the crust, and even the high assimilation during the magma upwelling. In this way, the mixed magma contains a wide range of crust-mantle composition characteristics and ore-forming element combinations. Therefore, the regional geochemical field depends to a great extent on the heterogeneity of crust and mantle components.