I. Overview
Different schools have different understandings of the tectonic units in this area. To sum up, they mainly include:
Li Siguang (1925 ~ 1955) based on the limited geological data obtained at that time, from the viewpoint of geomechanics, determined Qilian Mountain as the west wing of the mountain-shaped structural system of Qilu River, in which Lanzhou, Baiyin and Wuwei are part of the horseshoe-shaped shield of the mountain-A Ning shield, and Alashan area is the arc-shaped structural belt on the south side of the A Ning shield. Gansu Provincial Bureau of Geology and Mineral Resources (1989) further concretized the above ideas, and pointed out that Qilian Mountain can be further divided into: the fold belt in the west wing before arc and the reflex arc in the west wing. There are various structural systems in Qilian Mountain and Longshou Mountain, mainly including the Paleo-Hexi system, the west wing of Qilu River, the zigzag of Qinghai-Tibet (mainly distributed in Qilian Mountain and its south side), the Longxi system and the Alashan arc structure, as well as latitudinal and latitudinal structures.
Under the guidance of the "trough-platform" theory, as early as 1945, Huang briefly discussed the scope and geological structural characteristics of Alashan (Longshou Mountain) and Qilian Mountain in his book "Main Geological Structural Units in China", and thought that the former was a part of the Sino-Korean platform and a link between the Tarim platform and the North China platform, while the latter was a typical dominant geosyncline. After a lapse of 10 years, he divided the Qilian Mountains from south to north into: Nanshan geosyncline sedimentary belt, Nanshan geosyncline marginal sedimentary belt and piedmont depression sedimentary belt, and pointed out that the Mesozoic-Cenozoic basins in Qilian Mountains had the hope of finding oil. 10 years later, he again divided this area into: Alashan Tailong Corridor Transition Zone, North Qilian Mountain Fold Zone, Qilian Middle Uplift Zone, South Qilian Mountain Fold Zone, and Qilian Southern Edge Transition Zone. It is considered that the main folding period of North Qilian Mountain is Caledonian rather than Variscan. In the same year, it was pointed out that the North Qilian Mountain is an excellent geosyncline fold belt and the South Qilian Mountain is an geosyncline fold belt, both of which have the characteristics of multi-cycle structural development. At the end of 1970s, Huang et al. re-divided the area into Alashan uplift (a part of China-North Korea paraplatform), corridor transition zone, Qilian Mountain piedmont depression, North Qilian excellent geosyncline fold zone, Middle Qilian uplift zone and South Qilian fold zone. Based on the theory of multi-cycle trough platform structure, this paper makes a comprehensive and systematic discussion on the tectonic framework and its evolution of Qilian Mountain. Because of absorbing the theory of plate tectonics, their above division and discussion played a role in establishing models at that time and still have great reference value today.
According to the theory of geosyncline and the viewpoint of historical development and evolution, Tuguangchi also divided this area into Alashan uplift belt, corridor depression belt, northern Qilian Caledonian fold belt, middle Qilian Precambrian fold belt, southern Qilian Early Paleozoic-Mesozoic (or early Paleozoic-Triassic) depression belt, southern Qilian Caledonian fold belt and Indosinian fold belt (or
Scholars who hold fault block theory, such as Zhao Shenggui (1996), divide this area into Alashan Block, Longshoushan Fault Uplift, Hexi Corridor Basin, Yongchang Zhongning Continental Margin Fault Depression Belt, North Qilian Rift, Middle Qilian Fault Uplift, South Qilian Fault Depression and Northern Qaidam Fault Uplift. He put forward that Qilian area is dominated by silicon-aluminum orogeny (A-type subduction), while northern Qilian area is dominated by intermediate-basic volcanism in the early Early Paleozoic, and intermediate-acidic volcanism in the later period. Most volcanic rocks belong to calc-alkaline series, and only spilite porphyry series develops locally, which has certain reference value.
In the early 1970s, Yin Zanxun, Li Chunyu and Fu Chengyi took the lead in introducing plate tectonics theory into China. It was also Li Chunyu who took the lead in applying plate tectonic theory to study the history of geotectonic and geological development in this area, and divided the area into Alashan uplift belt, North Qilian fold belt (including corridor belt), Middle Qilian uplift belt and South Qilian fold belt. It is considered that North Qilian is an early Paleozoic ocean basin with complete ophiolite and blue color. 1982, based on the global structure of Asia, he thought that Qilian Mountain was actually a suture zone between China-Korea plate and Yangtze plate. Under his inspiration and guidance, many scholars began to set foot in this field. Quan Wang (1976) wrote that there are ancient oceans in this area; Xiao Xuchang (1978) has done a lot of field research in this area, and proposed that there are many stages of ophiolite in this area. Wu Hanquan (1980) studied the high-pressure metamorphic belt in North Qilian Mountains. Zhang Zhimeng (1980) proposed for the first time that there is a gully-arc basin system in Qilian Mountain, and the corridor is equivalent to a back-arc basin. Xia et al. (199 1) made an in-depth study of volcanic rocks in this area, which further proved that there was a trench-arc basin system in this area and it was subducted northward. Zuo (1986, 1987) thinks that North Qilian belongs to a "limited ocean basin"; Xu Zhiqin (1994) put forward a dynamic model of island arc proliferation in this area. Zhang Qi et al. (1997) systematically studied the ophiolite in North Qilian, and pointed out that the ophiolite in this area has diversity. Feng Yimin has also studied the Qilian Mountains since 1970s, and thinks that the Middle Qilian Mountains and Qaidam belong to the same plate, and there are two sets of rift-plate tectonic systems in the southwest margin of North China ancient land in the early Paleozoic. The orogenic belt is divided into subduction orogeny, collision orogeny and intracontinental orogeny, which is a composite orogenic belt. Tang Zhongli, Li and Xiong (1995) conducted a study on the Jinchang-Menyuan geoscience section in this area. They believe that the early Longshoushan fault is a low-angle normal fault, which links the formation and metallogenic model of super-large and large deposits with the tectonic background.
The above research work is the basis of our work.
Second, the tectonic framework of the southwest margin of the North China Plate.
From the viewpoint of historical evolution, guided by activity theory and system theory, based on the most basic geological facts such as sedimentary formation, magmatism, tectonism and mineralization in this area, and on the basis of a large number of previous research results, we systematically, comprehensively, multilaterally and comprehensively discussed the scope, nature, evolution of each tectonic unit in this area and its coupling relationship with the metallogenic system. Outline the Caledonian tectonic framework in this area (Figure 1- 1). From north to south, the tectonic units are: Longshoushan continental margin belt, Hexi corridor margin basin, northern Qilian suture belt, central Qilian discrete island arc terrane, southern Qilian back-arc basin and Qaidam block. Their main features are summarized as follows:
Figure 1- 1 Structural framework and metallogenic system in the southwest margin of ancient North China continent
(a) The continental margin of Longshou Mountain
It is bounded by Chaoshui Basin in the north, Hexi Corridor of Longshoushan deep fault in the south, Jinta-Ding Xin fault in the west, Tarim plate in the east and Yinchuan in the west, with a slightly east-west arc distribution. The main rock unit in this area is the former Great Wall Jilongshan Group (AnChL). Because of its age, the rock group suffered from multi-stage and multi-cycle metamorphism and deformation in the later period, and became fragmented and unclear in sequence, belonging to non-Smith strata. The lower part is Baijiazuizi Formation, and there are magnetite quartzite and magnetite amphibole in the west, which is called Dongdashan Formation. Original stone reduction (Tang Zhongli, Li,1995; Wang Chongli, 1994), the protolith formation is equivalent to volcanic-sedimentary formation, Baijiazuizi Formation is carbonate formation and basic volcanic formation, and Dongdashan Formation is clastic magnetite quartzite and basic volcanic formation. Considering that the basic volcanic rocks (amphibolite) are layered, which represents the earliest record of magmatic activity in this area, the measured age of 3056Ma(Sm-Nd method, average, Wang Chongli, 1994) should belong to the middle Archean eruption, indicating that the Longshoushan rock group has the nature of ancient continental nucleus. In addition, the age of metamorphic diabase dikes that cut through migmatite and were bent and deformed is (2486 16) Ma ~ (2796 56) Ma, with an average of 2600Ma (Wang Chongli, 1994), which is a proof of the Archean era. The REE distribution pattern of the rocks in the lower part of Longshoushan Group also shows that they belong to Archean products, because the REE distribution curve of Neoarchean rocks is much to the right (see discussion later). The upper part of Longshoushan rock group is mainly composed of intermediate-acid volcanic-clastic rocks, and its age is (2147 74) Ma (Rb-Sr isochron method, Northwest Geological Survey, 1993), which is equivalent to Paleoproterozoic. The Great Wall period deposits are missing in this zone, and the Jixian period deposits are complex continental clastic metastable deposits (Dunzigou Group), and the Qingbaikou period deposits are missing. Sinian deposits are glacial water deposits and carbonate-clastic deposits (Hanmushan Group), and the bottom of carbonate-clastic rocks contains phosphorus. The early Paleozoic was a uplift and denudation area, the late Paleozoic was a clastic rock-carbonate (coal-bearing) formation and molasses formation, and the Mesozoic and Cenozoic was a fluvial and piedmont clastic rock (molasses) formation. Caledonian and Variscan granites are also developed in this area, which should be the products of northward subduction of Qilian Ocean and its sub-ocean and intracontinental A-type subduction orogeny respectively (the azimuth mentioned here is only modern azimuth, which does not represent geological historical azimuth, the same below).
Dongdashan Iron Mine was formed in the active belt of the Middle-New Archean ancient continental margin. In the early Mesoproterozoic, Jinchuan ultrabasic complex containing nickel and copper was formed under cracking conditions. The (1508 3 1) Ma (SM-ND internal isochron method) age obtained by Tang Zhongli (1995) should be the time when the above magma left the mantle.
(2) marginal basin of Hexi Corridor
At present, the basin on the edge of Hexi Corridor is very narrow, with a nearly east-west zonal distribution. If we can consider the history of its formation and evolution, it is not difficult to infer that it was a vast continental margin basin at that time (Caledonian). The basement of continental margin basin is the southern extension of North China ancient land. The Cambrian continental side (north side) is a shallow shelf clastic rock unit (Dahuangshan Formation D) with flysch characteristics. Among them, there are wave-like scouring surfaces, imprints and staggered layers, and carbonate rocks and volcanic rocks (Heicigou Formation H) appear far from the continental margin, reflecting the characteristics of seawater deepening from north to south and increasing geological activities. The general trend of Ordovician is to inherit the characteristics of CAMBRIAN volcanic-sedimentary construction, but the difference is that volcanic activity in the basin is more intense and mature, and expanding oceanic crust-ophiolite appears in some areas (Tiger Mountain and Yushugou Mountain). The Silurian basin shrank, and graptolite clastic facies (Dirigou Formation Sa), sandstone shale formation (Xia Han Formation Sh) and Devonian Deyu Formation (Laojunshan Conglomerate) were formed in the later stage, which marked the disappearance of the basin and the beginning of orogeny.
(3) North Qilian suture zone
As a suture zone between Qaidam-Middle Qilian Plate and North China Plate, the North Qilian Orogenic Belt was first proposed by Li Chunyu and accepted by everyone. But the question is, is this suture zone the product of the opening and closing of the North China plate and the Qaidam plate (Tethys orogenic belt), or the result of the collision of the Qaidam-Central Qilian plate or Caledonian plate (Cordillera orogenic belt)? Is there a native ocean (Qilian Ocean) outside the small ocean basin in northern Qilian? The problem is not limited to this. As a suture zone, the North Qilian orogenic belt itself is complex and diverse, which is different from the east and the west, and there are many micro-landmasses in the west. So where did these micro-landmasses come from? What is its formation mechanism? The opinions of various families are also inconsistent. According to the principle of model comparison, we preliminarily think that the above-mentioned micro-landmasses and the middle Qilian micro-landmasses are the result of the southward subduction of Qilian Ocean, which led to their separation from the edge of Qaidam landmass. Its tectonic framework is like a multi-island tectonic scene in the southwest Pacific Ocean.
In short, although the internal structural units of the North Qilian suture zone are complex and diverse, they are mainly composed of the following units: micro-landmasses, melange belts, ophiolite, ridge island volcanic rocks, island arc (including continental margin arc and oceanic crust island arc) volcanic rocks and island arc deposits (Figure 1-2). The micro-landmass is mainly composed of Precambrian strata with Caledonian island arc granite on it. The specific lithology of the former is gray gneiss, greisen schist, marble (Anqiu of Beidahe Formation and Anqiu of Yemanshan Formation), extremely thick ophiolite melange (Aoyougou Formation), phyllite, clastic rock, locally mixed with carbonate rock, metamorphic sandstone and iron ore layer (Huashugou Formation Chh). In addition, the formation of mottled clastic rocks and carbonate rocks of Mesoproterozoic Tolaishan Group (Ch-JxT) and Qingbaikou Gongcha Group (QnG) are also seen. Two ophiolite melange belts in the north and south. The northern belt is mainly distributed in NW-SE direction along Jiugequan in Sunan and Bai Quan Gate, and extends to the Hanshan area in Changma, Yumen. The southern belt is large, mainly distributed in Magou-Qingshuigou-Xiangzigou-Guomisi-Qilian County-Jingyang Lingnan, with a width of about 20-25 km from north to south and an intermittent extension of nearly 500km, showing NW-SE direction. Eclogite was found in Qingshuigou, and the age of associated blueschist was 440 ~ 460 Ma (glaucophane, polycrystalline muscovite, 39Ar/40Ar method), and the age of blueschist in Jiuquan in the north was 447Ma (Wu Hanquan, unpublished data). The northern belt is called low-grade blue schist belt, and the southern belt is called high-grade blue schist belt. Ophiolite melange belt is mainly composed of continental margin arc (southern belt), oceanic crust island arc (northern belt), flysch accretionary wedge, high-grade (southern belt) and low-grade (northern belt) blueschist and ophiolite belt. There are three ophiolite belts in the suture zone, from south to north: Yushigou-Chuanlagou-Xiaobaibao ophiolite belt; Dacha Daban ophiolite (belt); Jiugequan-Bai Quan Gate Ophiolite Belt; The age of the above three ophiolite belts is Caledonian, while the age of ophiolite belts distributed in micro-landmasses is Mesoproterozoic. Caledonian ophiolite mostly has the characteristics of oceanic ridge or oceanic island basalt, and some of them coexist with boehmite, such as Dachadaban ophiolite (Zhang Qi, 1997). It is speculated that ophiolite was formed in ocean islands and arc basins. Considering that deep-sea sediments such as siliceous rocks in the North Qilian orogenic belt are abundant and volcanism is strong, ophiolite and blueschist structures are superimposed on accreted deep-sea sediments and volcanic arcs, ophiolite should belong to Cordillera type. The volcanic rocks in the continental margin arc and early sedimentary period (Neoproterozoic-Middle Cambrian) are equivalent to the Heilagou Formation, and the volcanic rocks in the continental margin arc (island arc rift type) are mainly distributed in Baiyin, Qingshuigou, Bai Liu Gully, Heishigou, Xiaoheilagou and Mianjian Gully. Because it developed in the weak zone above the ancient continental basement in North China, a series of continental alkaline basalts began to form. With the deepening of the division of continental margin arc basement, overflow forms the main body of marine basic volcanic rocks in this area, while in Baiyin and other places, because of the thick crust and the slow rise of basic magma, the lower crust melts deeply and produces silicon-rich magma. This silicon-rich magma first rises to form acidic volcanic rocks, and then the lower basic magma rises to form basic magma with the upper strata, forming a bimodal combination. These acidic volcanic rocks are the direct surrounding rocks of Baiyinchang copper and polymetallic massive sulfide deposits.
Figure 1-2 Schematic diagram of the internal structure of the suture zone between North China Plate and Qaidam-Central Qilian Plate (modified according to Zhang Qi 1997)
1- Precambrian; 2- Blue schist zone; 3- Ophiolite; 4- Alaska type rock mass; 5- peridotite-diorite body; 6- Early gully ophiolite; 7- Continental margin arc; 8—— oceanic crust island arc. No, ① ~ ⑨ is ophiolite: ①-Jiuquan; (2)- big fork and big board; ③—Bianmagou; ④ Yushigou; ⑤—Ice ditch; 6-Little Babel; ⑦-Bai Jing Temple; 8- Hushan; Pet-name ruby-Yushugou Mountain. A ~ G is Alaska-type rock mass: A- Salah rock mass; B- Youhulu Mountain; C— Zhamashigou; D— south of the ice ditch; East-Shuidong Gorge; Mu Bai Canyon; G- big beach; Laohushan peridotite-diorite type rock mass
In the early continental margin, island arc deposits were dominated by pyroclastic materials, in addition, island arc slope facies gravity flow, slump deposits and island arc flysch were also seen, and no river and lake facies deposits in the early rift valley were found. In the middle and late stage, the oceanic crust-type island arc volcanic rocks and island arc deposits are equivalent to some sewer groups (OY) and Bao Zhongqun (OZ), starting from Baiyin North in the east, passing through Shihuigou in Yongdeng County and Minle County West Road in the west, ending at Altun Fault, and distributed along Nanshan Corridor in the west. Island arc volcanic rocks are mainly tholeiite, calc-alkaline basalt, andesite (molecule of Yingou Group) and island arc alkaline olivine trachyte, trachyte, albite square boiling rock and albite trachyte (molecule of Bao Zhongqun). It shows that the Middle Ordovician island arc has matured. Island arc deposits are mainly composed of pyroclastic rocks, sedimentary rocks and algal limestone.
(4) Discrete island arc terrane in the central Qilian Mountains.
The discrete island arc terrane in the central part of the study area is NW-SE banded, starting from the eastern part of Lanzhou in the east, passing through Minhe, Ledu, Xining, Huangyuan and Shuleshan in Qinghai in the west, and also ending at Altun Fault, bounded by the northern margin fault of the central Qilian Mountains in the north, and adjacent to the back-arc basin by the southern margin fault of the central Qilian Mountains in the south. It is 70 ~ 80 km wide and about 1000km long. It is mainly manifested in the widespread development of Jinning and Caledonian intermediate acid magmatic rocks on the ancient basement, which are related to copper, tungsten, molybdenum, lead and zinc minerals.
(5) South Qilian back-arc basin
After the formal separation of the central Qilian island arc and the Qaidam plate in the middle and late Caledonian, the southern Qilian back-arc basin was formed, which was mainly Silurian volcano-normal deposit, with a large number of intermediate-basic volcanic eruptions in the west and Cambrian (Ordovician) ophiolite in the east.
(6) Qaidam block
Only Da Kenda Ban Group is scattered.