I. Stratigraphic sequence and lithostratigraphy
1. South Qinling tectonic belt
The strata in the southern Qinling tectonic belt are well developed, and the basement consists of Douling Group and Maotang Group (Xixia-Xichuan area of Henan Province), Wudang Group and Yaolinghe Group (Wudang Mountain area of northwest Hubei Province), or Yunxi Group and Yaolinghe Group (Ankang area of Shaanxi Province). The caprock includes Sinian-Cretaceous.
Douling Group is composed of biotite plagioclase gneiss, sericite quartz schist, graphite schist and marble, with terrigenous clastic rock-argillaceous rock-marl and a small amount of plagioclase amphibole, which is a shallow-sea volcanic-sedimentary formation. The metamorphic degree reaches amphibolite facies and local migmatization. Douling Group is not integrated under Maotang Group (Yaoyingzhai Formation), with a layer of metamorphic conglomerate in between.
Yaoyingzhai Formation, Wudang Group and Yunxi Group of Maotang Group are all volcanic-sedimentary formations dominated by quartz keratophyre or chronological keratophyre tuff. Matoushan Formation and Yaolinghe Formation of Maotang Group are mainly composed of basaltic spilite, including sericite schist, phyllite and a small amount of rhyolite porphyry. In Wudang Mountain area, the exposed area of Wudang Group is about four times that of Yaolinghe Group, so quartz keratophyre is absolutely dominant in this area. At present, there is great controversy about the age attribution and correlation of the above groups. However, according to the fact that Douling Group is obviously deeper than other groups and is not integrated under Yaoyingzhai Formation, it shows that Douling Group should be the oldest known outcropping rock in South Qinling, and its age belongs to Proterozoic, while Wudang Group and Maotang Group (Yaoyingzhai Formation) belong to Mesoproterozoic (Editorial Board of stratigraphic code, China, 1999).
The caprock deposits in South Qinling include Sinian-Cretaceous. At the same time, the formation nature and metamorphic degree of rocks often change greatly laterally. The Sinian-Ordovician in Xichuan Community and Liuba-Yunxian Community consists of non-metamorphic-shallow metamorphic limestone, dolomitic limestone, marble, sandstone, slate and phyllite, while in Ankang-Pingli area in the north of Ziyang-Zhuxi Community, the Cambrian (Donghe Group) is a set of volcanic-sedimentary rocks, with great vertical changes in thickness and lithology, mainly composed of slate. Donghe Group generally shows the characteristics of rift environment. The Cambrian and Ordovician in the south of Gaotan-Fang Bing Street area are composed of slate, phyllite, limestone siltstone and siliceous rocks. Among them, the Cambrian has high carbon content and is mixed with stony coal seams. Silurian is mainly distributed in Liuba-Yunxian community and Ziyang-Zhuxi community. The former is composed of thick phyllite and mica schist, while the latter is composed of sandstone and slate. The Silurian of Xichuan Group consists of argillaceous rocks, clastic rocks and carbonate rocks, with pyroclastic rocks at the bottom.
Figure 3-3 Schematic Diagram of Geological Structure Units in Qinling Mountains
(According to Zhang Benren et al. 1990)
1- four yuan; 2- North China Craton caprock; 3- South Qinling caprock; 4- Yangtze craton caprock; 5- yuan Grain Rain; 6- Taikoo Yu; 7- ophiolite formation; 8- Late Mesozoic granite; 9- Late Paleozoic-Early Mesozoic granite; 10- Early Paleozoic granite; 11-Mesoproterozoic granite; 12-basic-ultrabasic rock mass
Devonian is mainly distributed in Baiyun-Zhashui community (northern Devonian belt) and Liuba-Yunxian community (southern Devonian belt). Devonian system in both areas is flysch formation, which consists of extremely thick shallow metamorphic rocks-basically unchanged sand-argillaceous rocks and carbonate rocks. Among them, Qingshiya Formation of Middle Devonian in Baiyun-Zhashui Community is the main occurrence horizon of regional layered lead-zinc deposits, and Devonian in Liuba-Yunxian Community Building is the occurrence horizon of mercury-antimony deposits. The Devonian system of Xichuan Group consists of unaltered argillaceous rocks and carbonate rocks. The Devonian Liu Lingqun in Xiping area in the northwest of this area is equivalent to the Devonian in Baiyun-Zhashui community. Carboniferous is distributed in Xichuan community, Liuba-Yunxian community and Baiyun-Zhashui community, and consists of basically unchanged limestone with carbonaceous shale. The Permian and Triassic in South Qinling are limited to Xikou area in Zhen 'an, and are mainly composed of limestone with shale and sandstone. Cretaceous is a continental stratum, mainly distributed in Xichuan community, and is composed of siltstone and conglomerate along Shangdan fault.
2. North Qinling tectonic belt
The strata in the North Qinling structural belt are undeveloped, mainly including rock assemblages dominated by Proterozoic Qinling Group, Mesoproterozoic Kuanping Group, Early Paleozoic Fengdan Group and Erlangping Group, with the following characteristics:
Qinling Group is a huge lens-like distribution on the plane. It is not a single stratigraphic unit, but includes geological bodies and rocks with different ages, sources and properties (Zhang Guowei et al., 1998). It is mainly composed of biotite plagioclase gneiss, calcium silicate and graphite marble, with a small amount of basic rock wall plagioclase amphibole, and the original rock of Qinling Group in the west is amphibole. The Qinling Group mainly experienced three metamorphic stages: Early Proterozoic, Neoproterozoic and Caledonian, and the main metamorphic stage occurred in Neoproterozoic. From west to east, the degree of metamorphism gradually increases from low amphibolite facies (Zhaigen area) to granulite facies (Tongbai area). At present, it is generally believed that Qinling Group belongs to Early Proterozoic. For example, You Zhendong et al. obtained the whole-rock Th-Pb isochron age of 22 100 Ma in the biotite plagioclase gneiss in Banchang, Neixiang. Zhang et al. obtained the age of the intersection of zircon U-Pb unconformity lines in biotite plagioclase gneiss in Xixia, and the Sm-Nd age of plagioclase amphibolite in the same area is 1982Ma. It is noteworthy that Zhang Guowei et al. obtained the Sm-Nd age of 2973±295ma in amphibolite of Qinling Group in Fengdanxi Gorge. Because biotite plagioclase gneiss is clastic rock, the Qinling Group mainly belongs to early Proterozoic, but it may also contain a small amount of late Archean materials.
Kuanping Group is distributed in the northernmost part of the North Qinling Belt in a strip shape. Its lower part is mainly composed of plagioclase amphibole and greenschist whose original rock is tholeiite, its middle part is mainly composed of quartz schist whose original rock is composite sandstone, and its upper part is mainly composed of marble. Although Zhang Shouguang and others proposed that the Kuanping Group should be a rock-structure-stratigraphic unit, they still believed that the group had the above sequence. The metamorphic degree of this group of rocks is greenschist facies-epidote amphibolite facies. Zhang obtained the Sm-Nd isochron of the broad-flat greenschist, with a boundary age of 975±39Ma and a lower age of 920±59Ma, and ε nd (t) =+4.5 ~+6.5. Kuanping Group seems to have formed at the turn of Mesoproterozoic and Neoproterozoic. However, in recent years, others (Zhang, Liu Dunyi, etc. ) The zircon U-Pb method is also used to determine the older age values of the mica quartz schist in this group, such as1681~1974 Ma, and the tectonic-thermal events that occurred around Qinling Group 1000Ma also obviously affected Kuanping Group.
Fengdan Group and Erlangping Group are ophiolite formations distributed on the north and south sides of Qinling Group respectively. All the ages belong to Early Paleozoic, and the existing age data of Fengdan Group are: Sm-Nd, 402.6 ~17.4 Ma; Rb-Sr，447.8 ~ 4 1.5ma； Among them, the Fengdan Group in the southern belt is dominated by metamorphic volcanic rocks, containing neutral volcanic rocks and a small amount of basic dykes and miscellaneous sandstone. Erlangping Group in the northern belt is mainly composed of basic lava, containing a small amount of quartz keratophyre, covered with extremely thick complex sandstone, relatively pure carbonate rock and radiolarian siliceous rock. The basic lava of two sets of ophiolite has pillow structure, and generally suffers from greenschist facies metamorphism caused by spilite alteration. According to the existing research, two sets of ophiolite are mainly formed in island arc or back arc marginal sea environment.
Secondly, the evolution of chemical elements in sedimentation.
Because there are few stratigraphic units in North Qinling and many stratigraphic records are missing, all the stratigraphic units we see now are unconformity contact and discontinuous sedimentation, so the variation characteristics of sedimentary cycles among stratigraphic units are not obvious. Therefore, when discussing the sedimentary evolution in Qinling area, the chemical element evolution of sediments in South Qinling area is mainly discussed.
1. Evolution of sedimentary chemical elements in South Qinling
Many achievements have been made in the study of tectonic environment and sediment source of stratigraphic deposits in different periods in South Qinling, but there are still many uncertainties. According to the analysis of provenance and tectonic environment of Proterozoic Douling Group, Cambrian-Ordovician System and Silurian-Devonian System in the South Qinling structural belt by Zhang Benren et al. (1994), Li, th, Sc, La, Co and REE of metamorphic clastic rocks in Douling Group are obviously lower than those in the North Qinling Group at the same time, while (La is based on Cambrian-Ordovician System and Silurian System. Based on the analysis of the source and tectonic environment of Silurian-Devonian sediments, it is considered that since Neoproterozoic began to transform to the continental margin of Yangtze plate, the ocean gradually shrank with the northward subduction of Qinling oceanic crust. Since Silurian, especially Devonian, the South Qinling has moved closer to the North Qinling on the active continental margin of the North China plate, and the provenance of the South Qinling sedimentary basin has undergone fundamental changes. Except for some sediments from the passive continental margin in the northern Yangtze Craton, most sediments come from the North Qinling tectonic belt. According to the research results of Yan et al. (1997) on stratum abundance in South Qinling (Table 3-5), the chemical composition of stratum sedimentary rocks in different periods in this structural belt has the following evolution trends:
Evolution of main components of (1)
As can be seen from Table 3-5 and Figure 3-4, the contents of Al2O3, SiO2 and Na2O in sedimentary rocks were higher in Early Proterozoic-Neoproterozoic, obviously decreased in Sinian-Ordovician, obviously increased in Silurian, gradually decreased in Permian, and began to increase after Triassic.
The contents of CaO and MgO show an opposite trend to the above elements, but with the evolution of geological history, the value of CaO/MgO shows an increasing trend, with MgO being higher in Sinian-Ordovician and CaO being higher in Devonian-Triassic.
The content of K2O in the pre-Sinian system showed a stable fluctuation trend. The Sinian system suddenly increased, then decreased, and the Silurian system greatly increased. The changes after Silurian are similar to those of Al2O3 and SiO2.
The content change of FeO+Fe2O3 is basically synchronous with the evolution of Al2O3 and SiO2, but it changes greatly in Proterozoic, in which the content of Wudang Group in Mesoproterozoic is low, while the content of Maotang Group and Yaolinghe Group in Neoproterozoic is high.
The reasons for the evolution of the above main components are as follows: from early Proterozoic to early Neoproterozoic, the whole area was in an unstable stage of structural development, sedimentary rocks were mainly clastic rocks and argillaceous rocks, and the proportion of carbonate rocks was low, so the contents of CaO and MgO were less, and the contents of elements such as Al2O3, SiO2 and Na2O were more; Because Proterozoic Douling Group, Neoproterozoic Maotang Group and Yaolinghe Group contain more basic volcanic rocks, higher FeO+Fe2O3 content and lower SiO2 content, while Mesoproterozoic Wudang Group and Neoproterozoic Yanxi Group contain more acidic volcanic rocks, so the SiO2 content is higher and FeO+Fe2O3 content is lower. However, during the Sinian-Ordovician, the South Qinling has been transformed into a passive continental margin in the northern part of the Yangtze Platform, which is in a relatively stable tectonic environment, with weak volcanic activity and relatively stable sedimentary environment. The proportion of carbonate rocks in the strata has increased obviously, with magnesium carbonate (dolomite) as the main rock, followed by argillaceous rocks and clastic rocks, so the MgO content has increased obviously. By Silurian, due to the collision and docking of the north and south Qinling Mountains, the tectonic sedimentary environment and sediment sources changed strongly. Sedimentary rocks are mainly argillaceous and clastic rocks, which leads to the rapid increase of Al2O3, SiO2 and Na2O. With the stability of sedimentary environment, the deposits of clastic rocks and argillaceous rocks gradually decrease, and the carbonate deposits gradually increase, mainly calcareous carbonate rocks, so the CaO content gradually increases. In Permian, almost all limestone was deposited. During Jurassic-Cretaceous, influenced by Yanshan movement, the South Qinling generally uplifted, and the terrigenous clastic rocks of rivers and lakes were deposited in the intermountain basins of the depression. The content of Al2O3 and SiO2 increased, but the ratio of SiO2/Al2O3 increased, while the ratio of K2O/Na2O decreased, indicating that the maturity of sediments was low.
(2) Evolution of trace elements and ore-forming elements
As shown in Figure 3-4, the content evolution of iron group elements or compatible elements such as Ti, V, Cr, Co, Ni, Sc showed a wave-by-wave downward trend from Proterozoic to Mesozoic, during which there were four periods of rapid content growth, namely Proterozoic (Duling Group), Neoproterozoic (Maotang Group, Yaolinghe Group), Early Paleozoic Cambrian and Silurian, and the first three periods all had different degrees of basicity. The addition of volcanic materials is the main reason for the relative enrichment of these elements in strata. The contents of Cr and Ni in Paleoproterozoic strata are obviously higher than those in other periods, which indirectly indicates that the composition of basic volcanic rocks is different from those in other periods. The change of Silurian provenance may be the main reason for the increase of iron group elements
Figure 3-4 Time Series Evolution Diagram of Geochemical Elements in South Qinling Mountains
Table 3-5 Average chemical composition (wB) of strata in different periods of South Qinling orogenic belt
Note: content units: principal component 10-2, Au 10-9, others 10-6. Pt3yl: Yaolinghe Formation, Pt3yn: Yan Xi Formation, Pt3mt: Maotang Formation, Pt2wd: Wudang Mountain Formation, Pt 1dl: Douling Formation. NN is the number of samples.
(According to Yan, 1997)
Incompatible elements such as lithium, beryllium, thorium, strontium, barium, etc. It has different evolution laws, but it has increased significantly in Silurian strata. In addition, Th increased significantly in Mesoproterozoic (Wudang Group), Ba in Cambrian and Sr in Permian-Triassic.
The content of tungsten was low in Proterozoic-Ordovician, increased in Silurian, decreased in Permian and then increased. Among them, it is only enriched in Silurian (1.86× 10-6).
The abundance of tin is generally low, but it is higher only in Sinian (2.8× 10-6) and Silurian (2.6× 10-6), which is close to the average value of the crust (2.5× 10-6) and higher than that of Nanling Bos Group (3.4×)
Don't take CAMBRIAN as the boundary, it showed a gradual upward trend before and then a downward trend. Among them, Yaoling River Group (1.4× 10-6) and Cambrian (1.6× 10-6) are relatively rich.
The enrichment and evolution of copper, zinc and silver are similar to those of iron group elements, mainly concentrated in Silurian and pre-Silurian strata. Copper is relatively enriched in Douling Group (42× 10-6), Yaolinghe Group (50× 10-6) and Cambrian (36× 10-6).
Zinc content is higher in Yaolinghe Formation (100× 10-6) and Silurian (100× 10-6). The Cambrian silver content was high (0.084× 10-9).
The evolution of Pb is generally on the rise, but it is only enriched in Maotang Formation (22× 10-6), Devonian System (20× 10-6), Carboniferous System (20× 10-6) and Cretaceous System (23×/kloc-6).
Gold is mainly concentrated in Devonian (1.9× 10-9), and the content in other strata is low.
Arsenic is mainly enriched in the strata after Silurian, with Devonian being the most abundant (9.8× 10-6), followed by Triassic (8.6× 10-6). Antimony is enriched in Sinian (1.6× 10-6) and Devonian (0.99× 10-6) strata.
The variation characteristics of rare earth elements in sandy argillaceous rocks in South Qinling (Table 3-6) are that σ σσREE increased from Proterozoic to Neoproterozoic, decreased to Cambrian, increased to Ordovician, decreased obviously in Silurian and Devonian, increased obviously in Carboniferous and Permian, and decreased obviously in Mesozoic. The evolution law of σσREE in the South Qinling and Nanling orogenic belts is different from the law that the total amount of rare earth in the strata increases with time after Archean in Australia platform (Nansi and Taylor, 1976).
Table 3-6 Rare Earth Element Parameters of Sandy Mudstone in Different Ages in South Qinling
N: number of samples.
The negative Eu anomalies of sandy argillaceous rocks in different ages in South Qinling are relatively weak in Proterozoic (Douling Group) and Mesoproterozoic (Wudang Group), but they are obvious after Neoproterozoic (Eu/EU = 0.56 ~ 0.70), and the Eu/Eu* value is relatively stable. The values of (La/Yb)cn in different times are similar, and only the values of (La/Yb)cn in Wudang Group in Mesoproterozoic are low. In addition, the La/Th ratio was low in the early Mesoproterozoic, and increased obviously in the Neoproterozoic. The La/Th values of Silurian and Devonian are obviously lower than those of Cambrian and Ordovician. But since Carboniferous, it has been on the rise.
Judging from the evolution of rare earth elements in sandy argillaceous rocks in the above-mentioned times, the evolution of sedimentary elements in South Qinling has two abrupt periods, which are related to the changes of tectonic environment and sediment sources. The first mutation period was from Mesoproterozoic to Neoproterozoic, corresponding to Jinning Movement. At this time, the tectonic environment of the South Qinling changed from a relatively unstable active continental margin to a passive continental margin on the northern margin of the Yangtze plate, which led to the increase of σσREE in sediments (mainly siltstone) and the abnormal negative Eu. The second mutation occurred between Ordovician and Silurian, corresponding to Caledonian movement. At this time, the Yangtze River and the continental margin of North China began to approach and dock. Before Silurian, the sediments in South Qinling were replenished by the northern margin of Yangtze plate, and the ratio of La/Th of sandstone and mudstone was high (> 4). After Silurian, especially after Devonian, the sediments in South Qinling were mainly replenished by the North Qinling tectonic belt, and the La/Th value (3.35) was close to that of the North Qinling tectonic belt (< 4). During this provenance transition period, σ-σREE of sandy argillaceous rocks also decreased.
According to the evolution analysis of main components, trace elements and rare earth elements in sandy argillaceous rocks in the above times, combined with the characteristics of sedimentary formation, the evolution of stratigraphic elements in South Qinling from Paleoproterozoic to Mesozoic can be divided into four stages, and the characteristics of tectonic environment, sedimentary formation, ore-forming elements and ore-transporting elements in each stage are as follows:
(1) the unstable evolution stage of the early Proterozoic-Neoproterozoic continental margin
This area is located in the northern margin of Yangtze ancient land at present, and the sedimentary formations are mainly volcanic-sedimentary formations of terrigenous clastic rocks, argillaceous rocks, various volcanic rocks and carbonate rocks. Different from other stages, Al2O3, SiO2 and Na2O are high, Ca is low, iron group elements are rich, and Li and Be are poor.
In Paleoproterozoic, this area may be in the continental rift tectonic environment, forming terrigenous clastic rock-argillaceous rock-marl sedimentary formation, containing a small amount of basic volcanic rocks (Douling Group). The content of σσREE in sandy argillaceous rocks is low (122× 10-6), and the content of mineralization and migration elements such as As, Sb, Cu and Cl in the strata is high.
In Mesoproterozoic, the tectonic environment gradually changed into continental island arc environment, forming a set of volcanic-sedimentary formations of terrigenous clastic rocks and tholeiite-calc-alkaline series basalts (Wudang Mountain Group). Volcanic rocks are mainly acidic, and the strata are rich in large ionic elements Ba and Th. Sandy argillaceous rocks are characterized by low La/Th value and generally low content of ore-forming elements in strata.
In the early Neoproterozoic, the area was still an active continental margin tectonic environment, and Maotang Group and Feixi Group were formed in different places. The main lithology of Maotang Group is volcanic-sedimentary rock series of metamorphic acidic volcanic rocks (Yaojiazhai Formation and quartz hornblende) and metamorphic basic volcanic rocks (Matoushan Formation and spilite), which is characterized by being rich in iron, nickel, cobalt, manganese, titanium, scandium and vanadium, and relatively rich in integrated mineral elements such as lead, zinc and gold. Yanxi Group is mainly composed of metamorphic neutral volcanic rocks (keratophyre) and metamorphic acid volcanic rocks (quartz keratophyre), with high Zr, P and Sc, and relatively rich metallogenic element Zn.
In the middle Neoproterozoic, this area may have entered the stage of passive continental margin structure development. The main lithology of Yaolinghe Group formed in this period is metamorphic basic volcanic rocks (keratophyre-spilite) and metamorphic argillaceous rocks, in which sodium-rich and magnesium-rich volcanic rocks reflect the tectonic environment of marginal extension, and Yaolinghe Group is rich in Fe, Cr, Sc, Ti, V, P, Sr, B and metallogenic elements Cu, Mo and Zn.
(2) Late Neoproterozoic (Sinian)-Early Paleozoic (Ordovician) passive continental margin evolution stage.
Since Sinian, the South Qinling has been transformed into a passive continental margin in the northern part of the Yangtze platform, with strong rifting in the early stage and detachment in the later stage. Sinian sedimentary marine argillaceous rocks and carbonate rocks dominated by dolomite. The high content of MgO, K2O and K2O/Na2O in the rocks reflects the increase of maturity, and the ore-forming elements and ore-transporting elements as, Sb, Sn and B are relatively rich.
Volcanic sedimentary formations of argillaceous rocks, carbonate rocks and intermediate-basic volcanic rocks were formed in CAMBRIAN. The stratum generally contains pyrite, carbonaceous shale, black rock and siliceous rock, and is rich in elements such as barium, boron, sulfur, antimony, molybdenum, copper, zinc, gold and silver.
Ordovician lithology is mainly carbonate rocks, followed by argillaceous rocks, with high content of MgO, CaO and Cl, and most of them are low in mineral elements.
(3) Silurian-Triassic continental margin docking residual basin development and continental-continental collision orogeny evolution stage.
The docking of North Qinling and South Qinling occurred in Late Silurian-Devonian, but it did not enter the stage of land-land collision orogeny immediately after docking, and there were still residual basins developed. Silurian sediments are mainly argillaceous rocks and a small amount of clastic rocks, which are rich in iron-loving elements such as cobalt, chromium, nickel, scandium and vanadium and incompatible elements such as potassium, lithium and thorium, and relatively rich in ore-forming and transporting elements such as tungsten, tin, copper, zinc and boron.
In Devonian, clastic rocks, argillaceous rocks and carbonate rocks were mainly deposited in the residual sea basin. Sedimentary rocks are relatively rich in ore-forming elements such as arsenic, antimony, gold and tungsten, and ore-transporting elements such as boron, fluorine and sulfur ... From Carboniferous to Permian, South Qinling was still a marine deposit. Carboniferous consists of carbonate rocks and argillaceous rocks, rich in B. Permian is mainly high strontium limestone, and most elements are exhausted. Triassic inherited the sedimentary environment of Permian, deposited a large number of marine limestone and a small amount of argillaceous rocks, and the content of ore-forming element as was high.
(4) The evolution stage of continental basins after Triassic.
Affected by the Yanshan movement, the South Qinling uplifted as a whole, and the terrigenous clastic rocks of rivers and lakes were deposited in the intermountain basins of the depression. The Cretaceous ore-forming elements are mostly in the middle, and Pb and Au are relatively high.
From the enrichment of ore-forming and ore-transporting elements in various stages of sedimentary evolution in South Qinling, it can be known that the enrichment law is similar to that in Nanling area, and the ore-forming elements are mainly enriched in the early and middle stages of the transition period of each evolution stage. For example:
Douling Group (As, Sb, Cu and Cl are 8× 10-6, 0.45× 10-6, 42× 10-6 and 77× 10-6 respectively) was formed in the Paleoproterozoic rift in the early stage of the first evolution.
The abundance of Sinian system (As, Sb, Sn and B) formed in the rift basin on the continental margin in the early stage of the second evolution is 7.0× 10-6,10-6, 2.87× 10-6 and 44×, respectively. The abundance of S is 0.58× 10-6, 1.6× 10-6, 36× 10-6, 89× 10-6 and 0.084×/kloc respectively.
The abundances of tungsten, tin, copper, zinc and boron in Silurian in the transitional period of the second and third evolution stages are 1.86× 10-6, 2.6× 10-6, 32× 10-6 and/kloc-0, respectively.
The abundance of Devonian series (As, Sb, Au, W, B, F, S) formed in residual basins in the early stage of the third evolution stage is 9.8× 10-6, 0.99× 10-6, 0.0019×/kloc-respectively.
According to the relationship between the tectonic environment in different evolution stages of South Qinling and the enrichment of stratigraphic ore-forming elements, Mo and Ag are the most abundant in the sedimentary strata (Cambrian) formed in the passive continental margin tectonic environment. The main enrichment strata of tin, antimony, fluorine and chlorine (Sinian and Devonian) are the tectonic environment for the development of passive continental margins and residual basins. On the other hand, tungsten is enriched in sedimentary strata (Silurian and Cretaceous) formed under the sedimentary tectonic environment of residual sea basins and inland basins. The rich layers of copper and zinc are mostly formed in continental margin rift and passive continental margin tectonic environment.
2. Evolution of chemical elements in North Qinling sediments.
The strata in North Qinling include Proterozoic Qinling Group, Mesoproterozoic Kuanping Group, Early Paleozoic Fengdan Group, Erlangping Group and Early Mesozoic Triassic. Zhang Benren et al. (1994) used rare earth and trace elements to study the tectonic environment and sediment sources of stratigraphic deposits in different periods in Qinling Mountains. The results show that the original sandy argillaceous rocks of Qinling Group, Kuanping Group and Erlangping Group all have the characteristics of mixed components in LA/CE-SC/TH diagram. The source area of clastic rocks of Qinling Group should be similar to the double-mode suite composed of alkaline basic rocks and acid rocks produced in continental rift, but the source area is similar to alkaline basic rocks and acid rocks. The provenance of Kuanping Group is Taihua Group, the ancient land of North China, and Qinling Group, and the sedimentary tectonic environment is Qinling island chain and limited margin rift basin. The origin of Erlangping Group is various. Qinling Group in the south, Kuanping Group in the north, volcanic rocks in the lower part of Erlang Group and early volcanic rocks in Fengdan Group in the south can all provide provenance for it, and its tectonic environment is active continental margin. Fengdan Group is located in the south of Qinling Group, and its tectonic environment also has the characteristics of active continental margin environment.
According to the research results of Yan et al. (1997) on the stratum abundance in North Qinling (Table 3-7), the structural environment, reservoir-forming characteristics, enrichment characteristics of ore-forming elements and ore-transporting elements in different periods of this structural belt are briefly described as follows:
1) In Paleoproterozoic, this area was in a continental rift environment, forming a set of terrigenous clastic composite sandstone, carbonate rocks and basic volcanic rocks (Qinling Group). Compared with other formations, the value, CaO value and CaO/MgO value are higher. Incompatible elements Li and Zr, volatile elements Cl (13/kloc-0 /×10-6), ore-forming elements As( 12× 10-6) and Sb (.
2) In Mesoproterozoic, this area changed into a continental rift environment, and a set of clastic rocks and basic volcanic rocks were deposited in the Qinling island chain and the limited margin rift basin, followed by carbonate rocks and a small amount of neutral volcanic rocks (Kuanping Group). Compared with the Paleoproterozoic Qinling Group, the contents of SiO2 _ 2, Al _ 2O _ 3, CaO, Na _ 2O and K _ 2O are lower, while the contents of MgO and iron-affinity elements are higher. Ore-forming elements Au(2× 10-9), Cu(43× 10-6) and ore-transporting elements S(4 10× 10-6) and B (23×10)
3) In the early Paleozoic, this area developed into an active continental margin environment. On the north side of Qinling Group are basic volcanic rocks (spilite), followed by clastic rocks, acid volcanic rocks (Shi Ying hornblende) and a small amount of carbonate rocks (Erlangping Group). A stratum (Fengdan Group) is formed on the south side of Qinling Group, which is dominated by basic neutral volcanic rocks and contains a small amount of clastic rocks. Compared with Proterozoic strata, the contents of Al2O3 and Na2O, TFeO+MgO increased, while the contents of CaO and K2O, K2O/Na2O and SiO2/Al2O3 decreased. Trace elements are characterized by iron-rich elements such as Co, Cr, Ni, Mn, Sc and V, and poor pro-MagmaElemental elements such as Li, Be, Zr and Th, which reflect the influence of intermediate-basic volcanic rocks. The ore-forming elements in Fengdan Group are relatively rich in Cu(65× 10-6), Pb(46× 10-6) and S(340× 10-6).
4) Mesozoic (Triassic), since the early Paleozoic, the North China platform collided with the Yangtze block continuously, which kept the North Qinling area rising until it sank again in the late Triassic, forming sedimentary formations dominated by argillaceous materials, so the contents of SiO2 _ 2, al2o, K2O, K2O and K2O increased obviously, while the contents of MgO, CaO, Na2O and TFeO+MgO increased. At the same time, the contents of iron group elements such as cobalt, nickel, manganese, scandium and vanadium decrease, while the contents of pro-MagmaElemental elements such as lithium, beryllium, zirconium and thorium increase. The ore-forming elements are Au(2.7× 10-9), As( 12× 10-6), Sb(0.72× 10-6) and Pb (40×/kloc-0).
Judging from the tectonic environment of ore-forming elements enrichment in strata of different ages in North Qinling, the strata formed by island arc environment (Fengdan Group) have the most obvious enrichment of Cu and Pb. Tungsten, gold, silver and boron are the most abundant in the strata (Triassic) formed in the continental depression basin. Tin, arsenic, antimony and fluorine are not only enriched in the sedimentary strata in the continental margin rift environment (Douling Group), but also in the strata formed in the continental depression basin (Triassic).
Compared with the Nanling and South Qinling tectonic belts, basic volcanic rocks are developed in the North Qinling tectonic belt, and the outstanding stratigraphic geochemical substances are the strong enrichment of Fe-based elements such as Cr, Ni, Co, Ti, Mn, V, Sc, Cu and Au, which reflects the strong activities of the crust and lithosphere and the frequent exchange of mantle materials in this area.
Table 3-7 Average chemical composition of strata in North Qinling orogenic belt in different periods
Note: Content unit: principal component10-2; Gold, mercury, palladium and platinum: 10-9, others10-6; Pz 1dn: Feng Dan group, Pz 1er: Erlangping group, Pt2tw: Wan Tao group; Pt2kp: kuanping group, Pt 1qn: Qinling group.
(According to Yan, 1997)
According to the evolution characteristics of stratigraphic elements in the North and South Qinling Mountains, the crustal evolution of the East Qinling Mountains from Paleoproterozoic to Mesozoic was dominated by vertical growth in the early stage and horizontal growth after Neoproterozoic.