(Research Center of Bioenvironmental Geochemistry, Chinese Academy of Geological Sciences, Beijing 100037)

Yan Binggang

(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037)

Rao keqin

(Statistical Information Center of Ministry of Health, Beijing 100044)

Restricted by geological factors, the regional distribution of elements and element combinations in supergene parent materials shows the characteristics of zoning according to structural background; Under the influence of today's natural environment, supergene elements are recombined after migration, which has the characteristics of zoning according to geographical elements; The dispersion and aggregation of elements are also constrained by their own geochemical behavior. It is these comprehensive factors that determine that the geochemical zoning of China, which is dominated by river sediments, is distributed in the northeast-southwest direction. China is divided into four regions from northwest to southeast. The distribution of elements and element combinations in different geochemical regions highlights the differences of the leading roles played by the above-mentioned different factors in different regions. The results of factor analysis of a large number of data show that the regional distribution characteristics of elements and element combinations are also the reflection of the accumulation and dispersion laws of elements in different geochemical areas mentioned above.

Geological structure, geographical environment, geochemical elements

As we all know, the general distribution characteristics of various elements in nature are closely related to the types of rocks and rock combinations and their geological background. The provenance of elements is rocks, and the distribution of rocks and rock combination types is restricted by tectonic background and geological process. The crust differentiated from the primitive mantle has been continuously transformed in the long river of geological history, forming a graded framework of shallow Si-Al, deep Si-Mg and upper mantle Fe-Mg. At the same time, due to geodynamics and long-term tectonic activities, the combination of layered substances in the framework will also change to varying degrees. It can not only recombine surface materials, but also bring deep materials to the surface and transform the surface components of the crust. At present, the distribution of crustal primary biomass in horizontal zoning and vertical stratification is the result of long-term geological activities. In addition, the tectonic pattern also determines the migration and accumulation of surface materials. Among them, the uplift zone is dominated by denudation and migration, while the depression basin is an accumulation area. Geomorphological differentiation and climate change are also important factors that determine the speed and quantity of material migration.

Due to the strong transformation of Mesozoic and Cenozoic tectonic movements, the topography and geological structure of the crust in Chinese mainland are extremely complicated. The west is mountainous, and the east is mainly low hills and vast plains. Generally speaking, from west to east and from north to south, the landscape is stepped. Structurally, the eastern and western regions also show very different structural landscapes. In the western Himalayan tectonic domain, with the Tethys Trough retreating from north to south until it is closed, the southwest of China gradually rises from north to south. From the late Himalayan cycle, the tectonic movement south of the Yarlung Zangbo River was dominated by intense compression, folding and uplift, forming a towering folded mountain system. The high principle of southern Qinghai is the fault-block mountain system under extension, and a huge piedmont depression is formed in the north and south of Tianshan Mountain. Contrary to the situation in the west, the Himalayan tectonic period in eastern China is characterized by the continental margin fold belt, on which an extensional basin developed. In Mesozoic, the eastern part of China was dominated by low mountains and hills, and a small intermountain basin developed on the tectonic background of Daxing 'anling-Taihang-Wuling Mountain. To the west of this line, Sichuan and Ordos are dominated by depressions, forming a large depression belt in Ordos, Sichuan and central Yunnan. By the late Cretaceous, the eastern faults had subsided, forming a series of fault basins. Since Cenozoic, especially since Neogene, the topography of eastern China has changed from high to low, and even dropped to near sea level in some places. Due to the subsidence of the continental margin, the present coastal plain landscape has been created.

1 geochemical division and element distribution characteristics

1. 1 geochemical division

The complex geological background and geographical environment, as well as the climate change in the north and south, are all important factors that restrict the separation, migration and accumulation of chemical elements from the source area. Although the characteristics of rock geochemistry and regional geochemistry of river sediments are not exactly the same, they are closely related. The composition of rock materials is restricted by the genetic mechanism, that is, it depends on the paleoenvironment and paleogeodynamics at the time of formation. River sediments are the products of semi-weathering or weathering of rocks under the action of geological cycles, which are restricted by factors such as material source, climate, landform and hydrological conditions. Various rocks and minerals in the earth's crust are weathered and decomposed under supergene action. Some elements exist in ionic state or complex form, and some elements are separated from colloid or particle suspension and dispersed or enriched again after migration and transformation. Some soluble elements, such as sodium, calcium, magnesium, boron, strontium, etc. , has a strong migration ability, and most of them are taken away by aqueous solution. Therefore, the average content of these elements in river sediments is much lower than that in the crust. On the contrary, elements that are not easy to lose are relatively abundant in river sediments, and the average content often exceeds the average of the crust. Obviously, the regional geochemical characteristics characterized by the elemental composition of river sediments are the product of comprehensive factors including paleogeodynamics, paleoenvironment and modern environment. Different factors in different regions and the differences in the influence degree of each factor constitute a multi-environmental geochemical landscape with river sediments as the main body.

Figure 1 geochemical zoning map of river sediments in China

According to the data of 1: 200,000 regional geochemical survey with river sediments as the main body carried out by the Ministry of Land and Resources (former Ministry of Geology and Minerals) in recent 20 years, it can be clearly seen from the comprehensive geological background and modern environment that the geochemical division of river sediments in China is roughly northeast-southwest, and it can be roughly divided into the following four regions from northwest to southeast (see figure1):

Ⅰ. Northwest Inland Geochemical Area (Inland Area)

Ⅱ. Daxinganling-Northern Tibet Plateau Geochemical Region (Plateau Region)

Ⅲ. Three rivers in Northeast China (Heilongjiang, Songhua River and Wusuli River)-Three rivers in Southwest China (Jinsha River, Lancang River and Nujiang River) Geochemical Region (Sanjiang District)

Ⅳ. Southeast coastal geochemical belt (coastal zone)

Geochemical zoning characteristics under the influence of environmental factors.

As we all know, silicate, which is the main component of crustal rocks, is a very sensitive mineral to the environment. The changing environmental chemical factors are finally mainly reflected in the changes of silicate minerals. From the northwest border of China to the southeast coast, due to the overall decrease of pH value, silicate minerals also show regular changes, from physical weathering in the northwest to chemical and biological weathering in the southeast. Silicate minerals, such as feldspar, are completely decomposed into hydroxides of iron and aluminum by hydromica, montmorillonite and kaolinite, which reflects the macroscopic law of element migration and transformation in river sediments. The four geochemical divisions also reflect this obvious zoning feature. However, it must be pointed out that even in the same area, with the change of altitude, this vertical zoning phenomenon will occur.

1.2. 1 Northwest Inland Geochemical Area

This area is mainly in Xinjiang Uygur Autonomous Region. It belongs to arid area, with abundant sunshine, less rainfall and large evaporation. Except Altai Mountain and Tianshan Mountain, the annual precipitation is generally less than 200mm, and there is basically no surface runoff. Vegetation is scarce in the whole region, but plant residues are thoroughly decomposed, and humus is rarely accumulated except swamp and peat, which can be formed in low-lying and humid environment. Usually, in the arid and oxidizing environment with pH greater than 8.5, physical weathering is the main method, and silicate minerals such as feldspar and mica in rocks only begin to transform into hydromica and chlorite through hydration in rainy season. At the same time, a large number of chlorine, sulfur and iodine are precipitated and accumulated in weathering crust, soil and depressions, while little calcium, magnesium, sodium and potassium are lost. Compared with the other three areas, the contents of these elements in river sediments are closest to the parent rocks.

1.2.2 Daxinganling-Northern Tibet Plateau Geochemical Region

Inner Mongolia Plateau, Loess Plateau and Qinghai-Tibet Plateau are the main parts. It belongs to semi-arid area, and the surface water system is generally developed, mostly in the source area of rivers, and the evaporation is greater than the precipitation. When the pH value is usually in the range of 8.5 ~ 7.5, aluminosilicate and silicate minerals are decomposed under the action of H2O and CO2 to produce clay minerals such as hydromica, pyrophyllite and montmorillonite. Most chlorides and sulfates are dissolved, Cl- and sulfates are leached out, alkali metals and alkaline earth metals are released, sodium is partially taken out, calcium, magnesium and potassium are partially retained in the eluvial layer, and part of SiO2 _ 2 is transferred to the solution, often forming calcium-magnesium carbonate accumulation.

1.2.3 northeast Sanjiang-southwest Sanjiang geochemical region

Mainly distributed in plains and hilly areas. Abundant rainfall and developed surface water system. The annual precipitation is between 400-800 mm, which belongs to semi-humid zone. It is a weakly acidic environment with a pH value of 7.5 ~ 5.5. Almost all silicate and aluminosilicate groups in minerals are dissolved, and SiO2 is further released. Alkali metals and alkaline earth metals are strongly leached and lost, and SiO2, Al2O3 and Fe2O3 are precipitated, resulting in clay minerals such as kaolinite, halloysite or hydroxide containing no potassium, sodium and calcium salt groups. The contents of calcium, magnesium, sodium, strontium and potassium continued to decrease.

1.2.4 southeast coastal geochemical area

This area is located in the subtropical-tropical climate zone, and the landform is mainly low mountains and hills, with offshore plains only in coastal areas. Due to the influence of subtropical monsoon climate in the Pacific Ocean and Indian Ocean, the rainfall is abundant, and the annual precipitation is generally above 800 mm. The surface water system is developed, and most of it enters the sea through this area. Due to the high temperature and rainy weather, the pH value is usually less than 5.5. In the humid zone under the reducing environment, chemical and biological weathering is thorough and leaching is strong. Calcium, magnesium, sodium and potassium are all released and taken away, aluminosilicate is completely decomposed, Al2O3 and Fe2O3 are separated from SiO2, and a large amount of SiO2 is leached. Al _ 2O _ 3, Fe _ 2O _ 3 and part of SiO2 _ 2 are colloidal aggregates in acidic medium to form diaspore, limonite and opal. Calcium, magnesium, sodium and strontium are reduced to the lowest level.

1.3 Geological background and element distribution characteristics of geochemical zoning

1.3. 1 Northwest Inland Geochemical Area

The Junggar-Beishan-Altai Mountain area north of Tianshan Mountain belongs to the Hercynian fold system of Junggar-Altai. Paleozoic strata are mainly exposed, showing the sedimentary characteristics of active continental margin. Miscellaneous sandstone and siliceous rock are common, and their composition maturity and structure maturity are low. Taking siliceous rocks in Beishan area as an example, the chemical composition is between normal chemical sedimentary siliceous rocks and volcanic sedimentary siliceous rocks, which reflects that the source of siliceous rocks is not only related to supergene geological processes, but also closely related to geological processes in the deep crust. The analysis results of macro and trace elements also confirm this point. Table 1 shows that in addition to the high abundance of barium and strontium under supergene action, iron, manganese, titanium, vanadium, chromium, nickel, copper and other siderophiles from deep crust are also high, while sodium, potassium, magnesium and lead are low.

Table 1 Average value of trace elements in early Paleozoic siliceous rocks in Beishan area

Note: The oxide content is percentage, and the single element is mg/kg.

Granite accounts for about 25% in the area, mainly quartz diorite, plagiogranite, granodiorite and adamellite in the early Hercynian, and active continental margin granite in the late Hercynian, followed by alkaline magmatism, with a small amount of basic rocks and ultrabasic rocks. These types of rocks are infiltrated by mantle-derived materials. Meta-theory, whether it is the analysis of rock genesis or the direct analysis of local rocks, reflects that iron group elements such as vanadium, chromium, cobalt, nickel, iron, manganese and titanium are relatively rich, with relatively high contents of copper, calcium and magnesium and relatively low contents of potassium and sodium. In Paleozoic strata, volcanic rocks and pyroclastic rocks are generally mixed, mainly andesite and basalt, which are homologous products with corresponding plutonic rocks and have similar element combination characteristics.

1.3.2 Daxinganling-Northern Tibet Plateau Geochemical Region

North to Daxinganling-Tianshan Mountains, south to Himalayas, including Inner Mongolia Plateau, Loess Plateau and Qinghai-Tibet Plateau. From the analysis of the overall geological structure background in the plateau area, except Tarim platform, Gangdise Mountain and Kunlun Mountain have obvious characteristics of magmatic arc, which may be dominated by rift activities in the continental crust in different periods, forming a series of fold systems with different scales and times. Although rifting and stretching can also bring mantle-derived materials to the crust, on the whole, the crustal transformation process is far less intense than that of the active continental margin. This is especially obvious in magmatic activity. The exposed area of deep-melting granite in Qinba area is only 19%, and remelted granite from crust accounts for 8 1%. The former is rich in iron group elements and heavy rare earth elements, while the latter is rich in large ion pro-MagmaElemental and light rare earth elements. There are deep-seated magmatic rocks represented by gabbro, quartz diorite, granodiorite, monzonite, biotite granite, tholeiite and andesite combination in Gangdise belt and Kunlun belt of Qinghai-Tibet Plateau. There are Cenozoic aegirine, nepheline leucite, nepheline syenite, tetrahedrite, alkaline trachyte and alkaline basalt series of mantle-derived alkaline magmatism in southern Qinghai, but the Qinghai-Tibet Plateau is still in the crust in a large area. As far as sedimentary rocks and metamorphic rocks with an exposed area of about 80% are concerned, volcanic rocks and feldspathic hard sandstone are developed in advanced geosynclines such as Daxinganling-Tianshan Mountain, Kunlun Mountain and Gangdise Mountain, and lithic hard sandstone is accumulated in Tanggula and North Himalayan geosyncline. The sediments on the platform or geosyncline are often covered by Mesozoic and Cenozoic large continental basins, such as Tarim Basin, Qaidam Basin and Erlian Basin. Due to sedimentary differentiation, the newly-born deposits have undergone material reorganization, and the material composition of the crust surface layer has changed greatly. Compared with the original trough sediments, the content of iron group elements decreased, while the content of alkali metals, alkaline earth metals and halogen group elements increased. Although the dew point in the uncovered fold zone still retains the composition characteristics of the active structural zone, from the average level of the crust surface materials in the whole region, vanadium, chromium, cobalt, copper, iron, titanium and halogen elements are reduced.

1.3.3 northeast Sanjiang-southwest Sanjiang geochemical region

From the Sanjiang area in the northeast to the Sanjiang area in the southwest, the broadband obliquely passes through the central part of China, and the North China Platform and the Yangtze Platform are the main geological structures. There is an ancient crystalline basement exposed in the platform area, which is the product of the earliest incomplete differentiation of mantle. Ancient metamorphic rock series often contain basic and ultrabasic rock lenses, and gabbro walls penetrate. The average composition of the substance tends to be alkaline, and the contents of magnesium, calcium and mantle-derived components are obviously high, while the contents of potassium, sodium, boron and lithium of lithophile elements are relatively low. Table 3 records the element contents of Neoarchean amphibolite and gneiss in North China Platform. In the widely exposed platform-type marine sedimentary caprock, although stable sandstone and carbonate rocks are well developed, different sedimentary combinations can be formed in the subsidence area. From Sinian to Lower Paleozoic, phosphorus, iron, vanadium, cobalt, nickel, molybdenum, copper and other elements left by basement erosion were deposited and enriched in some areas, forming industrial deposits with a certain grade. Bauxite and manganese ore are also developed on the weathering crust of Upper Paleozoic. The exposed area of magmatic rocks is very small, not exceeding 10%, among which Permian continental tholeiite and Cenozoic alkaline basalt have obvious slow source characteristics, which are consistent with the overall characteristics of the distribution of ancient crystal basement elements in the whole region.

Table 2 Element Contents of Granites of Different Genetic Types *

Note: The oxide content is percentage, and the single element is μ g/g/g ... The deep source syntectic type is represented by Gangdise region, the alkaline-weakly alkaline type is represented by Qiangtang region, and the crust source remelting type is represented by Himalayan region. "*" is all iron.

Table 3 Element Content of Neoarchean Metamorphic Rock in Qian 'an, East Hebei Province

Note: the oxide content is percentage, and the single element is μ g/g/g ... "*? For all iron.

1.3.4 southeast coastal geochemical area

Coastal areas are controlled by the South China Fold System. The basic characteristics of the original rock composition are that the iron-loving elements are lower than those in the adjacent Sanjiang area, and the molybdenum and tungsten elements are obviously higher. The South China Fold System is actually a aulacogen formed on the basis of weak extension in many places. Although the exposed area of magmatic rocks is large, accounting for about 30%, the deep magmatic activity is very weak. The latest rift-type Quaternary mantle-derived alkaline basalt is only found in limited areas of Qiongyue, Fujian, Zhejiang and Taiwan provinces, which is not enough to affect the average composition of the crust surface in coastal areas. The crust-derived remelted granite, which is mainly composed of potash feldspar granite, biotite granite, diorite granite and granodiorite, is only the reorganization of the materials in the crust and does not change the overall composition of the crust. But it can gather elements such as tungsten, tin, bismuth, molybdenum, lead, zinc, mercury and arsenic, and even form large and super-large deposits. In Paleozoic aulacogen, coarse clastic materials accumulated rapidly are rarely seen, mainly fine clastic flysch deposits from the source area. During the migration of original biomass, relatively sufficient sedimentary differentiation brought a certain number of active elements and transition elements, and weak active and inactive elements mostly stayed in the denudation area, which may also be one of the reasons for the low content of weak active and inactive elements in this area.

Enrichment characteristics of element 2

This paper focuses on the regional distribution characteristics of 24 elements closely related to crop growth and life sciences. Firstly, the single-element average is calculated from several hundred to 2,000 single-element total data in each1:200,000 international framework by removing abnormal values for many times, that is to say, these values only represent the macro-regional background and do not reflect local anomalies; Then calculate the national average on the basis of every1:200,000 international frames. Table 4 clearly reflects the crustal abundance values of various elements and the average content of river sediments in China.

Table 4 Comparison Table of Average Contents of Sediments and Crusts in Chinese Rivers (mg/kg)

Note: The background value is the national average.

Through further analysis, the regional distribution characteristics of each factor are still very distinct. If the elements with similar distribution characteristics are classified into one category, they can be divided into the following eight categories.

2. 1 sodium, calcium, magnesium, potassium and strontium

The high values of alkali metal and alkaline earth metal elements are widely distinguished. Except K, the other four elements [Na (> 2.04%), Ca (> 5.0%), Mg (> 1.70%) and Sr (> 346.75 mg/kg)] are concentrated in the arid and semi-arid areas in the north and northwest of China, and their content changes gradually decrease from northwest to southeast. The high range of strontium and sodium also extends to the northeast of China. Generally speaking, the southern half of China is mostly a low-value area of calcium, sodium and strontium, and their contents are lower than their background values respectively. Although the high-value area of magnesium also appears in the northern region, it is slightly different from the distribution of calcium, sodium and strontium. Only in Taihang Mountain, Yanshan Mountain and Liaodong Peninsula is the middle-high content area (0.93% ~ 1.70%), while southern Hunan and western Hubei are at the same level. China is generally at a low K level, with high-value areas (> 2. 14%) and low-value areas (< 1.92%) distributed in a star shape. Relatively speaking, Zhejiang, Jiangxi, Fujian, Jilin, southern Liaoning and northeastern Inner Mongolia have large high-value areas, and the contents in northwest, southwest and central and southern regions are generally low, but there are small high-value to high-value areas in southern Hainan Island and southwestern Guangxi.

2.2 chromium, iron, manganese, nickel, cobalt, vanadium and titanium

The high and medium value areas (higher than the national average) of iron group elements are distributed in the northeast, from Yunnan-Guizhou Plateau to Changbai Mountain in Jilin Province, especially in parts of southwest, south-central and north China. There are also high-value areas of iron, vanadium, manganese and cobalt in northern Xinjiang. Generally speaking, there are two low-value areas (lower than the national average) of the above seven elements in China, and their contents are lower than their background values respectively. One extends from the Qinghai-Tibet Plateau and southern Xinjiang to the eastern part of Inner Mongolia, and the other is distributed in southern China and the southeast coast, forming two low-value areas that are roughly parallel, but the scale is quite different. Except for titanium, there is no obvious low value distribution in the southeast coast and South China.

2.3 Copper, zinc, lead and mercury

Generally speaking, these elements show a decreasing trend from southeast to northwest, that is, the high and median areas are generally distributed in southwest-northeast direction. The high and medium value areas are mainly concentrated in the southwest, southeast and central areas of China, south of Qinling-Dabie Mountain and east of Sanjiang Basin in eastern Tibet. In addition, copper occurs in northern Xinjiang and Taihang Mountains, and zinc occurs in Changbai Mountain, Xiaoxing 'anling and Daxing 'anling in different scales. The low-value areas with copper and zinc contents lower than the average background value appear in Inner Mongolia to northern Tibet, Zhejiang to Guangxi, Inner Mongolia to northern Tibet, Shandong Peninsula and Guangdong to Guangxi coastal areas respectively. Most of the southeast, south-central and southwest regions of China belong to the high-value to medium-value regions of mercury (> 72 ug/kg) and lead (> 32.5 mg/kg), and they are distributed in a large area. The contents of mercury and lead in northern China are generally lower than the average background value, which are 43mg/kg and 24.73mg/kg respectively.

2.4 Arsenic and Cadmium

High (> 20.76 mg/kg) and medium (15.3 1 ~ 20.76 mg/kg) arsenic areas are mainly distributed in Tibet, Yunnan, Guangxi, Hunan and Guangdong provinces. Especially in Tibet, the arsenic content is almost higher than its average background value (11.31mg/kg). In Inner Mongolia and Xinjiang, although the arsenic content is high in a small area, it is generally lower than the average background value in other areas.

The distribution range of high cadmium content (> 0.32 mg/kg) is similar to that of As, but it is concentrated. High-value areas are mainly distributed in Yunnan, Guizhou and Guangxi, but there are also sporadic median to high-value areas of different scales in northern Hubei, southwestern Shaanxi and western Zhejiang. Qinghai-Tibet Plateau, Xinjiang, Inner Mongolia, Northeast China, North China and Southeast Coast all belong to low cadmium areas below the background value.

2.5 fluorine, boron and lithium

These elements are widely distributed in river sediments in China. The contents of boron, fluorine and lithium in most areas except Northeast China, North China, Northwest China, Southeast Coast of Shandong Peninsula and Hainan Island were higher than the average background values of 45.02mg/kg, 465mg/kg and 37. 1mg/kg, respectively. However, the enrichment degree of these elements and the geographical distribution of their high values and median values are not exactly the same. The distribution ranges of high and median values of boron and lithium are closer. Boron is highly enriched in Tibet, and lithium is high in Tibet, from the eastern Yunnan-Guizhou Plateau to western Hubei and Jianghan Plain. The high fluorine area is scattered in the south of China, which is similar to the distribution of lithium, but it is not obviously enriched in Tibet.

2.6 phosphorus

There are 1 band with high value (> 10.30 mg/kg) and 1 band with medium value (8.78 ~ 10.30 mg/kg) in China, and there are two bands with low value lower than the background value (647mg/kg). High-value areas are distributed in Altai Mountain, Daxing 'anling Mountain and Changbai Mountain. The middle belt extends intermittently from Yunnan-Guizhou Plateau to the southeast of Northeast China, showing an obvious northeast-southwest distribution. There are two low-value phosphorus areas: one is concentrated in the southeast and southeast coastal provinces of China and Hainan Island, and the other is from the Qinghai-Tibet Plateau through Gansu and western Inner Mongolia to western Liaoning.

2.7 aluminum

Most areas in China belong to the median area where the aluminum content is close to the average (6.52%) or slightly higher than the average. Only Yunnan, Fujian, Guangdong, Liaoning, Kyrgyzstan, Heilongjiang, northeastern Inner Mongolia, northern and eastern Xinjiang, southwestern Shanxi, eastern Sichuan, southern Tibet and the border areas of Henan, Hubei and Anhui have relatively high aluminum content [w (al) > 8%]. The median area of aluminum is mainly concentrated in Daxing 'anling, Changbai Mountain and southeast China. Inner Mongolia Plateau, Loess Plateau, Qinghai-Tibet Plateau, Xinjiang and Sichuan Basin all belong to low aluminum areas. Generally speaking, the average content of aluminum tends to increase gradually from northeast to southwest.

2.8 molybdenum

Macroscopically, there are two areas with high background value of NE-SW molybdenum in eastern China (> 1.90 mg/kg). One is located in the southeast coast, concentrated in Zhejiang, Fujian, Guangdong and other provinces, and distributed in pieces; The other extends intermittently from the eastern part of Yunnan-Guizhou Plateau to the northeast, passing through western Hunan, western Hubei, western Shandong and eastern Sichuan, until southern Shaanxi. In addition, there are discontinuous high background value areas in Northeast China, Xinjiang, Tibet and Qinghai. Inner Mongolia, Shaanxi, Gansu, Ningxia, western Sichuan, Liaoning, Shandong, Henan, Jiangsu, Anhui and Jiangxi form two low molybdenum belts which are roughly arranged in parallel.

3 combination of elements and spatial distribution

The distribution pattern of elements today is the result of the comprehensive action of element characteristics, regional geological background and natural environment. If we only discuss the regional geochemical characteristics marked by river sediments qualitatively, it is difficult to clearly distinguish the relationship between complex factors. However, the combination of elements in river sediments formed by various factors can still reveal the overall characteristics of regional geochemistry. The results of principal component multivariate correlation analysis between elements in different regions clearly show that five principal components can represent the cumulative contribution rate of 24 elements, but the overall correlation of each principal component is not very close. Therefore, we use orthogonal rotation and oblique rotation to analyze the results of principal component analysis step by step. See Table 5 for the rotation results. From the results of orthogonal and oblique rotation, we can judge the corresponding relationship between 24 elements and 5 factors, and analyze which factors play a leading role in the regional distribution of different element combinations.

3. 1 factor 1 is positively correlated with vanadium, chromium, cobalt, copper, iron, titanium, manganese and nickel.

From the analysis of geological background, these eight elements mainly come from the deep crust or upper mantle, or the remnants of early primitive mantle differentiation; Judging from the current environment, it belongs to elements that are difficult to migrate, have similar chemical characteristics, and are easy to aggregate and disperse during weathering. Therefore, the abundance of iron group elements in inland areas and Sanjiang areas is higher than that in plateau areas and coastal areas. Generally speaking, the dominant factor affecting the regional distribution characteristics of such elements in river sediments is the geological background, not the environment.

Table 5 orthogonal factor rotation: the orthogonal factor solution model of maximum variance rotation (VARIMAX)

Note: * shows obvious correlation.

3.2 Factor 2 is positively correlated with potassium, sodium, magnesium, aluminum, calcium, phosphorus and fluorine.

These seven elements are mainly concentrated in the upper crust, and their total average content in the surface layer of the crust is higher than that in the corresponding layer. Under the control of their own habits and behaviors, from northwest to southeast, with the change of natural environment, the leaching migration ability is enhanced, and the alkali metal and alkaline earth metal elements are gradually decreasing. This shows that their migration and accumulation are mainly restricted by the environment at that time, and the influence of parent rocks supplying river sediments is only in a subordinate position.

3.3 Factor 3 is positively correlated with boron, mercury, lithium and arsenic, and negatively correlated with strontium.

During the endogenous process, the contents of boron and lithium gradually increased from ultrabasic rocks to basic, neutral and acidic rocks, especially pegmatite formed by gas-hydrothermal solution after magmatic period. Under the supergene action, boron and lithium minerals are easy to decompose and precipitate, and are adsorbed by clay minerals, and the adsorption amount is proportional to the content of clay minerals. The content of river sediments is closely related to the modern environment, and the changing trend of low in the north and high in the south is roughly consistent with the regional distribution of clay mineral content. Arsenic and mercury are easy to be enriched after magmatic period, often accompanied by supergenesis and difficult to migrate. The sediment content in the northern river is lower than that in the southern river, which basically reflects the original conditions. The strontium content in pegmatite is the lowest, which is easily replaced by magnesium in supergene, and the content in river sediments changes from high in the north to low in the south. For different reasons, these five elements are combined and associated with factor 3.

3.4 Factor 4 has obvious positive correlation with molybdenum, lead and zinc.

These three elements often occur in hydrothermal solution associated with igneous rocks, and the magmatic activity in South China has created a good opportunity for the enrichment of these elements. As far as molybdenum is concerned, the content of granite in South China is not high (0.3 ~ 2.8 μ g/g), but it often accumulates in deposits rich in organic matter and iron sulfide under the supergene action of strong reduction. Lead and zinc are often closely related in the endogenous process. In the supergene process, carbonate is encountered in neutral environment to generate stable carbonate minerals, and sulfide precipitation is accompanied by reduction. It is some characteristics of * * * that make these three elements rise and fall in river sediments.

3.5 Factor 5 is only significantly related to cadmium.

No matter in magmatic rocks or hydrothermal solution after magmatic period, cadmium has not been obviously enriched, and it is often parasitic in zinc minerals and becomes a rare dispersed element. The migration ability of cadmium in supergene is very weak, and it is easy to stay in place and be adsorbed by clay minerals. It is precisely because of its unique habits that cadmium alone constitutes a single system.

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