Journal of Geosciences of China
Vol.1 No.1, Dec.1999
http://www.geosciences.net
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GEOCHEMICAL FIELD OF ELEMENTS AND ITS GEOLOGICAL IMPLICATIONS Guoneng CHEN Department of Earth Sciences, Zhongshan University, Guangzhou, 510275,China
Abstract The in-situ hypothesis on the origin of granite reveals the concentration-dispersion trends of various chemical elements in endogenic processes. Based on this as well as the periodic table of elements, the Geochemical Field of Elements (GFE) concept is derived. It unravels the close relation between the configuration of elements and their spatial distribution, exhibiting the geological sections of granite bodies, of the continent and of the earth. In addition, Geochemical Field of Elements shows the cyclic process of continental materials, and demonstrates that melting within the crust is a key link in the formation and evolution of the continental crust.
Key words:
in-situ melting / geochemical field of elements / melting interface / continental material cycle / shelled or layered structure of the earth
Introduction
as the concentration-dispersion trends of elements in endogenic process corresponding to the periodic law, and the spatial distribution pattern of elements in the Earth resulted in thereof. From the definition above, it is easy to distinguish GFE from the traditional concept of Geochemical Classification of Elements (GCE for short) (Zhao and Zhang, 1988; Roster and Lange, 1972). GFE is based on the research of material evolution of the crustal interior started from a geological angle, revealing the concentration-dispersion trends of elements in endogenic process and the spatial distribution pattern of elements concealed in the periodic table. GCF, on the other hand, is on the affinity of elements started from a chemical angle, revealing the natural assemblage of elements (Zhao and Zhang, 1988; Roster and Lange, 1972).
Granites are the rocks that belong to the continent and compose the main part of the upper crust. Thus their origin should be solved first of all in the studies of material evolution of the crustal interior (Chen, 1997). A new model about the origin and mineralization of magmatic granite referred to In-situ Melting Hypothesis has been advanced in the recent years (Chen, et al, 1996). The strong points of the hypothesis are as follows: 1.Most granites originate from the change of material in property (from solids to melts) caused by the variation of temperature within the crust, and resulting in the redistribution and reorganization of elements. 2.The variation of chemical and isotopic compositions of granite reflects merely the material evolution of magma system, but not the sources of magma. 3.The granite formed by in-situ melting is in a layer form. Batholiths are nothing than the protruding part of the upper limits of the granite layers (melting interface, MI for short), and their size and shape mirror only the geometric relation between the MI and the plane of denudation. 4.The ore-forming elements of granite, as its rockforming ones, are also derived from the rocks in situ. On the basis of the studies above, and of the data of numerous granite bodies and the related ore-deposits, it is found that some elements used to concentrate in the upper part of a magma layer, some in the bottom, and some move out of the magma system (Chen, et al, 1996). From this the concept of Geochemical Field of Elements (GFE for short) is derived, which is defined
1. Introduction to the Geochemical Field of Elements According to the concentration-dispersion trends of elements in endogenic process unraveled by the Insitu Melting Hypothesis (Chen. et al., 1996), GFE is divided into four fields with the help of the periodic table, namely, Magma Field, Hydrothermal Field, Medium Field and Gas Field (Fig.1).
1.1 Magma Field Elements stayed mainly below MI (melting interface) during the melting-crystallizing process belong to the Magma Field (Fig.1). They are largely oxyphile. Though with a dual nature of oxyphile and chalcophile affinity in the nature, Fe, Co and Ni display rather strong oxyphile character during the
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Guoneng CHEN: Geochemical Field of Elements _______________________________________________________________________________
Fig.1 Geochemical Field of Elements crystallization of granitic magma, usually incorporated into the melanocratic and accessory minerals. Magma Field is further divisible into the upper and the lower subfields: the former lies at the center of GFE and the latter at its periphery.
Therefore, Al is considered as an element with dual nature of both the upper and the lower subfields. (2) The upper subfield Elements in this subfield Include Group IA of the periodic table from Li to Cs, and Be, Ba, Si, B as well as the rare element family in Zavaritsky's GCF (except LREE). These elements usually tend to concentrate toward the upper part of magma layer during the melting-crystallizing process. Therefore, their contents commonly decrease from top downwards in a matured rock body or in the latest member of a composite batholith that is regarded as the product of multi- melting (Chen et al., 1996).
(1) The lower subfield Elements in this subfield, including the siderophile, Mg, Ca, Sr and LREE, tend to move toward the lower part of magma layer. The reasons are (Chen et al., 1996; Chen, 1993). (a) During the melting process, they are generally released in a later stage and constitute the main part of residua, thus migrate downward under gravitation; (b) During crystallization of magma, minerals made up by these elements are usually crystallized at an earlier stage, thus migrate downward again. The downward motion of Sr and LREE is due to the fact that they usually replace Ca in calcium-bearing minerals For the reasons above, the content of these elements generally increases from top downward in a highly evolved granite body, which can be observed in the rock bodies of liu Ao-ling, of Dajishan, of Xiao Taoyuan, etc in South China (Chen et al., 1996). Statistics on numerous granite bodies indicates that the vertical variation trend of Al is different from one to another (Chen et al., 1989). Such dual character of Al is also demonstrated by its concentration factor in different rock series: decreasing from basic to acid member in the calc-alkaline series and with the opposite trend in the alkaline series (Liu et al., 1985).
1.2 Hydrothermal Field Hydrothermal Field is located in the lower part of GFE. With W as a dividing line, elements on the upper right of the field are strongly chalcophile while those in the lower left are strongly oxyphile. Under sulfur-rich conditions, the former are usually not incorporated into silicate minerals but concentrated in hydrothermal solution and expelled from magma system when decompression of the system. During the upward moving with the hydrothermal solution, these elements are mostly precipitated as sulfide accumulation, fixed again in the covering rock-strata above MI. Uranium and thorium in the lower left also tend to concentrate in hydrothermal solution though their oxyphile character is stronger. When the medium is rich in sulfur, uranium occurs in the form of U4+, which
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Journal of Geosciences of China
Vol.1 No.1, Dec.1999
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can extensively replace Zr, Hf and REE isomorphically in magmatic stage. Uranium will disperse in accessory minerals (Chen et al., 1996; Liu et al., 1985) and at this time occur as an element of Magma Field. Under oxidation condition, uranium can form complex cation (UO2) 2+, i.e. uranyl, which cannot replace other cations due to its large ionic radius, and is therefore concentrated in the hydrothermal solution. Multiple melting usually causes a rise in oxygen fugacity in the magma system (Chen et al, 1996), which may explain why uranium mineralization is often much younger than the mother rock.
This field includes all the inert gas elements that used to not take part in most chemical processes, and after expelled from magma system, mostly to migrate into the atmosphere. In other words, the atmosphere is their destination in the melting-crystallizing process. Hydrogen, a special element, was originally placed in Medium Field on its behavior in hydrothermal stage (Chen et al., 1996). However, If in consideration of the distribution of elements in the universe (Chen and Yao, 1999), hydrogen should belong to Gas Field. For the reason, a little improvement of GFE on the original one (Chen et al., 1996) is made in this paper (Fig.1).
1.3 Medium Field This field lies in the right upper part of GFE and includes C, N, O, P and all the members of Group A. These elements are also expelled from the magma system after satisfying the crystallization of silicate minerals. Different from those of Hydrothermal Field, these elements are hardly fixed in the lithosphere in the form of solid mineral after expelled from the magma system, instead, move into the hydrosphere. In other words, during the hydrothermal process, these elements are generally not to form deposits but to act as a medium or a carrier for transportation of ore-forming materials. After the precipitation of ore-forming elements, the medium will flow into the hydrosphere. Some studies have shown that if the magma is enriched in volatile, phosphorus tends to concentrate toward the volatile component during the crystallization of magma (Liu et al., 1985). Nevertheless, the enrichment of P in hydrothermal deposits is seldom observed. For the reason, P is temporally regarded as an element of Medium Field.
2. Implications of GFE in Ore-Searching It has been proven by the observation of numerous ore-deposits and their relevant granite bodies in South China that the mineralization is usually zoned as follows. (1) From the top of an ore-bearing granite body downwards, the enrichment zone of REE used to be located at a relatively deeper part with the light members beneath the heavy (Chen et al., 1996; Chen et al., 1989). Over the REE zone is the enrichment zone of Ni-Ta, with Ta positions higher and Ni below (Fig.2). This is in consistence with the fact that the mineralization varies from Ta Ni yttrium earths cerium earths along with the increase of the exposed area of ore-bearing granite bodies (Chen et al., 1989). (2) Elements, W and Mo are generally found in the cover of a granite body nearby the contact plane (Fig.2). In tungsten-molybdenum veins, the enrichment section of Mo commonly underlies that of W (RGMIM, 1985).
Fig.2 Sketch showing mineral zoning in No.430 rock body The left is a plan (after Wang et al., 1988) while the right is a profile (not to scale)
(3) Elements Cu, Au, Ag and Zn are usually above W and Mo, located farther from the contact plane (Chen et al., 1989; Wang et al., 1988) (Fig.3). (4) The precipitation of Hg needs the temperature much lower than that of the other transition elements; thus Hg is usually located in the farthest place in
hydrothermal stage (Wang et al., 1988) (Fig.3). By turning GFE anti-clockwise with Gas Field at the upper part, it can be found that the zoning of transition elements in and above an ore-bearing granite body is clearly reflected in GFE by a zigzag path (Fig.4).
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Guoneng CHEN: Geochemical Field of Elements _______________________________________________________________________________
Fig.3 Sketch showing mineral zoning (Modified after Wang et al., 1988) (a).Ore zoning in Qibaoshan Mine, Hunan Province (b).Ore deposits and geochemical anomaly related with the concealed rock body of Longxianggai, Guangxi Province
1.Cu-Zn-Sn deposit; 2.Pb-Zn-Sn deposit; 3.Zn-Sn deposit; 4.W occurrence;5.skarn (corresponding to the projection of the top of the rock body); 6.Hg deposit; 7.geochemical anomaly; 8.heavey mineral anomaly
Fig.4 Distribution of transition elements above and below MI as compared with their positions in GFE
(Dispersed elements are bracketed)
3. GFE and the Material Cycle of Continental Crust
GFE 90 anti-clockwise. Hence the Magma Field below MI represents the granitic llayer, the Hydrothermal Field should present the covering rock strata, which is called consolidated-metamorphic layer. Materials of this layer are mainly derived from two sources: originally the sediments that have consolidated or even metamorphosed; and later, the elements of Hydrothermal Field expelled from the magma system during the melting-crystallizing processes.
As mentioned previously, granites forming through in-situ melting are layer-like. MI (the melting interface) marks the interface of the two phases, melts and solids while melting, and now is represented by the contact plane between granites and their surrounding rock-strata. The position of MI in GFE lies between Magma Field and Hydrothermal Field with the former below and the latter above by turning
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Journal of Geosciences of China
Vol.1 No.1, Dec.1999
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_______________________________________________________________________________ The destination of the elements of Gas Field is the atmosphere and of those of Medium Field is the hydrosphere after expelled from magma system. Therefore, the dividing line between Gas Field and Magma Field (granitic layer) in GFE should represent the weathering interface (WI), and that between Gas Field and Medium Field represent the upper limit of accumulation of sediments, i.e. the accumulation interface (AI). Besides, the line between Medium Field and both, Magma Field (granitic layer) and Hydrothermal Field (consolidated-metamorphic layer), should present the oxidation-reduction interface (ORI). Oxygen and sulfur are respectively located above and below ORI in GFE (Fig.5), which is consistent with the satiation that the sulfides underground change into oxides or hydroxides. The layer between AI and ORI is designated as the
soft accumulative layer on the earth's surface, in which the elements of Medium Field are evidently most abundant and active. This layer is mainly originated from that the elements in both Medium and Gas Fields interact with those in Magma Field and Hydrothermal Field under the normal atmosphere temperature and pressure. In the upper-left of GFE, only WI is present, i.e. granite layer directly contacts with Gas Field (Fig.5). This means that deep-seated granite will eventually be exposed to the surface to experience weathering and erosion. In fact, the orogen and uplift region are often the place where granite is exposed. Besides, the above phenomenon can also be explained as that the primitive sediments on the earth's surface should have been derived from weathering and erosion of the primitive igneous rocks.
Fig.5.The layered structure of the continental crust unraveled by GFE AI is for the accumulation interface, WI for the weathering interface, ORI for the oxydation-reduction interface and MI for the melting interface
On the right-hand side, there are AI and ORI. Accumulation regions on the earth's surface are often represented in landscape by plain or lowland where the important elements of biosphere as C, N, O and P, and the halogens F, Cl, Br and I are abundant. Thus it is readily understandable that the right-hand side of GFE should represent the accumulation region while the upper left, the denudation. Compared the layers and interfaces in GFE (Fig.5) with those in a common crustal profile (Fig.6), it may be seen that GFE does exactly reflect the structure o f continental crust.
Apart from the layered structure, GFE unravels the cyclic process of the crustal material. As shown by the arrows in Fig.5 and Fig.6, the old igneous rocks are subjected to weathering in the denudation region, and then to be transported to and finally deposited in the lowlands, forming the accumulation layer. Through diagenesis, sediments in this layer change into rocks, constituting the main part of the consolidated-metamorphosed layer. Once the geotemperature going up, part of the consolidatedmetamorphosed layer changes into the magma layer by melting, and during the process, gravitation makes the heavy materials move downwards, thus
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Guoneng CHEN: Geochemical Field of Elements _______________________________________________________________________________
Fig.6 Geological interpretation of GFE WAL is for weathering-accumulation layer; CML for the consolidated-metamorphosed layer; GL1 and GL2 respectively for the older and younger granite layer; and F for fault (other symbols as Fig.5)
compositions with that of granite, a vertical zoning pattern of the elements below the Earth’s surface can roughly be observed (Table 1).
the new igneous rocks forming through the process are more acid and alkaline than the old ones. After uplifted to the Earth's surface, the new igneous rock layer formed by crustal melting is again subjected to weathering and erosion. A crustal material cycle is thus formed. It may see that melting or remelting of the preexisting rocks should be the key link for the formation of continental crust in the above-mentioned cycle. And missing this link in the oceanic cycle, on the other hand, should be the reason that a granite layer is hardly formed in the evolutionary process of the oceanic crust.
Table 1. Average contents of the lower magma field elements in major igneous rock (ppm)
4. GFE and the Shelled or Layered Structure of the Earth Magma Field is divisible into the upper and lower fields, which sheds lights not only on the directions of element migration during the melting-crystallization process, but also on the spatial distribution of elements within the Earth. The three elements Fe, Ni and Co are located in the center of GFE, constituting also the core of the Earth. The dividing line between the upper and the lower magma fields represents interface between the granite layer and the underlying layer. From this interface downward should be the area where elements of the lower magma field are concentrated. Of course the concentration areas are different from element to element. Assume that the ultrabasic rock are from the mantle and the basic from the lower crust, then we compare their chemical
Element
Ultrabasic rock
Basic rock
Intemediate rock
Granite
AI Ca Sr Mg Mn Sc Ti V Cr Co Ni Fe
4500 7000 10 259000 1500 5 300 40 2000 200 2000 98500
87600 67200 440 45000 2000 2.4 9000 200 200 45 160 85600
88500 46500 800 21800 1200 2.5 8000 100 50 10 55 58500
77000 15800 300 5600 600 3 2300 0 25 5 8 27000
After Vinogradov (1962)
As shown in table 1, the highest contents of Al and Sr is in the intermediate rock, and Ca, Mn, Ti and V in the basic while the others in the ultrabasic. Hence, although we know very little about the material composition of the various layers of the Earth, yet we can believe that the average chemical composition is different depending on depth. Relevantly, a quarter of cross section from the core to the granite layer of the entire Earth is clearly exhibited in GFE (Fig.7).
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Journal of Geosciences of China
Vol.1 No.1, Dec.1999
http://www.geosciences.net
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Fig.7 Relation between the magma field and the shelled structure of the Earth
Conclusions
References
The periodic table has described the regularity of the structural variations of elements whereas GFE established in this paper unravels both the concentration-dispersion trends in endogenic process and the spatial distribution pattern of elements concealed in the periodic table. The In-situ Melting Hypothesis (Chen et al., 1996) can be regarded as the geological interpretation of GFE. The three geological sections displayed in GFE, i.e. the section of a granite body, of the continental crust and of the globe, exhibit the unity between the microscopic structure of material and their macroscopic distribution in the nature. GFE has also unraveled both the cyclic process of the continental material and the possible formation way of the continental crust. The "continental crust" formed in somewhere of the lithosphere could be due to where was once or even more times subjected to melting in geological history, leading to the redistribution and re-organization of elements and the formation of granitic rocks. According to the mechanism of Plate Tectonics, such situation is not able to occur within the oceanic crust thus, in which no granite layer can be formed. This research is financially supported by both the Science Fund and Doctoral Fund of National Education Ministry of China
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