1. Paleotectonic Environment Analysis of Metamorphic Volcanic Strata
Since Pearce and Cann (197 1, 1973) put forward a geochemical method to distinguish basalts with different tectonic backgrounds and establish a so-called "structure-magma discrimination map", a large number of papers on geochemical characteristics of volcanic rocks and intrusive rocks and their tectonic environments have supported the understanding that chemical composition can limit the tectonic background of magma origin. In addition, many related diagrams for distinguishing the genetic type, magmatic origin and tectonic background of magmatic rocks by using macro elements, trace elements and rare earth elements have been formed, and these diagrams have been successively included in Diagrammatical Discrimination of Original Rock of Metamorphic Rocks (Wang Renmin et al., 1987) and Using Geochemical Data: Evaluation, Representation and Interpretation (Hugh R. Rorison,/kl). However, it should be emphasized that most of the diagrams are summarized from experience, and many of them have their basic principles and application scope. However, some diagrams formed by active elements are not applicable to high-grade metamorphic volcanic rocks transformed by strong metamorphism. Therefore, when using related diagrams to judge the tectonic environment of metamorphic volcanic rocks, we should use them carefully on the basis of understanding the basic principle and application scope of diagrams.
Of course, simply or mechanically applying various tectonic-magmatic discriminant maps may lead to wrong judgments or even wrong conclusions, because the control of tectonic environment on the geochemical characteristics of rocks is very complicated. Only on the basis of correctly understanding and grasping the internal relationship between tectonic environment and rock geochemistry, through systematic analysis of the characteristics of major elements, trace elements and rare earth elements of a rock assemblage, and through comparison with the rock geochemical data in the known tectonic environment, can a reasonable conclusion be drawn. Among them, the discrimination marks of trace elements and rare earth elements may be more effective, and the discrimination effect of various cobwebs is better than that of binary or ternary discrimination diagrams of minority elements (Zhang Benren, 200 1).
An important problem in distinguishing paleotectonic environment by geochemical characteristics is that geochemical signs and diagrams used to distinguish tectonic environment are mainly based on the study of rocks in Mesozoic and Cenozoic plate tectonic environments, and whether they can be directly applied to the study of early Precambrian tectonic environment, that is, whether plate tectonic mechanism also played a role in early Precambrian is still controversial. The homogenization view holds that this tectonic mechanism can be traced back to Archean, while the non-homogenization view holds the opposite view. However, in recent years, a large number of Archean tectonic studies tend to think that there is a plate tectonic mechanism in Archean, especially in New Archean, but the characteristics of plate tectonics may be different from those of today's plate tectonics. These arguments remind us that we should be more cautious in judging the early Precambrian tectonic environment by geochemical markers, but we should not give up eating because of choking.
2. Paleotectonic environment analysis of clastic sedimentary rocks.
(1) Relationship between geochemical characteristics of clastic sedimentary rocks and tectonic environment: For clastic sedimentary rocks, the tectonic environment is also closely related to the geochemical characteristics of rocks. The tectonic environment restricts the provenance of sedimentary materials, controls the weathering, hydrodynamic transport, sorting and deposition of provenance, and thus controls the geochemical characteristics of clastic sedimentary rocks. For example, the material source in the island arc environment is mainly the newly born immature crust in the island arc area. Due to the large topographic difference, strong hydrodynamic transport and weak sorting, the clastic sedimentary rocks formed have relatively low K2O/Na2O value, low ∑REE and light rare earth elements, and low light rare earth elements. However, the sedimentary basin with passive continental margin environment originated from mature crust, with weak hydrodynamic force and strong sorting function during transportation. The fine clastic materials formed are characterized by high K2O/Na2O value, high ∑REE and LREE content, high LREE/HREE and high Eu/Eu* (high Wedepohl, 1995). Because the geochemical characteristics of clastic sedimentary rocks are mainly controlled by the chemical composition of provenance, weathering and denudation, transportation and sedimentation, the analysis of its sedimentary environment should also start from these aspects. Of course, for advanced metamorphic strata, it is necessary to exclude the influence of metamorphism.
(2) Chemical maturity and archaization degree of clastic sedimentary rocks: maturity of clastic sedimentary rocks is one of the important signs of sedimentary energy balance, which reflects the structural stability or instability of clastic sedimentary construction. In metamorphic clastic sedimentary rocks, the chemical indicators that determine the maturity of sediments are SiO _ 2 content and al2o 3/SiO _ 2 ratio, which reflect the contents of chronological, clay and feldspar in clastic sedimentary rocks. Generally, quartzite and mudstone/shale represent the two end members with the highest degree, while the sedimentary formation composed of quartzite-mudstone represents the sediments in the stable environment or craton sedimentary basin. Therefore, for metamorphic sandstone, the higher the SiO2 content, the lower the Al2O3/SiO2 ratio and the higher the maturity. Another useful chemical maturity is (Na2O+K2O) content and Na2O/K2O ratio. The former is also a measure of feldspar content, while the latter reflects the relative proportion of plagioclase and potash feldspar. According to modern weathering theory, plagioclase decomposes first, and Na, Ca and Sr are rapidly lost during weathering, so higher Na2O content means lower maturity of clastic sediments.
The chemical composition of metamorphic clastic sedimentary rocks can determine the archaization degree of its source area, which can be measured by chemical change index (CIA) or chemical weathering index (CIW). CIA = Al2O3/(Al2O3+Cao *+Na2O+K2O) (molar ratio) (naisbitt and Yang,1982); Ciw = Al2O3/(Al2O3+CaO*+Na2O) (molar ratio) (Harnois, 1988), where CaO* is the amount of apatite after deducting CaO (assuming that P2O5 is all in apatite). For clastic sedimentary rocks which suffered from high grade metamorphism, K2O may be active during metamorphism, so it is more appropriate to evaluate its weathering by chemical weathering index (CIW). Generally, the CIW value of fresh mafic felsic magmatic rocks is between 40 and 65 (high and Wedepohl, 1995), and that of shale rocks is between 80 and 90. The high weathering index, high weathering degree and low denudation rate of rocks in the source area indicate that the tectonic environment is relatively stable.
Chemical change index (CIA) or chemical weathering index (CIW) was originally used to explain the weathering degree of rocks, but in fact, it is also influenced by the composition of source rocks. The chemical change index or chemical weathering index of felsic source rocks is greater than that of mafic source rocks. Also affected by particle sorting during transportation, the CIW/CIA value of miscellaneous sandstone and sandstone is usually lower than that of mudstone or shale (Gaohe Wedepohl, 1995). In addition, the diagenetic process and subsequent metamorphism may also have an impact on this.
Triangular diagrams of (Cao *+Na2O)-Al2O3-K2O (Figure 5-4-4a) and (Cao *+Na2O+K2O)-Al2O3-(FeO *+MgO) (Figure 5-4-4b) can illustrate the weathering trend. These two maps show the weathering trend of average granite and average gabbro. The chemical composition of clastic rocks, especially mudstone, can be projected on the maps to infer the past weathering, and the deviation from the weathering trend may be caused by diagenesis and metamorphism.
Figure 5-4-4 (Cao *+Na2O)-Al2O3-K2O triangle (A) (according to Nesbitt and Young, 1982, 1989) and (Cao *+Na2O+K2O)-Al2O3-(FeO *+MgO).
In Figure A, the further weathering trend of granite is also shown. In Figure B, A and B represent the average weathering trends of granite and gabbro respectively, C is the diagenetic and/or metasomatism trend of kaolin transforming into illite with high K+/H+ ratio fluid, and D is the diagenetic and/or metasomatism trend of kaolin transforming into chlorite with high Mg2+/H+ ratio fluid. The chemical composition is expressed in molar ratio. CaO* represents CaO related to the silicate part of the sample, and FeO * = FeO*=FeO+0.8998×Fe2O3.
(3) Discriminating diagrams of tectonic environment: There are few diagrams that discriminate tectonic environment by using constant elements. K2O/Na2O-SiO _ 2 discriminant diagram of sandstone and mudstone (Figure 5-4-5) is commonly used in many documents, which can discriminate three tectonic settings: passive continental margin (PM), active continental margin environment (ACM) and ocean island arc. When using this graph, if the clastic sedimentary rocks are rich in carbonate components, it is necessary to convert the analysis data into data without CaCO3.
Fig. 5-4-5 Discrimination Diagram of K2O/Na2O-SiO _ 2 for Sandy Mudstone
(According to Roser and Korsch, 1986, quoted in Rorison, 1998)
Bhatia (1983) analyzed and summarized a large number of geochemical data of sandstone in different structural positions in modern times, and distinguished sandstone in four tectonic environments: ocean island arc, continental island arc, active continental margin and passive continental margin. The main chemical parameters used are: 1. 1 13× Fe.
As for trace elements, the diagrams of La-Th-SC and Th-SC-Zr/ 10 (Figure 5-4-7) designed by Bhatia and Crook( 1986) can also distinguish the miscellaneous sandstone of ocean island arc, continental island arc, active continental margin and passive continental margin.
Fig. 5-4-6 Distribution Map of Main Chemical Components for Discrimination of Structural Environment of Sand and Sandstone
(According to bhatia, 1983)
A- ocean island arc (black square); B- continental island arc (black triangle); C- active continental margin (black asterisk); D- passive continental margin (black dot)
For shale or argillaceous rocks, trace elements can also be analyzed by the spider web diagram standardized by post-Archaean craton shale (PAAS) or North American shale (NASC) (Figure 5-4-2). Compared with PAAS, the shale formed by ocean island arc has the characteristics of multi-element loss, while the shale in continental island arc and active continental margin has higher pro-MagmaElemental concentration and wide curve profile, and the samples in passive continental margin are similar to PAAS, with a gentle trend.
Fig. 5-4-7 (a) La-Th-SC discriminant diagram and Th-SC-Zr/ 10 discriminant diagram of complex sandstone.
(According to bhatia et al., 1986)
Ocean island arc; B- continental island arc; C- active continental margin; Passive continental margin
In a word, the research on the tectonic environment of clastic sedimentary rocks is not as deep as that of magmatic rocks. In the discussion, it is necessary to systematically analyze, sort out and verify the major elements, trace elements and rare earth elements in order to draw a correct conclusion. For clastic sedimentary rocks reconstructed by high grade metamorphism, the influence of metamorphism should also be considered.