Yishui Maojiayao Fortune Telling _ Yishui Maojiayao Fortune Telling?

Genesis of Perilla Granodiorite: A Case Study of Mashan Rock Mass

Archean granitic rocks have many genetic models, and partial melting, magma mixing and surrounding rock contamination may all form granitic rocks. At present, most genetic models are based on the geochemical characteristics of Archean granitic rocks and experimental petrological data. According to literature statistics, among various genetic models, partial melting and magmatic differentiation are still dominant in the discussion of granite genesis, while the discussion of magma mixing and surrounding rock contamination is less. Xu Huifen (1998) and Su (1999) studied the genesis of perilla granodiorite (Mashan rock mass and part of Xueshan rock mass) in this area. They believe that perilla granodiorite was formed by partial melting evolution of metamorphic supracrustal rocks (biotite amphibole gneiss, etc.). ) Yishui Group. This section will discuss the genesis of perilla granodiorite (mainly Mashan rock mass) from geological characteristics, geochemistry and isotope geochemistry.

I. Genetic characteristics of geological occurrence

The perilla granodiorite in this area is represented by Mashan rock mass, which is mainly distributed in the south of Baijiaping-Luojiazhuang and the north of Maojiayao-Dongyuan, and is nearly equiaxed. In addition, there are also small areas exposed to the east of Shishan Guanzhuang and Li Hsing, which are triangular. This occurrence reflects the characteristics of magma domes and rock plants, which are consistent with the characteristics of different sizes of domes and rock plants in Erin-Ehrt Root Lake Complex in Lake Superior in North America and Perilla tonalite in eastern Hebei.

The surrounding rocks of perilla granodiorite in this area are mostly metamorphic supracrustal rocks of Yishui Group. This kind of rock has also undergone metamorphic transformation of granulite facies in many places (such as the north of Yanjiaguanzhuang and the Jiuziling area of Beixiazhuang), with different degrees of deep melting, but no perilla granodiorite. The fusion veins are mostly felsic, which is obviously different from the composition of perilla granodiorite. The relationship between perilla granodiorite and Yishui supracrustal rocks in different places shows that perilla granodiorite is not the product of deep melting of Yishui supracrustal rocks, and their relationship should be intrusive contact.

Many inclusions with different lithology often occur in perilla granodiorite in this area, some of which are large in scale, and the boundary with rock mass is sometimes blurred, so some researchers regard this feature as evidence that rock mass is the product of deep melting of supracrustal rocks (Xu Huifen,1998; Su et al., 1999). However, careful study of its composition and distribution characteristics shows that the inclusions in Mashan rock mass can be basically divided into two types, and their distribution has certain regularity. One kind of inclusion is large in scale, slender or lenticular, and its lithology is mainly composed of granulite and gneiss. The boundary between them and the rock mass is sometimes clear and sometimes unclear (due to simultaneous deformation and transformation with the rock mass), and they are mostly distributed at the edge of the rock mass. The other kind of inclusions is small, only a few centimeters to dozens of centimeters, mainly oval, and its internal foliation is often inconsistent with that of rock mass (see Figure 3-4); This small-scale inclusion is mainly composed of basic components (pyroxene amphibole, lherzolite granulite, plagioclase amphibole, etc.). ) and dispersed in the rock mass.

In the center of the main part of Mashan rock mass (Mashan area), the deformation is weak, the inclusions are few, and only small lumps are occasionally seen, which is basic; At the edge of rock mass, deformation is strengthened, gneiss is obvious, and the number and scale of inclusions are increased, mainly composed of granulite and gneiss. The deformation characteristics and inclusion distribution characteristics are similar to many Archean TTG rock series in the world. Such as southwest Greenland (meyers,1986; There are two kinds of inclusions in typical Archean TTG rock series, such as NcGregor, 1979) and Limpopo zone in South Africa (J.M.Barton et al., 1992). One is small (5 ~ 30cm), oval or spherical, and generally consists of a single lithology (mainly plagioclase amphibole, amphibole, pyroxene amphibole, lherzolite, garnet-bearing lherzolite, etc.). ), no directional distribution, can appear in different parts of the rock mass, and the boundary with the rock mass is clear. The other is generally large in scale (usually several meters to tens of meters, in some cases thousands of meters), layered and banded, with the same permeability foliation as rock mass, and concentrated on the edge of oval or dome rock mass. Generally speaking, the former is the source rock or residue of the TTG rock series, and the latter represents the Archean supracrustal rock series when the TTG rock mass was emplaced (Peng, 1993). Mashan rock mass is similar to this feature. Small massive basic inclusions may represent the remnants of its deep source rocks, while large granulite-gneiss inclusions only represent the inclusions on the crust captured during the intrusion of rock mass.

The second is the genetic information reflected by geochemical characteristics.

Geochemical composition not only reflects the characteristics of chemical composition of rock mass, but also is used to discuss the genesis of granitic rocks. Zhang Yuquan et al. (1995) divided granite into three genetic types according to its geological and geochemical characteristics: mantle-derived type (M type), crust-mantle-derived type (CM type) and crust-derived type (C type). Projecting the analysis results of major and trace elements of Mashan rock mass in this area into the relevant discriminant diagram, it can be seen that Mashan rock mass is mainly located in the CM type and M+CM type areas (Figure 3-48), and nearly half of the samples in the δEu and K-K/Rb diagrams of Na2O+K2O are located in the M area (Figure 3-48). It can be seen that Mashan pluton is mainly crust-mantle derived granite, which contains a certain amount of mantle-derived materials and a certain amount of crust-derived materials. Most of these granites are formed in the transition zone between the upper mantle and the lower crust.

Fig. 3-48 Schematic Diagram for Identifying Rock Source Area of Mashan Rock Mass

(Zhang Yuquan et al., 1995)

Archean granitic rocks are widely distributed and have complex genesis, which can be generally divided into TTG (tonalite, augite and granodiorite) and granite intrusive body (Condie, 1993). The former has a strongly fractionated rare earth model (5 < (La/Yb) n ≤ 150), low HREE content, similar differentiation degree between LREE and HREE, and Eu anomaly has no obvious distribution characteristics of rare earth elements. The difference between the light and heavy rare earths in the latter is moderate, and the HREE is gentle, but there is no obvious difference, and there are different degrees of negative Eu anomalies (EU/EU * = 0.2 ~ 0.7). There are obvious differences in rare earth characteristics between them. The characteristics of rare earth elements in Mashan rock mass have been discussed in the third section of this chapter. Most samples (the first and second types mentioned above) have obvious light and heavy rare earth fractionation, (La/Yb)n = 19.66( 10 sample average), and there is no obvious Eu anomaly, EU/EU * =1.000. These characteristics are consistent with the characteristics of rare earth elements in Archean TTG rock series. There are also a few samples in Mashan rock mass (the third kind mentioned above), and the fractionation of light and heavy rare earths is weak. (La/Yb) n =13.48 (the average of 6 samples), and the Eu is obviously negative anomaly, and EU/EU * = 0.49 (the average of 6 samples), which is similar to Archean granite. A large number of geochemical and experimental petrological studies show that the REE characteristics of Archean TTG-like granitic rocks were formed under the condition that basaltic rocks were partially melted and amphibole, clinopyroxene and garnet were the main residual phase minerals (Barker and Arthur, 1976 and1979; Jahn et al.,1981; Rapp et al.,1991; Martin, 1994). However, Archean granitoids are considered to be related to partial melting of pre-existing crustal materials (Sylvester, 1994). There are two different rare earth models in Mashan rock mass in this area, which shows that the main part of Mashan rock mass is formed by partial melting of basic rocks, and a small part may be related to the addition of crustal materials caused by the capture of supracrustal rocks in Yishui rock group during rock intrusion. The trace element characteristics of Mashan rock mass also prove this point. Pearce diagrams of most samples show that Archaean TTG rock series has three peaks (see Figure 3- 17), and only a few samples (Eu negative anomaly) have obvious losses relative to Rb and th, which is consistent with the characteristics of crustal remelting granite.

The REE characteristics of the main body of Mashan rock mass are similar to those of plagioclase gneiss or granulite in its surrounding rocks or inclusions (Figure 3-49), and some researchers regard this similarity as evidence that this rock mass is the product of deep melting of Yishui supracrustal rocks (Xu Huifen et al.,1998; Su et al., 1999). The author believes that this similarity cannot explain the genetic relationship between them. As we all know, different minerals have different distribution patterns of rare earth elements. The fractionation of light and heavy rare earths in feldspar changes greatly, and its outstanding feature is that it has obvious Eu positive anomaly; However, amphibole, garnet and other minerals have low light rare earth content and high heavy rare earth element content. If the influence of some minor minerals on the distribution of rare earth elements in rocks is not considered, the difference in the distribution of rare earth elements in major rock-forming minerals is the internal reason for the high concentration of light rare earth elements in some crust-derived granites, strong fractionation of light and heavy rare earth elements and abnormal europium. Especially in the case of low melting point. If Mashan rock mass is formed by partial melting of plagioclase gneiss or granulite in surrounding rock, the melt should be richer in light rare earth than the source rock because felsic material melts first, with heavy rare earth loss and abnormal negative europium. But this is not the case. Therefore, the similarity of rare earth elements in plagioclase gneiss in rock mass and surrounding rock can not explain the genetic relationship between them.

Fig. 3-49 Rare Earth Contrast Diagram of Mashan Rock Mass (A) and Surrounding Rock Oblique Gneiss (B)

The experimental data of partial melting of amphibole in Rapp( 199 1) are plotted in "A", where A is the melting experimental sample and B is the melt composition.

Rapp et al. (199 1) conducted partial melting experiments on four basic volcanic rocks in different regions of the United States under different temperatures and pressures to simulate the genesis of Archean TTG rocks. Among them, the original rock of sample No.2 is flat tholeiite. Under the conditions of t = 1000 ~ 1050℃ and p = 2.2 ~ 3.2 GPA, the melt composition is tonalite; As shown in figure 3-49-a, the rare earth mode of parent rock and melt is obviously fractionated. It can be seen that the rare earth distribution pattern of its melt is basically consistent with the rare earth characteristics of the main part of Mashan rock mass in this area. It can be considered that the main body of Mashan rock mass is probably the product of partial melting of basic rocks, rather than the product of partial melting of plagioclase gneiss in Yishui rock group.

Three. Genetic information reflected by isotope geochemistry

Because the crustal material is relatively richer in Rb and nd than the mantle material, the continental crust and the mantle magma source region have completely different Nd and Sr isotopic compositions. According to the isotopic composition of Sm-Nd and Rb-Sr, the material source and genesis of Mashan rock mass can be further discussed.

The primitive depleted mantle evolved to Neoarchean, and its εNd(t) value was between +3 and +4.5. The Sm-Nd isochron age of Mashan pluton is 2688Ma (see Chapter 6 for details), and its εNd(t) value is +3. 1 17, which is close to the loss mantle value at that time. In order to further illustrate this problem, the author selects gray gneiss with different origins in the same period for comparison. The terminal (T) value of the gray gneiss (2700Ma) in the source region of the depleted mantle in Taishan area is +3.3 (Jiang Boming et al., 1988). At least a part of the gray gneiss (2700~2772Ma) in the Bangfim metamorphic complex in Brazil is reconstructed from ancient siliceous alumina (Teixeira, 1996), and its εNd(t) values are -0.3 1 and -2.04. Obviously, the εNd(t) value of Mashan pluton is very similar to that of Taishan grey gneiss from deficit mantle, but obviously different from that of Brazil. This shows that Mashan pluton mainly comes from the source region of depleted mantle. Mashan rock mass is formed by partial melting of basic rocks. Combined with the Nd isotopic characteristics, it can be shown that these basic rocks mainly come from the source region of the depleted mantle, and it is impossible to stay in the crust for a long time, because their εNd(t) values are close to those of the depleted mantle at that time.

For Sr isotopes, a low ISr value (less than 0.70 15 for Neoarchean rocks) usually means that the rocks originated from the mantle or have a short crustal residence time. High ISr values usually indicate that rocks come from ancient silicon-aluminum shells. The Rb-Sr isochron age of Mashan rock mass is 2383 Ma, and the ISR value is 0.703. From the ISr value alone, its value is high, showing the genetic characteristics of the crust; The ISr value is calculated according to 2383Ma, which obviously cannot represent its original state. In order to eliminate the influence of the late transition age, the εSr(t) and εNd(t) values of each sample are calculated with 2688Ma(Sm-Nd isochron age) (see Chapter 6 for the original analysis data) and projected on the εNd(t) vs εSr(t) diagram. Mashan rock samples (ε nd (t) =+2.45 ~+3.37, ε Sr (t) =-4.19 ~-60.31) are all distributed in the second quadrant of the figure, that is, the range of mantle arrangement; Unlike Phanerozoic mixed granite with crust and mantle, most of which are located in the mantle array (the second quadrant) and the end members of the crust (the fourth quadrant), almost all samples of Mashan rock mass are located in the mantle array (Figure 3-50), which indicates that its main body comes from the mantle source area. Mashan pluton belongs to normal 18O granite, and its lower end (that is, δ 18O is between +6.0 and +8.5) may be related to mantle magma separation or anatexis.

Fig. 3-50 Nd-Sr Isotopic Relationship Diagram Mantle Source Region and Crust-Mantle Mixed Granite

(Except the data in this area, other data are quoted from Zhu Bingquan and others 1998)

YDY-mantle-derived granite area of Kerguelen Islands in Indian Ocean: HN- South China crust-mantle mixed granite area; XZ crust-mantle mixed granite area in Lhasa terrane, Tibet

According to the above discussion of geology, geochemistry and isotope geology, we can draw the following conclusions: Mashan rock mass is an intrusive granodiorite body, and some rock masses were polluted to varying degrees when Yishui captured xenoliths; The main body of Mashan rock mass is produced by partial melting of basic rocks, and its parent rock mainly comes from the source region of depleted mantle and stays in the crust for a short time.