Black mica

The biotite in this area mainly occurs in biotite amphibolite gneiss-granulite metamorphic from intermediate-acid rocks, biotite (garnet) syenite potassium gneiss metamorphic from argillaceous rocks and various perilla granites. It is also found in amphibole amphibole plagioclase granulite metamorphic basic rocks, but the content is less. The distribution of biotite in various rocks is uneven and its content changes greatly. In biotite amphibolite gneiss and biotite (garnet) syenite (adamellite) gneiss, the highest content of biotite can reach more than 20%, which is usually distributed in directional aggregates or bands, and the band direction is the same as that of gneiss. The particle size of biotite is generally 0.4 ~ 0.5 mm, and a few extend to 65438 0 mm; Polychromaticity is obvious, Ng is brown and Np is light yellow. In garnet-bearing perilla granite and general perilla granite, biotite is a large porphyritic crystal or fine aggregate that penetrates or surrounds pyroxene and amphibole. Some biotite is related to garnet and apatite. In addition, there are some small pieces of biotite wrapped in garnet. Therefore, biotite has at least three different stages of formation, that is, early biotite envelops garnet, middle biotite and garnet are formed, and late biotite meets amphibole. The early and late biotite is small flake, and the middle biotite forms large flake, which has deformation characteristics such as tearing, crystal bending and wave extinction.

The EPMA analysis results of biotite are shown in Table 4-6. The total oxide content is between 96% and 98%, and it is estimated that the crystal water content is more than 2%. The xmg [100 mg/(mg+Fe2+)] value of biotite is between 40 and 67, belonging to magnesite biotite and iron biotite. The XMg values of biotite of different rock types are obviously different. For example, the XMg value of biotite in mafic biotite amphibole plagioclase granulite (samples numbered 1 ~ 3 and 5 in Table 4-6) is 63.98 ~ 67.26, which is the highest value in biotite in this area. The XMg value of biotite in perilla granite (sample No.7 ~18 in Table 4-6) varies from 40 to 50. The XMg value of biotite (sample No.4 in Table 4-6) in biotite amphibolite gneiss of neutral protolith [w (SiO _ 2) = 57%] is 56.28, which is between the biotite in the above two types of rocks. It can be seen that the XMg value of biotite in mafic rocks gradually decreases from intermediate acid rocks to acid rocks, that is, from mafic biotite to iron biotite. It can also be seen intuitively from the contents of FeO and MgO in biotite that with the increase of rock acidity, the MgO content in biotite decreases and the FeO content increases accordingly. The argillaceous metamorphic rocks in this area, such as biotite in biotite garnet syenite potash gneiss (sample No.6 in Table 4-6), are different from biotite in other metamorphic supracrustal rocks. This biotite has lower XMg value, higher Al2O3 and FeO content and lower MgO content, which seems to be closer to the iron biotite in perilla granite.

Table 4-6 Chemical Composition Analysis Results of biotite (wB/%)

Note: electron probe analysis results: 1 ~ 4 (serial number, the same below) and 7. Tested by the detection room of China Geo University (Beijing). The rest were tested by Institute of Deposit Geology, Chinese Academy of Geological Sciences. 1 ~ 3. It occurs in Erhui plagioclase granulite and is collected from Guanzhuang, Shishan; 4. Produced from Heiyun Erhui plagioclase granulite, collected from Guanzhuang, Shishan; 5. It occurs in biotite amphibole granulite inclusions in Xueshan rock mass; 6. Pomegranate potassium gneiss from Heiyun West Line in Beixiazhuang; 7 ~ 10. It occurs in biotite amphibole granodiorite and Mashan rock mass; 1 1 ~ 14. it comes from perilla granite and xueshan rock mass; 15 ~ 16. from perilla adamellite, Niuxinguanzhuang; 17 ~ 18. It comes from garnet-bearing perilla granite and Caiyu rock mass.

In this paper, the classification diagram of Mg-(Al ⅵ+Fe 3++Ti)-(Fe 2++Mn) biotite and the classification diagram of Mg/(Mg+∑ Fe+Mn) biotite are used to treat biotite in various rocks as. What is more prominent is that biotite components in granulite and lherzolite granulite are very concentrated (Figure 4- 1 1), and most of them belong to magnesian biotite; The biotite in Mashan rock mass and other rock masses mostly belongs to iron biotite (Figure 4- 10).

Fig. 4-mg-(al Ⅵ+Fe 3++ti)-(Fe 2++Mn) classification diagram of10 mica.

1- granulite and lherzolite gneiss; 2- Mashan rock mass; 3- Snow Mountain rock mass; 4- Niuxinguanzhuang rock mass; 5— Cai Yu Rock Body

Figure 4- Taxonomy of biotite +0 1

1-granulite and lherzolite granulite; 2- cassiterite garnet gneiss; 3- Mashan rock mass; 4- Snow Mountain rock mass; 5- Niuxinguanzhuang rock mass; 6— Cai Yu Rock Body

According to the diagram of TiO _ 2-Fe×100/(Fe+mg) (Figure 4- 12), the metamorphic phase formed by biotite can be judged. The biotite in granulite and lherzolite gneiss in this area basically belongs to granulite facies, while some biotite in Mashan rock mass and Xueshan rock mass belongs to granulite facies, and most biotite in other rocks belongs to amphibolite facies, which is basically consistent with the results obtained from garnet and amphibole.

Fig. 4- TiO _ 2-[Fe× 100/(Fe+Mg)] diagram of biotite.

(The legend is the same as Figure 4- 1 1)

In mafic rocks, the composition of large directional biotite (0.6mm) and small non-directional biotite has no obvious change from the core to the edge, and the XMg values are all around 64, and there are basically no composition bands. The large biotite (deformed) and small authigenic biotite in Xueshan rock mass and Niuxinguanzhuang rock mass have no obvious changes, but the XMg value of biotite in Mashan rock mass [the sample with the serial number of 7 ~ 10 in Table 4-6] clearly distinguishes 40 and 50 groups. In addition, there are two kinds of biotite in garnet-bearing perilla granite, one is wrapped in garnet (sample number 18), and the other is massive biotite containing garnet (sample number17). Their XMg values are 50.02 and 40.49, respectively. The former is relatively poor in FeO and rich in MgO, while the latter is relatively rich in FeO and poor in MgO. Assuming that the garnet composition is fixed, the higher the XMg value of biotite means the lower the formation temperature. In addition, the Na and Al ⅵ of the former are much higher than that of the latter, indicating that the pressure of the former is higher than that of the latter. Therefore, among the above three types of biotite, there are obvious differences in composition between the first and second types of biotite, and the pressure conditions formed over time decrease and the temperature increases. However, due to the lack of analytical data of the third type of biotite, no obvious composition change was found between the second type and the third type of biotite.

From Mashan rock mass (sample No.7 ~ 10)-Xueshan rock mass (sampleNo. 12 ~ 14)-Niuxinguanzhuang rock mass (sampleNo. 15 and 16), the XMg value of biotite contained therein.

Using biotite-garnet minerals in perilla granite, it is estimated that the temperature of biotite formation is about 750℃, indicating the peak temperature of biotite formation.