The structure of metamorphic rocks can be divided into four categories according to their genesis.

1. Redundant structure

Due to incomplete recrystallization, the mineral composition and structural characteristics of the original rock are partially preserved, and the structure formed in this way is called rewriting constitutive model. Residual structure is common in shallow metamorphic rocks, but it can also appear in deep metamorphic rocks under the condition of uneven temperature and pressure distribution. Residual structure is an important basis for restoring original rock. In addition, the formation of residual structures is also related to the properties of original rocks. Generally speaking, the coarser the particle size of the original rock, the more stable the mineral composition and the easier it is to form the residual structure.

The name of residual structure can be prefixed with "residual" before the original rock structure.

(1) protolith is the residual structure of magmatic rocks, including afterglow green structure, residual granite structure, residual tuff structure and porphyritic structure. The most common is the embryonic patchy structure.

◎ Porphyry structure: After the basic extrusive rocks are metamorphic, the matrix may recrystallize into amphibole and plagioclase embedded with each other, but plagioclase or pyroxene phenocrysts in the original rocks are sometimes replaced by mineral aggregates such as amphibole, epidote, chlorite, sericite and calcite (Figure 7-6a); But the phenocryst shape of plagioclase or pyroxene still exists. However, after the metamorphism of acid extrusive rocks such as quartz porphyry and rhyolite, the mineral components in the matrix may have been completely transformed into quartz, sericite, alunite and chlorite. , but the porphyry is still well preserved.

Fig. 7-6 mottled structure (a) and residual sand structure (b)

(a) chlorite schist (Wutai, Shanxi Province, single polarized light, d=2mm): The pyroxene phenocrysts in the original rocks are the illusion of chlorite replacement, with a small amount of residue in the core, and the matrix has become chlorite and timely (cited by He Tongxing and others,1980); (b) Metamorphic gravelly chronotropic complex sandstone (Zhoukoudian, Beijing, orthogonal polarization, d=6.4mm): detritus is chronotropic and quartzite, and cements are sericite, biotite and chronotropic (quoted by You Zhendong and Wang Fangzheng, 199 1).

(2) The protolith is the residual structure of sedimentary rocks. In metamorphic rocks whose original rocks are sand and conglomerate, the shape of gravel or sand particles is often retained, but the cement has become sericite, chlorite and other minerals or has obvious recrystallization, which is called residual gravel structure or residual sand structure. In some metamorphic conglomerates, although sometimes the material composition inside the gravel may change after high-temperature recrystallization and strong deformation, the outline of the gravel is still clearly visible (Figure 7-6b).

2. Crystal structure

Metamorphic structure is the structure formed by recrystallization and metamorphic crystallization of rocks in solid state. Because it is different from the crystallization condition of magma from molten melt, the metamorphic structure is similar to the crystal structure of magmatic rocks in appearance, but it has many different characteristics: ① The mineral particles of metamorphic structure grow almost at the same time, and the porphyritic crystals and meta-matrix are formed at the same time, even a little later, which is obviously different from the porphyritic structure of magmatic rocks; ② There are many inclusions in metamorphic minerals, especially in phenocrysts; (3) The degree of automorphism of minerals in metamorphic structures does not represent the order of crystallization, but represents the size of mineral crystallization ability.

According to the degree of mineral automorphism in metamorphic rocks, this sequence is called metamorphic series. Under the condition of regional metamorphism, protoliths with different compositions have different metamorphic series, but the main trend is similar. The sequence is generally: sphene → rutile → garnet → tourmaline → staurolite → kyanite → andalusite → epidote → pyroxene → amphibole → magnetite → timely → plagioclase → orthoclase.

Metamorphic texture is the most common texture in metamorphic rocks, which can be further divided according to its granularity, shape and relationship.

(1) According to the relative size of metamorphic minerals

◎ Isogranular metamorphic structure: a structure in which the grain sizes of major metamorphic minerals are almost equal.

◎ Unequal-grained metamorphic structure: The main metamorphic minerals in rocks vary in size and change constantly.

◎ Porphyry metamorphic structure: There are massive metamorphic minerals in the fine mineral aggregate, which are called porphyry (Figure 7-7).

Figure 7-7 Speckle Crystal Structure (Single Polarization, 23x)

(quoted from Le Changshuo, 1984)

Andalusite slate, porphyritic crystals become andalusite.

(2) According to the absolute size of metamorphic mineral grains.

When a rock has an equigranular crystal structure, it can be divided into:

◎ Coarse-grained metamorphic structure: the average diameter of main mineral particles is greater than 3 mm ..

◎ Medium-grained metamorphic structure: the average diameter of main mineral particles is1~ 3 mm. ..

◎ Fine-grained structure: the average particle size of main mineral particles is less than1mm.

The grain size of metamorphic crystals generally increases with the increase of metamorphic degree, but under the same metamorphic conditions, the grain size of metamorphic crystals in different regions is not exactly the same, which is mainly related to the nucleation ability and growth rate of minerals. In the process of recrystallization, fluid will inhibit the formation of crystal nucleus, increase diffusion and increase particle size. In addition, the sensitivity of original rock properties to metamorphic reaction also affects the grain size of metamorphic rocks. For example, in a rhythmic layer in which the original rock is composed of sandstone, shale and limestone, because shale and limestone are sensitive to metamorphism, coarse particles are easily formed during metamorphism, while sandstone is insensitive to metamorphism, and its particle size increases little or even maintains its original particle size.

(3) according to the morphology of metamorphic minerals

◎ Granular metamorphic structure: also known as metamorphic crystal structure, metamorphic minerals are mainly composed of granular minerals (near equiaxed).

◎ Hornstone texture: The cryptocrystalline metamorphic texture (micro-granite porphyry texture) formed by contact thermal metamorphism of argillaceous rocks is called hornstone texture. The rock with this structure is gray-black, uniform, dense, hard and massive, like a horn, hence the name.

Scale metamorphic structure: most metamorphic minerals are flaky. Due to the different types of metamorphism, flaky minerals can be oriented, such as chlorite in chlorite schist; It can also have no directional arrangement, such as muscovite in greisen.

Columnar metamorphic structure: Most metamorphic minerals are always slender columnar minerals, such as amphibole and pyroxene. If the metamorphic minerals are fibrous minerals (generally the aspect ratio is greater than 1:5), they are called fibrous metamorphic structures, such as sillimanite and actinolite.

If rocks are composed of minerals with different shapes, the structural description of metamorphic rocks is: particle size+secondary mineral morphology+primary mineral morphology. For example, if a gneiss has a medium grain size, the secondary mineral biotite is scaly, and the main mineral feldspar is granular, its structure can be called medium-grained scaly granular metamorphic structure or medium-grained scaly granulized gneiss structure.

If the rock has a porphyritic crystal structure, the structure of the matrix is sometimes described. Such as the andalusite hornfel in Zhoukoudian, the whole rock is porphyritic metamorphic structure and the matrix is hornfel structure.

3. Debris structure

The structure produced by rock mechanical failure is called cataclastic structure. It can be further divided into:

◎ Breccia structure: when the original rock is fine, if the rock is slightly broken, breccia-like rock fragments will be formed, which is formed under the condition of tensile stress.

◎ Fragmentation structure: Most minerals in the rock are fractured or broken, and their edges are finely ground, but their original shapes are still retained. Some minerals have irregular serrated and angular edges. Flexible minerals (such as biotite) are distorted (Figure 7-8).

Figure 7-8 Fragmented Structure (Fragmented Rock)

◎ Fragment structure: When the rock is strongly broken, there are still large mineral fragments in the broken ultrafine particles, which are very similar to "phenocrysts" and are called fragments. They have irregular tearing edges, cracks and edge fragments. The broken fine particles are very similar to the matrix in magmatic rocks or sedimentary rocks, and are called "broken matrix" or matrix. This structure is often accompanied by foliation. If the directional pressure is strong and lasts for a long time, the broken spots can be eyeball-shaped, and the matrix part of the rock has obvious foliation, which is called broken eyeball structure.

◎ Mylonite structure: When the stress is very strong, almost all mineral particles are broken into particles, which are directional and have a flowing structure, leaving a small amount of slightly larger rigid mineral (such as timely) fragments, but they are often round and eyeball-shaped.

4. metasomatic texture

In metamorphism, due to the action of chemically active fluid phase, the original minerals are dissolved and replaced by new minerals, resulting in a structure called metasomatic texture.

False structure of metasomatism: During metasomatism, the primary mineral is replaced by another new mineral with different chemical composition, but the crystal form of the primary mineral remains, such as chloritization of garnet and serpentine of olivine.

◎ Metasomatic porphyritic structure: In the process of metasomatism, it can sometimes develop into quite large porphyritic crystals, which are called metasomatic porphyritic crystals. At the edge of migmatization or granite intrusion, metasomatic porphyry of albite or potash feldspar may appear in the area where alkaline solution is active. The differences between them and porphyritic rocks and ordinary porphyritic crystals are as follows: ① The distribution of porphyritic crystals in rocks is uneven and their sizes are different; ② It is often associated with metasomatic veins with similar components; (3) often cutting the schistosity of surrounding rock or other early structures; ④ metasomatic minerals sometimes exist in metasomatic phenocrysts in the form of inclusions; ⑤ The phenomenon of pushing apart the schistosity of metamorphic rocks is not obvious.