There are 20 kinds of common metamorphic minerals in metamorphic rocks, and the main optical characteristics of the thin slices are as follows:
(1) aluminum-rich minerals
Under the condition of chemical series, it shows that the original rock is argillaceous rock or the original rock is rich in argillaceous (tuffaceous) felsic rock.
1. corundum (Crd)
The flake is colorless or light blue, with columnar, plate-like or granular crystals, no cleavage, cracks, proper protrusion height (plate I- 1), and grade I gray interference color; However, due to its high hardness and thick thickness, it can reach class II blue interference color (plate I-2). Corundum is one of the typical high-temperature minerals, which is commonly found in rocks rich in A 12O3 but lacking in SiO2 _ 2.
2. Andalusite (and)
The thin slice is colorless, sometimes slightly pink-colorless, with weak polychromatic, and the cross section is square rhombic column (plate I-3), in which two groups of nearly orthogonal cleavage can be seen, and a group of (columnar) cleavage can be seen on the cylinder (plate I-4) with protrusions in the middle; The highest interference color is grade I yellow, with parallel symmetrical extinction and negative ductility. Crystals often contain inclusions such as carbon, which are diagonally distributed in the cross section and are called kyanite (Plate Ⅰ-5). It is often one of the typical minerals of argillaceous rocks that have undergone low-temperature and low-pressure regional metamorphism (low-pressure greenschist facies) or low-level thermal contact metamorphism (andalusite (sodalite-green curtain) amphibole facies).
3. Kyanite
The thin slice is colorless, sometimes slightly blue, and is plate-like (plate I-6) extending along the C axis, showing {100} cleavage, with {00 1} lateral cleavage developed and protruding at the correct height. The interference color is I-level top, ng ∧ C ≈ 30, but Np is almost vertical on the cross section. It is one of the typical minerals of medium-pressure metamorphic facies series.
4. sillimanite
Flaky colorless, usually fibrous, bundle-like aggregate (plate I-8), {00 1} cracks and develops, making the crystal "bamboo-like"; The center height is raised, the interference color is Grade II (Plate I-9), the extension direction is parallel extinction, the cross section is symmetrical extinction, and the ductility is positive. It is one of the typical metamorphic minerals with low pressure and high temperature.
(2) minerals rich in calcium
There are many indications that the original rock is carbonate rock or replaced by carbonate (calcium-rich) fluid.
1. zoisite
Zoisite belongs to the orthorhombic system, and all the flakes are colorless columnar or granular, which are highly prominent (plate II- 1), and there are abnormal interference colors (dark blue, brown or indigo blue, plate II-2) in the middle and lower part of Grade I interference color. Usually formed by plagioclase alteration, it also exists in medium-high pressure metamorphic facies series.
2. pumice stone
The flake is colorless or light yellow, light brown, slightly multicolored, columnar, granular or radial aggregate (plates II-3 and 4), with positive height protruding, grade I gray interference color, often with abnormal brown or blue interference color, sometimes with uneven interference color on the same section, sometimes with annular structure, parallel extinction and negative ductility. It is commonly found in calcareous rocks that have undergone low-temperature and low-pressure regional metamorphism (low-pressure greenschist facies-amphibole facies) or low-temperature thermal contact metamorphism (andalusite amphibole facies-common amphibole amphibole facies).
3. Andalusite (Sc)
The pigment flakes are colorless or turbid, columnar or granular, and the protrusions may have negative middle-low protrusions, depending on the composition. Two groups of nearly orthogonal cylindrical perfect cleavage (plates II-5 and 6) have parallel extinction and negative ductility, and the highest interference color can reach Grade III (calcium-rich andalusite), while sodium-rich andalusite is Grade I gray, and special speckle interference color can sometimes be seen in those with high interference color. It is often one of the representative minerals of calcareous rocks that have undergone regional metamorphism at medium-low temperature and low pressure (low pressure greenschist facies-amphibole phase) or medium-high heat contact metamorphism (common amphibole-pyroxene amphibole phase).
4.diopside
The flakes are colorless, short columnar or granular aggregates, and the height is raised. Two groups of nearly orthogonal cleavage can be seen on the cross section (plates ⅱ-7 and 8), and only one group of cleavage can be seen on the longitudinal section, with interference color grade ⅱ; The extinction angle is large, ng∧c = 38° ~ 48°, often less than 40. It occurs in calcareous and ferromagnesian rocks that have undergone regional metamorphism at medium temperature and low pressure (low pressure amphibolite facies) or advanced thermal contact metamorphism (pyroxene amphibolite facies).
(iii) Fe-Mg minerals
There are many indications that protoliths are basic rocks or sedimentary rocks with similar compositions.
1. chlorite
The flakes are light green, with weak polychromatic, flaky or scaly aggregates (plate III- 1), with positive and low protrusions, and the interference colors are grade I gray to grade I yellow. Chlorophyll has an abnormal interference color of dark blue or rust brown (plate ⅲ-2), which is close to parallel extinction. Formed under the condition of low temperature and low pressure metamorphism, it is one of the representative minerals of low pressure greenschist facies.
2. epidote
The flakes are yellow-green, multicolor (plate III-3), columnar or granular, highly prominent, with strong interference colors (orange, green, blue-green or crimson, plate III-4) of class II to III shadows, uneven distribution of interference colors within particles, and parallel extinction of cylinders. Formed under the condition of low temperature and low pressure metamorphism, it is one of the representative minerals in greenschist facies, and is also common in altered rocks.
3. Hard chlorite
The flakes are gray to dark green, multi-colored, flaky or absinthe-like aggregates (plates III-5 and 6), with high protrusion, I-level interference color, oblique extinction and negative ductility. It is one of the representative Fe-Mg minerals at low temperature and low pressure.
4. actinolite
Flaky light green-yellow-green, weakly polychromatic (plate III-7), columnar, fibrous or radial aggregate with amphibole cleavage, protruding in the middle, interference color slightly lower than tremolite, and the highest is from the end of Grade I to the middle of Grade II (plate III-8), ng ∧ c = 1 1. It is commonly found in metamorphic mineral assemblages rich in iron and magnesium at low pressure and low temperature.
5.tremolite
The flake is colorless, columnar or radial, with amphibole cleavage (two groups of oblique cleavage), medium-high protrusion (plate ⅳ- 1), and the highest interference color is orange (plate ⅳ-2), ng ∧ c = 16 ~ 2 1. It is one of the common minerals in calcareous and ferromagnesian rocks that have undergone low-pressure and low-temperature metamorphism.
6. glaucophane
Dark blue, special multicolor, ng- dark blue, nm- reddish purple, NP- colorless or light blue-green, light yellow-green, long column (plate ⅳ-3, 4), hornblende cleavage, medium-high protrusion, the highest interference color is less than one level, and the extinction angle is small, ng ∧ c = 5 ~ 7. Glaucophane is one of the typical minerals with low temperature and high pressure (blue schist facies).
7. staurolite
The flake is bright yellow (plate ⅳ-5), with obvious multicolor, columnar or granular crystals, often containing a large number of matrix inclusions and showing a sieve-like metamorphic structure, with appropriate protruding height, and the top interference color is Grade ⅰ (plate ⅳ-6); The longitudinal plane is parallel extinction and positive ductility, and the transverse plane is symmetrical extinction. Sometimes you can see twins interspersed on a cross. It is one of the representative Fe-Mg minerals that are metamorphic at medium temperature and low pressure.
8. Cordierite (rope)
Flakes are colorless, mostly irregular and granular, plum blossom-shaped or spindle-shaped (plate IV-7), with negative low and positive low protrusions, and the interference color is grade I gray, and twins (triple, hexa-crystal and multi-flake twins) are often found (plate IV-8). Biaxial crystal (which can be distinguished from synchrotron) has both positive and negative optical properties, and its optical axis angle is large. The refractive index and birefringence of cordierite are higher than that of alkali feldspar, and the interference color is slightly higher than that of alkali feldspar. It is common in iron-magnesium rocks that have undergone medium-low temperature regional metamorphism (medium-low pressure amphibolite facies) or thermal contact metamorphism (pyroxene amphibolite facies).
9.Omp (omp)
Flake is colorless to light green (plate ⅴ- 1), columnar or granular, protruding from a suitable height, with pyroxene cleavage, and the highest interference color is blue-green (plate ⅴ-2), ng ∧ c = 39 ~ 43. It is one of the characteristic minerals in eclogite (under high temperature and high pressure).
(4) Magnesium-rich minerals
There are many indications that the original rocks are mafic or magnesium-rich rocks.
1. serpentine
The flake is colorless or light yellow-green, leaflike or fibrous aggregate (plates V-3, 4), with positive low protrusions, grade I gray interference color and nearly parallel extinction. Serpentine and fibrous serpentine have positive ductility, while serpentine has negative ductility. It is one of the typical low-temperature (altered) magnesium-rich metamorphic minerals.
2. talc (Tc)
The sheet is colorless and flaky, with positive and low protrusions (plate V-5) and weak flash protrusions. The interference color can be as high as Grade III (Plate V-6) (higher than muscovite, but difficult to distinguish only by interference color), with parallel extinction, positive ductility, a few oblique extinction but small extinction angle. Talc is often associated with magnesium-rich minerals (different from muscovite) and is one of the typical low-temperature magnesium-rich metamorphic minerals.
(5) Other minerals
The composition of these minerals varies greatly, and different mineral types appear under different original rocks and metamorphic conditions. Therefore, its accurate identification also needs data such as chemical composition analysis.
Garnet (Ga)
The flake is colorless, pink or yellowish brown, equiaxed or irregular (plates ⅴ-7 and 9), with extremely high protrusions, no cleavage, full extinction under orthogonal lens and isotropic body. When calcium-bearing eclogite molecules participate, there are common optical anomalies, such as weak grade I gray interference color and band structure (plate V-8). It often contains a large number of matrix inclusions, which constitute a structure including metamorphism, sieve metamorphism and residual bundle.
Second, the structural characteristics of metamorphic rocks
(1) Crystal structure
Metamorphic texture is the main structure of metamorphic rocks with thorough metamorphism. The observation and description of metamorphic structures are usually carried out from three aspects: the size of mineral particles, crystallization habits and morphology, and the relationship between minerals, and then comprehensively described in the following order: absolute size+relative size+crystallization habits and morphology+metamorphic structures.
1. Size of mineral particles
It can be divided into two aspects: the absolute size and relative size of main mineral particles (the particle size of most mineral particles is not necessarily directly related to mineral types and crystallization habits);
(1) According to the absolute size of mineral particles, it is divided into:
◎ coarse crystal structure: > 3 mm;
◎ Medium grain crystal structure:1~ 3 mm;
◎ Fine crystal structure: 0.1~1mm;
◎ Microcrystalline structure: < < 0.1mm.
(2) According to the relative size of mineral particles (particles with content > 50%), it can be divided into:
◎ Isogranular structure;
◎ Unequal grain crystal structure;
◎ porphyritic crystal structure.
When a rock has a porphyritic metamorphic structure, it corresponds to the structure of metamorphic matrix. Its structure is described as: xx metamorphic matrix porphyritic metamorphic structure, or xx metamorphic matrix porphyritic metamorphic structure. For example, "variegated metamorphic structure of micro-scale granular metamorphic matrix"; Or it is described as "porphyritic crystal structure, and the metamorphic matrix has a granular crystal structure with medium-fine particles and unequal particles".
2. Crystallization habit and morphology of mineral particles
◎ Granular metamorphic structure: The rocks are mainly composed of granular minerals (feldspar, quartz, calcite, etc.). ). According to the shape of mineral grain edges, it can be further divided into suture granular metamorphic structure (plate ⅵ- 1) and mosaic granular metamorphic structure (see Figure 3-2).
◎ Scale metamorphic structure: The rocks are mainly composed of flaky minerals such as mica, chlorite and talc (Plate VI-2). As for the arrangement and orientation of these flaky minerals, it does not affect the structural names, but they can be different structures. If the orientation is arranged, the rock is schistose structure; If these flaky minerals are evenly distributed but lack directionality, then these rocks are huge. If there is no orientation, the distribution is uneven, which may be a patchy structure.
◎ Fibrous metamorphic structure: The rocks are mainly composed of long columnar, needle-like or fibrous minerals (Figure VI-3), such as actinolite, tremolite, sillimanite and wollastonite.
Many metamorphic rocks are usually composed of metamorphic minerals with different forms, and their structural names are generally named in the order of "less before and more after". For example, the metamorphic minerals in a metamorphic rock are mainly muscovite, followed by chronological, so the structure of the rock is "granular metamorphic structure"; On the contrary, when it is the main metamorphic mineral and the content of muscovite is relatively low, the rock structure is called "scale granular metamorphic structure" (plate VI-4).
3. The relationship between mineral particles
◎ Including metamorphic structure: some non-oriented fine mineral particles (guest crystals) are wrapped in larger metamorphic minerals (main crystals) (plate VI-5). It should be noted that the guest crystal formed earlier than the host crystal, otherwise it should belong to the metasomatic perforation structure.
◎ Sieve metamorphic structure: many small grains (oriented or non-oriented) are wrapped in large metamorphic minerals, which makes the main crystal sieve (plate VI-6).
◎ Residual structure: Fine mineral particles wrapped in large metamorphic minerals (usually porphyritic) are arranged in parallel (straight line, curved line or even twisted) direction (Figure VI-7), and are intermittently connected with the same mineral at the outer edge of large metamorphic minerals (usually metamorphic matrix).
(2) deformed structure
1. Fragment structure
According to the degree of brittle deformation, that is, rupture → crushing (granulation) → recrystallization or metamorphic crystallization, it can be divided into different types:
◎ Fragmented structure: the broken base (granular particles, the particle size of which is smaller than that before deformation) accounts for 10% ~ 50%, and there are often remnants of original rock structure, relatively complete mineral particles and aggregates (Figure 3-3).
◎ Fragment structure: base breaking (
2. Mylonite structure
Mylonite structure is the product of ductile deformation, which often has the characteristics of synmetamorphic deformation, that is, recrystallization and/or metamorphic crystallization exist at the same time of deformation (the crystallization at this time is called "stress growth" and the formed minerals are called "stress minerals"). Different structural types usually have transitional characteristics:
◎ Early mylonite structure: the broken matrix (crystal, generally more than 0.5mm) does not exceed 50%, and the structure is formed due to ductile flow; There are many fracture points, which are usually oriented and produce intragranular deformation (microstructure or fabric).
◎ Mylonite structure (plate ⅶ- 1): mainly broken matrix (microcrystal, generally less than 0.5mm), with foliation due to ductile flow and a small amount of broken spots; Fragments are usually oriented and produce intragranular deformation (microstructure or structure).
◎ Ultra-mylonite texture: there are basically no broken spots, and the broken base is fine (similar to aphanitic), showing a "flow" orientation.
(3) metasomatic texture
Different types of metasomatic texture can reflect the degree of metasomatic metamorphism and the changing characteristics of its composition. Metasomatic texture can be found in most metamorphic rocks, but it is more developed in gas-hydrothermal metasomatic metamorphic rocks (altered rocks), contact metasomatic metamorphic rocks and migmatite, which is one of the important signs to distinguish metamorphic rocks from magmatic rocks, sedimentary rocks and divide the genetic types of metamorphic rocks.
Metasomatic texture usually develops from the edge of mineral particles, cracks (metasomatic erosion structure) (plate VII-2) or central perforation (metasomatic perforation structure) (plate VII-3). With the increase of metasomatism intensity, new metasomatic minerals gradually replace the residual minerals until the residual minerals are completely replaced, leaving only their illusion (metasomatic illusion structure) (plate VII-4). Therefore, different types of metasomatic texture are the intuitive basis for judging metasomatic intensity, and also one of the important signs for dividing metasomatic metamorphic zone (altered intensity zone) and migmatite zone (intensity zone).
III. Genetic types and classification of metamorphic rocks
Metamorphic rocks can be divided into five types according to their genesis: contact metamorphic rocks (which are also divided into thermal contact metamorphic rocks and contact metasomatic metamorphic rocks), dynamic metamorphic rocks, regional metamorphic rocks, migmatites and metasomatic metamorphic rocks.
(1) thermal contact metamorphic rocks
These rocks are classified according to chemical series (five protoliths) and physical series (three contact metamorphic facies) (Table 3-2). These five protoliths (series) are felsic rocks, argillaceous rocks, carbonate rocks (calcareous or magnesium), basic rocks and magnesium rocks. The corresponding thermal contact metamorphic rocks are hornblende, porphyry slate, (contact) marble, (contact) schist and (contact) gneiss.
The representative rock type of thermal contact metamorphic rocks is amphibole. Thermal contact metamorphic rocks, except marble, can be named hornfels when their metamorphic minerals are dispersed or have non-directional structure. When naming, before the basic name "amphibole", the characteristic metamorphic minerals and main mineral combinations are listed, that is, before the basic name, they are named according to the general naming principles and order of metamorphic rocks, such as: cordierite amphibole (plate VII-5), andalusite amphibole (plate VII-6), garnet diopside amphibole, etc.
The contact schist or contact gneiss of thermal contact metamorphic rocks with directional structure (generally inherited crystalline foliation) is named according to the naming principle of regional metamorphic schist and gneiss. For example, sillimanite andalusite mica (contact) schist exposed in Zhoukoudian, Beijing, where the study of thermal contact metamorphism is very mature.
(2) Dynamic metamorphic rocks
There are many classification schemes for dynamic metamorphic rocks, which are different from structural rocks or fault rocks. At present, deformation characteristics-deformation structure (representing deformation characteristics and strength, deformation strength is expressed by granulation degree and broken matrix content), recrystallization and metamorphic crystallization degree (expressed by new mineral content) are generally preferred as the main classification basis (Table 3-3).
Table 3-2 Classification Table of Thermal Contact Metamorphic Rock
Table 3-3 Classification of Dynamic Metamorphic Rocks
The naming of dynamic metamorphic rocks is to first determine the basic rock name according to the deformation characteristics, and then further name it according to the original rock or mineral composition. Such as granite cataclastic rock and felsic mylonite.
(3) regional metamorphic rocks
There are many kinds of regional metamorphic rocks. When identifying unknown rocks, we should start with the structural structure of rocks, and make a rough classification (sub-classification) according to hand samples and structural characteristics.
1. Rock type with directional structure
According to the type of directional structure, it can be preliminarily divided into four subcategories:
Slate;
Phyllite;
Schist;
Gneiss
2. Rock types that usually have no directional structure.
Quartzite;
Marble.
These two types of rocks are generally massive structures, and sometimes banded structures are developed. When it has schist structure (generally formed by strong deformation and crystallization), it should be classified as schist
3. Rock types with or without directional structure (further classified according to composition)
Metamorphic rocks (felsic series, ferromagnesian series);
◎ plagioclase amphibolite (felsic series, mafic series);
◎ granulite (felsic series, mafic series);
◎ Eclogite (Fe-Mg series, Mg series).
The above rocks belong to the basic type (subclass) of regional metamorphic rocks. After further microscopic observation of their rock slices, they should be named in detail. For naming principles and order, please refer to chapter 1 of this article.
Among regional metamorphic rocks, schist and gneiss are common and diverse, and their species division and detailed nomenclature can refer to different classification tables (Table 3-4, Table 3-5 and Table 3-6). Most rock names listed in the table are basic names.
Table 3-4 Classification of schists according to columnar mineral types
Table 3-5 Classification of mica schists
Table 3-6 Classification of Gneiss Based on Feldspar Types
Gneiss are named according to the types of feldspar (Table 3-6). When further naming, flaky and columnar minerals and characteristic metamorphic minerals are added before the basic name. Such as: blue biotite plagioclase gneiss (plate VII-7), corundum-sillimanite potassium gneiss (plate VII-8), etc.
(4) Mixed rocks
The migmatite is formed by the interaction and mixing of new veins (usually similar in composition to granitic rocks and relatively light in color) and residual matrix (usually residual regional metamorphic rocks and relatively dark components), so its formation mode is special and it has a special metamorphic structure-migmatite structure; The prevalence of metasomatism (the development of metasomatic texture) reflects the extent to which the original rocks (metamorphic rocks) have been transformed. Therefore, the main marks of migmatite classification are: the composition and proportion of matrix and vein, and the transformation degree of original rock and rock structure. Based on this, four basic types of migmatite can be divided (Table 3-7), which correspond to the migmatite (strength) zone.
Table 3-7 Classification and main characteristics of migmatite
(5) metasomatism of metamorphic rocks
The main basis for distinguishing metasomatic metamorphic rocks (also known as gas-liquid metamorphic rocks or altered rocks) is the symbiotic combination and mineral content of metasomatic (altered) minerals. In thin section identification, attention should be paid to the influence of metasomatism on rock naming. Under the action of gas-hydrothermal metasomatism, the characteristics (composition and structure) of original rocks are constantly changing, and rocks with different metasomatism degrees can also show good zoning (that is, alteration zones) in space.
The basic naming method of metasomatic metamorphic rocks corresponds to the division of alteration (intensity) zones in principle (Table 3-8).
Table 3-8 Division of Alteration Zones and Naming of Gas-liquid Metamorphic Rocks