(1) Basalt Basalt is dark in color, black or dark green and dark brown. The main minerals are Labrador and pyroxene, and olivine is common. Among them, clinopyroxene is the main pyroxene, orthopyroxene is rare, and metamorphic pyroxene with small optical axis angle can be seen in the matrix. The secondary mineral is brown amphibole, and biotite is rare. The accessory minerals are magnetite and apatite. Common mottled structure and glass matrix mottled structure, the matrix is mostly intergranular structure (photo 4-44,51), interstitial structure (photo 4-43), interstitial structure (photo 4-26,45) and spherical structure (photo 4-3 1 ~ 33). Sometimes there are no spots. Common stomata and almond structure. As mentioned above, basalts are further named, and the common ones are Dingyi Petrochemical olivine basalt (photo 4-15,27) and olivine basalt (photo 4- 13, 14, 19, 20,24,25,35,44). Photo 4-49 shows the stomatal basalt collected in Fujian, China. Some of its stomata are as long as 10 ~ 20 cm, which is irregular. After the rock is altered, it can be named altered basalt, or it can be directly prefixed with altered minerals, such as carbonized basalt. The transition from basalt to andesite is also common in nature, which can be called andesite basalt (photos 4- 18, 22, 28, 39, 52, 53). Its microscopic characteristics are generally less olivine in phenocrysts and more plagioclase and amphibole except pyroxene in phenocrysts. Its meaning is different from basaltic andesite in TAS diagram.
(2) tholeiite
Ic basalt does not have tholeiite structure, but its main feature is that the chemical composition of W (SiO _ 2) is relatively high, but the alkali content is low, and w(Na2O+K2O) is mostly 2% ~ 4%, which is a typical representative of subalkaline basalt. The main mineral components are plagioclase (with plagioclase as the phenocryst and plagioclase as the matrix), ordinary pyroxene and calcium-poor pyroxene (hypersthene and metamorphic pyroxene), and Hy standard minerals appear (this is the identification sign of tholeiite). Olivine is generally not seen, and sometimes occurs as phenocrysts (as unbalanced minerals). It is usually corroded into a harbor shape and surrounded by calcium-poor pyroxene, forming a changeable pyroxene or hypersthene reaction edge, which can be distinguished from alkaline olivine basalt. In addition, in tholeiite (which often forms a cross-shaped character with alkali feldspar), time-dependent interstitial substances can be found (photo 4- 17), or time-dependent q standard minerals can be found in tholeiite, which can be distinguished from most basalts by CIPW calculation. The rock has a porphyritic structure, and the matrix is intergranular structure, interstitial structure (photos 4- 16, 17, 23) or dense structure, and sometimes there are no spots or few spots. Common pore structure (photo 4-54) and almond-shaped structure (photo 4-4 1). According to the naming principle of igneous rocks mentioned above, tholeiite can be further named, such as blowhole (or almond) tholeiite (photo 4-4 1, 54) and so on.
When there are timely or Q standard minerals in tholeiite, it can be called timely tholeiite. When tholeiite contains more than 5% olivine (mostly phenocrysts), it can be called olivine tholeiite. Its chemical composition is unsaturated silica, which contains neither Q standard minerals nor Ne standard minerals.
Tholeiite is widely distributed and exposed in ocean islands, deep-sea basins and continental interior.
(3) Rough metaphysics
Sun Nai, Zhou Xinmin and others suggested replacing coarse-grained basalt with "coarse-grained basalt" to distinguish it from the abbreviation of coarse-grained basalt. It is characterized by high degree of crystallization, mineral particle size > > 0.5 mm, fully crystalline structure, and mineral components can be recognized by naked eyes. There are many characteristic intergranular structures (coarse-grained structures) or diabase structures and sub-diabase structures (that is, the average particle size of pyroxene is less than or close to the average length of plagioclase, and plagioclase is partially wrapped or alternately distributed by pyroxene). Coarse basalt is a transitional type between gabbro (or diabase) and basalt (photo 4- 1 1, 12).
(4) Alkaline olivine basalt
Alkaline olivine basalt is a typical representative of alkaline basalt and widely distributed. The main minerals are intermediate-basic plagioclase, titanium-bearing augite and more olivine, followed by alkali feldspar. There is little or no perillene. Porphyry structure. Porphyry crystals include olivine, titanium pyroxene and middle labradorite. The matrix consists of olivine, calcium-rich titanaugite, intermediate plagioclase and interstitial materials, such as intergranular structure, interwoven structure or hidden structure, glassy glow green structure (photo 4-56), etc. The interstitial materials are mainly composed of albite-rich, zeolite and potash feldspar, and sometimes a small amount (< 5%) of feldspar-like and alkali-rich brown basalt glass such as nepheline appears. There is no seasonality and his cross-cultural relationship with alkali feldspar. The pyroxene phenocrysts are characterized by rich titanium, and have common ring-shaped and sand-bell-shaped structures (photos 4-6 and 56). Speckled olivine has no pyroxene reaction edge, and the two are co-bonded. Plagioclase phenocrysts usually have alkaline feldspar rings. Does not contain calcium-poor pyroxene and orthopyroxene. All the above features are the identification marks that are different from tholeiite. In addition, the common xenoliths and deep xenoliths in alkaline basalts (photo 1- 10) are also important signs to distinguish tholeiite. There are three sources of xenoliths: ① Single mineral xenoliths broken from homologous rocks, such as olivine (photo 4-57), enstatite (photos 4-55, 58), diopside (photo 4-59) and chromium-containing spinel (photos 4-55, 61); ② Single mineral xenocrysts in the Si-Al layer, such as Yingshi (photo 4-62), feldspar (photo 4-63) and carbonate minerals (photo 4-64). (3) The earliest megacrysts came from deep places, such as augite, garnet, amphibole, plagioclase and ilmenite. Most xenoliths are not autogenous, but clastic. Except olivine, all kinds of reaction marginal structures and even xenoliths react to form fine mineral aggregates (photo 4-60). The reaction edge with this characteristic can be easily distinguished from phenocrysts (such as pyroxene) with the same composition in the same piece. The difference between olivine xenocrysts without reaction edge and olivine phenocrysts in the same slice is based on the characteristic Kenke zone of olivine xenocrysts (like twins, it is formed by mantle-derived olivine under the action of internal dynamics, which is equivalent to the timely deformation zone, photo 4-57). The main rock types of deep xenoliths are spinel lherzolite, lherzolite (photo 4-66) and harzburgite (photo 4-65), followed by garnet lherzolite, sometimes dunite, pyroxenite and eclogite. They have reaction edge structures such as pyroxene and spinel in contact with alkaline basalt, while olivine has no reaction edge and has a common Kenke zone. Sometimes broken peridotite fragments can also be seen in basalt (photo 4-67). When the phenocrysts of rocks are mainly titanium pyroxene, it can be called titanium pyroxene alkaline basalt, which has been fished in the deep seabed of the western Pacific Ocean (photo 4-56).
(v) High alumina basalt
High alumina basalt is a kind of basalt with W (Al2O3) > 16% ~ 17%. Its occurrence, mineral composition and chemical composition are transitional rocks between tholeiite and alkaline basalt. The phenocrysts are composed of Labrador feldspar, hypersthene and olivine, and the reaction side structure is common. Most of them are distributed in island arcs and active continental margins, especially in the andesite belt around the Pacific Ocean. Symbiosis with andesite, high calcium dacite and rhyolite.
(6) Glass-based basalt
Hyaluronate has a characteristic glass-based mottled structure, and its matrix is mainly composed of a large number of dark brown volcanic glass, in which plagioclase and olivine microcrystals in different directions are often evenly distributed. The phenocrysts are olivine, plagioclase and pyroxene (photos 4-30, 42, 47). Yitong basalt in Jilin (a good stone casting material) has a glass-based porphyritic structure composed of plagioclase and brown volcanic glass (photo 4-29).
In addition to the above-mentioned common main types, it is often mentioned in the literature: oceanic rocks in transition to picrite. This kind of rock has many names, such as dark rock rich in picrite, which is also synonymous with picrite basalt (in fact, the latter is not basic lava). IUGS believes that oceanite is actually picrite basalt, not limited to the ocean, so it is suggested to use this term carefully. Followed by sodium trachyte and potassium trachyte. Among them, sodic trachyte includes Hawaiian rock and anorthite basalt characterized by plagioclase. Potassium trachyte is an olive trachyte. In addition, there are plateau basalts, also known as flood basalts, which originally refer to large-scale basalt thick-layer accumulation produced in continental crust, ejected along cracks and close to horizontal rock flow. Its lithology is dominated by tholeiite, coexisting with high alumina basalt and alkaline basalt, while others are dominated by alkaline olivine basalt. Obviously, it has evolved into the term of rock assemblage.
(7) spilite
Spilite is a special type of basic lava rich in sodium, and its chemical composition w(Na2O) is generally above 4%. The mineral composition is sodium plagioclase or albite, augite or diopside and its altered minerals (chlorite, actinolite, epidote and hematite, etc.). Sodium plagioclase crystals often have a hollow skeleton structure (long skeleton crystals have serrated edges and hollow glass filling inside, etc.). , photos 4-34, 68, 69). Generally does not contain olivine or a small amount of serpentine olivine. It often has pillow structure, stomata and almond structure; There are many characteristics of spilite structure (photos 4-34 and 68). Unlike interstitial structure, intergranular structure and concealed structure, plagioclase in spilite structure has high sodium content, poor shape, irregular and serrated edges, and most of the minerals filled in plagioclase lattice are massive and cotton-wool micropyroxene and its altered minerals (chlorite, epidote, calcite, calcite, etc.). Sometimes, we can also see spherical structure (photo 4-69) or emerald structure. Occipital bodies are often developed at the top and edge of spilite flow, and the occipital nuclei are mostly coarse-grained sodalite, which tapers to the glassy edge or spherulite edge, and its spherulite diameter is about 65438±0mm, which is composed of radial albite. Some occipital nuclei are spilite with tholeiite at the edge. This shows that spilite is not the alteration product of ordinary basalt. In addition to pillow structure, stomata and almond-shaped structure can also be seen. Spilite is often associated with keratophyre and quartz keratophyre. Most scholars believe that spilite is the product of marine (underwater) volcanic eruption and is formed by alteration or slight metamorphism.