1. Mineral composition and its characteristics
The main mineral assemblage is the same as gabbro, including plagioclase and pyroxene, followed by olivine, sometimes amphibole, biotite, and occasionally syenite and orthoclase. The accessory minerals are ilmenite and magnetite. When there is vitreous body, it is mostly brown or orange yellow or orange red. Plagioclase is mainly Labrador or Betonite, and sometimes intermediate feldspar or even albite can be seen. The grade of porphyritic plagioclase is higher than that of matrix plagioclase, which can generally differ by 15 ~ 20, and twins generally develop. The pyroxene is calcium-rich augite or diopside, and can also be calcium-poor variant pyroxene or hypersthene. Ordinary pyroxene can be phenocryst or matrix, while metamorphic pyroxene only appears in matrix, while hypersthene only appears phenocryst. Olivine usually appears in the matrix in the form of automorphic phenocrysts or heteromorphic particles. Under the influence of early changes, it can be completely or partially transformed into brownish red illite and iron minerals, and can also be replaced by brownish red chlorite or chlorite under the influence of later fluids.
Fig. 2- 15 authigenic plagioclase of olivine trachyte (Shandong, single polarization, d =4. 8mm) is irregularly distributed, filled with olivine, pyroxene and magnetite particles, and some secondary chlorite, showing intergranular structure.
2. Structure and its characteristics
The structure of basalt is mainly porphyritic structure and cryptocrystalline texture, non-speckled, matrix intergranular structure, concealed structure, tholeiite structure and glass structure.
The intergranular (or coarse-grained) structure is characterized in that many fine particles of pyroxene or magnetite are filled in the triangular gap composed of relatively authigenic long slab plagioclase microcrystals (Figure 2- 15).
The interstitial structure is filled with glass in the microcrystal gap of dense plagioclase. If there is chlorite filling, it is called interlayer structure.
Intergranular interlayer structure (tholeiite structure) is a transitional structure between intergranular structure and interlayer structure. In addition to granular pyroxene and magnetite, there is also vitreous in the triangular gap composed of authigenic plagioclase microcrystals (Figure 2- 16).
The vitreous structure is almost entirely composed of vitreous, which is the product of rapid cooling. When more than 5% of the porphyritic crystals develop, it can be called glass-based porphyritic structure (Figure 2- 17).
Fig. 2- 16 The phenocrysts of Dingyi basalt (Fangshan, Nanjing, single polarization, d =4mm) are olivine and pyroxene from Dingyi Petrochemical Company, and the matrix is composed of basic plagioclase, pyroxene microcrystal, magnetite and glassy, with porphyritic structure.
Fig. 2- 17 glass-based basalt (Zhangjiakou, Hebei, single polarization, d =4. 8mm) has a glass-based porphyritic structure with phenocrysts, olivine and basic plagioclase, and the matrix is composed of orange glass.
3. Species division and its characteristics
(1) classified by structural structure
It can be divided into coarse-grained basalt, glass basalt, pore basalt and almond basalt.
Coarse xuanyan is fully crystalline, fine-grained, partially medium-grained and porphyritic, and the matrix has intergranular structure (see Figure 2- 15). If the mineral composition cannot be identified by the naked eye, it is called fine-grained basalt.
Basalt is grayish black, fine and dense, and its particle size is generally
Basalt glass is a kind of dense glassy basalt without water. Glass can be recrystallized.
(2) Classification by mineral composition
Common olivine basalt, pyroxene basalt, etc.
Olive basalt is a kind of basalt rich in olivine. If all the olivine becomes olivine, it can be called Dingyi basalt (see Figure 2- 16).
Pyroxene basalt is a kind of basalt rich in pyroxene.
(3) Classification by chemical composition
It can be divided into tholeiite, high alumina basalt and alkaline basalt, and its characteristics are shown in Table 2-6.
Sodium-rich basic lava ejected from the seabed of spilite is generally green or light green aphanitic rock, which often has spilite structure and pillow structure. The phenocrysts are mainly composed of albite and pyroxene, and the matrix is composed of chlorite, actinolite, epidote, chlorite, calcite and serpentine. Iron-bearing minerals are dispersed. Sometimes residual pyroxene or pyroxene illusion can be seen. Sometimes there are basic plagioclase residues, often with variegated structure or interwoven structure (Figure 2- 18).
Fig. 2- 18 spilite slate plagioclase (Yingyangguan, Guangxi, single polarization, d =2. 2mm) is non-oriented, and its lattice is filled with granular epidote, magnetite, flaky chlorite and a small amount of carbonate minerals.
Table 2-6 Chemical Composition Classification and Characteristics of Basalt
Spilite is often associated with keratophyre and quartz keratophyre, and this rock combination is called spilite-keratophyre (or quartz keratophyre) rock series (or suite). Most of spilite occurs in geosyncline fold belt, which is generally considered as the product of submarine volcanic eruption. It often coexists with ultramafic-mafic intrusive complex and radiolarian flint, forming a special rock combination (complex system) called ophiolite suite or ophiolite suite. Qilian Mountain in China, Yarlung Zangbo River in Tibet, Tianshan Mountain and northwestern Guangxi are all developed.
As for the genesis of spilite, it is generally believed that it was formed by submarine metasomatism in the mid-ocean ridge.
4. Secondary changes of basalt
Under the supergene action, dark minerals and light minerals in basalt are unstable, some are transformed into clay minerals, and some form iron oxide. Finally, it becomes a mixture of diaspore, silicon dioxide and iron oxide, forming so-called bauxite or laterite, such as laterization of Emeishan basalt in some areas. Sometimes a good bauxite layer can be formed.
5. Distribution and occurrence of basalt
Basalt is widely distributed on the earth and often occurs in the form of thick rock flow. Basalt is not only widely distributed in continental crust (such as North America, South America, India, Siberia and China); The oceanic crust of the Pacific Ocean, the Atlantic Ocean and the Indian Ocean is almost completely covered by them. Basalt is almost five times as much as all other lava, so it is the most common lava in nature. They can exist not only in ridges and islands in the ocean, but also in continental margins and stable continents.
Basalt is widely distributed in China, and it is produced in almost every geological period. For example, there are Precambrian Xiong 'er basalts in Rudong Qinling and a large number of Caledonian basic lava activities in Chyi Chin mountainous area, which constitute the spilite-quartz keratophyre series in Baiyinchang, Gansu and Liushanyan, Tongbai, Henan. During Hercynian period, there was a set of flood rock series composed of Permian Emeishan basalt in Yunnan, Guizhou and Guichuan, southwest of China. Tertiary basalt flows are developed in Nanjing, Liuhe and Hainan, tertiary basalt flows are developed in Hannuoba, Hebei, which is famous at home and abroad, and quaternary basalt flows are developed in Datong and south-central Daxing 'anling, Shanxi.
6. Related minerals and industrial uses
Minerals related to basalt include copper ore and some secondary deposits. For example, the natural copper of Jinguashi in Taiwan Province Province, China is produced in the pores of basalt; There are also copper deposits in Baiyinchang, Gansu Province and Liushanyan spilite quartz keratophyre series in Tongbai, Henan Province. Iceland spar, agate and clay minerals are also produced in basalt.
Basalt is a good raw material for casting stone, and the cast stone products made of basalt have strong acid resistance, wear resistance, compression resistance and insulation properties, and have many industrial uses. In addition, basalt fiber can be made of basalt and used in concrete.
The compressive strength of basalt is about 3500×105 ~ 5000×105 pa, and that of porous basalt is slightly lower. When the columnar joints develop, they can often form water bodies and permeable bodies, and can also become oil and gas channels; In addition, basalt can also be the bottom and cover of some oil and gas fields. For example, the bottom of some oil-bearing sections in Liaohe area is basalt.