Basalt can occur in many tectonic environments, such as mid-ocean ridges, island arcs, back-arc basins, ocean islands and continental rift zones. At the same time, basalt also appears on terrestrial planets and the moon. Basalt magma comes from the mantle, and the chemical composition of basalt and its mantle-derived inclusions and xenoliths is of great significance to understand the composition and process of deep mantle materials.
(A) the basic characteristics
Basalt is mainly composed of augite and basic plagioclase, and also contains volcanic glass. See Table 7-2 for common minerals in basalt. Fresh basalt is black, dark gray and grayish green after weathering, and strongly oxidized basalt is purplish red. Pore structure and almond-shaped structure of rocks are developed, mainly in the form of porphyritic and non-porphyritic cryptocrystalline texture, and the matrix is mostly microcrystalline-aphanitic.
Table 7-2 Summary of Common Minerals in Basalt
◎ Olivine: It is more common in basalt than gabbro, because olivine is often the first mineral to crystallize in mafic magma (unless under very high pressure). Due to the rapid cooling of magma, olivine crystallized first can't react with SiO2 _ 2 in the melt or the reaction is incomplete, so it often exists in the form of phenocrysts, but its edge will be eroded more or less.
◎ Pyroxene: It is commonly found in basalt, and high-titanium pyroxene is usually found in alkaline basalt, with obvious polychromatic color and well-developed zone. Besides augite, tholeiite also contains low calcium augite (enstatite or variable augite). The variable augite usually only appears in the matrix, which is different from other augite with a small angle of 2V(< 30°). The enstatite usually occurs only in the form of phenocrysts, but does not exist in the matrix.
◎ plagioclase: it is basic plagioclase, and plagioclase in porphyry and matrix is the crystallization product of two generations. The An molecule of the first crystallized phenocrysts is higher, reaching bainite, and the grade of crystallites crystallized later is generally lower than that of phenocrysts 10 ~ 20, mainly Labrador stone.
◎ amphibole and biotite: rare in basalt, only found in phenocrysts, often with blackened edges and melting erosion.
(2) Structure and occurrence
Basalt is generally porphyritic with a small amount of porphyritic cryptocrystalline texture or glassy structure. For basalt with porphyritic structure, the matrix structure can also be explained by the ground-series phase diagram (see chapter 5). Because the matrix is solidified on the surface (PH2O = 105 Pa), plagioclase crystallites crystallize earlier than pyroxene crystallites, so pyroxene is often filled in the gaps between plagioclase crystallites. Due to the different condensation rates of magma, besides pyroxene, there may be volcanic glass and magnetite. , forming different matrix structures:
◎ Intergranular structure: also known as coarse-grained structure and granular structure. A large number of granular pyroxene and magnetic mineral particles are filled in the gaps between irregularly arranged slender plagioclase microcrystals. The rock is fully crystalline and formed under the condition of slow cooling (Figure 7-7b).
Figure 7-7 Structure of Basalt under Different Cooling Conditions
◎ Intergranular structure: plagioclase is arranged in disorder, and the material filled in the gap of plagioclase is aphanitic-glassy. It reflects the rapid cooling speed (Figure 7-7a).
◎ Intergranular concealed structure: also known as tholeiite structure. Fillers include pyroxene, magnetic minerals and vitreous. It is a structural type between the first two structures.
Under the condition of rapid cooling, plagioclase microcrystals cannot crystallize out, and the matrix is completely composed of volcanic glass, which is called glass-based porphyritic structure (Figure 7-7c). If there are no porphyritic crystals or less than 5% porphyritic crystals in the rock, it is a glassy structure.
Basalt has common pore structure, massive structure, almond-shaped structure, and sometimes slag structure, rope structure and columnar joint structure. Near-surface lava flow, the surface structure is mainly composed of rope lava and slag lava. Basalt erupting on the seabed and underwater often has a special pillow structure.
Pahoehoe lava: It is characterized by smooth, undulating and wrinkled surface, which is caused by surface quenching of molten magma that is still plastic during movement. Generally, rope-like lava is curved or arranged in a chain along the flow direction, and the arc points to the flow direction of lava at most. It is formed by basalt with high fluidity, cooled outside and still in a molten state inside. The porosity can exceed 20% of the total volume. Rope lava near the surface is easy to be covered, and it is sometimes difficult to distinguish it from pillow lava formed underwater when it is denuded and exposed to the surface in the later stage (see Figure 3-22).
◎ aalava: "aa" is a Hawaiian word pronounced "ah", which is used to describe the lava flow with rough surface. This lava flow is full of porous and prickly lava fragments, which are called "slag blocks". Slag lava is called flower lava because the surface lava is constantly consolidated during the lava flow, and the consolidated surface layer becomes brittle and fractured with the lava flow, forming a "slag block", which is bonded with the liquid lava flow to form flower lava.
The end of rope lava can be transformed into slag lava. Rope lava can only be formed in basaltic lava with low viscosity, while slag lava can be formed in basalt and its evolved magma (with high viscosity).
◎ Pillow lava: Pillow lava is ellipsoid and superimposed together. Ellipsoid surface is glassy, with emission structure inside, which is round and pillow-shaped. It is formed by rapid cooling and condensation of lava in water.
Compared with other evolutionary magma, basaltic melt has the characteristics of low viscosity and low volatile matter, and usually erupts from the crater or quietly overflows to the surface in Hawaiian style, forming a lava flow with low aspect ratio (below 1: 50). Basalt can also erupt in Stromboli style, forming low-explosive pyroclastic rocks. Basalt erupts in water, usually forming pillow lava or sheet rock flow, which may contain basaltic clastic rocks. When basaltic magma comes into contact with surface water or groundwater, its sudden expansion will form a shulte-style eruption, forming a ring or a low crater.
Basalt is generally produced in the form of lava, forming lava flow, lava blanket, lava platform or shield-shaped volcanic cone, which is widely distributed; In a few cases, pyroclastic rocks are formed, and pyroclastic cones are formed in the crater. Columnar joints usually develop on the cross section of thick basalt, which is mainly caused by the uniform shrinkage of lava after cooling.
(3) Classification and nomenclature of mafic volcanic rocks
Mafic volcanic rocks that can quantitatively count the mineral composition or identify porphyritic minerals can be classified and named according to the actual mineral content in the QAPF classification diagram (Figure 4-2 1). Vitreous or aphanitic rocks can only be classified by TAS volcanic rock classification table according to their chemical composition (Figure 4-22).
1. Classification and nomenclature of oil fields and petrography
The field name of basalt is mainly based on the composition of porphyry and the structure of rock. For example, olivine phenocrysts in olivine basalt are abundant; Odin basalt olivine phenocrysts were fossilized by Odin; Stomatal or almond basalt has a stomatal (or almond) structure; The matrix of glass-based basalt is glassy structure; The matrix of coarse basalt has a fully crystalline coarse basalt structure.
Petrographic nomenclature of basalt mainly combines mineral identification and content statistics in hand specimens and thin slices. Can be divided into the following four common types (Table 7-3).
Table 7-3 Classification and Naming of Petrography of Xuanwu Rock
(According to Robin 20 10, modified)
Petrographic characteristics of basalt-related rocks are as follows:
Picrite): SiO2 _ 2 is lower than basalt (ultrabasic rock) and contains more MgO. It contains more olivine and less plagioclase.
Basalt andesite: The mineral type is similar to basalt, but plagioclase contains more albite. See Chapter 8 for details.
◎ Coarse basalt, kyanite and alkaline basalt: usually containing alkali feldspar and feldspar.
2. Classification and nomenclature of chemical constituents
The grain size of many basalts is so fine that it is difficult to distinguish minerals under the microscope. Usually, this kind of basalt can only be named by chemical composition, that is, it can be directly mapped by chemical composition of rocks or converted into CIPW standard minerals for detailed naming. Among them, TAS volcanic classification map (Figure 4-22) is the most commonly used method. According to this, it can be divided into alkaline basalt series and subalkaline basalt series.
◎ Alkaline basalt series: in the TAS diagram, it is located in the basalt area and above the alkaline-subalkaline. No matter whether nepheline exists or not, the separated crystals have evolved into fused trachyte and acoustic rock with high alkalinity. Alkaline olivine basalt is the representative of alkaline basalt series.
◎ Subalkaline basalt series: in the TAS diagram, it is located in the basalt area, below the alkaline-subalkali boundary, including calc-alkaline basalt and tholeiite, and separated and crystallized into low-alkali fused dacite or rhyolite. Among them, tholeiite series has obvious iron-rich trend in the process of differentiation, while calc-alkaline series has no iron-rich trend, but evolves to alkali-rich direction (Figure 4- 13). Tholeiite is the representative of subalkaline basalt series.
More detailed species classification of basalt can be divided into different series and rock types according to Hy, Q, Ol and Ne. Calculated according to CIPW standard minerals (see Applied Petrology of Magmatic Rocks edited by Qiu Jiaxiang for details, 199 1).
3. Classification of tectonic environment
Basalt can be divided into different types according to its tectonic environment, such as mid-ocean ridge basalt, ocean island basalt, continental overflow basalt, continental rift basalt and basalt related to subduction zone. Basalt with different tectonic backgrounds has different characteristics of major elements and trace elements. According to the geochemical characteristics of basalt, the paleotectonic environment can be distinguished. Tholeiite occurs in various tectonic environments, while calc-alkaline basalt usually occurs in island arc and active continental margin environment related to subduction.
(4) Common species and genera
1. Subalkaline series
The chemical composition is characterized by rich CaO, Al2O3, MgO, FeO, Fe2O3 and poor alkali (about 4% K2O+Na2O). Na2O in rocks is generally greater than K2O, CaO is relatively stable (about 10%), and MgO and TFeO change greatly. Generally speaking, the contents of K2O and Na2O in subalkaline calc-alkaline basalts are higher, while the contents of CaO, TFeO and MgO are lower and the content of al2o 3 is higher. When al2o 3 is 365,438+06% ~ 65,438+07%, it can be called high alumina basalt. Compared with calc-alkaline basalt, tholeiite is alkali-poor, especially K2O and TiO2 _ 2. The ratio of TFeO/MgO increases with the increase of SiO2 _ 2. Compared with continental tholeiite, oceanic tholeiite is slightly rich in MgO and CaO, while the latter is slightly rich in SiO2. The former is obviously lower in K2O (K2O < 0.3%) and has a higher Na/K ratio (> 10), while the latter is relatively rich in K2O and relatively poor in Na2O with a Na/K ratio (1. 1 ~).
◎ tholeiite: it is a representative of subalkaline basalt, and its chemical composition is characterized by high SiO2 _ 2 (average > 49%) and low alkali content, and K2O+Na2O is mostly 2% ~ 4%. Minerals are characterized by the appearance of low calcium pyroxene (enstatite or variable pyroxene). Low calcium pyroxene can exist as the proliferation edge of phenocrysts, matrix minerals or olivine phenocrysts (Figure 7-8a). The crystallization order of phenocrysts is olivine → plagioclase → augite, and there is no olivine in the matrix. Synchrosite can appear in the matrix of tholeiite, which is called synchrosite tholeiite. Synchrosite crystals come from the magma melt that evolved in the latest stage. Porphyry crystals or standard minerals containing olivine are called olivine tholeiite, and when the Ol content reaches 25% ~ 40%, they are called picrite basalt. When it occurs in standard minerals, it is called timely tholeiite. Generally tholeiite does not contain mantle-derived xenoliths, but there are some exceptions. For example, tholeiite xenoliths are found in Niutoushan, Fujian, China.
◎ Calcareous Basalt: Compared with tholeiite, the iron-rich trend is not obvious, and its evolution trend is different from tholeiite in layered rock mass. It usually occurs in the island arc or orogenic environment related to subduction and coexists with typical calc-alkaline andesite, dacite and rhyolite.
◎ High-alumina basalt: Al2O3 > 16.5% basalt, whose occurrence, mineral composition and chemical composition are between tholeiite and alkaline basalt, equivalent to medium-potash arc basalt. Porphyry minerals are usually plagioclase, olivine, augite and magnetite, and occasionally amphibole. Plagioclase is a high-grade Labrador plagioclase.
◎ Coarse xuanyan: also known as granulite, its mineral composition is similar to tholeiite, but its crystallinity is better. The matrix has a coarse-grained structure, and the particles can be distinguished by naked eyes. The difference between trachyte and diabase lies in its prominent occurrence.
◎ Vitreous salt: It is named because of its special glass-based mottled structure. Its matrix is mainly brown basalt glass with a small amount of plagioclase microcrystals. Porphyry crystals include pyroxene, basic plagioclase and olivine.
◎ spilite: a marine extrusive rock, the main minerals are sodium plagioclase and chlorite (Figure 7-8b), epidote, chlorite, calcite, sericite and metallic minerals, and sometimes the remains of basic plagioclase and pyroxene, and the matrix is cryptocrystalline texture. It belongs to tholeiite series and is chemically characterized by high Na2O content. Its mineral composition and chemical composition are different from those of normal basalt. Spilite is often associated with keratophyre and quartz keratophyre, which is called spilite keratophyre system or spilite keratophyre formation. It is generally believed that spilite is formed by low-grade metamorphism after the interaction between basalt and sodium-rich seawater.
Figure 7-8 tholeiite and spilite
2. Alkaline series
The mineral composition and chemical composition of alkaline basalt are quite different, and the outstanding feature is that it is rich in alkali, with K2O+Na2O > 5%, up to 9%. Generally speaking, Na2O>K2O, rarely K2O >Na2O. Compared with tholeiite, alkaline basalt (Figure 7-9) is characterized by containing a large number of alkali feldspar, alkaline dark minerals and titanaugite, and olivine appears in the matrix, which does not contain chronotropic. If the rock alkalinity is high, feldspar will appear. Compared with calc-alkaline basalt, it is rich in TiO 2 (> 2%) and has high alkali content. Rock often has glass-based variegated structure and glass-based interlaced structure.
Common species and genera of alkaline basalt series are as follows:
Alkaline olivine basalt: It is a representative of alkaline basalt and contains feldspar minerals such as nepheline. Microscopically, the augite in alkaline basalt is mostly lavender titanium-bearing augite, which has obvious polychromatic characteristics and well-developed zonal zones. Ordinary pyroxene phenocrysts usually appear before plagioclase, and the crystallization order of phenocrysts is olivine → ordinary pyroxene → plagioclase. Olivine is common in both phenocrysts and substrates. The formation pressure is high and the focal depth is large.
◎ fayalite: The standard mineral NE > 5%, OL < 5%, without Hy, is obviously unsaturated SiO2 _ 2 with high alkali content. It is composed of basic plagioclase, clinopyroxene and feldspar-like, and may contain a little olivine. Monocline pyroxene is mainly titanium pyroxene, and feldspar-like materials are nepheline and leucite. According to different types of feldspar, they are named nepheline alkaline basalt, leucite alkaline basalt and so on.
Fig. 7-9 Microscopic sketch of alkaline basalt (24 times) (according to Mohouse, 1959)
◎ Kyanite: standard mineral NE > 5%, OL > 5%, also alkaline basalt, with obviously unsaturated SiO2 _ 2, high alkali and rich feldspar. The main minerals are basic plagioclase, olivine, clinopyroxene and feldspar. Different from alkali xuanyan, it is rich in olivine (> 5%), up to 25%. Porphyry crystals are olivine and pyroxene, and feldspar-like crystals mainly exist in the matrix. According to the types of feldspar, it can be divided into nepheline kyanite and leucite basalt.
Alkaline basalt usually contains mantle-derived inclusions and xenolith megacrysts (chapter 6), which is a window to study the composition of mantle materials. Giant crystal minerals such as blue corundum, zircon and garnet are important gem raw materials.
3. Potassium xuanyan series
According to the division of Table 4- 13 in Chapter 4, the rocks with the same SiO2 _ 2 content as basalt and basaltic andesite are trachyte and basaltic trachyandesite, which are characterized by orthoclase and often coexist with intermediate-acid trachyandesite, potassium-rich dacite and potassium-rich rhyolite. K-basaltic rock series belongs to the extensional rock series of orogenic belt and has the characteristics of low titanium. The identification of potash basalt series rocks is mainly verified by SiO _ 2-K2O diagram and AFM diagram. In mineralogy, augite and low calcium augite are different from alkaline basalt series. Meen( 1992) thinks that this is related to the crystallization differentiation of pyroxene in magma of high-pressure magma chamber (reflecting the thick crust and deep Moho surface). Potassium basaltic rocks are commonly found in the following three tectonic environments (Gill, 20 10): (1) the extended rift environment of ocean island arc and back-arc basin, such as Izu-Ogawara-Mariana island arc system, which is often related to calc-alkaline volcanic activity; (2) Continental magmatic arc rift zones, such as the Cascade region in the western United States, are related to low-potassium, medium-potassium and high-potassium calc-alkaline volcanic rocks; (3) In the environment after the collision between Qinghai-Tibet Plateau and Alps, potash basaltic rocks are related to the thinning of lithosphere and the extension and collapse of orogenic belt.
◎ K-trachyte: A kind of rock belonging to K-trachyte series (see Table 4- 12), also known as olivine trachyte, is a K-metamorphic species of basaltic trachyte. In chemical composition, SiO _ 2 is less than 57%, and it is relatively rich in alkali, Al2O3, K2O and pro-MagmaElemental macro ions. K2O/Na2O is generally close to 1, Fe2O3/FeO is relatively high, while TiO2 _ 2 is relatively low, and nepheline standard mineral molecule (ne) can appear. Porphyry minerals include olivine, clinopyroxene and plagioclase, in which clinopyroxene is rich in Ca and poor in Ti and Fe, and plagioclase is a tholeiite with olivine-like edges, and sometimes leucite phenocrysts can be seen. The matrix is mainly composed of anorthite, plagioclase and clinopyroxene, and often contains vitreous.
See Table (7-4) for the main identification features of different series of basalts.
Table 7-4 Main Identification Features of Basalt
(modified according to Hyndman, 1985)