Common types of acid rocks

◎ Granite: It is a kind of crystalline granular acid plutonic rock with flesh red to light gray (Figure 7-2 1 A). The main mineral components are Yingshi (20% ~ 50%) and feldspar (60% ~ 70%). The secondary minerals are biotite, amphibole or pyroxene, and the content is generally 5% ~ 10%. The accessory minerals are magnetite, ilmenite, zircon and apatite. , and the content is < 1%.

Microscopically, granite usually has a semi-automorphic equigranular structure, dark minerals crystallize earlier and the crystal form is relatively complete, and feldspar is partially automorphic, and it is generally abnormal when it is in time (Figure 7-2 1B). Feldspars are mainly potassium feldspar (orthoclase and microcline) and plagioclase, and microcline often has lattice twins.

There are image granites in Suzhou granite. Microscopically, it is mainly composed of potash feldspar and quartz, and the content of dark minerals is very low. Rocks are usually porphyritic in structure. When it meets feldspar, it forms a wedge-shaped pattern texture, so it is called Wen Xiang granite (Figure 7-2 1C, d).

Figure 7-2 1 granite

Granodiorite: It is often associated with granite, and is common at the edge of granite rock mass. The content of SiO2 _ 2 is about 65%, and the timely content is more than 20%. The content of plagioclase (plagioclase or intermediate feldspar) is higher than that of alkali feldspar, and the dark minerals are amphibole and biotite. The accessory minerals are sphene, apatite, magnetite and zircon. Grayish green or dark gray (Figure 7-22A). Plagioclase often has obvious banded structure. Generally, it is a semi-autogenous granular structure and a point structure (Figure 7-22B).

◎ tonalite is a crystalline intermediate-acid plutonic rock, and its mineral composition is generally similar to quartz diorite. It is mainly composed of plagioclase (intermediate feldspar and plagioclase), Yingshi and biotite (Figure 7-23A). Plagioclase usually has a banded structure. Besides biotite, dark minerals sometimes contain amphibole and pyroxene. Plagioclase accounts for 60% of the total feldspar, while alkali feldspar (orthoclase) accounts for less than 65,438+00% of the total feldspar, which often occurs in the form of interstitial products. The common accessory minerals are apatite, sphene and magnetite.

◎ Longer (Ordovician) ringspot granite: it is a special variety of granite, red or gray, and its main feature is special porphyritic structure (longer ringspot structure). That is, the variegated crystal is oval-shaped flesh-red potash feldspar surrounded by a circle of gray-white feldspar (anorthite) shells (Figure 7-23 B). Potash feldspar phenocrysts are single crystals with box twins, or a collection of several irregular and fan-shaped crystals. The matrix minerals of this granite are mainly quartz, potash feldspar, biotite and sometimes amphibole. The accessory minerals are zircon, apatite and metal minerals. It was mainly formed in Precambrian and distributed in Miyun, Beijing, Chicheng, Hebei, Shangrao, Jiangxi, Huanren, Liaoning, Shangzhou, Shaanxi and Zhangzhou, Fujian.

Figure 7-22 Granodiorite

Figure 7-23 tonalite and Longer Porphyry Granites

Granite porphyry is a common acidic hypabyssal rock. The mineral composition is basically the same as granite. It has a porphyritic structure and the matrix has a micro-granite structure (Figure 7-24A). The phenocrysts are mainly coesite and feldspar, but there are also biotite and amphibole. Timely phenocrysts often have good crystal shapes (Figure 7-24B). Potassium feldspar is orthoclase or feldspar. Biotite and amphibole sometimes darken their edges.

Figure 7-24 Granite Porphyry

2. Acid eruption rocks

◎ Rhyolite: It is formed by condensation on the surface of granitic magma, and rhyolite structures are often developed (Figure 7-25A, b). Generally, it is dark red, meat red, grayish yellow and other colors. It usually has a patchy structure. The porphyritic crystals are mainly Queshi (Qz) and sanidine (San), with occasional plagioclase and biotite. The matrix is dense aphanitic or glassy, showing fine spherical and glassy structure (Figure 7-25C, D).

Figure 7-25 Rhyolite

Rhyolites are widely distributed and often coexist with ignimbrite. Related metallic minerals are lead, zinc, silver, gold and uranium, and common nonmetallic minerals are zeolite, montmorillonite, kaolinite, pyrophyllite, alunite and fluorite.

◎ dacite: It is a volcanic rock with chemical composition and mineral composition equivalent to granodiorite. Light color, gray-gray white. Porphyry structure: Porphyry crystals are mostly neutral plagioclase, Yingshi and alkali feldspar, and porphyry plagioclase belts are developed (Figure 7-26); The matrix is fine-grained feldspar, quartz, etc. , containing a small amount of biotite and amphibole, etc. The matrix is usually mosaic structure, glass-based interwoven structure and glassy structure. Dacite often coexists with rhyolite, trachyte and andesite, forming a very thick volcanic rock series.

◎ ignimbrite: The rock is dense and looks like lava (Figure 7-27A), but it has pyroclastic structure. It is mainly composed of rock debris, crystal debris, mud debris and glass debris. (fig. 7-27B, c), its particle size is less than < 2mm. Plastic chips and glass chips are often squashed and elongated, bypassing rigid chips and crystal chips in parallel, forming a pseudo-corrugated structure (Figure 7-27D).

Generally speaking, tuff is a kind of high viscosity acidic, intermediate acidic and alkaline molten slurry rich in volatile matter. When it rises to the near surface, it expands and bubbles when milk boils due to the sudden drop of external pressure. The bubble wall became thinner and thinner, and finally exploded. The bubble wall is broken, the molten slurry is crushed, and a large number of molten slurry gushes out of the crater, suspended in the gas in a hot state, forming a volcanic ash flow or pyroclastic flow flowing along the hillside, and then the volcanic ash flow quickly accumulates. Under the action of heat and heavy load, plastic fragments (that is, mud fragments) and glass fragments are squeezed and deformed, and they fuse with each other to form rocks.

Figure 7-26 dacite

(Jiangning, Jiangsu)

Figure 7-27 ignimbrite

◎ Acidic glassy volcanic rocks: it is a general term for obsidian, turpentine and perlite. Its composition is 80% ~ 100% glassy, which is the product of rapid condensation of rhyolite and dacite magma on the surface. The main difference among obsidian, turpentine and perlite is different in water content, among which the water content of obsidian is very low (< 1%), turpentine is very high (4% ~ 10%) and perlite is in the middle (3% ~ 4%).

—— Obsidian: black or dark brown, conchoidal fracture, glass luster (Figure 7-28A). The composition is similar to that of granite, but they are all made of glass, and sometimes pinnate microcrystals and microcrystals appear in the glass (Figure 7-28B).

-Turpentine rock: dark gray, dark brown tone, luster and structure very similar to turpentine, with conchoidal fracture. There are some microcrystals or spots in the glass matrix.

—— Perlite: dark gray-black, pearly luster and pearly crack structure. Sometimes, the color of pearl particles is different from that of surrounding cements. The particles are brownish red and the cements are light gray green. Under single polarization, there are arc-shaped crack structures and a small amount of potassium feldspar speckles (Figure 7-28C), and between orthogonal polarization, the glass is completely extinguished (Figure 7-28D).

Figure 7-28 acidic glassy volcanic rocks