Common ultrabasic rocks

Ultra-basic rocks are generally dark in color, often dark green, light black, brown and green, mostly medium-coarse dense blocks with high density. The main minerals are olivine and pyroxene, the secondary minerals are amphibole and mica, and the secondary minerals are magnetite, ilmenite, chromite, spinel, garnet and apatite. Serpentine is easy to appear on the surface of olivine. Serpentine is first explained along the edges and cracks of olivine, and then spread all over the whole, leaving only the illusion of olivine. After serpentine is formed, fresh rocks often take the form of authigenic or semi-authigenic granular structure, reactive edge structure and network structure.

The common rock types are dunite, peridotite, olivine pyroxenite and pyroxenite.

Figure 7- 1 pure sandstone

◎ dunite: The rocks are almost entirely composed of olivine (Figure 7- 1A), with a very small amount of orthopyroxene, clinopyroxene and amphibole, and the auxiliary minerals are chromite, magnetite, ilmenite and spinel. Under the microscope, olivine is fully self-shaped or semi-self-shaped, showing a blue-purple interference color of Grade II between orthogonal polarizations (Figure 7- 1B). The original dunite is olive green, yellow-green and light grayish green, brown or grayish brown after limonite or Dingyi petrochemical, and dark green or grayish black after serpentine. It has been found in Tibet, Inner Mongolia, Shaanxi and other places.

◎ peridotite: fresh rocks are olive green, sometimes dark green and grayish black, with high density. It has a granular structure and a massive structure. It is mainly composed of olivine and an indefinite amount of pyroxene. Generally speaking, olivine accounts for 40% ~ 90%, and pyroxene accounts for 10% ~ 40%. Olivine is often forsterite and forsterite, while pyroxene is orthopyroxene and clinopyroxene. The rock also contains a small amount of amphibole, biotite, plagioclase, chrome spinel, ilmenite and so on. When the olivine content in the rock is high and the pyroxene content is less than 10%, it is called peridotite (Figure 7-2A, b), and when the pyroxene content reaches 10% ~ 40%, it is called pyroxene peridotite (Figure 7-2C, d). According to the different composition of pyroxene, it can be divided into clinopyroxene (mainly clinopyroxene Cpx), orthopyroxene peridotite (mainly orthopyroxene Opx) and lherzolite (the content of clinopyroxene and orthopyroxene is almost equal). Microscopically, the cleavage of pyroxene is well developed, with more cracks in olivine, lower interference color of pyroxene and higher interference color of olivine (Ol) (Figure 7-2D). It is produced in Inner Mongolia, Hebei, Sichuan and Jiangsu.

Figure 7-2 peridotite and pyroxene peridotite

(Fangshan, Liuhe, Jiangsu)

◎ Olivine pyroxene: It is mainly composed of orthopyroxene, clinopyroxene and olivine, with pyroxene content of 60% ~ 90%, olivine 10% ~ 40%, and a small amount of amphibole and metal accessory minerals. According to the types of pyroxene, it can be divided into olivine pyroxene, olivine monoxene and olivine double pyroxene. It is produced in Tibet, Inner Mongolia and Hebei.

◎ Pyrite: The content of SiO2 _ 2 is often above 45%, which is ultramafic rock. The rocks are mostly dark green to black (Figure 7-3A, B), with authigenic or semi-authigenic granular structure and massive structure. Almost all of them are composed of pyroxene, with clinopyroxene and orthopyroxene accounting for 90% ~ 100%, and a small amount of olivine, amphibole, biotite, chromite, magnetite and ilmenite. Observed under single polarization, pyroxene is yellow or brown with cleavage development (Figure 7-3c). Between orthogonal polarizations, pyroxene is yellow and red interference color grade II (Figure 7-3D). According to the types of pyroxene, pyroxene can be divided into plagioclase pyroxene, clinopyroxene and dihalite. According to the secondary minerals, pyroxenite can be divided into amphibole pyroxenite and biotite pyroxenite. It is produced in Sichuan, Hebei, Gansu and Ningxia.

Figure 7-3 pyroxenite

Kimberlite: It is an ultra-shallow subvolcanic rock, once called breccia mica peridotite, and it is a breccia potash ultramafic rock. It was originally seen in a place called Kimberly in South Africa, hence its name. Kimberlite usually occurs in rock pipes and walls. The color is dark, mostly green. The content of silica is usually less than 35%. The mineral composition is very complex, including primary minerals such as olivine, phlogopite, garnet and diamond, altered minerals such as serpentine, chlorite and carbonate, and minerals from other rocks and xenoliths in the deep crust. There are usually porphyritic structures and breccia structures (Figure 7-4A). Porphyry crystals are mainly olivine and phlogopite (Phl). Olivine is round and is usually influenced by strong serpentine and carbonate alteration. The matrix has a micro-mode structure and consists of olivine, phlogopite, magnetite and chromite (Figure 7-4B).

Kimberlite is the most important diamond parent rock, and valuable primary diamond deposits often occur in rock pipes. The area of rock pipes is generally < < 10000m2, and they often appear in groups. Among them, kimberlite with porphyritic structure and rich in coarse olivine is rich in diamond, while kimberlite with rich in phlogopite is in micro-mode structure and poor in diamond. Kimberlite is distributed in Shandong, Liaoning, Guizhou, Hubei, Henan and other places in China. Kimberlite in Shandong produces valuable diamonds.

Figure 7-4 Kimberlite

Picrite porphyrite: A subvolcanic rock with porphyritic texture. The phenocrysts are serpentine olivine and augite, and the matrix is augite, bronze augite, biotite and a lot of glass (Figure 7-5).

Figure 7-5 picrite porphyry

( www.largeigneousprovinces.org )

2. Ultra-basic rocks spew out rocks

The mineral composition of extrusive rocks of ultrabasic rocks is similar to that of intrusive rocks of ultrabasic rocks, which are rich in iron and magnesium minerals such as olivine and pyroxene, and sometimes contain a certain amount of glassy. Most of them are fine-grained, aphanitic and glassy, with dark color and color ratio greater than 90. Ultrabasic extrusive rocks are rarely distributed in nature. At present, the discovered rock types include picrite, Komat rock, Maimeiqi rock, peridotite, Limbaugh rock and pyroxenite.

Picrite: ultrabasic or ultramafic volcanic rocks rich in olivine. Dark granular structure. Its chemical composition is characterized by silica < 47%, Na2O+K2O < 2%, MgO > 18%. The mineral components are mainly olivine (50% ~ 75%) and pyroxene (< 40%), sometimes containing a small amount of basic plagioclase and amphibole (< 10%), and the auxiliary minerals are ilmenite, magnetite and apatite (Figure 7-6).

◎ Comati Rock: It was first discovered in the Comati Valley of barberton Mountain in 1969, hence its name. The original intention is limited to ultramafic lava with prickly structure at the top of pillow flow in Archean greenstone (Figure 7-7A, B).

Figure 7-6 picrite

Komatiite is mainly composed of phenocrysts (or skeleton crystals) of olivine and pyroxene, a small amount of chrome spinel and glass matrix. It has pillow structure, clastic structure and typical spiny (fishbone structure or feather structure) structure (Figure 7-7C, D), which is characterized by long, thin and serrated olivine speckles and is the product of quenching crystallization. In chemical composition, the typical komatiite is characterized by MgO > 18%, Cao/Al2O3 > 1 and low alkali.

Figure 7-7 Komatiite

Maimeiqi rock (glassy dunite): It is an ultrabasic magma with a glassy porphyritic structure equivalent to pure peridotite. It was first discovered in the Maimechi River valley in Siberia, Russia, hence its name. It is mainly composed of olivine phenocrysts and black glass matrix, which sometimes contains a small amount of titanium-containing augite microcrystals and magnetite. If the pyroxene content is high, it can be called Limbaugh rock. In chemical composition, it is characterized by MgO > 18%, TiO > 1%, Na2O+K2O < 1%.