Andesite

Classification：Igneous Rock

Neutral volcanic eruption rocks are common rocks in subduction areas, mainly plagioclase and hornblende, usually dark gray, and may contain plagioclase (usually neutral plagioclase), black mica and hornblende.

Andesite is a neutral calc alkaline eruption rock. The composition of the diorite is equal to that of the diorite. The word Andesite is derived from Andes in western South America Andes. It is distributed in the circum Pacific active continental margin and island arc area. The occurrences are dominated by continental central eruption, and are often associated with volcanic clastic rocks of corresponding components. Some rock clocks and rock needles emit out of the phase. The height of Andesite volcanoes is the largest, generally higher than 500~1500 meters, and some individuals can reach more than 3000 meters.

Composition structure:
Andesite a neutral calc alkaline eruption rock. Its composition is equivalent to diorite. A dark gray, pale rose, dark brown and other colors. Porphyritic structure. The color rate of Andesite is generally 20~35. The hand specimens are gray, black, red, purple and brown, and are green and spotted after alteration. The speckles are mainly plagioclase and dark minerals. Plagioclase is mainly composed of medium feldspar and elongated stone, often with girdle and erosion structure. The common dark minerals are pyroxene (pyroxene, pyroxenite), amphibole and biotite. The matrix is mainly composed of interlaced structure and Anshan structure (glass interlaced structure). It consists of microcrystals, pyroxene, chlorite and anosic glass, and alkaline feldspar.
Quartz is rare and only fills in microcrystalline space. The accessory minerals are mainly apatite and iron oxide. Stomata, massive structures, and some stomata are filled with calcite, quartz and chlorite, forming almond structure.

Physical and chemical properties:
The content of SiO in Andesite changed greatly (52 to 63%), with an average content of 58.17%. 98.5% of Andesite SiO is supersaturated, and the standard mineral quartz appears (mostly less than 15%). Andesite can be divided into two types according to the content of SiO2: 52 to 57% are basalt Andesite, 57 to 63% are Andesite, and Andesite's Litman index, that is, the ratio of (KO+NaO) 2/ (SiO-43), is generally less than 3.3, and is calc alkaline, and Andesite mean chemical composition is SiO2=52.4%, Al2O3=17.17%, CaO=7.92%. Low, CaO is higher, total alkalinity is less than 5.5%, Na2O>K2O is characteristic. Andesite is not easy to distinguish from basalt. Generally speaking, SiO2>52% is <40% with Andesite and vice versa.

The cause of formation:
Andesite is the most widely distributed volcanic rock in the orogenic belt. It is named for the large number of developed Andes in the Americas. Most of them are rock cover, rock flow and rock clock emplacement. The action of the heated fluid often produces the rock rock of the green pan. Its related minerals are copper, lead, zinc and gold (silver). The word Andesite is derived from Andes in western South America Andes. It is distributed in the circum Pacific active continental margin and island arc area. The occurrences are dominated by continental central eruption, and are often associated with volcanic clastic rocks of corresponding components. Some rock clocks and rock needles emit out of the phase. The height of Andesite volcanoes is the largest, generally higher than 500~1500 meters, and some individuals can reach more than 3000 meters.
From island arc to active continental margin, the alkalinity of Andesite generally increases and potassium quality increases. The Andesite class in the orogenic uplift area, with the strengthening of tectonic activity, evolves in the direction of the multidirectional rhyolite, while in the depression, with the weakening of the tectonic activity, it often evolves to the rough and even the rock.
There are usually 3 views on the causes of Andesite. It is considered that Andesite is a product of basalt magma differentiation, and the main basis is that Andesite is often symbiotic with basalt, and the initial value of 87Sr/86Sr is similar. Assimilation suggests that Andesite is the result of the assimilation of basaltic magma into the granitic continental crust. The main basis is that the Andesite component is between basalt and granite, and Andesite is mainly distributed in the continental crust. From the point of view of plate tectonics, the Andesite pulp originated from the subduction of the oceanic plate under the continental plate, and the oceanic crust and its upper complex sediments were influenced by high temperature and high pressure, converted into amphibolite and quartz eclogite, and then formed Andesite pulp through partial melting, and the magma ascended into the mantle wedge zone and reacted with the mantle rock after it was reacted to Cheng Huishi. The rock, then partially melted, can form Andesite pulp; water and hydrated oceanic crust in the ocean sediments are removed from the subduction to a certain depth, rising to the top of the mantle wedge area, making the mantle rich in water, and the partial melting of the rich water mantle can also form a Andesite. The experimental data show that the melting point of Andesite is the lowest when the pressure is 3 x 109 kPa, and the molten peridotite can be produced by partial melting of peridotite from 1 to 1.5 x 109 pa. The third Andesite Genesis views are accepted by most people.

Main origin:
Any member of a large class of rocks occurring in most volcanic regions of the world. It mainly occurs as surface deposits, followed by dykes and small rock plugs. Many Andesite deposits are not normal lava flows, but rock breccias, mud flows, tuff and other clastic rocks. For example, the pumice tuff near Rome and the pumice tuff in the Eifel area of Germany. Andesite, most of the Cordillera mountains (Ping Hangshan) from Andean mountains, North America and China, are mainly composed of Andesite. In fact, there are plenty of such rocks in the volcanoes along the rim of the Pacific Rim. Volcano (Pelee), St. Vincent and greanin's Sue Free Iyer volcano, Crater Volcano, pan ladder volcano, Mt Fuji, Bobo Carter Peter volcano (Popocatepetl), the volcano, Shasta, Hu De, and Adams volcano, have erupted a large number of rock rocks.

Subclassification:
The Andesite class can be divided into 3 subclasses:
1., quartz containing Andesite, namely dacite, is sometimes regarded as an independent class.
2. common hornblende and biotite Andesite;
3. pyroxene Andesite. Dacite contains primary quartz, and quartz may be small round packets or crystals, or just as tiny particles filled with gaps in the matrix. Ordinary amphibole and biotite Andesite are rich in feldspar, usually pale pink, yellow or gray. Pyroxene Andesite is the most common Andesite type. The amount of pyroxene can be compared with that of basalts, which is deeper, denser and more basic.

Common types:

Rough rock:
An extrusive rock that is equal to two long rocks and is situated between trachyte and Andesite. The rough Andesite is white, gray, yellow or red, often porphyritic and rough pore structure, massive structure. Phenocrysts are mainly composed of plagioclase (feldspar, feldspar) and dark minerals. In general, Shi Banjing has a feldspar edge, forming a regular edge structure, or a gap between alkaline feldspar and plagioclase microcrystals. The matrix has interlacing structure and glass based interlacing structure. The matrix minerals are mainly plagioclase and alkaline feldspar, and often contain different amounts of vitreous.
It is the product of volcanic eruption during tectonic movement from stable to stable stage, which is relatively stable in late orogeny or tectonics. Its magma is mainly derived from the upper mantle influenced by deep faults. Coarse anite is symbiotic with basalt, Andesite and rhyolite, or with alkaline basalt, rough rock and rock rock. The main occurrence is central eruption, most of which are produced by the interbeds of lava and volcanic clastic rocks. Mesozoic volcanic rocks in Jiangsu and Anhui, China are common coarse andesites, and are related to Fe, Cu and pyrite deposits.

Porphyritic rock:
Mineral composition is mainly composed of pyroxene, quartz, hornblende and biotite. The main chemical components are SiO265~71%, Al2O315.5~18.6%, followed by Fe2O3 and CaO below 3~5%, and Na2O and K2O as 1~3%, and a small amount of TiO2, P2O5, Mn2O3 below 1%. The structure of black and white appearance like rice named. The specific gravity is 2.60~2.66, and its hardness is 5~6. Its appearance is bright, usually with black, red and gray cornerstones, and its surface is scattered like white plagioclase particles of rice grains. After a long time of natural weathering, the rock is easily transformed into a loose porous material, which is characterized by strong adsorbability and easily dissolving and releasing trace mineral elements in water. It is a neutral volcano extrusive rock formed during the early Mesozoic, which was cooled, solidified or crystallized by volcanic lava at the early stage of Cenozoic. In addition, the weathered porphyritic Andesite is mainly located in the mountains of the mountains composed of massive Andesite. The geological age is about five million years ago. It belongs to the Andesite mass in the early Miocene in the middle of the third period and the weathered porphyry Andesite of the volcanic erupted Andesite through the river and redeposited. Mainly in the eastern coastal mountains, Mizuho, Chi Mei Rui to source.

Changes in nature:
The Andesite formed by magma cooling will be in different environments.
There will be different changes. It will be classified into two ways, one is the change of chemical composition, the other is the change of physical structure.

Changes in chemical composition:
It mainly consists of dissolution, substitution and interchangeability of A and B components. Dissolution is the so-called leaching, the solution takes some ions away from the mineral; the substitution effect is that when some ions are taken away from the mineral, they are filled with new ions; the exchange of A and B minerals is a new mineral. This is the reason why rock weathering, mass exchange and metamorphism are commonly used. Changes in B, physical structure: when the temperature, pressure condition changes or ions change in the rock, it will affect the bond length and the arrangement position between the atoms. This is the change in the so-called mineral structure, such as the graphite again high pressure environment, can become a diamond. Although its composition has not changed, the way of bond is quite different. The physical properties of it have also changed. This is the cause of metamorphic rock. Any substance will change with the change of temperature and pressure.

Physical changes of Andesite:
(1) Andesite will become shale after long time erosion, transportation and deposition, and finally deposited into a quiet water body.
(two) if Andesite is buried to about 300~500 C at a temperature of about 2 ~ 6Kb, it will form schist.
(three) Andesite acid by hydrothermal dissolution, formation of altered Andesite temperature is about 100~200 DEG C under the condition, also called propylite.
(four) Andesite is often altered to become bluetstone under hydrothermal action. This alteration product is a fine aggregate of epidote, chlorite, albite, sericite, quartz and carbonate minerals. It is an important prospecting marker for lead, zinc and silver deposits.
The transition relationship between Andesite and basalt is often present, and it is often symbiotic in the form of production. It is difficult to distinguish them by means of naked eye, chemical analysis or microscope identification. At this time, the transitional rocks can be classified as basalt - Andesite or Anshan - basalt.

Andesite line:
A lithofacies geogeographic demarcation line on the active continental margin and separated by different rock series. It is also called the Marshall line. There is a series of tholeiite represented by ophiolite suite on one side of this line, and there are calc alkaline magmatism series mainly composed of Anshan volcanic rock, quartz diorite and granodiorite on the land side. The formation of Andesite line is the result of plate subduction. At the rim of the Pacific Rim, the Andesite line is located roughly from Alaska to the Japanese island arc, the Mariana Trench, the silk disulfide islands, the Bismarck islands, Fiji and Tonga islands to New Zealand and Chatham island.
The porphyrite series, represented by the ophiolite suite, is the lithofacies geographical boundary between the calc alkaline series magmatic rocks dominated by Anshan quality volcanic rocks, quartz diorite and granodiorite. It is also called the Marshall line. It can be regarded as a boundary line with Andesite on the active continental margin. On this line, there is no Andesite on the ocean side and Andesite on the side of the land. The formation of Andesite line is the result of plate subduction. When the oceanic plate subducts at the trench, it forms the La basalt series and some deep-sea sediments of the oceanic crust. In the depth of 150~250 km, the local melting melting occurs, forming a calc alkaline Andesite series of magma and erupting along the fissure to form a volcanic island arc. Therefore, the Andesite line is most obvious at the active continental margin. The rim of the Pacific rim is mainly located in the east line of Alaska to New Zealand and Chatham Island, through the Japanese island arcs, Mariana Trench, silk disulfide islands, the Bismarck islands, Fiji and Tonga islands.