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Granodiorite

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Granodiorite is intrusive igneous rock that have phaneritic textured.The grain sizes are visible to the naked eye.Granodiorite formation is slow cooling crystallization below Earth’s surface. It is similar to granite and diorite, but It have more plagioclase feldspar than orthoclase feldspar.According to the QAPF diagram, granodiorite has a greater than 20% quartz by volume, and between 65% to 90% of the feldspar is plagioclase. A greater amount of plagioclase would designate the rock as tonalite.

Group: Plutonic.

Colour: Typically light-coloured.

Texture: Phaneritic (medium to coarse grained).

Mineral Content: Quartz, plagioclase, with lesser orthoclase, biotite (these separate it fromdiorite) and amphibole ( hornblende) (plagioclase always greater than 2/3 of total feldspar).

Silica (SiO 2) content – 63%-69%.

Name origin: The name comes from two related rocks to which granodiorite is an intermediate: granite and diorite. The gran- root comes from the Latin grānum for “grain”, an English language derivative. Diorite is named after the contrasting colors of the rock.

Intrusive Equivalent: Rhyodacite.

Structure: Massive, confining.

Major minerals: Plagioclase, Potassium Feldspar, Quartz, Biotite

Accessory minerals: Muscovite mica, Hornblende, Pyroxene

Chemical Composition and Classification

Granodiorite composition is felsic intermediate rock. It is igneous equivalent of dacite. Granodiorite contained a large amount of sodium (Na) and calcium (Ca) rich plagioclase, potassium feldspar, quartz, and minor amounts of muscovite mica as the lighter colored mineral components and biotite, amphiboles frequently inside of hornblende more abundant  than the granite. Mica can be found in nicely-fashioned hexagonal crystals, and hornblende may appear as needle-like crystals. Minor amounts of oxide minerals inclusive of magnetite, ilmenite, and ulvöspinel, in addition to some sulfide minerals may also be present.

Granodiorite is in section 4 of the QAPF diagram.

Formation of the Granodiorite Rock

Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet’s mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.

Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock (called country rock); the magma cools slowly and, as a result, these rocks are coarse-grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes. When the magma solidifies within the earth’s crust, it cools slowly forming coarse textured rocks, such as granite, gabbro, or diorite.

The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth’s surface.

What is the difference between Granite and Granodiorite?

These rocks are both classified as granitic, because they both are rich in quartz. Granite contains mostly potassium feldspars and has a low percentage of dark iron and magnesium minerals. In contrast, granodiorite contains more plagioclase (calcium and sodium) feldspar than potassium feldspar and has more dark minerals. Thus it is a darker color than granite. Chemical and x-ray analysis of granite and granodiorite can be used to “fingerprint” these rocks, telling their exact composition and where they may have formed.

Where is It Located

Granodiorite, like diorite, is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas.

Uses of The Rock

  • Granodiorite is most often used as crushed stone for road building.
  • It is also used as construction material, building facade, and paving, and as an ornamental stone.
  • The portico columns of the Pantheon in Rome are formed from single shafts of granodiorite, each 12 metres tall by 1.5 metres in diameter.

Facts About The Rock

  • One of the most abundant igneous rocks is granodiorite.
  • This rock has some features of the acidic granites and some features of the intermediate rocks.
  • Granodiorite is an attractive, coarse-grained rock. The crystals making up the mass of the rock can easily be seen with the naked eye.
  • The main minerals in granodiorite are feldspar, quartz, hornblende, augite and mica.
  • There are two main color varieties of granodiorite. One is pink because of the color of most of the feldspar in the rock. White granodiorite contains pale-colored feldspar.
  • This rock looks similar to granite. When its minerals are examined and the total silica content worked out, it can be seen that it is an intermediate, not an acid rock.
  • In many types of igneous intrusions, granodiorite can be found, especially those formed at some depth below the surface of the Earth.
  • The vast batholith in southern California covers a surface area of ​​more than 7700 sq km. Much of it is made of granodiorite.
  • Because of its coloring and crystalline appearance, granodiorite is used for ornamental purposes.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Wikipedia contributors. (2018, October 22). Granodiorite. In Wikipedia, The Free Encyclopedia. Retrieved 12:42, April 11, 2019, from https://en.wikipedia.org/w/index.php?title=Granodiorite&oldid=865243097

Cite this article as: Geology Science. (2020). Granodiorite. [online] Available at: http://geologyscience.com/rocks/igneous-rocks/granodiorite/ [22nd January 2020 ]
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