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In my old textbooks, granites used to be part of the metamorphic rocks.
Is this still true or not ? If not fully true, to which extent ?
Could someone point me towards clear examples or definitions showing they are considered not metamorphic rocks any more ?

If not metamorphic, please which part of the rock classification do they belong to exactly ?

Thanks, Gilbert.

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  • $\begingroup$ Hello Treblig, it may help to give background with a reference/quote if you happen to be able to $\endgroup$ – JeopardyTempest Dec 5 '18 at 16:46
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    $\begingroup$ Generally Granite and Gneiss can have the same constituent minerals and even similar crystal sizes but the granite name is reserved for rock that cooled from magma. Alternatively Gneiss is a metamorphic rock that formed from existing rocks, containing granite minerals, that have undergone significant pressure and some heating. Gneiss can usually be identified from Granite by the presence of some layering in the rock, often with the biotite mica's plate like crystals showing the layering. $\endgroup$ – user824 Dec 5 '18 at 17:54
  • $\begingroup$ For starters, if you search 'is granite a metamorphic rock' google already tells you that granite is composed of mainly quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. Some of these rocks are usually found inside other rocks or ores. $\endgroup$ – Eevee Dec 7 '18 at 14:07
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This is the key:

used to be part

It is important to look at this from a historical point of view. Up until the 1950s and early 1960s, there was no agreement to how granites form. This became the granite controversy.

On the one hand, experimental work showed that granites crystallise from magma, but this magma had to be derived from basalt-like magmas. This led to two problems:

  1. Where is all the basalt from which the granite was supposed to form?
  2. What happened to all the rock that was there before the granite (aka the "room problem")?

This led people to think that granites formed by metasomatic replacement of other rocks. Essentially, you have other things than were not granite and then they were transformed to granite through various geological process, without ever crystallising from magmas. This is what people say when they talk about granites as metamorphic rocks.

But, this is not the case and later research demonstrated that granites from by crystallisation of magma and the process of metamorphic formation of granites (by metasomatism, or "granitisation") does not actually exists. There were (and still are) many questions left unanswered on exactly how this happens, but we know that this does happen and granites are indeed igneous rocks. In fact, there's a great paper published just this week in the journal Nature, showing modelling of how some of these things can form.

Nevertheless, granite formed can be sometimes related to metamorphism. Extreme cases of hot metamorphic can cause melting of (previously sedimentary and then metamorphic) rocks to form magma that eventually crystallises as granites. This is known as migmatisation, and is commonly used to explain the widespread occurrence of "leucogranites" in the Himalayas for example.

Additionally, some high-grade metamorphic rocks contain quartz, feldspars, micas, and amphiboles. The metamorphic texture (foliation and lineation) is not always obvious, making it easy to mistake metamorphic gneisses as igneous granites.

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Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartzand feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock.

Source from: https://docsbay.net/granits

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According to google, granite is an igneous rock. And as I said in my comment, it has several different other rocks that are formed within it, such as quartz or feldspar.

Some people may tell you that granite is a sedimentary rock, but based on the way it is formed (by magma cooling beneath the earths' surface) it is considered an igneous rock.

More information at geology.com.

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