right|thumb|250px|A sample of granulite-facies metamorphic rock of [[felsic composition, with garnet porphyroblasts]]

Granulites are a class of high-grade metamorphic rocks that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. A common type of granulite found in high-grade metamorphic rocks of the continents contains pyroxene, plagioclase feldspar and accessory garnet, oxides and possibly amphiboles. Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino- and orthopyroxene in a metabasite (metamorphed basalt) defines the granulite facies.

A granulite may be visually quite distinct with abundant small pink or red pyralspite garnets in a 'granular' holocrystalline matrix. Concentrations of garnets, micas, or amphiboles may form along a linear pattern resembling gneiss or migmatite banding.

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|<small>Diagram showing metamorphic facies in pressure-temperature space. The domain of the<br /> graph corresponds to circumstances within the Earth's crust and upper mantle.</small>

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Formation

Granulites form at crustal depths, typically during regional metamorphism at high thermal gradients of greater than 30&nbsp;°C/km. In continental crustal rocks, biotite may break down at high temperatures to form orthopyroxene + potassium feldspar + water, producing a granulite. Other possible minerals formed at dehydration melting conditions include sapphirine, spinel, sillimanite, and osumilite. Some assemblages such as sapphirine + quartz indicate very high temperatures of greater than 900&nbsp;°C. Some granulites may represent the residues of partial melting at extraction of felsic melts in variable amounts, and in extreme cases represent rocks that all constituent minerals are anhydrous and thus look as if they did not melt at ultrahigh temperature conditions. Therefore, very high temperatures of 900 to 1150&nbsp;°C are even necessary to produce the granulite-facies mineral assemblages. Such high temperatures at crustal depths only can be delivered by upwelling of the asthenospheric mantle in continental rifting settings, which can cause the regional metamorphism at the high thermal gradients of greater than 30&nbsp;°C/km.

Granulite facies

The granulite facies is determined by the lower temperature boundary of 700 ± 50&nbsp;°C and the pressure range of 2–15&nbsp;kb. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite and Al<sub>2</sub>O<sub>3</sub>-rich pyroxenes.

Transition between amphibolite and granulite facies is defined by these reaction isograds:

:amphibole → pyroxene + H<sub>2</sub>O

:biotite → K-feldspar + garnet + orthopyroxene + H<sub>2</sub>O.

Hornblende granulite subfacies is a transitional coexistence region of anhydrous and hydrated ferromagnesian minerals, so the above-mentioned isograds mark the boundary with pyroxene granulite subfacies – facies with completely anhydrous mineral assemblages. To the German petrologists granulite means a more or less banded fine-grained metamorphic rock, consisting mainly of quartz and feldspar in very small irregular crystals and usually also containing a fair number of minute, rounded, pale-red garnets. Among English and American geologists the term is generally employed in this sense.

The granulites are very closely allied to the gneisses, as they consist of nearly the same minerals, but they are finer-grained, have usually less perfect foliation, are more frequently garnetiferous, and have some special features of microscopic structure. In the rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a closely fitted mosaic. The individual crystals never have perfect form, and indeed traces of it are rare. In some granulites they interlock, with irregular borders; in others they have been drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between the larger. This is especially true of the quartz and feldspar which are the predominant minerals; mica always appears as flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary schistosity, and they may be aggregated into bands in which case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals.