Granophyre ( ; from ''

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

'' and ''porphyry'') is a subvolcanic rock that contains

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

and alkali

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

in characteristic angular intergrowths such as those in the accompanying image. The

texture Texture may refer to: Science and technology * Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface characteristics with waves shorter than road roughness * Texture ...

is called granophyric. The texture can be similar to micrographic texture and to the coarser graphic intergrowths of quartz and alkali feldspar common in

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silici ...

. These textures document simultaneous crystallization of quartz and feldspar from a silicate melt at the eutectic point, perhaps in the presence of a water-rich phase. They may also be formed by crystallization when the magma is significantly undercooled, not necessarily under eutectic conditions. Granophyres typically are intrusive rocks that crystallized at shallow depths, and many have compositions similar to those of

s. A common occurrence of granophyre is within layered igneous intrusions dominated by rocks with compositions like that of

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

. In such occurrences, the granophyre may form as an end product of fractional crystallization of a parent mafic

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

, or by melting of rocks into which the mafic magma was emplaced, or by a combination of the two processes. Granophyre may also form as the uppermost stratigraphic layer resulting from melting of upper-middle crustal rocks by a

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...

impact. For example, the upper layer of the Main Mass of the 1850 Ma Sudbury Structure is composed of fine-medium grained granitic rocks with abundant granophyric textures.

References

Igneous petrology Subvolcanic rocks Porphyritic rocks {{Petrology-stub

See also

References