Granophyre ( ; from ''

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, 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 coo ...

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

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

and alkali

feldspar Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

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

texture Texture may refer to: Science and technology * Image texture, the spatial arrangement of color or intensities in an image * Surface texture, the smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface c ...

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 silicic c ...

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

. 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 (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

, 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 rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

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-rock-stub Igneous petrology Subvolcanic rocks Porphyritic rocks

See also

References