Granophyre ( ; from ''
granite'' and ''porphyry'') is a
subvolcanic rock A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at depths less than within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks and plutonic ro ...
that contains
quartz and alkali
feldspar in characteristic angular intergrowths such as those in the accompanying image.
The
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 silicic com ...
. 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
granites.
A common occurrence of granophyre is within layered igneous intrusions dominated by rocks with compositions like that of
gabbro. In such occurrences, the granophyre may form as an end product of
fractional crystallization Fractional crystallization may refer to:
* Fractional crystallization (chemistry), a process to separate different solutes from a solution
* Fractional crystallization (geology)
Fractional crystallization, or crystal fractionation, is one of the ...
of a parent mafic
magma, 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 Natural satellite, moon. When the ...
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.
See also
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References
Igneous petrology
Subvolcanic rocks
Porphyritic rocks
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