Restite
Encyclopedia
Restite is the residual material left at the site of melting during the in place production of granite
through intense metamorphism
.
Generally, restite is composed of a predominance of mafic
mineral
s because these are harder to melt (see Bowen's reaction series
). Typical minerals are amphibole
, biotite
, pyroxene
, ilmenite
or other iron oxide
s and some plagioclase
feldspar
. When chunks of restite are caught up within the granite it is known as a restite inclusion or enclave.
of sedimentary origin;
The melt reaction produces a granitic melt and solid orthopyroxene and cordierite.
Cordierite in restite inclusions is unstable at low pressures; this reverts to Al
-rich mica
and quartz during ascent with the entraining magma
. Orthopyroxene, unstable at low temperatures, reverts to an assemblage of biotite plus quartz. Restite feldspars will typically be a sodic plagioclase. Thus, restite inclusions in S-type granites will be a recrystallised granoblastic
textured
inclusion of biotite-muscovite
-feldspar-quartz.
If the restite minerals are carried with the magma, as the minerals become thermo-barometrically unstable during ascent, they will react back with the magma to form biotite from orthopyroxene, and feldspar or mica from cordierite. These reactions also involve consumption of significant quantities of water, and hence, will preclude the generation of a hydrothermal solution.
assemblage. Similar to the reactions occurring in S-type granites, the restite minerals will revert to hornblende
and plagioclase upon ascent, resorbing water and precluding generation of hydrothermal solutions.
Porphyry copper deposits are generally associated with I-type granites which are not restite mediated.
and igneous differentiation
processes.
Restite acts as a form of buffer within magma, acting as a reservoir primarily of water and water-adsorbent minerals, which may prevent or retard a granitic magma from attaining water saturation
. This is analogous to the behaviour of a chemical buffer solution
or mineral redox buffer
except in this case it is a mineral-water exchange.
This process occurs by hydration
of ferromagnesian minerals, particularly hornblende
, which may adsorb up to 5% H2O, and by conversion of pyroxene to hornblende during melting or fractionation at temperatures below the pyroxene stability field. This process is envisaged as, for instance, pyroxene-bearing restite inclusions 'soaking up' water and being converted to hydrous hornblende-bearing inclusions.
Secondly, restite acts as a compositional buffer, providing elements to the surrounding magma as it is melted and ground up by erosive forces within the ascending magma. Restite can, in large enough amounts, retard the compositional changes of a magma either via providing more reagent
s or physically trapping crystals within the magma.
Generally, restite is not present within magmas in large amounts and thus the effects of the above processes are not usually profound. However, it is likely that, particularly for S-type granite which is formed by wholesale anatexis
(melting) of metasedimentary rocks, restite mediated melting and fractionation is crucial to the composition and behaviour of these magmas.
In magmas which do not have a restite component, such as most M-type granites, some A-type granites, and most basaltic magmas, it is much easier for these magmas to achieve more dramatic fractional crystallization effects.
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
through intense metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
.
Generally, restite is composed of a predominance of mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s because these are harder to melt (see Bowen's reaction series
Bowen's reaction series
Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be found together while others are almost never associated with one another...
). Typical minerals are amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
or other iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s and some plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. When chunks of restite are caught up within the granite it is known as a restite inclusion or enclave.
S-type restite reactions
Restite in S-type granites is produced from the melting, within the crust, of a typical metamorphic mineral assemblage of high-pressure gneissGneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
of sedimentary origin;
- biotite + quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
+ feldspars --> OHHydroxylA hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
-bearing melt + orthopyroxene + cordieriteCordieriteCordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
+ residual feldspars
The melt reaction produces a granitic melt and solid orthopyroxene and cordierite.
Cordierite in restite inclusions is unstable at low pressures; this reverts to Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
-rich mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
and quartz during ascent with the entraining magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
. Orthopyroxene, unstable at low temperatures, reverts to an assemblage of biotite plus quartz. Restite feldspars will typically be a sodic plagioclase. Thus, restite inclusions in S-type granites will be a recrystallised granoblastic
Granoblastic
Granoblastic is an anhedral phaneritic equi-granular metamorphic rock texture. Granoblastic texture is typical of quartzite, marble and other non-foliated metamorphic rocks without porphyroblasts. Characteristics defining granoblastic texture include: grains visible to the unaided eye, sutured...
textured
Rock microstructure
Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
inclusion of biotite-muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
-feldspar-quartz.
If the restite minerals are carried with the magma, as the minerals become thermo-barometrically unstable during ascent, they will react back with the magma to form biotite from orthopyroxene, and feldspar or mica from cordierite. These reactions also involve consumption of significant quantities of water, and hence, will preclude the generation of a hydrothermal solution.
I-type restite reactions
Restite reactions in I-type granites are essentially similar, but due to the mafic and granitic source rocks, the restite assemblage is predisposed to produce an orthopyroxene + clinopyroxene + plagioclase +/- garnetGarnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
assemblage. Similar to the reactions occurring in S-type granites, the restite minerals will revert to hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
and plagioclase upon ascent, resorbing water and precluding generation of hydrothermal solutions.
Porphyry copper deposits are generally associated with I-type granites which are not restite mediated.
Importance of restite
Restite is an important constituent in fractional crystallisationFractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
and igneous differentiation
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
processes.
Restite acts as a form of buffer within magma, acting as a reservoir primarily of water and water-adsorbent minerals, which may prevent or retard a granitic magma from attaining water saturation
Saturation (chemistry)
In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
. This is analogous to the behaviour of a chemical buffer solution
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
or mineral redox buffer
Mineral redox buffer
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions at which a rock forms and evolves can be important for interpreting the rock history...
except in this case it is a mineral-water exchange.
This process occurs by hydration
Mineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
of ferromagnesian minerals, particularly hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, which may adsorb up to 5% H2O, and by conversion of pyroxene to hornblende during melting or fractionation at temperatures below the pyroxene stability field. This process is envisaged as, for instance, pyroxene-bearing restite inclusions 'soaking up' water and being converted to hydrous hornblende-bearing inclusions.
Secondly, restite acts as a compositional buffer, providing elements to the surrounding magma as it is melted and ground up by erosive forces within the ascending magma. Restite can, in large enough amounts, retard the compositional changes of a magma either via providing more reagent
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
s or physically trapping crystals within the magma.
Generally, restite is not present within magmas in large amounts and thus the effects of the above processes are not usually profound. However, it is likely that, particularly for S-type granite which is formed by wholesale anatexis
Anatexis
Anatexis in geology, refers to the differential, or partial, melting of rocks, especially in the forming of metamorphic rocks such as migmatites.-Optimum Temperature Conditions for Crustal Melting:...
(melting) of metasedimentary rocks, restite mediated melting and fractionation is crucial to the composition and behaviour of these magmas.
In magmas which do not have a restite component, such as most M-type granites, some A-type granites, and most basaltic magmas, it is much easier for these magmas to achieve more dramatic fractional crystallization effects.