Restite is the residual material left at the site of melting during the ''in place'' production of

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 ...

. Generally, restite is composed of a predominance of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s because these are harder to melt (see Bowen's reaction series). Typical minerals are

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

or other

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

s and some

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

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 ...

. 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

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

, of a typical metamorphic mineral assemblage of high-pressure

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

origin; :biotite +

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 ...

+ feldspars → OH-bearing melt + orthopyroxene +

cordierite Cordierite (mineralogy) or iolite (gemology) 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: to . A high-tempera ...

+ 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-rich

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

and quartz during ascent with the entraining

. 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 Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

-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 Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

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 The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron (Fe ...

+ plagioclase +/-

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

assemblage. Similar to the reactions occurring in S-type granites, the restite minerals will revert to

hornblende Hornblende is a complex silicate minerals#Inosilicates, 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. Hornblende minerals are common ...

and plagioclase upon ascent, resorbing water and precluding generation of hydrothermal solutions.

Porphyry copper Porphyry copper deposits are copper ore bodies that are formed from hydrothermal circulation, hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those flui ...

deposits are generally associated with I-type granites which are not restite mediated.

Importance of restite

Restite is an important constituent in fractional crystallisation and

igneous differentiation In geology, igneous differentiation, or magmatic 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. The sequence of ...

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 A buffer solution is a solution where the pH does not change significantly on dilution or if an acid or base is added at constant temperature. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solution ...

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 (or equivalently, oxygen fugacities) at which a rock forms and evolves can be ...

except in this case it is a mineral-water exchange. This process occurs by hydration of ferromagnesian minerals, particularly

, 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 In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

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 (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term " partial melting" refers to the p ...

(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 Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% ...

magmas, it is much easier for these magmas to achieve more dramatic fractional crystallization effects.

References

* White, A.J.R., 2001. Water, Restite and Granite Mineralisation, ''Australian Journal of Earth Sciences'', 48, pp 551-555. Petrology Igneous rocks Metamorphic rocks

S-type restite reactions

I-type restite reactions

Importance of restite

See also

References