Restite
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Restite is the residual material left at the site of melting during the in place production of magma.
Generally, restite is composed of a predominance of mafic minerals because these are harder to melt (see Bowen's reaction series). Typical minerals are amphibole, biotite, pyroxene, ilmenite or other iron oxides and some plagioclase feldspar. When chunks of restite are caught up within the granite it is known as a restite inclusion or enclave.
Restite in S-type granites is produced from the melting, within the Earth's crust, of a typical metamorphic mineral assemblage of high-pressure gneiss of sedimentary origin;
- biotite + quartz + feldspars → hydroxyl-bearing melt + orthopyroxene + cordierite + 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 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.
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 +/- garnet 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.