Editing Granite (section)

==Origin==
[[File:Xenolith, Yosemite National Park (19446829471).jpg|thumb|Mafic [[Enclave (geology)|enclave]] in granite rock, at [[Yosemite National Park]]]]
Granite forms from silica-rich ([[felsic]]) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock of the lower [[Earth's crust|crust]], rather than by decompression of mantle rock, as is the case with [[basalt]]ic magmas.{{sfn|Philpotts|Ague|2009|pp=15–16}} It has also been suggested that some granites found at [[convergent boundaries]] between [[tectonic plate]]s, where [[oceanic crust]] [[subducts]] below continental crust, were formed from [[Mélange|sediments]] subducted with the oceanic plate. The melted sediments would have produced magma [[intermediate magma|intermediate]] in its silica content, which became further enriched in silica as it rose through the overlying crust.<ref>{{cite journal |last1=Castro |first1=Antonio |title=The off-crust origin of granite batholiths |journal=Geoscience Frontiers |date=January 2014 |volume=5 |issue=1 |pages=63–75 |doi=10.1016/j.gsf.2013.06.006|doi-access=free |bibcode=2014GeoFr...5...63C }}</ref>

Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon.{{sfn|Blatt|Tracy|1996|p=128}} Further fractionation reduces the content of iron, calcium, and titanium.{{sfn|Blatt|Tracy|1996|p=172}} This is reflected in the high content of alkali feldspar and quartz in granite.

The presence of granitic rock in [[island arc]]s shows that [[fractional crystallization (geology)|fractional crystallization]] alone can convert a basaltic magma to a granitic magma, but the quantities produced are small.{{sfn|Philpotts|Ague|2009|p=378}} For example, granitic rock makes up just 4% of the exposures in the [[South Sandwich Islands]].<ref>{{cite journal |last1=Baker |first1=P. E. |title=Comparative volcanology and petrology of the atlantic island-arcs |journal=Bulletin Volcanologique |date=February 1968 |volume=32 |issue=1 |pages=189–206 |doi=10.1007/BF02596591|bibcode=1968BVol...32..189B |s2cid=128993656 }}</ref> In continental arc settings, granitic rocks are the most common plutonic rocks, and batholiths composed of these rock types extend the entire length of the arc. There are no indication of magma chambers where basaltic magmas [[Igneous differentiation|differentiate]] into granites, or of [[cumulate rock|cumulates]] produced by [[mafic]] crystals settling out of the magma. Other processes must produce these great volumes of felsic magma. One such process is injection of basaltic magma into the lower crust, followed by differentiation, which leaves any cumulates in the mantle. Another is heating of the lower crust by [[underplating]] basaltic magma, which produces felsic magma directly from crustal rock. The two processes produce different kinds of granites, which may be reflected in the division between S-type (produced by underplating) and I-type (produced by injection and differentiation) granites, discussed below.{{sfn|Philpotts|Ague|2009|p=378}}

===Alphabet classification system===
[[File:Mineralogy igneous rocks EN.svg|thumb|left|upright=1.4|Mineral assemblage of igneous rocks]]
The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was. The final texture and composition of a granite are generally distinctive as to its parental rock. For instance, a granite that is derived from partial melting of metasedimentary rocks may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based.

The letter-based Chappell & White classification system was proposed initially to divide granites into [[I-type granite|I-type]] (igneous source) granite and S-type (sedimentary sources).<ref>{{cite journal |last1=Chappell |first1=B. W. |last2=White |first2=A. J. R. |title=Two contrasting granite types: 25 years later |journal=Australian Journal of Earth Sciences |date=2001 |volume=48 |issue=4 |pages=489–499 |doi=10.1046/j.1440-0952.2001.00882.x|bibcode=2001AuJES..48..489C |s2cid=33503865 |s2cid-access=free |url=https://faculty.uml.edu//nelson_eby/Research/A-type%20granites/Chappell%20and%20White%20S%20and%20I%20type%20granites.pdf |url-status=live |archive-url= https://web.archive.org/web/20221022054006/https://faculty.uml.edu/nelson_eby/research/a-type%20granites/chappell%20and%20white%20s%20and%20i%20type%20granites.pdf |archive-date= Oct 22, 2022 }}</ref> Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks.

I-type granites are characterized by a high content of sodium and calcium, and by a [[strontium isotope]] ratio,  <sup>87</sup>Sr/<sup>86</sup>Sr, of less than 0.708. <sup>87</sup>Sr is produced by radioactive decay of <sup>87</sup>Rb, and since rubidium is concentrated in the crust relative to the mantle, a low ratio suggests origin in the mantle. The elevated sodium and calcium favor crystallization of hornblende rather than biotite. I-type granites are known for their [[porphyry copper]] deposits.{{sfn|Philpotts|Ague|2009|p=378}} I-type granites are orogenic (associated with mountain building) and usually metaluminous.{{sfn|Blatt|Tracy|1996|p=185}}

S-type granites are sodium-poor and aluminum-rich. As a result, they contain [[mica]]s such as biotite and muscovite instead of hornblende. Their strontium isotope ratio is typically greater than 0.708, suggesting a crustal origin. They also commonly contain [[xenolith]]s of metamorphosed sedimentary rock, and host [[tin]] ores. Their magmas are water-rich, and they readily solidify as the water outgasses from the magma at lower pressure, so they less commonly make it to the surface than magmas of I-type granites, which are thus more common as volcanic rock (rhyolite).{{sfn|Philpotts|Ague|2009|p=378}} They are also orogenic but range from metaluminous to strongly peraluminous.{{sfn|Blatt|Tracy|1996|p=185}}

Although both I- and S-type granites are orogenic, I-type granites are more common close to the convergent boundary than S-type. This is attributed to thicker crust further from the boundary, which results in more crustal melting.{{sfn|Philpotts|Ague|2009|p=378}}

A-type granites show a peculiar mineralogy and geochemistry, with particularly high silicon and potassium at the expense of calcium and magnesium<ref>{{cite book |last1=Winter |first1=John D. |title=Principles of igneous and metamorphic petrology |publisher=Pearson Education |year=2014 |location=Harlow |isbn=9781292021539 |page=381 |edition=Second ; Pearson new international}}</ref> and a high content of high field strength cations (cations with a small radius and high electrical charge, such as [[zirconium]], [[niobium]], [[tantalum]], and [[rare earth element]]s.){{sfn|Philpotts|Ague|2009|p=148}} They are not orogenic, forming instead over hot spots and continental rifting, and are metaluminous to mildly peralkaline and iron-rich.{{sfn|Blatt|Tracy|1996|p=185}} These granites are produced by partial melting of refractory lithology such as granulites in the lower continental crust at high thermal gradients. This leads to significant extraction of hydrous felsic melts from granulite-facies resitites.{{sfn|Blatt|Tracy|1996|pp=203–206}}<ref>{{cite journal |last1=Whalen |first1=Joseph B. |last2=Currie |first2=Kenneth L. |last3=Chappell |first3=Bruce W. |title=A-type granites: geochemical characteristics, discrimination and petrogenesis |journal=Contributions to Mineralogy and Petrology |date=April 1987 |volume=95 |issue=4 |pages=407–419 |doi=10.1007/BF00402202|bibcode=1987CoMP...95..407W |s2cid=128541930 |url=http://www.gt-crust.ru/jour/article/view/579 }}</ref> A-type granites occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica.<ref>{{cite journal |last1=Cottle |first1=John M. |last2=Cooper |first2=Alan F. |title=Geology, geochemistry, and geochronology of an A-type granite in the Mulock Glacier area, southern Victoria Land, Antarctica |journal=New Zealand Journal of Geology and Geophysics |date=June 2006 |volume=49 |issue=2 |pages=191–202 |doi=10.1080/00288306.2006.9515159|s2cid=128395509 |doi-access=free |bibcode=2006NZJGG..49..191C }}</ref> The rhyolites of the Yellowstone Caldera are examples of volcanic equivalents of A-type granite.<ref>{{cite journal |last1=Branney |first1=M. J. |last2=Bonnichsen |first2=B. |last3=Andrews |first3=G. D. M. |last4=Ellis |first4=B. |last5=Barry |first5=T. L. |last6=McCurry |first6=M. |title='Snake River (SR)-type' volcanism at the Yellowstone hotspot track: distinctive products from unusual, high-temperature silicic super-eruptions |journal=Bulletin of Volcanology |date=January 2008 |volume=70 |issue=3 |pages=293–314 |doi=10.1007/s00445-007-0140-7|s2cid=128878481 }}</ref>

M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle.<ref>{{cite journal |last1=Whalen |first1=J. B. |title=Geochemistry of an Island-Arc Plutonic Suite: the Uasilau-Yau Yau Intrusive Complex, New Britain, P.N.G |journal=Journal of Petrology |date=1 August 1985 |volume=26 |issue=3 |pages=603–632 |doi=10.1093/petrology/26.3.603|bibcode=1985JPet...26..603W }}</ref> Although the fractional crystallisation of basaltic melts can yield small amounts of granites, which are sometimes found in island arcs,<ref>{{cite journal |last1=Saito |first1=Satoshi |last2=Arima |first2=Makoto |last3=Nakajima |first3=Takashi |last4=Kimura |first4=Jun-Ichi |title=Petrogenesis of Ashigawa and Tonogi granitic intrusions, southern part of the Miocene Kofu Granitic Complex, central Japan: M-type granite in the Izu arc collision zone |journal=Journal of Mineralogical and Petrological Sciences |date=2004 |volume=99 |issue=3 |pages=104–117 |doi=10.2465/jmps.99.104|bibcode=2004JMPeS..99..104S |doi-access=free }}</ref> such granites must occur together with large amounts of basaltic rocks.{{sfn|Philpotts|Ague|2009|p=378}}

H-type granites were suggested for hybrid granites, which were hypothesized to form by mixing between mafic and felsic from different sources, such as M-type and S-type.<ref>{{cite journal |last1=Castro |first1=A. |last2=Moreno-Ventas |first2=I. |last3=de la Rosa |first3=J.D. |title=H-type (hybrid) granitoids: a proposed revision of the granite-type classification and nomenclature |journal=Earth-Science Reviews |date=October 1991 |volume=31 |issue=3–4 |pages=237–253 |doi=10.1016/0012-8252(91)90020-G|bibcode=1991ESRv...31..237C }}</ref> However, the big difference in rheology between mafic and felsic magmas makes this process problematic in nature.{{sfn|Philpotts|Ague|2009|pp=104-105}}

===Granitization===
[[File:Migmatite (Morton Gneiss Complex, Archean, ~3.524 Ga; Cold Spring Granite Company quarry, Morton, Minnesota, USA) 3 (41897958622).jpg|thumb|Migmatite featuring felsic minerals, at [[Morton Gneiss|Morton Gneiss Complex]]]]
Granitization is an old, and largely discounted, hypothesis that granite is formed in place through extreme [[metasomatism]]. The idea behind granitization was that fluids would supposedly bring in elements such as potassium, and remove others, such as calcium, to transform a metamorphic rock into granite. This was supposed to occur across a migrating front. However, experimental work had established by the 1960s that granites were of igneous origin.{{sfn|Philpotts|Ague|2009|p=511}} The mineralogical and chemical features of granite can be explained only by crystal-liquid phase relations, showing that there must have been at least enough melting to mobilize the magma.<ref>{{cite book |last1=McBirney |first1=Alexander R. |title=Igneous petrology |date=1984 |publisher=Freeman, Cooper |location=San Francisco, Calif. |isbn=0877353239 |pages=379–380}}</ref>

However, at sufficiently deep crustal levels, the distinction between metamorphism and crustal melting itself becomes vague. Conditions for crystallization of liquid magma are close enough to those of high-grade metamorphism that the rocks often bear a close resemblance.{{sfn|McBirney|1984|pp=379–380}} Under these conditions, granitic melts can be produced in place through the partial melting of metamorphic rocks by extracting melt-mobile elements such as potassium and silicon into the melts but leaving others such as calcium and iron in granulite residues. This may be the origin of ''[[migmatite]]s''. A migmatite consists of dark, refractory rock (the ''melanosome'') that is permeated by sheets and channels of light granitic rock (the ''leucosome''). The leucosome is interpreted as partial melt of a parent rock that has begun to separate from the remaining solid residue (the melanosome).{{sfn|Philpotts|Ague|2009|p=44}} If enough partial melt is produced, it will separate from the source rock, become more highly evolved through fractional crystallization during its ascent toward the surface, and become the magmatic parent of granitic rock. The residue of the source rock becomes a [[granulite]].

The partial melting of solid rocks requires high temperatures and the addition of water or other volatiles which lower the [[solidus temperature]] (temperature at which partial melting commences) of these rocks. It was long debated whether crustal thickening in orogens (mountain belts along [[convergent boundaries]]) was sufficient to produce granite melts by [[radiogenic heating]], but recent work suggests that this is not a viable mechanism.<ref>{{cite journal |last1=Clark |first1=Chris |last2=Fitzsimons |first2=Ian C. W. |last3=Healy |first3=David |last4=Harley |first4=Simon L. |title=How Does the Continental Crust Get Really Hot? |journal=Elements |date=1 August 2011 |volume=7 |issue=4 |pages=235–240 |doi=10.2113/gselements.7.4.235|bibcode=2011Eleme...7..235C }}</ref> In-situ granitization requires heating by the asthenospheric mantle or by underplating with mantle-derived magmas.<ref>{{cite journal |last1=Zheng |first1=Y.-F. |last2=Chen |first2=R.-X. |title=Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins |journal=Journal of Asian Earth Sciences |date=2017 |volume=145 |pages=46–73 |doi=10.1016/j.jseaes.2017.03.009|bibcode=2017JAESc.145...46Z |doi-access=free }}</ref>