Granitoid

A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar.^[1] Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites.^[2] As only two of the three defining mineral groups (quartz, plagioclase, and alkali feldspar) need to be present for the rock to be called a granitoid, foid-bearing rocks, which predominantly contain feldspars but no quartz, are also granitoids.^[2] The terms granite and granitic rock are often used interchangeably for granitoids; however, granite is just one particular type of granitoid.

Granite rock hand sample

Granitoids are diverse; no classification system for granitoids can give a complete and unique characterization of the origin, compositional evolution, and geodynamic environment for the genesis of a granitoid. Accordingly, multiple granitoid classification systems have been developed such as those based on: geochemistry, modal content, emplacement depth, and tectonic regime.

Generalizations

 

Illustration of continental collision as a result of convergence

There are several generalizations that apply to majority of granitoids. Typically, granitoids occur where orogeny thickens continental crust either by subduction yielding a continental arc or by convergence yielding continental collisions.^[3] Generally, the evolution to granitoid magmas requires a thermal disturbance to ascent though continental crust.^[3]  Most granitoids are generated from crustal anatexis, the partial melting of the crust;  however the mantle may contribute both heat and material.^[3]  Granitoids can occur coeval with volcanic rocks that have equivalent chemical composition ( granites-rhyolites, syenite-trachyte, dacite-granodiorite etc.) however, these extrusive rocks are often eroded so just the plutonic rocks outcrop.^[3]  Granitoids can form in all tectonic environments.^[3]

There are numerous exceptions to these generalizations.^[4][3]  For example, granitoids can form in anorogenic  environments, a granitoid source rock can be from the mantle (ex. intraplate hotspots) and the melting mechanism can be radiogenic crustal heat.^[4][5][6]

References

1. "USGS: Mineral Resources On-Line Spatial Data: Granitoid".
2. Streckeisen, A. (1974). "Classification and Nomenclature of Plutonic Rocks: Recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks". Geologische Rundschau (in German). 63 (2): 773–86. Bibcode:1974GeoRu..63..773S. doi:10.1007/BF01820841. S2CID 130569261.
3. Winter, John D. (2014). Principles of igneous and metamorphic petrology. Harlow. p. 402. ISBN 978-1-292-02153-9. OCLC 890442947.{{cite book}}: CS1 maint: location missing publisher (link).
4. Clarke, D. B. (1992) Granitoid Rocks, Chapman  & Hall, London.
5. Pitcher, W. S. 1982. Granite type and tectonic environment. In Hsu, K. J. (ed.) Mountain Building Processes, 19–40. London: Academic Press.
6. Barbarin, Bernard (1990). "Granitoids: Main petrogenetic classifications in relation to origin and tectonic setting". Geological Journal. Wiley. 25 (3–4): 227–238. doi:10.1002/gj.3350250306. ISSN 0072-1050.