Granitoid

Granite rock hand-sized sample

A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental crust. ^[1] These rocks are primarily composed of quartz, plagioclase, and alkali feldspar. ^[2] Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites. ^[3] 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. ^[3]

Nomenclature and classification

The terms granite and granitic rock are often used interchangeably for granitoids; however, granite is just one particular type of granitoid.

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 composition, emplacement depth, and tectonic regime.

Generalizations

Illustration of continental collision as a result of convergence

There are several generalizations that apply to the majority of granitoids. Typically, granitoids occur where orogeny thickens continental crust either by subduction yielding a continental arc or by convergence yielding continental collisions. ^[4] Generally, the evolution to granitoid magmas requires a thermal disturbance to ascent though continental crust. ^[4]   Most granitoids are generated from crustal anatexis, the partial melting of the crust;  however the mantle may contribute both heat and material. ^[4]   Granitoids can occur coeval with volcanic rocks that have equivalent chemical composition (granite–rhyolite, syenite–trachyte, granodiorite–dacite etc.) however, these extrusive rocks are often eroded so just the plutonic rocks outcrop. ^[4]   Granitoids can form in all tectonic environments. ^[4]

There are numerous exceptions to these generalizations. ^{[5] [4]}   For example, granitoids can form in anorogenic environments, a granitoid source rock can be from the mantle (for example, at intraplate hotspots) and the melting mechanism can be radiogenic crustal heat. ^{[5] [6] [7]}

References

1. Tomás, R.; Cano, M.; Pulgarín, L. F.; Brotóns, V.; Benavente, D.; Miranda, T.; Vasconcelos, G. (2021-11-01). "Thermal effect of high temperatures on the physical and mechanical properties of a granite used in UNESCO World Heritage sites in north Portugal". Journal of Building Engineering. 43: 102823. doi:10.1016/j.jobe.2021.102823. hdl: 10045/115630 . ISSN   2352-7102.
2. "USGS: Mineral Resources On-Line Spatial Data: Granitoid".
3. 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.
4. Winter, John D. (2014). Principles of igneous and metamorphic petrology. Harlow. p. 402. ISBN   978-1-292-02153-9. OCLC   890442947..
5. Clarke, D. B. (1992) Granitoid Rocks, Chapman  & Hall, London.
6. Pitcher, W. S. 1982. Granite type and tectonic environment. In Hsu, K. J. (ed.) Mountain Building Processes, 19–40. London: Academic Press.
7. Barbarin, Bernard (1990). "Granitoids: Main petrogenetic classifications in relation to origin and tectonic setting". Geological Journal. 25 (3–4). Wiley: 227–238. Bibcode:1990GeolJ..25..227B. doi: 10.1002/gj.3350250306 . ISSN   0072-1050.