Granitoid
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.<ref>Template:Cite journal</ref> These rocks are primarily composed of quartz, plagioclase, and alkali feldspar.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites.<ref name="Streckeisen-1974">Template:Cite journal</ref> 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.<ref name="Streckeisen-1974" />
Nomenclature and classificationEdit
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.
GeneralizationsEdit
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.<ref name="Winter-2014">Template:Cite book.</ref> Generally, the evolution to granitoid magmas requires a thermal disturbance to ascent though continental crust.<ref name="Winter-2014" /> Most granitoids are generated from crustal anatexis, the partial melting of the crust; however the mantle may contribute both heat and material.<ref name="Winter-2014" /> 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.<ref name="Winter-2014" /> Granitoids can form in all tectonic environments.<ref name="Winter-2014" />
There are numerous exceptions to these generalizations.<ref name="Clarke-1992">Clarke, D. B. (1992) Granitoid Rocks, Chapman & Hall, London.</ref><ref name="Winter-2014" /> 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.<ref name="Clarke-1992" /><ref>Pitcher, W. S. 1982. Granite type and tectonic environment. In Hsu, K. J. (ed.) Mountain Building Processes, 19–40. London: Academic Press.</ref><ref>Template:Cite journal</ref>