Agranitoid 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 ofquartz,plagioclase, andalkali feldspar.[2] Granitoids range from plagioclase-richtonalites to alkali-richsyenites and from quartz-poormonzonites to quartz-richquartzolites.[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]
The termsgranite andgranitic 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, andtectonic regime.
There are several generalizations that apply to the majority of granitoids. Typically, granitoids occur whereorogeny thickenscontinental crust either bysubduction yielding acontinental arc or byconvergence yielding continental collisions.[4] Generally, the evolution to granitoidmagmas requires a thermal disturbance to ascent though continental crust.[4] Most granitoids are generated fromcrustal anatexis, the partial melting of the crust; however the mantle may contribute both heat and material.[4] Granitoids can occur coeval withvolcanic rocks that have equivalent chemical composition (granite–rhyolite, syenite–trachyte,granodiorite–dacite etc.) however, these extrusive rocks are ofteneroded so just theplutonic 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 inanorogenic environments, a granitoid source rock can be from the mantle (for example, at intraplatehotspots) and the melting mechanism can beradiogenic crustal heat.[5][6][7]
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