|
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.[1] These rocks are primarily composed ofquartz, 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] 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 content,[jargon] emplacement depth, and tectonic regime. Generalizations 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
|
.png){kind=small}
