Metamorphism of Granitoid Rocks

Granitoid rocks comprise granite, alkali-feldspar granite, granodiorite and tonalite. They constitute the largest portion of the continental crust. Granitoid gneisses, commonly migmatites, are in fact the dominant rock type of the continents. Because the main constituents — alkali-feldspar, plagioclase, quartz, biotite, muscovite, hornblende — are found over a wide range of P-T conditions, this rock group is not a very useful indicator of metamorphic grade and is therefore largely neglected in textbooks on metamorphic petrology. Unlike wet sedimentary rocks, granitoid rocks will enter the metamorphic realm in a predominantly dry state. In order to start metamorphic reactions some hydration is necessary. The access of a water-rich fluid will be facilitated by tectonic activity. Also, in the absence of penetrative deformation, granitoid rocks retain remarkably well their original igneous structures.