Metamorphism of Granitoids

Abstract

Granitoid rocks comprise granites, alkali-feldspar granites, granodiorites and tonalites, and constitute a large portion of the continental crust. 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.