Charnockite

(named for the founder of Calcutta, J. Charnock), a hypersthene granite typical of Precambrian assemblages that contain migmatites. Charnockite is composed of orthoclase, oligoclase, quartz, and hypersthene; it also often includes the garnets almandine and pyrope, as well as biotite, magnetite, and other minerals. Two types of charnockites are distinguished: ferrohypersthene charnockite and garnet charnockite.

The composition of charnockite varies. It is currently believed that ferrohypersthene charnockites occur in relatively shallow fades, whereas garnet charnockites are typical of more deep-seated metamorphic facies. The formation of garnet charnockites whose outcroppings are observed in the most deeply eroded Precambrian shields (Indian, Sinian, Aldan, Baltic, and Canadian), is distinguished. The origin of charnockites is linked with magmatism and metamorphism; charnockites are associated with anorthosites and other plutonic rocks, forming the charnockite-anorthosite series.

The term “charnockite” was first proposed by the British geologist T. Holland in 1900 for the intrusive rocks of the Madras area, which contain ferruginous hypersthene as the chief mineral.