CHARNOCKITE

any member of a series of metamorphic rocks with variable chemical composition, first described from the state of Tamil Nadu in southern India and named for Job Charnock. The term is often limited to the characteristic hypersthene granite of the series. Charnockite occurs all over the world, most often in deeply eroded Precambrian basement rock complexes. Members of the series contain characteristic minerals that distinguish them from other rocks of comparable bulk chemical composition. Hypersthene is typical for all members of the series. The alkali feldspar may be intermediate between ordinary microcline and orthoclase, a fine microperthitic texture being common; the plagioclase feldspar is usually antiperthitic. Dark colour and clouding of the feldspars are typical features of these rocks, as is a bluish tint in quartz. Some charnockites contain a brownish-green hornblende, often rather rich in titanium. The garnet characteristic of these rocks is rich in pyrope. The charnockite series originally was assumed to have developed by the fractional crystallization of a silicate magma (molten material). Subsequent studies have shown, however, that many, if not all, of the rocks are metamorphic, formed by recrystallization at high pressures and moderately high temperatures.