Charnockites of Manbhum and the Charnockite Problem

The ultrabasic rocks and norites of Manbhurn are not to be included in the charnockite series. They belong to the earlier sedimentary-igneous sequence that underwent intense deformation and recrystallization before the charnockites (sensu stricto) came into being. The basic charnockites (pyroxene granulites) are derived as a result of metamorphic convergence at granulite facies from cafemic sediments and norites. These are isofacial with diopside-calc-granulite, khondalite and leptynite. Influx of granitic material converted the pyroxene granulites to a series of rocks of intermediate composition and ultimately to a granite. A part of all these varieties are hypersthene bearing with the features characteristic of the group.

The charnockite habit, including the index mineral, was impressed on the rocks by the pre-granite metamorphism, retained in the successive products of granitization, and obtained by the intrusive granite by crystallization under identical PT environment.

All hypersthene granites in this typical milieu, and the granulite facies rocks are charnockitic, and have a dry mineral assemblage association together with other members of the charnockite series. In the term 'charnockite' is to stay, it should be used as a synonym of hypersthene granite in the typical set-up and association. The term 'charnockite series' should be dropped.