Charnockite

Charnockite

(1) Charnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene (usually hypersthene), as an end-member of the charnockite series (Classification of Igneous Rocks, 2nd ed., 2002, by R. W. Le Maitre et al).

(2) The Charnockite suite or series is a group of igneous rocks, variably metamorphosed of wide distribution and great importance in India, Ceylon, Madagascar and Africa.

The name was given by Dr T. H. Holland from the fact that the tombstone of Job Charnock, the founder of Calcutta, is made of a block of this rock. The charnockite series includes rocks of many different types, some being acid and rich in quartz and microcline, others basic and full of pyroxene and olivine, while there are also intermediate varieties corresponding mineralogically to norites, quartz-norites and diorites. A special feature, recurring in many members of the group, is the presence of strongly pleochroic, reddish or green hypersthene. They may be named by adding orthopyroxene or specifally hypersthene to the normal igneous nomenclature (e.g. hypersthene-granite), but specific names are in widespread use such as norite, mangerite, enderbite, jotunite, farsundite, opdalite and charnockite (in the strict sense); equivalents of gabbro, monzonite, tonalite, monzodiorite, monzogranite, granodiorite and granite.

Many of the minerals of these rocks are schillerized, as they contain minute platy or rod-shaped inclusions, disposed parallel to certain crystallographic planes or axes. The reflection of light from the surfaces of these inclusions gives the minerals often a peculiar appearance, e.g. the quartz is blue and opalescent, the feldspar has a milky shimmer like moonshine, the hypersthene has a bronzy metalloidal gleam. Very often the different rock types occur in close association as one set forms bands alternating with another set,or veins traversing it, and where one facies appears the others also usually are found.

The term charnockite in this sense is consequently not the name of a rock, but of an assemblage of rock types, connected in their origin because arising by differentiation of the same parent magma. The banded structure which these rocks commonly present in the field is only in a small measure due to plastic deformation, but is to a large extent original, and has been produced by flow in a viscous crystallizing intrusive magma, together with differentiation or segregation of the mass into bands of different chemical and mineralogical composition. There have also been, of course, earth movements acting on the solid rock at a later time and injection of dikes both parallel to and across the primary foliation.

A striking fact is the very wide distribution of rocks of this group in the southern hemisphere; but they also, or rocks very similar to them, occur in Norway, France, Germany, Scotland and North America, though in these countries they have been mostly described as pyroxene granulites, pyroxene gneisses, anorthosites, &c. They are usually of Proterozoic age (late Precambrian).

In India they form the Nilgiri Hills, the Shevaroys, the Biligirirangan Hills and part of the Western Ghats, extending southward to Cape Comorin and reappearing in Ceylon. While, the granulite facies metamorphism is dated as 2.5 Ga in Nilgiris, Shevroys, Madras (Chennai) regions, the granulite facies event transforming the granitic gneisses into charnockite in the southern part of the South Indian granulite terrain is dated as 550 Ma. Although they are certainly for the most part igneous gneisses (or orthogneisses), rocks occur along with them, such as marbles, scapolite limestones, and corundum rocks, which were probably of sedimentary origin.

References

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