Charnockite

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Charnockite (IPA: /ˈtʃɑː(r)nəkʌɪt/) is a series of foliated metamorphosed igneous rocks 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.

Many of the minerals of these rocks are schillerized, as they contain minute platy or rod-shaped enclosures, disposed parallel to certain crystallographic planes or axes. The reflection of light from the surfaces of these enclosures 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 consequently is 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 crushing, but is to a large extent original, and has been produced by fluxion 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. In fact, the history of the structures of the charnockite series is the history of the most primitive gneisses in all parts of the world, for which we cannot pretend to have as yet any thoroughly satisfactory explanations to offer.

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 regarded as being of Archean age (preCambrian), and in most cases this can be definitely proved, though not in all. It is astonishing to find that in spite of their great age their minerals are often in excellent preservation.

In India they form the Nilgiri Hills, the Shevaroys, the Biligirirangan Hills^[1] 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.

[edit] References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.