Charnockite (/ˈtʃɑːrnəkaɪt/) is anyorthopyroxene-bearingquartz-feldspar rock formed at high temperature and pressure, commonly found ingranulitefacies’metamorphic regions,sensu stricto as anendmember of the charnockite series.[1]
Thecharnockite suite orseries is a particularly widespread form ofgranofels. Granofels are one of the few non-foliated rocks to form under relatively high temperatures and pressures. This combination is generated only deep in thecrust bytectonic forces that operate on a grand scale, so granofels is a product of regional, rather than contact,metamorphism. It is formed mostly from thegranite clan of rocks, or occasionally from thoroughly reconstitutedclays andshales. It is of wide distribution and great importance in India,Sri Lanka,Madagascar and Africa. It was named by geologistT. H. Holland in 1893 after the tombstone ofJob Charnock, inSt John's Church inKolkata, India, which is made of this rock.[2][3]
The charnockite series includes rocks of many different types, some beingfelsic and rich inquartz andmicrocline, others mafic and full ofpyroxene andolivine, while there are also intermediate varieties corresponding mineralogically tonorites, quartz-norites anddiorites. A special feature, recurring in many members of the group, is the presence of a stronglypleochroic, reddish or green orthopyroxene (ferroanenstatite formerly known ashypersthene).[4] The alkali feldspars in the group are generallyperthites, with intergrowths ofalbite andorthoclase ormicrocline, while the plagioclases are generallyantiperthites and intermediatemesoperthites occur too. Rocks of the charnockite series may be named by adding orthopyroxene to the normaligneous nomenclature (e.g. orthopyroxene-granite), but specific names are in widespread use such as norite,mangerite,enderbite,jotunite,farsundite,opdalite and charnockite (in the strict sense); equivalents ofgabbro,monzonite,tonalite, monzodiorite,monzogranite,granodiorite and granite.[1] The BGS classification[5] is unambiguous, replacing local names with standard igneous nomenclature prefixed by 'charnockitic'. Thus, 'charnockite' becomes 'charnockitic granite' and rocks with the traditional names 'mangerite' and 'enderbite' become 'charnockitic monzonite' and 'charnockitic tonalite' respectively.
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All charnockites were once thought to be igneous, but it is now recognized that many are metamorphic, because, despite the high temperatures and pressures, the originalprotolith never actually melted. However, some orthopyroxene-bearing granites with distinct igneous features exist, and these rocks also fall within the definition of charnockite.[6]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, thefeldspar 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 onefacies appears the others also usually are found.[4]
The term charnockite in this sense is consequently not the name of a rock, but of an assemblage of rock types. The assemblage is connected by origin, the differentiation of the same parentmagma. The banded structure which these rocks commonly present in the field is only in a small measure due toplastic 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.[4]
The charnockites are widely distributed in the southern hemisphere. They, or rocks very similar to them, also occur in Norway, France, Sweden, Germany, Scotland and North America, though in these countries they have been mostly described as pyroxenegranulites, pyroxenegneisses,anorthosites, or other names.[7] They are typically ofProterozoic age.
In India they form theNilgiri Hills, theShevaroys, theBiligirirangan Hills[8] and part of theWestern Ghats, extending southward toKanyakumari and reappearing inSri Lanka.[9]
A commercial variety calledgreen ubatuba is found in Brazil.[10]
While the granulite facies metamorphism is dated as 2.5 Ga (billion years ago) in Nilgiris, Shevaroys, 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 (million years ago).[citation needed] Although they are certainly for the most part igneous gneisses (or orthogneisses), rocks occur along with them, such asmarbles,scapolitelimestones, andcorundum rocks, which were probably ofsedimentary origin.[9]
This article incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Charnockite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 5 (11th ed.). Cambridge University Press. pp. 947–948.