"Rock of the Month # 9, posted for March 2002" --- Sample 1856 (photo by Karyn Gorra).

Charnockite is a term coined by Holland (1900) to describe a type of granitic rock, usually of a distinctive greenish-yellow hue, found at Madras, in Madras state on the southeast coast of India (the official name of city today is Chennai, and the state is Tamil Nadu). The colour, unusual for a granite, is due at least in part to fine alteration and fracture fillings in the feldspars. Charnockites typify the deep continental crust of the Earth, and they are commonly found in granulite-facies metamorphic terranes. As with the broader family of granites, charnockites have been ascribed diverse origins, spanning a range of metamorphic and igneous derivations (Kilpatrick and Ellis, 1992).

The sample in the photograph is from the Pangnirtung region of southeast Baffin Island, just north of the Arctic Circle in Canada's eastern Arctic region, the territory of Nunavut. It is clearly a crystalline granitic rock, composed largely of two feldspars, perthite and andesine, but in addition it contains roughly 5 volume percent orthopyroxene (hypersthene). Minor minerals are quartz, myrmekite, hornblende, biotite, magnetite and clinopyroxene, with traces of apatite, sulphides and zircon. In strict terminology, the rock is a hypersthene monzonite (mangerite), estimated colour index 9 percent. Baffin charnockites are of Proterozoic age (Pidgeon and Howie, 1975; Jackson and Morgan, 1978).

Charnockites are found on every continent but, since exposures are limited to deep, high-grade terranes, they are unfamiliar in many regions. Thus in the Grenville province of North America, they are well-developed in parts of Labrador, Quebec and the Adirondacks of New York, but uncommon in Ontario. The following literature survey of 723 articles on charnockites hints at their worldwide occurrence. The MINLIB bibliography undoubtedly stresses the occurrences in the Indian subcontinent, yet southern India may well provide the world's best and most accessible region for the study of these interesting rocks. They have received well-deserved attention in the past 20 years (see, e.g., Radhakrishna, 1993; Bhattacharya et al., 2001; Rickers et al., 2001).

References

BHATTACHARYA,S, KAR,R, MISRA,S and TEIXEIRA,W (2001) Early Archaean continental crust in the Eastern Ghats granulite belt, India: isotopic evidence from a charnockite suite. Geol.Mag. 138, 609-618.

HOLLAND,TH (1900) The charnockite series, a group of Archaean hypersthenic rocks in Peninsular India. Geol.Surv.India Memoir 28, part 2 (1900), reprinted in `Granulites of South India' (Radhakrishna,BP, Ramakrishnan,M and Mahabaleswar,B editors), Geol.Soc.India Memoir 17, Pichamuthu volume, 502pp., 7-27 (1990).

JACKSON,GD and MORGAN,WC (1978) Precambrian metamorphism on Baffin and Bylot Islands. In `Metamorphism in the Canadian Shield' (Fraser,JA and Heywood,WW editors), Geol.Surv.Canada Pap. 78-10, 367pp., 249-267.

KILPATRICK,JA and ELLIS,DJ (1992) C-type magmas: igneous charnockites and their extrusive equivalents. Proc. 2nd Hutton Symposium on the Origin of Granites and Related Rocks (Brown,PE and Chappell,BW, editors), Trans.Roy.Soc.Edinburgh, Earth Sciences, 83 parts 1-2, 507pp., 155-164.

PIDGEON,RT and HOWIE,RA (1975) U-Pb age of zircon from a charnockitic granulite from Pangnirtung on the east coast of Baffin Island. Can.J.Earth Sci. 12, 1046-1047.

RADHAKRISHNA,BP (editor) (1993) Continental Crust of South India. Geol.Soc.India Memoir 25, 379pp., map and sections.

RICKERS,K, MEZGER,K and RAITH,MM (2001) Evolution of the continental crust in the Proterozoic Eastern Ghats belt, India and new constraints for Rodinia reconstruction: implications from Sm-Nd, Rb-Sr and Pb-Pb isotopes. Precambrian Research 112, 183-210.