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Journal of Petrology | Volume 43 | Number 11 | Pages 2143-2170 | 2002
© Oxford University Press 2002

A Metamorphosed, Early Archaean Chromitite from West Greenland: Implications for the Genesis of Archaean Anorthositic Chromitites

HUGH ROLLINSON1,*, PETER W. U. APPEL2 and ROBERT FREI3,4

1GEMRU, UNIVERSITY OF GLOUCESTERSHIRE, FRANCIS CLOSE HALL, SWINDON ROAD, CHELTENHAM GL50 4AZ, UK
2GEOLOGICAL SURVEY OF DENMARK AND GREENLAND, THORAVEJ 8, DK-2400 COPENHAGEN NV, DENMARK
3GEOLOGICAL INSTITUTE, UNIVERSITY OF COPENHAGEN, ØSTER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK
4DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, DK-1350, COPENHAGEN K, DENMARK

An early Archaean (>3·81 Ga) chromitite–ultramafic layered body from the Ujaragssuit nunât area, west Greenland, may represent the Earth’s oldest chromitite. The layered body occurs as a large xenolith (800 m x 100 m) entrained within tonalitic gneisses and preserves primary igneous layering and textures. New Re–Os and Pb–Pb isotope results support the view that it has been metamorphosed twice, in the early and late Archaean at ~3·75 Ga and ~2·8 Ga. Mineral chemistry and textures indicate that the chromite compositions preserve two different evolutionary trends. There is a main magmatic trend in which Cr/(Cr + Al) ratios remain relatively constant but in which there is strong enrichment in Fe3+, Fe2+ and Ti with progressive differentiation. This trend is a composite of magmatic-liquidus, magmatic-cooling and subsolidus re-equilibration processes. A second trend is defined by chromites from harzburgites in the upper part of the layered body. These chromites show magmatic replacement textures in which Fe-rich chromites are altered to aluminous chromites. Chromites showing magmatic replacement textures are thought to have formed by reaction with a late, interstitial melt during the solidification of the layered body. The close association between the Fe3+–Cr-chromites of the main trend and Al-rich chromites of the type found in other Archaean megacrystic anorthosites suggest a magmatic-genetic relationship between the two types of chromite. We propose that anorthositic chromites form in an Fe-rich basaltic melt derived from a komatiitic, boninitic or basaltic parent magma through reaction between the melt and early-formed Fe-rich chromite.

KEY WORDS: chromite; anorthosite; Archaean; Greenland; Re–Os


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