A metamorphosed, early Archaean chromitite from west Greenland: Implications for the genesis of Archaean anorthositic chromitites

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 × 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 Fe ³+, Fe ²⁺ 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 Fe ³⁺-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.