Age and Petrogenesis of Archaean Chromitite from the Seqi Ultramafic Complex, Southern West Greenland

The chromite deposits of the Seqi Ultramafic Complex are part of layered ultramafic bodies that occur within Mesoarchaean tonalitic orthogneiss in the 3.2–3.0 Ga Akia Terrane of the North Atlantic Craton, southern West Greenland. The calculated Al2O3-melt and TiO2-melt concentrations of the parental melt are within the range of 12.22–12.60 and 0.34–0.41 wt%, respectively. Seqi chromitites exhibit the patterns of PGE normalised with chondrite with positive Os, Ir and Ru anomalies and negative PPGE anomalies. These geochemical characteristics indicate that the Seqi chromitites formed as part of cumulates originated from a parental komatiitic magma that experienced crustal contamination. The ReOs isotope data present a minimum Re-depletion model age (TRD) of 3124–3014 Ma for the Seqi chromitite, which is the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains occur within the Seqi chromitites, yielding a UPb weighted mean age of 2985.43 ± 4.26 Ma, which is identical to the peak of regional granulite-facies metamorphism of the Akia Terrane. This study presents the first dating of the formation age of the Seqi chromitite of >3124 Ma.