EVIDENCE FOR THE EMPLACEMENT OF CA. 3.0 GA MANTLE-DERIVED MAFIC-ULTRAMAFIC BODIES IN THE UKRAINIAN SHIELD

Abstract

The Archean bedrock of the Ukrainian Shield is divided into western and central blocks by a N–W trending tectonic zone known as the Golovanevsk suture zone. Within this zone, there are numerous mafic-ultramafic bodies including the chromitite-bearing Lipovenki and Kapitanov massifs. The chromitites of these massifs possess mineralogical and textural features and platinum-group element systematics similar to those typically observed in ophiolitic podiform chromitites. Although there is no field evidence for an ophiolitic structure, the geochemical evidence strongly suggests an origin in the oceanic lithosphere. The Re/Os ratios of the chromites and whole-rock chromitites are extremely low, and consequently, their 187Os/188Os ratios have increased little since crystallization. Osmium isotope model ages indicate formation at approximately 3.0Ga, suggesting that the massifs represent portions of the Archean oceanic lithosphere.