Abstract

Komatiites occur in many Archaean and Proterozoic greenstone belts metamorphosed to greenschist or upper amphibolite facies conditions. However, komatiites have so far not been reported from high-grade metamorphic environments (upper granulite facies conditions). Here we report the occurrence of Mg-, Cr-and Ni-rich ultramafic rocks (26 to 31% MgO, 2800 to 3800 ppm Cr, 1400 to 1800 ppm Ni) with unfractionated PGE patterns (8.15 to 12.36 ppb Pt, 6.17 to 13.49 ppb Pd, Pd/Ir ~2 to 6) in the high-grade polymetamorphic Central Zone of the Limpopo Belt, South Africa. The composition of the rocks overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, except that the Central Zone samples are markedly enriched in highly incompatible trace elements and show negative Nb anomalies. Based on these data, we interpret the rocks to represent contaminated komatiites of Archaean age – the first such manifestation in the Limpopo Belt and in a high to ultra-high grade (≥900°C, 10 kbar) metamorphic environment.

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