H. MOURI, W.D. MAIER AND G. BRANDL 55 Introduction Komatiites occur in many greenstone belts metamorphosed to greenschist or upper amphibolite facies metamorphic conditions. The highest P-T conditions reported so far for ultramafic rocks of possible komatiitic compositions were recorded from the Ruby Range in Montana, USA (710°C at 5 to 7 kbar) (Desmarais, 1981) and from the Archaean Sargur Terrane, Karnataka, South India (700°C at 8 kbar) (Srikanantappa et al., 1985). In this paper we report the occurrence of ultramafic rocks with strong geochemical komatiitic affinities from the Central Zone of the Limpopo Belt – the first such manifestation in a high- to ultra-high grade (T≥900°C, 10 kbar) metamorphic environment. In support of our interpretations, we present major and trace element data of the rocks, including platinum-group elements (PGE). The PGE are particularly useful as a diagnostic tool as they tend to be immobile under most conditions, and show marked fractionation during partial melting and magma crystallization, thereby resulting in characteristic element distribution patterns for a range of different rocks. General characteristics of komatiites Komatiites are highly magnesian (>18 weight % MgO) ultrabasic lavas (Arndt et al., 2008 and references therein), they were first recognized in 1969 in the Barberton Greenstone Belt of South Africa (Viljoen and Viljoen, 1969). Since then, many studies have shown that the rocks occur predominantly in Archaean or Proterozoic greenstone belts within cratons. Komatiites have been subdivided into four main types: (i) Al-depleted or Barberton-type, characterized by relatively low Al 2 O 3 /TiO 2 ~10, and REE patterns with a slight relative enrichment in LREE to MREE (La/Sm N >1) and MREE to HREE (Gd/Yb N >1). They are represented, e.g., by the c. 3.5 Ga komatiites of the Komati Formation in the Barberton greenstone belt of South Africa (Viljoen and Viljoen, 1969) and the 2.9 Ga komatiites from the Southern Cross belt in the Yilgarn Craton of Western Australia (Perring et al., 1996). (ii) Al-undepleted or Munro-type komatiites have higher Al 2 O 3 /TiO 2 ~20, and are relatively depleted in LREE (La/Sm N <1), but have chondritic Gd/Yb. They are found at many localities globally, e.g., in the ~2.7 Ga Abitibi Greenstone Belt of Canada (Pyke et al., 1973), the Eastern Goldfields Superterrane of the Yilgarn Craton of Western Australia (Kambalda komatiite formation) (Gresham and Loftus- Hills, 1981), and the Belingwe Greenstone Belt of Zimbabwe (e.g. Nesbitt et al., 1977). (iii) Relatively Ti-depleted and silica-enriched komatiites, e.g., in the Commondale greenstone fragment, South Africa (Wilson, 2003) and (iv) Ti- and Fe- rich komatiites, e.g., from the Karasjok and Lapland Greenstone Belts, ON THE POSSIBLE OCCURRENCE OF KOMATIITES IN THE ARCHAEAN HIGH-GRADE POLYMETAMORPHIC CENTRAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA H. MOURI Department of Geology, University of Johannesburg, South Africa e-mail: hmouri@uj.ac.za W.D. MAIER Department of Geology, University of Oulu, Finland e-mail: wolfgang.maier@oulu.fi G. BRANDL Council for Geoscience, Limpopo Unit, P.O. Box 620, Polokwana, 0700, South Africa e-mail: gbrandl@geoscience.org.za © 2013 June Geological Society of South Africa 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. SOUTH AFRICAN JOURNAL OF GEOLOGY, 2013, VOLUME 116.1 PAGE 55-66 doi:10.2113/gssajg.116.1.55