A new class of silica enriched, highly depleted komatiites in the southern Kaapvaal Craton, South Africa

The 3.33 Ga Commondale Ultramafic Suite, located in the southeastern part of the Kaapvaal Craton, represents a previously unrecognized compositional variant of Archaean komadites. Textures, compositions and structures of cooling units are unlike komatiite flow successions described in greenstone belts in the Kaapvaal Craton and elsewhere in the world. Olivine phenocrysts are the most magnesian known from any natural magmas with core compositions >Fo(96). Both orthopyroxene and olivine are present as spinifex and as granular crystals. Geochemically these are some of the most depleted magmas ever recorded with extreme depletion of Ti (Primitive Mantle normalized Ti <0.5), Al2O3/TiO2 60-74. Total REE are 0.85-4.1 ppm compared with 5-12 ppm for komatiites from Munro Township and 12-19 ppm for Gorgona Island. Primitive Mantle normalized [Gd/Yb] is 0.25-0.33. Primitive Mantle normalized Nb and Ta are equally depleted in these rocks at <0.05. At the same time, these lavas are enriched in silica relative to Barberton komatiites with the same magnesium content. Liquid compositions are 29-31 wt.% MgO and Mg#. is >0.9. These lavas could have been derived from multiple fractional melting events in a plume axis source, but given that most komatiites are considered to have formed in plume environments, it seems anomalous that similar compositions have not been described from other Archaean greenstone belts. An alternate mechanism may be that these komatiites formed in an unusual tectonic setting that may have involved water. Such a setting could be subduction-related melting of a depleted mantle wedge. (C) 2003 Elsevier B.V. All rights reserved.