Mafic and ultramafic extrusives of the Barberton Mountain Land in terms of the CMAS system

Evolutionary trends among the mafic and ultramafic extrusives of the Onverwacht Group are revealed by subprojections within the CMAS tetrahedron. The parental magma of the Onverwacht extrusives had a composition close to that of the ultramafic rocks occurring at the base of the pile. The development of this parental magma is satisfactorily accounted for by partial melting of a four-phase lherzolite mantle at depths of 90–100 km or more. Polybaric olivine and orthopyroxene fractionation were important in controlling the development of mafic and ultramafic komatiite-type extrusives and associated tholeiitic basalts occurring at stratigraphically higher levels. Attention is drawn to the presence of nepheline normative extrusives in the upper formations of the Onverwacht Group. The accumulation and preservation of 15,000 m of mantle-derived material required the presence of a pre-Onverwacht floor.