Mashikiri

Nephelinite lavas represent the earliest Karoo igneous activity at the northern end of the Lebombo monocline and extend westwards into the northern Soutpansberg region, where they overlie sandstones and volcanoclastic sediments and are succeeded by olivine-rich basalts. They extend for some 80 km and attain a maximum thickness of about 170 m. They are referred to as the Mashikiri Formation by Cleverly and Bristow (1979). The lavas commonly carry stellate glomeroporphyritic aggregates of clinopyroxene in a fine-grained or glassy matrix with small phenocrysts of olivine and nepheline apparent in some flows. Small plugs, sills and dykes of coarser nephelinite with abundant nepheline occur to the west of the flows. The pyroxene phenocrysts are titaniferous augite which is frequently overgrown by aegirine-augite while olivine, rare nepheline and titano-magnetite are also phenocrystal. Nepheline and clinopyroxene are the principal matrix phases with minor aegirine-augite, opaque oxides, apatite and rarely biotite and sodic amphibole. Analyses of most of these phases are given by Bristow (1984) who also presents rock analyses, including trace element data, and discusses at length petrogenetic models. The nephelinites are discussed in a regional context by Duncan et al. (1984). Strontium isotope data are given and considered by Bristow et al. (1984).