Archean melt inclusions in zircon from quartzite and granitic orthogneiss from South Africa; magma compositions and probable sources of protoliths

Results are presented of analyses of melt inclusions in >3.0 Ga zircon grains from three lithologies in South Africa: the Orange Grove Quartzite of the Witwatersrand Supergroup, quartzite of the Beit Bridge Group and the S-type Singelele orthogneiss, the latter both of the Central Zone of the Limpopo Belt. In each case, the presence of melt inclusions indicates that the zircon is magmatic in origin. No melt inclusions occur in metamorphic overgrowths in some grains. Melt inclusions in detrital zircon grains from the two quartzite units indicate that both tonalitic-trondhjemitic and granitic magmas were being emplaced in the source areas at about 3.2 Ga. Granitic magmatism at that time is among the earliest recognized in southern Africa. Inclusions in zircon grains from the Singelele orthogneiss are compositionally similar to the host rock, showing that they are a magmatic phase rather than a xenocrystic phase.