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Chapter 5.2 The Ancient Gneiss Complex of Swaziland and Environs: Record of Early Archean Crustal Evolution in Southern Africa

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Publisher Summary This chapter discusses the record of early Archean crustal evolution in Southern Africa. Southern Africa preserves one of the most complete and detailed records of early Precambrian crustal evolution, and the Ancient Gneiss Complex (AGC) of Swaziland and the related rocks along the southern margin of the 3.5–3.2 Ga Barberton Greenstone Belt (BGB) have played a prominent role in models for the early evolution of continental crust. The AGC is separated from the BGB by a large granitoid sheet-like pluton some 3 Ga in age and known as the Mpuluzi Batholith. In northwest Swaziland, however, small inliers of AGC gneisses occur in faulted and sheared contact with BGB rocks, and this, together with the presence of a tectonic wedge of tonalitic gneiss in the lower Onverwacht Group of the BGB, suggests that the two units were in direct contact prior to about 3.0 Ga. Layered gneisses of the Ngwane Gneiss are characterized by the alternation of medium- to fine-grained light and dark colored layers ranging in thickness from a few mm to 50 cm, as a consequence of variations in the amount of hornblende and biotite. Detrital zircons from Dwalile metagreywacke samples display typical rounding ascribed to sedimentary transport, but preserve several features such as oscillatory zoning that suggest a primary magmatic origin.