THE EARLY ARCHEAN NONDWENI GREENSTONE-BELT, SOUTHERN KAAPVAAL CRATON,SOUTH-AFRICA .2. CHARACTERISTICS OF THE VOLCANIC-ROCKS AND CONSTRAINTS ON MAGMA GENESIS

The early Archaean Nondweni greenstone belt is dominated by mafic volc anic rocks which have a wide range in compositions. Primary volcanic s tructures and compositional characteristics point to this succession b eing unique in Archaean geology and an important indicator of controls of magma formation and environment of deposition. Compositionally the Nondweni lavas are broadly similar to those of the Barberton Sequence (high values for CaO/Al2O3, Ti/Y and low Al2O3/TiO2). They are also d istinctly different in that SiO2 content is markedly higher and incomp atible trace element ratios are also different. The high silica conten t has promoted the subdivision of komatiitic and basaltic rocks into t he following categories: komatiite, komatiitic andesite, komatiitic ba salt, basalt and basaltic andesite. The recognition of the distinct su ite of komatiitic andesites is aided by the lower Zr/Ti ratios than th ose lithologies of equivalent MgO content reported from other greensto ne belts, and SiO2 contents as high as 58%. These characteristics sugg est similarity to Recent high-Mg andesites. None of the lava compositi ons exceed 23% MgO (anhydrous) and pyroxene, rather than olivine, domi nates the crystallization assemblage. Olivine spinifex textures are en tirely absent in the Nondweni greenstone belt and instead, pyroxene (o r amphibole after pyroxene) spinifex is abundantly developed and is ch aracterised by spectacular radial conical structures which have their apices towards.the tops of the flow units. Some of the spinifex cone s tructures attain heights of up to 6 m. Branching pyroxene crystals and feathery textured plagioclase typify the spinifex textures of the kom atiitic andesites. The common occurrence of coned and branching spinif ex textures in a wide range of magma compositions, indicates that thes e were controlled by thermal characteristics and depositional environm ent for the flow units rather than solely by composition. This may hav e been facilitated by emplacement in shallow water rather than in deep water, as envisaged for the Onverwacht Group of the Barberton Sequenc e. Shallow-water depositional environments in the Nondweni Group are s upported by sedimentary lithofacies. Well preserved pillow structures also show a wide range in textural forms. Sizes of pillows, degree of welding of pillow margins, development of hyaloclastite and formation of shelf structures, formed by repeated filling of lava tubes, are rel ated to both compositional type as well as physical controls such as e xtrusion rate and nature of the substrate. Komatiites seldom form pill ows and very large pillow formation is restricted to the komatiitic an desite lava type. Komatiite flow successions are typified by spinifex textured units, as well as by massive and highly brecciated aphyric fl ow units. Unaltered pyroxene and plagioclase characterise some of the massive flow units and pillowed basalts of the Witkop Formation. The p resent paper is the second part of a series on the Nondweni greenstone belt. Part I (Wilson and Versfeld, 1994) describes the stratigraphy, sedimentology, mineralization and depositional environment. There are strong indications that the tectonic setting near the southern margin the of Kaapvaal Craton may have been different to that of the Barberto n greenstone belt. A present day analogue to the Nondweni tectonic set ting may be an ensialic back-arc basin adjacent to a continental margi n.