The early Archaean nondweni greenstone belt, southern Kaapvaal Craton, South Africa, Part II. Characteristics of the volcanic rocks and constraints on magma genesis

Abstract

The early Archaean Nondweni greenstone belt is dominated by mafic volcanic rocks which have a wide range in compositions. Primary volcanic structures and compositional characteristics point to this succession being 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/Al 2O 3, Ti/Y and low Al 2O 3/TiO 2). They are also distinctly different in that SiO 2 content is markedly higher and incompatible trace element ratios are also different. The high silica content has promoted the subdivision of komatiitic and basaltic rocks into the following categories: komatiite, komatiitic andesite, komatiitic basalt, basalt and basaltic andesite. The recognition of the distinct suite of komatiitic andesites is aided by the lower Zr/Ti ratios than those lithologies of equivalent MgO content reported from other greenstone belts, and SiO 2 contents as high as 58%. These characteristics suggest similarity to Recent high-Mg andesites. None of the lava compositions exceed 23% MgO (anhydrous) and pyroxene, rather than olivine, dominates the crystallization assemblage. Olivine spinifex textures are entirely absent in the Nondweni greenstone belt and instead, pyroxene (or amphibole after pyroxene) spinifex is abundantly developed and is characterised by spectacular radial conical structures which have their apices towards the tops of the flow units. Some of the spinifex cone structures attain heights of up to 6 m. Branching pyroxene crystals and feathery textured plagioclase typify the spinifex textures of the komatiitic andesites. The common occurrence of coned and branching spinifex textures in a wide range of magma compositions, indicates that these were controlled by thermal characteristics and depositional environment for the flow units rather than solely by composition. This may have been facilitated by emplacement in shallow water rather than in deep water, as envisaged for the Onverwacht Group of the Barberton Sequence. Shallow-water depositional environments in the Nondweni Group are supported 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 related to both compositional type as well as physical controls such as extrusion rate and nature of the substrate. Komatiites seldom form pillows and very large pillow formation is restricted to the komatiitic andesite lava type. Komatiite flow successions are typified by spinifex textured units, as well as by massive and highly brecciated aphyric flow units. Unaltered pyroxene and plagioclase characterise some of the massive flow units and pillowed basalts of the Witkop Formation. The present 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 of Kaapvaal Craton may have been different to that of the Barberton greenstone belt. A present day analogue to the Nondweni tectonic setting may be an ensialic back-arc basin adjacent to a continental margin.