THE EARLY ARCHEAN NONDWENI GREENSTONE-BELT, SOUTHERN KAAPVAAL CRATON,SOUTH-AFRICA .2. CHARACTERISTICS OF THE VOLCANIC-ROCKS AND CONSTRAINTS ON MAGMA GENESIS
Ah. Wilson et Ja. Versfeld, THE EARLY ARCHEAN NONDWENI GREENSTONE-BELT, SOUTHERN KAAPVAAL CRATON,SOUTH-AFRICA .2. CHARACTERISTICS OF THE VOLCANIC-ROCKS AND CONSTRAINTS ON MAGMA GENESIS, Precambrian research, 67(3-4), 1994, pp. 277-320
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.