Tectonostratigraphy, structure and metamorphism of the Archaean Ilangwe granite - greenstone belt south of Melmoth, Kwazulu-Natal.
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Date
1997
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Abstract
The mapped area, measuring about 400m2, is situated along the southern margin of the
Archaean Kaapvaal Craton south of Melmoth in KwaZulu-Natal and comprises greenstones
and metasediments forming a narrow, linear E-W trending and dominantly northerly
inclined belt flanked to the north and south by various granitoids and granitoid gneisses
which have been differentiated for the first time in this study. This belt is here referred to as
the ILANGWE GREENSTONE BELT.
The lIangwe Belt rocks are grouped into the Umhlathuze Subgroup (a lower metavolcanic
suite) and the Nkandla Subgroup (an upper metasedimentary suite). The former consists
of:
(a) the Sabiza Formation: a lower amphibolite association occurring along the
southern margin of the greenstone belt;
(b) the Matshansundu Formation: an eastern amphibolite-BIF association;
(c) the Olwenjini Formation: an upper or northern amphibolite-banded chert-BIF
association.
whereas the latter is sub-divided into:
(a) the Entembeni Formation: a distinctive phyllite-banded chert-BIF association
occurring in the central and the eastern parts of the belt;
(b) the Simbagwezi Formation: a phyllite-banded chert-amphibolite association
occurring in the western part of the belt, south-east of Nkandla;
(c) the Nomangci Formation: a dominantly quartzite and quartz schist formation
occurring in the western part of the belt, south-east of Nkandla.
The contacts between the six major tectonostratigraphic formations are tectonic.
In the eastern sector of the lIangwe Belt, the lowermost metasedimentary formation, the
Entembeni Formation, cuts across both the Sabiza and Matshansundu Formations (the
lower formations of the Umhlathuze Subgroup) in a major deformed angular unconformity
referred to as the Ndloziyana angular unconformity. In the central parts of the belt, the
Entembeni Formation structurally overlies the Olwenjini Formation in what seems to be a
major local unconformity (disconformity). In the western sector of the belt, the Simbagwezi
Formation occurs as a structural wedge between the lower and upper formations of the
Umhlathuze Subgroup. That is, it structurally overlies the Sabiza Formation and structurally
underlies the Olwenjini Formation. The uppermost metasedimentary unit, the Nomangci
Formation occurs as a complex series of finger-like wedges cutting and extending into the
Simbagwezi Formation and in each case showing that the Nomangci Formation structurally
underlies the Simbagwezi Formation. This structural repetition of lithological units is
suggestive of normal dip-slip duplex structures.
Palimpsest volcanic features, such as pillow structures and minor ocelli, indicate that many
of the amphibolitic rocks represent metavolcanics, possibly transformed oceanic crust. This
is also supported by limited major element geochemistry which suggests that the original
rocks were ocean tholeiites. Evidence suggests that the talc-tremolite schists and the
serpentinitic talc schists represent altered komatiites. The nature of the metasediments
(represented by banded metacherts, quartzites and banded iron formations) and their
similarity to those of the Barberton, Pietersburg and Nondweni greenstone complexes
suggests that they were formed in relatively shallow water environments.
The lIangwe magmatism is represented by different types of granitoids and granitoid
gneisses and basic-ultrabasic intrusive bodies. Based on similar geochemical and
mineralogical characteristics and on regional distribution, mutual associations and contact
relationships, these granitoids and granitoid gneisses can be divided into three broad
associations, viz:
(a) The Amazula Gneiss - Nkwa/ini Mylonitic Gneiss - Nkwalini Quartzofeldspathic
Flaser Gneiss Association: a migmatitic paragneiss and mylonitic to
flaser gneiss association of older gneisses of Nondweni age occurring in several
widely separated areas and intruded by younger granitoids.
(b) The early post-Nondweni Granitoids comprising the Nkwalinye Tonalitic
Gneiss (a distinctive grey gneiss intrusive into the greenstones and older
gneisses) and the Nsengeni Granitoid Suite (an association of three granitoid
units of batholithic proportions flanking the greenstone belt and intrusive into the
greenstones, older gneisses and Nkwalinye Tonalitic Gneiss).
(c) The late post-Nondweni Granitoids comprising the Impisi-Umgabhi Granitoid
Suite, a batholithic microcrystic to megacrystic association of five granitoid
phases/units occurring to the north and south of the greenstone belt and intrusive
into the greenstones, older gneisses and early post-Nondweni granitoids.
Limited major element geochemistry suggests that the granitoids and granitoid gneisses
are of calc-alkaline origin and are of tonalitic, granodioritic, adamellitic and granitic
composition. An igneous derivation from material located possibly at the lower crust or
upper mantle is suggested.
At least three major episodes of deformation (01, O2 and 03) have been recognized in the
greenstones. During 01, a strong penetrative S1 tectonic foliation developed parallel to the
So primary layering and bedding. This period was characterized by intense transpositional
layering, recumbent and isoclinal intrafolial folding with associated shearing,thrusting and
structural repetition of greenstone lithologies. These processes took place in an essentially
horizontal, high strain tectonic regime.
The first phase of deformation (OG1) in the migmatitic and mylonitic gneisses was also
characterized by recumbent and isoclinal intrafolial folding and is remarkably similar to the
01deformational phase in the lIangwe greenstones.
Structural features of the first phase of deformation suggest that it was dominated by
formation of fold nappes and thrusts and was accompanied by prograde M1 medium-grade
middle to upper amphibolite facies metamorphism.
During D2 deformation, the subhorizontal D1 structures were refolded by new structures with
steeply inclined axial planes. This resulted in the formation of superimposed Type 3
interference folding in the amphibolitic rocks and large-scale, E-W trending, doublyplunging
periclinal folds in the metasediments. These periclinal folds have steeply inclined
and overturned limbs and are characterized by narrow, closed elliptical outcrop patterns
well-defined by extensive banded ironstones and metacherts.
The second phase of deformation in the granitoids (DG
2) was characterized by steeply
plunging and steeply inclined small-scale tight to isoclinal similar folds. Large-scale folds
are not present in the granitoids.
Evidence suggests that the second phase of deformation was a major compressional event
which resulted in the large-scale upright, flattened flexural folds. It was accompanied by
widespread regional greenschist metamorphism and the intrusion of the early postNondweni
granitoids.
The third phase of deformation produced steeply plunging small-scale folds on the limbs
and axial planes of the pre-existing large-scale F2 folds and upright open folds in the
granitoid terrain. This episode was characterized by the emplacement of the late postNondweni
granitoids (along the D2 greenstone boundary faults) and is associated with two
significant events of prograde M3 upper greenschist facies metamorphism and retrograde
M3 lower greenschist facies metamorphism.
Post-D3 deformation is characterized by late cross-cutting faults and the emplacement of
younger basic - ultrabasic bodies.
Description
Thesis (Ph.D.)-University of Natal, 1997.
Keywords
Greenstone belts--KwaZulu-Natal., Geology, Structural--KwaZulu-Natal., Geology, Stratigraphic--Archaean., Metamorphism (Geology)--KwaZulu-Natal., Geology--KwaZulu-Natal., Theses--Geology.