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

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

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466



Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



Fig. 5.2-1. Simplified and schematic geological map of Swaziland and environs showing major

Precambrian rock units and locations of dated samples. Based on 1:250,000 Geological Map of

Swaziland (1982). Locations of dated samples mentioned in the text are shown, but sample prefixes

such as AGC and BA left off. Modified from Kröner et al. (1989). (For legend see next page.)



5.2-1. Introduction



467



Fig. 5.2-1. (Continued.)



(Fig. 5.2-1; see Hunter, 1970) and which are summarized below. The AGC is a typical early

to mid-Archean terrain comprising granitoids and multiply deformed granitoid gneisses of

the tonalite-trondhjemite-granodiorite (TTG) suite (Hunter et al., 1978) and interlayered

amphibolites, most of which are probably derived from gabbroic dykes (Hunter et al., 1984;

see also Fig. 5 in Jackson, 1984). This tectonic sequence of siliceous and mafic gneisses

is also known as the Bimodal Suite (Hunter, 1970), now renamed as Ngwane Gneiss (NG,

Wilson, 1982), and is the oldest part of the AGC (see Fig. 5.2-2(a)). The complex also

includes remnants of greenstone belt supracrustal assemblages that vary in size from xenoliths a few centimetres long to inliers several kilometres across. The largest of these is the

Dwalile greenstone remnant in southwestern Swaziland (Fig. 5.2-1).

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 (Hunter, 1974; Barton et al., 1983; see Fig. 5.21). In northwest Swaziland, however, small inliers of AGC gneisses occur in faulted and

sheared contact with BGB rocks (e.g., near Piggs Peak, see Fig. 5.2-1), and this, together

with the presence of a tectonic wedge of tonalitic gneiss in the lower Onverwacht Group

of the BGB (De Wit et al., 1987) suggests that the two units were in direct contact prior to

about 3.0 Ga.



468

Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



Fig. 5.2-2. Field photographs of rocks from the Ancient Gneiss Complex in Swaziland. (A) Banded tonalitic-trondhjemitic Ngwane Gneiss

with layer-parallel leucocratic veins and cross-cutting pegmatites. Sample AGC 150 with a maximim age of 3644 ± 4 Ma was taken from the

darkest and most homogeneous portion of this roadcut along the asphalt road immediately east of ‘The Falls’ and some 7 km northeast of

Piggs Peak (see Fig. 5.2-1 for location). (B) Mylonitized tonalitic gneiss (tight isoclinal folds in leucogranite veins still preserved) from contact

zone of Ngwane Gneiss with ultramafic rocks of the Barberton Greenstone Belt. Small waterfall at ‘The Falls’ below homestead and due west

of sample AGC 150. (C) Ductile shear zone in Ngwana Gneiss E of Bloemendal, central Swaziland. (D) Contact between isoclinally folded

Ngwane Gneiss (bottom) and foliated but non-layered Tsawela Gneiss (top). Small tributary of Ngwempesi River, central Swaziland. (E) Tight

fold in metagreywacke of Dwalile Supracrustal Suite. This is location of sample AGC 40 shown in Fig. 5.2-1. (F) Garnet–sillimanite gneiss

(khondalite) from metapelite sequence of sedimentary inlier in Ngwane Gneiss at Shiselweni, central Swaziland. This is near location of sample

AGC 155 shown in Fig. 5.2-1.



5.2-1. Introduction



Fig. 5.2-2. (Continued.)

469



470



Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



The AGC has its widest distribution in a broad belt through Swaziland (Fig. 5.2-1), and

the area so far studied in most detail centers around the town of Mankayane (Hunter, 1970;

Hunter et al., 1978; Jackson, 1984). The NG represents the oldest unit of the AGC as suggested by its structural evolution (Jackson, 1984; Jackson et al., 1987) and geochronology,

and its preserved metamorphic grade is upper amphibolite-facies. However, at least part

of the suite has been to granulite-facies as shown by inliers of >3.3 Ga metasedimentary

rocks with high-grade mineral assemblages indicating temperatures of 700–900 ◦ C and

pressures of 6–7.5 kbar (Kröner et al., 1993; Condie et al., 1996). The NG gneisses and

greenstone remnants are intruded by the Tsawela Gneiss (TG, see Fig. 5.2-1), a weakly

to well foliated tonalitic to trondhjemitic pluton which was emplaced after the oldest NG

gneisses and greenstones had already been deformed at least once (Jackson, 1984; see

Fig. 5.2-2(d)). Age relationships are discussed below.

Scattered remnants of mafic–ultramafic metavolcanic rocks associated with BIF and

clastic metasedimentary rocks occur throughout the NG and are infolded with the granitoid

gneisses (Fig. 5.2-2(e)). They are collectively referred to as the Dwalile Supracrustal Suite

(DSS, Wilson, 1982; Jackson, 1984). The Dwalile greenstone remnant is the largest of

these. Jackson (1984) considered these rocks, some of which are lithologically somewhat

similar to the Onverwacht Group greenstones (BGB), but of higher metamorphic grade,

to be younger than the NG on structural grounds. However, direct field evidence for this

interpretation was not provided, since all contacts are tectonic.

Supracrustal rocks of dominantly sedimentary origin are common in the Mkondo Valley

of central Swaziland and in the southern part of the country and were previously included

in the AGC. The precise correlation of these rock-types is still debated, but there is a possibility that they represent metamorphic equivalents of the post-3.0 Ga Pongola Supergroup

(Wilson and Jackson, 1988), and this is perpetuated in the Geological Map of Swaziland

(1982). Other granitoid gneisses, locally garnetiferous and containing metasedimentary relicts, have been given local names (e.g., Mahamba and Nhlangano gneisses, see Fig. 5.2-1

and Geological Map of Swaziland, 1982) and are most likely late Palaeo- to Mesoarchean

in age. Although originally included in the AGC, they are not further considered here.



5.2-2. FIELD RELATIONSHIPS AND ORIGIN OF COMPONENTS OF THE AGC

Layered gneisses of the Ngwane Gneiss are characterized by the alternation of mediumto fine-grained light and dark coloured layers ranging in thickness from a few mm to 50 cm,

as a consequence of variations in the amount of hornblende and biotite. Planar foliation is

commonly defined by aligned hornblende and/or biotite laths. Quartzofeldspathic veins and

pegmatites of several generations are common, and the earliest are thin (∼2–5 mm wide)

veinlets arranged parallel to the dominant foliation and occasionally displaying intrafolial

folds. Subsequent generations cross-cut the foliation but may be locally attenuated and/or

folded (Fig. 5.2-2(a); see also Hunter et al., 1992).

Three groups of chemically distinct quartzofeldspathic gneiss have been recognized in

the NG (Hunter et al., 1984; Hunter, 1991). The most common has Rb and (Nb + Y)



5.2-2. Field Relationships and Origin of Components of the AGC



471



contents similar to Phanerozoic subduction-related granites (Pearce et al., 1984) but is distinguished from them by heavy rare earth element (HREE) and high field strength element

(HFSE) depletion. The second group comprises high-SiO2 gneisses with large negative

Eu-anomalies, high Th/Ba ratios, enriched contents of HFSE and flat HREE slopes. The

third type is characterized by strongly fractionated REE patterns, small to large positive

Eu-anomalies, and high contents of Ba and Sr (Hunter et al., 1984).

There have been few detailed studies on the structural evolution of the AGC, but descriptions of various field relationships are summarized on the published 1:50,000 sheets of the

Geological Survey of Swaziland. A more detailed analysis of the Ngwane and Tsawela

Gneisses and the Dawlile greenstone remnant in southwestern Swaziland was provided by

Jackson (1984), who demonstrated that these rocks reflect evolution from early homogeneous ductile strain at upper amphibolite to granulite grade, to late inhomogeneous brittle

strain, indicating deformation at successively higher crustal levels. This uplift probably occurred along large-scale N- and NW-directed shear zones some of which have spectacular

exposures (Fig. 5.2-2(b,c)) and resulted in a vertical displacement estimated at ∼20 km

(Jackson, 1984).

Geochronological data discussed below demonstrate that most of the NG has a complex history and that gneisses at outcrop scale may range in age between 3.64 and 3.2 Ga

(Kröner et al., 1989) and are intimately interlayered without any obvious structural break.

This is similar to the situation in the early gneiss terrain of West Greenland (see Nutman et

al., this volume) and is best explained through the mechanism of banded gneiss formation

as proposed by Myers (1978).

Jackson (1984) mapped the NG, the Dwalile greenstone remnant and the Tsawela gneiss

in the Mankayane area (Fig. 5.2-1) and recognized a large overturned antiformal structure

in these rocks. The cumulative strain pattern in these repeatedly and ductilely deformed assemblages indicates flexural flow, preceded and followed by flattening. High strains during

the early phases of deformation resulted in highly attenuated folds (e.g., Fig. 5.2-2(d,e))

boudins, dyke contacts and other primary igneous structures. Such gently dipping, highstrain structures result from lateral translation of soft, hot, middle to lower continental crust

as documented by Myers (1978) in West Greenland. Jackson (1984) also concluded that

this style of deformation differs from that induced by diapirism around the margins of the

BGB.

The boot-shaped, amphibolite-facies Dwalile greenstone remnant occurs in the extreme

SW of Swaziland (Fig. 5.2-1), south of the small village of Dwalile. It was mapped by

Jackson (1984) who concluded from a structural study that contacts between rocks of the

multiply deformed Dwalile and Ngwane rocks are everywhere conformable, but that the

Dwalile suite structurally overlies the Ngwane gneisses and may therefore be younger.

The structurally lowest rocks in the foliated suite are magnesian schists and serpentinites,

interlayered with, and followed by, komatiitic to andesitic metavolcanic rocks and tuffs,

now largely preserved as amphibolite, amphibole schist and actinolite schist. Structurally

overlying this mafic–ultramafic suite are metasedimentary rocks comprising metaquartzite,

metagreywacke (Fig. 5.2-2(e)), metapelite, calc-silicate gneiss and BIF. The metasedimentary rocks are locally interlayered with the metavolcanic rocks, often with sharp contacts.



472



Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



Although this could be the result of early thrusting, similar to what has been inferred in the

lower parts of the BGB (De Wit et al., 1983), Jackson (1984) and Kröner and Tegtmeyer

(1994) interpreted many of these contacts as non-tectonic, suggesting a primary sequence

of interbedded komatiitic–basaltic lavas and predominantly clastic sediments.

Tegtmeyer (1989) and Kröner and Tegtmeyer (1994) chemically analysed and dated

several units of the Dwalile sequence, and these and additional data are summarized in

Kröner et al. (1993); the age data are reviewed below. Peak metamorphic conditions at

550–600 ◦ C and ∼4 kbar were estimated from mineral compositions, and the coexistence

of quartz, staurolite, andalusite, muscovite, biotite and chlorite in one semi-pelitic sample

supports this estimate. The Dwalile mafic and ultramafic rocks display remarkable similarities with those of the BGB, and Tegtmeyer (1989) suggested on the basis of REE data that

some Dwalile meta-komatiites and metabasalts reflect up to 10% contamination with older

continental crust, a feature which is supported by the Sm-Nd isotopic data.

The Tsawela gneisses occur in the Mankayana area of central Swaziland (Fig. 5.2-1) and

comprise tonalites with 63–69% SiO2 , and MgO and CaO contents reflecting the relative

abundances of hornblende and biotite (Hunter, 1993). Chondrite-normalized REE display

steep light REE and flat to gently sloping heavy REE patterns that are reminiscent of those

of the Kaap Valley Pluton at Barberton (Hunter et al., 1978; Hunter 1993).



5.2-3. GEOCHRONOLOGY AND IMPLICATIONS FOR GNEISS–GREENSTONE

RELATIONSHIPS

There is a large number of published age data on the AGC. However, with few exceptions, these do not resolve age differences within the AGC or the chronological relationship

with the BGB as discussed below. Barton et al. (1980) reported a 10-point Rb-Sr wholerock isochron age of 3555± 111 Ma with a 87 Sr/86 Sr initial ratio of 0.6999± 16 for banded

Ngwane gneisses from a quarry near the Njoli Dam in northeast Swaziland (AGC 136, see

Fig. 5.2-1). Carlson et al. (1983) analysed samples from widely scattered rock types including Ngwane gneisses, Dwalile greenstones and younger granitoid intrusives by the Sm-Nd

method and obtained a best-fit line fulfilling isochron criteria with an age of 3417 ± 34 Ma

and εNd(t) = 1.1 ± 0.6. These authors argued that the excellent collinearity of their data

points supports derivation of all analysed rock types from an isotopically homogeneous

source and that the calculated age reflects the time of igneous emplacement.

However, Kröner et al. (1989) demonstrated with U-Pb single zircon SHRIMP ages that

igneous emplacement of the NG precursors occurred over a time span of at least 400 My.

The above Sm-Nd ‘isochron’ age of 3417 ± 34 Ma is therefore suspect and does not provide evidence for the AGC as a whole to be time-equivalent or to be significantly younger

than the BGB, the more so since Carlson et al. (1985) revised their age to ‘approaching

3550 Ma’ (no analytical data and errors given). The 3555 Ma Rb-Sr age of Barton et al.

(1980) is also suspect, since Kröner et al. (1989) dated zircons from one of the gneisses at

the Njoli Dam quarry (AGC 136, see Fig. 5.2-1) and obtained a magmatic emplacement

age of 3214 ± 20 Ma.



5.2-3. Geochronology and Implications for Gneiss–Greenstone Relationships



473



Direct evidence for pre-3500 Ma ages in the AGC was provided by Compston and

Kröner (1988), who SHRIMP-dated single zircons from a tonalitic Ngwane Gneiss north

of Piggs Peak (AGC 150 in Fig. 5.2-1) that reflect four distinct age groups, interpreted

as igneous and metamorphic episodes, and whose oldest group yielded a precise age of

3644 ± 4 Ma (Fig. 5.2-3). The other groups are defined by mean ages of 3580, 3504, and

3433 Ma (see Compston and Kröner, 1988, for detailed discussion).

There is no doubt that the 17 oldest zircon grains of this sample crystallized at

3644 ± 4 Ma, on the evidence of the excellent replication of age within and between

grains. The crucial question is what geological event do these grains represent? Compston and Kröner (1988) suggest two alternative possibilities. (1) Magmatic crystallization

in the original gneiss protolith, the zircon age would reflect emplacement and freezing of

the tonalite pluton prior to its deformation, and all younger zircon ages would represent

later growth or alteration due to metamorphism. Points consistent with this interpretation



Fig. 5.2-3. Concordia diagram for all analysed zircons from banded tonalitic gneiss sample

AGC 150. Error boxes are 1-sigma. The oldest magmatic episode at 3644 ± 4 Ma (2σ ) produced

the dominant type of zircon which probably precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss occurred within the oldest grains at 3504 and

∼3433 Ma. Whole new grains also grew at these times. The post-3644 Ma growth is interpreted

as due to episodic deformational and metamorphic events that transformed the original tonalite pluton into a foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of

younger zircon of different colour and texture, dated at 2986 and 2867 Ma. Euhedral, finely-zoned

whole grains having the 2986 Ma age are also present, evidently contributed by thin felsic veins

associated with the nearby Mpuluzi granite (Compston and Kröner, 1988).



474



Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



are the uniformity of the 3644 Ma ages, the relic igneous characteristics seen in some grains

(subhedral morphology and internal euhedral lamellae), and the large proportion of grains

that have this age. (2) Xenocrysts older than the tonalite. In this case, zircons characterizing emplacement of the tonalite magma would be one of the other groups that generate the

continuum of 207 Pb/206 Pb ages down to ca. 3400 Ma, as shown in Fig. 5.2-3. For example, the magmatic grains could be the group at 3504 ± 3 Ma, whereas the younger group

at 3433 ± 4 Ma could be interpreted as new growth or recrystallization of zircon that accompanied the development of gneissic foliation at that later time. There is no objective or

conclusive way of choosing between these alternatives, but Compston and Kröner (1988),

using estimates for zircon solubility in the original tonalite melt, favoured 3644 Ma as the

magmatic emplacement age. If this interpretation is correct, this is the oldest emplacement

age yet reported for a crustal rock in the African continent.

Further indications for the antiquity of the NG gneisses come from zircon ages summarized below, and for a 3538±3 Ma old tonalitic gneiss wedge within the lower Onverwacht

Group (Theespruit Formation) in the southern part of the Barberton Mountain Land (Armstrong et al., 1988). Lastly, there is a zircon age of 3570 ± 6 Ma for a granite clast in

the Moodies conglomerate of the BGB (Kröner and Compston, 1988), believed to be derived from erosion of the AGC terrain during greenstone basin evolution (Eriksson, 1980;

Jackson et al., 1987).

The oldest date so far reported from the supracrustal assemblage of the BGB is a

207 Pb/206 Pb mean zircon age of 3547 ± 3 Ma for strongly deformed felsic metavolcanic

samples of the Theespruit Formation, exposed in the Steynsdorp greenstone remnant (BA

39 and 40 in Fig. 5.2-1) and in tectonic contact with the rest of the BGB (Kröner et al.,

1996). These rocks are ∼100 My older than those of the Komati Formation in the main

belt and constitute the oldest components of the BGB, named Steynsdorp suite by Lowe

and Byerly (this volume) and onto which successively younger units were tectonically and

magmatically accreted (Kröner et al., 1996).

The available isotopic data therefore indicate that at least parts of the NG are older

than the oldest parts of the BGB, but direct evidence for a basement-cover relationship is

lacking. Age relationships and chemical similarities between felsic volcanic rocks of the

BGB and TTG gneiss domes surrounding the BGB suggest a genetic relationship, and the

presence of zircon xenocrysts up to 3.7 Ga in age in samples of both the felsic volcanics

and the granitoids suggests that older crust was involved in their formation (Kröner et al.,

1996).



5.2-4. NGWANE GNEISS

Apart from the 3.64 Ga zircon age for the tonalitic gneiss in northern Swaziland, there

are several other pre 3.5 Ga ages for similar rocks of occurring in the NE and central parts

of the country. Sample AGC 55 is a tonalitic gneiss collected close to the contact with the

Dwalile greenstone remnant (Fig. 5.2-1). The igneous zircons provided nearly concordant

ages with a mean of 3521 ± 23 Ma, and one zircon xenocryst has a slightly discordant



5.2-5. Dwalile Supracrustal Suite



475



207 Pb/206 Pb age of 3683 ± 10 Ma (Kröner and Tegtmeyer, 1994). This suggests that mater-



ial of similar age as exposed in northern Swaziland was involved in the generation of 3.5 Ga

gneisses and that at least some of the Ngwane gneisses are not juvenile but represent, at

least in part, remelts of earlier crustal material. This is also suggested by the whole-rock

Nd isotopic systematics for many Ngwane gneiss samples which provide depleted mantle

Nd mean crustal residence ages of 3.6–3.7 Ga (Kröner et al., 1993).

Further indications for the antiquity of the Ngwane gneisses come from a nearconcordant SHRIMP zircon age of 3563 ± 3 Ma (Fig. 5.2-4(a)) for a tonalitic gneiss

in central Swaziland (AGC 200 in Fig. 5.2-1; Kröner et al., 1989) and for a concordant

SHRIMP age of 3505 ± 24 Ma (Fig. 5.2-4(b)) for a trondhjemitic gneiss from the Njoli

Dam area of NE Swaziland (AGC 185 in Fig. 5.2-1; Kröner et al., 1989). Similar to the

3.64 Ga gneiss from NW Swaziland, the zircon population of this sample shows a complex

pattern of Pb-loss, even within single grains, which gives rise to exceptionally large scatter in the Concordia diagram (Fig. 5.2-4(b)). A small, clear, euhedral zircon population in

this sample is slightly discordant (symbols with broken lines in Fig. 5.2-4(b)) and defines

a distinctly younger age group with a Concordia intercept at 3166 ± 4 Ma. These grains

represent thin leucocratic lit-par-lit veins that cut the banded gneiss and reflect an event of

leucogranite magmatism that is also widespread elsewhere in the AGC.

Many TTG gneisses mapped as Ngwane Gneiss in the field, and resembling those mentioned above, yielded a surprising variety of igneous emplacement ages between 3455

and 2745 Ma, as summarized by Kröner et al. (1993). This either implies that the Ngwane

Gneiss is an extremely heterogeneous assemblage in which the various members are not all

genetically related, or many gneisses so far labelled as Ngwane Gneiss on account of their

field appearance do not belong to this unit. Gneisses as young as 3200 Ma have acquired

a fabric that is macroscopically indistinguishable from that of much older varieties. This

attests to the fact that intense ductile deformation has affected many of the Palaeoarchean

rocks of the AGC at various times and has often obliterated earlier structures (Jackson,

1984).

All samples of Ngwane Gneiss with zircon ages between 3.66 and 3.2 Ga have Sm-Nd

isotopic compositions that can be fitted to a common regression line on a 143 Nd/144 Nd

versus 147 Sm/144 Nd diagram, defining an “age” of 3760 ± 210 Ma and εNd(t) = +2.7

(Kröner et al., 1993). The most plausible interpretation of this linear relationship is that all

these rocks were derived from a crustal protolith which separated from its mantle source

some 3.7–3.8 Ga ago. The consistency of the age defined by the above reference line with

the individual Nd model ages led Kröner et al. (1993) to suggest that the Ngwane gneisses

probably inherited their Nd isotopic composition from their (common?) protolith(s).



5.2-5. DWALILE SUPRACRUSTAL SUITE

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. Kröner and Tegtmeyer (1994) reported detrital



476



Chapter 5.2: The Ancient Gneiss Complex of Swaziland and Environs



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