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Chapter 5.3 An Overview of the Geology of the Barberton Greenstone Belt and Vicinity: Implications for Early Crustal Development

Chapter 5.3 An Overview of the Geology of the Barberton Greenstone Belt and Vicinity: Implications for Early Crustal Development

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482



Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



Fig. 5.3-1. General geology of the Barberton Greenstone Belt and vicinity. Abbreviations: IF: Inyoka

Fault; GGF: Granville Grove Fault.



5.3-2. General Geology of the BGB



483



5.3-2. GENERAL GEOLOGY OF THE BGB

5.3-2.1. Stratigraphy

The BGB includes volcanic, sedimentary, and shallow intrusive rocks ranging in age from

>3547 to <3225 Ma (Figs. 5.3-1 and 5.3-2). The rock have traditionally been divided into

three main lithostratigraphic units: from base to top, the Onverwacht, Fig Tree, and Moodies Groups (Viljoen and Viljoen, 1969a, 1969b; Lowe and Byerly, 1999). The Onverwacht

Group is composed largely of mafic and ultramafic volcanic rocks with subordinate felsic

volcanic flow units and tuffs. Mostly thin, interbedded sedimentary units marking breaks in

eruptive activity have been widely silicified to form impure cherts. The Onverwacht Group

is diachronous in age across the belt. Rocks in the southern part of the belt range from

>3547 to ∼3260 Ma and exceed 10 km in stratigraphic thickness. Ultramafic and mafic

rocks fringing the western and northern edge of the belt, which may be as little as 1000 m

thick, appear to range from ∼3330 Ma to perhaps as young as 3240 Ma (summarized in

Lowe, 1999b). The Fig Tree group is a transitional unit up to ∼1800 m thick composed

of interlayered volcaniclastic strata, marking the final stages of greenstone belt volcanism,

and terrigenous clastic units eroded from uplifted portions of the underlying greenstone

succession. It was deposited between ∼3260 and 3225 Ma. The post-3225 Ma Moodies

Group is composed of up to 3000 m of coarse, quartzose and feldspathic sandstone and

chert-clast conglomerate derived by erosion of underlying greenstone units and uplifted

plutonic rocks.

The present discussion is focused on the southwestern half of the contiguous greenstone

belt, mostly in South Africa and west of road R40 from Barberton to Swaziland, and the

surrounding deeper-level (TTG) plutons, gneisses, migmatites, and supracrustal xenoliths

(Fig. 5.3-1), which together comprise the Barberton Granite-Greenstone Terrain. Rocks in

the northeastern part of the belt, beyond Fig. 5.3-1, have not been well studied and many

show greater strain than those in the study area. BGB rocks in Swaziland (Fig. 5.3-1) are

as yet poorly known. The Jamestown and Nelshoogte Schist Belts (Fig. 5.3-1) have been

studied locally (e.g., Anhaeusser, 1972, 1985, 2001) but stratigraphic and age relationships

are poorly resolved. These more highly strained belts are not considered here.

5.3-2.1.1. Structure

Although well preserved in many respects, rocks of the BGB have been subject to multiple

episodes of folding, faulting, and metamorphism (e.g., de Wit, 1982; de Ronde and de Wit,

1994; Lowe et al., 1999). Today, the supracrustal sequence is broken up into tectonic blocks

by both large and small faults: the major faults discussed here are shown in Fig. 5.3-3(A).

Both bedding and fault planes throughout the belt have generally been rotated to vertical or

subvertical dips. Over wide areas, small-scale strain effects such as cleavage, foliation, and

lineation, are absent or have been partitioned into more ductile units, resulting in the wide

preservation of textural features down to a few microns across. A more detailed summary

of the structures and structural development of the BGB is presented later in this paper.



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Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



5.3-2.1.2. Alteration

Virtually all BGB rocks have been altered at temperatures in excess of 300 ◦ C (Xie et al.,

1997; Cloete, 1999; Tice et al., 2004). However, many units display original textures and

sedimentary structures down to a few microns across, generally as a consequence of early

silicification and the local partitioning of strain into adjacent more ductile units, such as serpentinized ultramafic lavas. However, widespread metasomatic alteration has obliterated all

but the most refractory primary minerals in most rocks. Preserved detrital and early diagenetic minerals in sedimentary units include chromites, zircon, apatite, rutile, coarse quartz,

carbonates, iron oxides, and barite. The bulk of the original sediments have been replaced

by microquartz (chert) or carbonate or have been recrystallized into fine mosaics of quartz,

carbonate, sericite, chlorite, and other alteration products. Igneous units have been similarly altered, although, again, textures and structures are locally preserved with remarkable

clarity. Peridotitic komatiites are commonly altered to mosaics of serpentine, magnetite,

tremolite, and chlorite. Primary chromite is commonly preserved and rare enclaves show

primary orthopyroxene, augite, fresh olivine (Smith and Erlank, 1982), and even optically

glassy melt inclusions (Kareem and Byerly, 2002, 2003). Extreme metasomatic alteration

of komatiitic and basaltic flow rocks marks the tops of most volcanic sequences. These

altered rocks commonly underlie regionally developed silicified sedimentary units (cherts)

and appear as brownish carbonate-rich rocks or greenish cherts. The remaining mineralogy

is dominated by Cr-rich sericite. They are generally cut by complexly anastomozing quartz

veins. These zones have variously been interpreted as sea-floor flow-top alteration zones

(Lowe and Byerly, 1986a; Lowe et al., 1999), hydrothermal alteration zones (Duchac and

Hanor, 1987; Hanor and Duchac, 1990), or shear zones marking major thrust faults (de

Wit, 1982, 1983).

Basaltic volcanic and volcaniclastic units are typically altered to assemblages of albite, tremolite, chlorite, and occasionally epidote and sphene. Primary pyroxenes and

plagioclase are preserved locally. Coarser-grained volcaniclastic units may be substantially replaced by iron-rich dolomite and magnesite. Felsic volcanic and volcaniclastic

units, where best preserved, contain primary zircon, apatite, quartz, and less commonly

plagioclase, hornblende, and sphene. More typically, these rocks are altered to lithologies

dominated by microcrystalline quartz and sericite.



Fig. 5.3-2. (Next page.) Generalized stratigraphies of the principal tectono-stratigraphic suites in the

BGGT (modified from Lowe, 1999c, Fig. 2). Within the study area, the Kromberg suite is an overlap

assemblage deposited on the Songimvelo suite. The outcrop areas of the Steynsdorp, Songimvelo,

and Kromberg suites include rocks of the individual suites as well as overlap assemblages deposited

during formation of adjacent, younger suites. Ages with asterisks (∗) below stratigraphic columns

are from xenocrysts within the magmatic rocks on that block. Ages with daggers (†) are from detrital

zircons and rock fragments within sedimentary units. Age with number symbol (#) is from a gneiss

block within a shear zone along the southern edge of the Songimvelo Block. Ages with double

asterisks (∗∗) are from the Theespruit Formation of the Steynsdorp suite south of the Komati Fault

on the west limb of the Onverwacht Anticline.



5.3-2. General Geology of the BGB



485



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Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



Major and trace element compositions of BGB rocks have been used with moderate

success in petrogenetic studies (e.g., de Wit et al., 1987a; Vennemann and Smith, 1999;

Byerly, 1999). In the most highly altered lithologies, immobile elements (Al, Ti, Zr and

Cr) and REE have proven useful in the identification of the protoliths (e.g., Duchac and

Hanor, 1987; Lowe, 1999a, 1999b).

5.3-3. TECTONO-STRATIGRAPHIC SUITES

While the classical Onverwacht, Fig Tree, and Moodies Groups provide a useful lithostratigraphic framework for the BGB, regional usage of these names conveys the impression of a layer-cake type stratigraphy that obscures the complex and diachronous

evolution of the belt. In order to better characterize the development of the BGB and surrounding granitoid intrusive units, we here divide rocks of the Kaapvaal Craton in the

BGB area into five major architectural elements here termed tectono-stratigraphic suites

(Figs. 5.3-1 and 5.3-2). These suites correspond generally to the tectono-stratigraphic

blocks of Lowe (1999c) and the “magmato-tectonic” events of Poujol et al. (2003) and

Moyen et al. (this volume). Each suite consists of a sequence of volcanic and sedimentary rocks and related deeper-level TTG plutons, gneisses, and migmatites. These form the

basic architectural building blocks of the BGGT. They are not simply fault-bounded structural blocks or exotic terranes that have been assembled tectonically. While some suites are

in fault contact, in many instances, volcanic or sedimentary rocks of one suite lie at least

locally with apparent conformity on strata of earlier suites.

The BGGT tectono-stratigraphic suites include (Figs. 5.3-1 and 5.3-2):

(1) the Steynsdorp suite, which includes pre-3.5 Ga rocks of the BGB, exposed in the

southernmost and southeasternmost parts of the belt, and the 3509 Ma Steynsdorp

Pluton;

(2) the Songimvelo suite, which includes much of the south-central part of the BGB and

the adjacent ∼3445 Ma TTG plutons and metamorphic units;

(3) the Kromberg suite, which includes volcanic and sedimentary rocks of the Kromberg

Formation;

(4) the Umuduha suite, occupying the central part of the BGB; and,

(5) the Kaap Valley suite, which includes the BGB north of the Inyoka Fault and the 3236

to ∼3225 Ma Kaap Valley, Nelshoogte, and Badplaas TTG Plutons.

The general relationships among these tectono-stratigraphic suites are shown schematically

in Fig. 5.3-4. We have not included the Steyndorp and Songimvelo suites in a single suite,

block, or domain as do Kisters et al. (2003) and Moyen et al. (2006), which they term the

Stolzburg domain, because, although they may have been metamorphosed and structurally

affected together by later events, they clearly represent geochronologically and stratigraphically separate eruptive and intrusive cycles.

5.3-3.1. Steynsdorp Suite

The Steynsdorp suite includes rocks older than 3500 Ma, including the Theespruit Formation and the 3509 Ma Steynsdorp Pluton in the Steynsdorp Anticline and the Theespruit



5.3-3. Tectono-Stratigraphic Suites



487



Fig. 5.3-3. (A) Map showing the major faults in the BGB discussed in the text. (B–F) Outcropping greenstone-belt rocks of the Barberton tectono-stratigraphic suites (black). (B) Steynsdorp suite

(STS). (C) Songimvelo suite (SVS). (D) Kromberg suite (K). (E) Umuduha suite (US). (F) Kaap

Valley suite (KVS).



Formation in the Onverwacht Anticline south of the Komati Fault (Fig. 5.3-3(B)). Locally, komatiitic and basaltic rocks of the Sandspruit Formation may represent older parts

of this suite. In the Onverwacht Anticline, the Theespruit Formation includes a variety



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Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



Fig. 5.3-3. (Continued.)



of metamorphosed and faulted felsic volcanic rocks, felsic breccias, banded cherts, and

metamorphosed mafic and komatiitic volcanic rocks (Fig. 5.3-5). These are separated from

rocks of the less altered Komati Formation by the Komati Fault (Figs. 5.3-1, 5.3-3(A)).

Felsic volcanic rocks have yielded ages of 3531 ± 10 Ma (Armstrong et al., 1990) and

3511 ± 3 Ma (Kröner et al., 1992). Gneissic TTG blocks with crystallization ages of



5.3-3. Tectono-Stratigraphic Suites



489



Fig. 5.3-3. (Continued.)



3538 ± 6 Ma (Armstrong et al., 1990) and 3538 + 4/−2 Ma (Kamo and Davis, 1994)

have also been brought up along faults.

In the Steynsdorp Anticline, the Steynsdorp suite includes a thick sequence of mafic

and felsic schists representing altered volcanic units and interlayered metaquartzites representing metamorphosed cherts assigned to the Sandspruit and Theespruit Formations



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Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



Fig. 5.3-4. Schematic diagram showing inferred pre-deformation relationships of the tectono-stratigraphic suites making up the BGGT. Modified from Lowe (1999c, Fig. 4). The magmatic center for the Kromberg suite is shown schematically on the right side of the diagram.



5.3-3. Tectono-Stratigraphic Suites



491



Fig. 5.3-5. Rocks of the Theespruit Formation, Steynsdorp suite, west limb of the Onverwacht anticline. (A) Layered tuffaceous aluminous metasediments. The coarser layers are composed of large

andalusite porphyroblasts, not primary conglomeratic material. (B) Aluminous metasediments showing layering that may represent primary cross laminations. Pen in both figures is 15 cm long.



(Viljoen and Viljoen, 1969a) and the associated Steynsdorp Pluton. Theespruit schists and

cherts show tight isoclinal folds with strong axial plane cleavage (Kisters and Anhaeusser,

1995b). Theespruit felsic schists have been dated at 3548 ± 3 to 3544 ± 3 Ma (Kröner et

al., 1996).

The composite Steynsdorp Pluton includes older tonalitic-trondhjemitic gneisses and

younger foliated granodiorite (Kisters and Anhaeusser, 1995b; Kröner, et al., 1996), both

dated between 3502 ± 2 and 3511 ± 4 Ma (Kamo and Davis, 1994; Kröner et al.,

1996). The late-stage granodiorites are clearly intrusive into the oldest portions of the

BGB (Kröner, et al., 1996). These rocks also contain zircon xenocrysts 3553 ± 5 to

3531 ± 3 Ma, the same age as felsic tuffs of the Theespruit Formation (Kröner et al.,

1996). The somewhat younger, weakly foliated Vlakplaats granodiorite, 3450 ± 3 Ma,

intrudes the Komati Formation in the Steynsdorp Anticline. It is notable for zircons

3702 ± 1 Ma, the oldest xenocryts reported from the BGGT or Kaapvaal Craton, indicating that the source region was not simply mafic crust but crust that was compositionally

and geochronologically complex (Kröner et al., 1996).



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Chapter 5.3: An Overview of the Geology of the Barberton Greenstone Belt and Vicinity



The contact between the Theespruit and Komati Formations, marking the contact between the Steynsdorp and Songimvelo suites, is poorly exposed but was termed a “plane

of décollement” by Viljoen et al. (1969) and a tectonic contact by Kröner et al. (1996).

Kisters and Anhaeusser (1995b) noted the contrast in style and intensity of deformation

across this contact.

The northern extent of the pre-3.5 Ga Steynsdorp suite beneath the younger rocks is

unknown, although similar-aged intrusive units are recorded from the Ancient Gneiss Complex in northeastern Swaziland (Kröner et al., 1991b) and xenocrystic zircons older than

3.5 Ga are present in younger felsic volcanic units in the Songimvelo, Umuduha, and Kaap

Valley suites (Fig. 5.3-4).

5.3-3.2. Songimvelo Suite

Classic sections of the Komati and Hooggenoeg Formations (Viljoen and Viljoen, 1969a,

1969b) totaling ∼6 to 7.5 km thick in the Onverwacht Anticline, Kromberg Syncline,

and Steynsdorp Anticline (Fig. 5.3-3(C)) form the Songimvelo suite (Fig. 5.3-6). In these

sections, Songimvelo suite rocks are overlain with apparent conformity by rocks of the

Kromberg suite.

5.3-3.2.1. Komati Formation

The Komati Formation (Viljoen and Viljoen, 1969a; Viljoen et al., 1983) includes peridotitic and basaltic rocks between the Komati Fault and the Middle Marker, a distinctive

chert marking the base of the Hooggenoeg Formation. The type section on the west limb of

the Onverwacht Anticline is about 3500 m thick and composed almost exclusively of komatiitic and subordinate basaltic volcanic rocks (Viljoen and Viljoen, 1969a; Dann, 2000).

Although de Wit et al. (1987b) suggested that the layered komatiites are vertical dikes

within a sheeted dike complex and Grove et al. (1997) interpreted them as horizontal sills

intruded into the deep Archean crust, most workers have supported their origin as ultramafic, high-magnesian lava flows (e.g., Cloete, 1999; Dann, 2000).

Komatiites are composed of high magnesian olivines and pyroxenes, and chromites, set

in a fine-grained matrix that was probably glass. Olivine and pyroxene in komatiitic flow

units commonly display spinifex textures formed through magma quenching (Fig. 5.3-7(A)

and (B)). Pillows are usually poorly developed but do occur in komatiitic basalts (Fig. 5.37(C)). Chromites are commonly preserved, both morphologically and compositionally, in

even the most altered rocks. However, primary olivine and pyroxene minerals are rarely

preserved. Olivine is largely pseudomorphed by serpentine and magnetite, and pyroxene

and glass by tremolite and chlorite, respectively.

The Komati Formation contains only one known sedimentary layer, a 5–10 cm thick

layer of felsic tuff (Fig. 5.3-7(D)), and, except at the top, lacks alteration zones that might

represent breaks in the eruptive sequence. Their absence probably reflects the short time

required for eruption of the 3.5 km of komatiitic lavas. Dann (2000) reports an age on the

felsic tuff in the middle part of the formation of 3481 ± 2 Ma.



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Chapter 5.3 An Overview of the Geology of the Barberton Greenstone Belt and Vicinity: Implications for Early Crustal Development

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