Ancient Gneiss Complex Research Articles

Gneiss-greenstone relationships in the Ancient Gneiss Complex of southwestern Swaziland, southern Africa, and implications for early crustal evolution

The Ancient Gneiss Complex (AGC) of Swaziland is made up of a variety of Archaean gneisses derived from granitoid intrusions of TTG composition, emplaced between 3.6 and 3.2 Ga ago, and infolded amphibolite-grade greenstone remnants locally referred to as Dwalile Supracrustal Suite (DSS). The greenstones were considered to postdate the oldest phases of the AGC, known as Ngwane gneiss, and this is now confirmed by conventional UPb zircon geochronology and SHRIMP single zircon ages on detrital grains from Dwalile clastic metasediments, suggesting that these rocks were derived from a granitoid source some 3.42 to 3.57 Ga in age. Trondhjemitic Ngwane gneiss structurally underlying the greenstones has a zircon age of 3521±23 Ma and contains a 3683±10 Ma old xenocryst. This together with Nd T DM model ages between 3.7 and 3.9 Ga and a pronounced negative Euanomaly for the metasediments strongly argues for the presence of very old crustal components in the AGC. Nd isotopic systematics of Dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of Ngwane type, and resulting Nd model ages (3.60–3.99 Ga) are likely to be too old. The major and trace element chemistry of the Dwalile metavolcanics is remarkably similar to that of Onverwacht Group rocks in the nearby Barberton Greenstone Belt (BGB) and this, together with a similar age for these two suites, points to a genetic link and a similar mechanism of formation. Our data as well as field relationships do not favour an intra-oceanic origin for the DSS, as has been inferred for the Onverwacht Group, nor do they support a juvenile origin for the exposed Ngane gneisses of the AGC. The Ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification, particularly through remelting and deformation, gave rise to the various compositional varieties now seen in the Ngwane suite.

Geochemical studies of Archaean granitoid rocks in the Southeastern Kaapvaal Province: implications for crustal development

Recently completed mapping of the Archaean granitoid terrain immediately to the south of Swaziland and extending into northern Natal reveals that tonalitic-trondhjemitic-granodioritic (TTG) lithologies are dominant. The first recognized generation of TTG magmas is preserved as complexly deformed layered gneisses comparable in both mineralogy and chemistry to the bimodal suite in the Ancient Gneiss Complex of Swaziland. These TTG gneisses are characterized by heavy rare-earth element (REE) and Y depletion and strongly fractioned REE patterns, having either no or a small positive Eu anomaly. The most widespread TTG magmas are represented by the Anhalt Granitoid Suite that builds a major, tabular batholith. Rb-Sr and Pb isotopic data indicate that the Anhalt Granitoid Suite was emplaced at ∼3.25 Ga. This age suggests emplacement of the suite was contemporaneous with the intrusion of the Kaap Valley and Stolzburg plutons into the northwestern flank of the Barberton Sequence, 100 km to the North. It is proposed that the Anhalt Granitoid Suite was generated as a consequence of thrusting directed N or NW associated with crustal shortening during the initial stages of continental collision that ultimately lead to the closure of an oceanic basin in which the Onverwacht Group of the Barberton Sequence accumulated. Final stages of collision were marked at ∼3.1 Ga by the intrusion of tabular, potassic granite batholiths, the main locus of emplacement of which lies to N and NW of that of the Anhalt Granitoid Suite. Subsequent to the establishment of thick continental crust as a result of repeated granitoid plutonism prior to ∼ 3.1 Ga, an extensive depositional basin developed in which the Pongola Sequence accumulated. Dominantly potassic granitoids having geochemical characteristics of Phanerozoic with-in plate granites intrude the Pongola Sequence as (i) gneiss domes, (ii) a tabular batholith and (iii) sharply discordant, megacrystic plutons. The source of these granitoid magmas is uncertain but partial melting of the pre-Pongola sialic basement is considered to be probable.

Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe

Stratigraphic and structural data from the Archean Zimbabwe craton suggest that a major detachment surface exists within the Belingwe greenstone belt. The surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex, older volcanic-sedimentary rocks, and a shallow-water sedimentary sequence in the footwall. Rocks dated at ca. 2.7 Ga above the detachment surface form the proposed Mberengwa allochthon. The regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets, not indigenous magmas erupted within autochthonous continental rifts. Magmatic rocks of the Mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts. Comparison of the Zimbabwe craton with the Proterozoic Birrimian terranes of west Africa leads us to suggest that Precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere.

The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited: A single zircon ion microprobe study

Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (< 3453 ± 6Ma) could be younger than the (structurally) overlying mafic and ultramafic volcanics of the Komati Formation (3482 ± 5Ma). A major structural discontinuity may therefore exist between the two formations. (2) An age of 3538 ± 6Ma established for a tectonic wedge of tonalitic gneiss within the Theespruit Formation confirms the presence of a sialic basement and deformed unconformity below that unit. The tonalitic gneiss is the oldest unit yet recorded within the greenstone belt, equal in age to the older components of the adjacent Ancient Gneiss Complex. (3) The interpreted ages of felsic volcanic rocks from both the Hooggenoeg (3445 ± 8Ma) and Theespruit Formations and the nearby Theespruit Pluton (3437 ± 6Ma) are essentially the same, and corroborate field and geochemical evidence that the felsic units were probably cogenetic and emplaced simultaneously as high-level equivalents of trondjhemite-tonalite plutons that intrude the greenstone belt at its southwestern extremity. (4) Felsic-intermediate volcanic-volcaniclastic rocks locally separating the two major groups (the Fig Tree and Moodies Groups) which overlie the Onverwacht Group record a second major peak of tonalitic magmatism in the Barberton terrain at about 3250 Ma. This is close to the age of the Kaap Valley tonalite pluton which intrudes the Barberton Greenstone Belt at ca. 3226 Ma along its northwestern margin. The present results indicate the Barberton Greenstone Belt is part of an allochthonous sequence containing major tectonic and stratigraphic breaks, with a protracted history; of which the last 200 million years, at least, evolved within a tectonically active convergent environment.

Growth of early Archaean crust in the Ancient Gneiss Complex of Swaziland as revealed by single zircon dating

We present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the Ancient Gneiss Complex (AGC) of Swaziland, southern Africa, that enable us to document a more extended history of early Archaean crustal growth in the eastern Kaapvaal craton than previously documented. The oldest rocks appear to be strongly flattened tonalitic-trondhjemitic orthogneisses dated as 3644 ± 4 Ma (Compston and Kröner, 1988) which are in faulted contact with rocks of the Barberton Greenstone Belt (BGB) in an AGC enclave in northwest Swaziland and which also record subsequent thermal-metamorphic-plutonic events at ≈ 3580, 3500, 3430, 3200 and 3000 Ma. These rocks are tectonically interbanded with ≈ 3200 Ma old granodiorites and K-rich leucogranites, thereby providing evidence for a severe and important post-3.2 Ga deformation event that is also inferred for ≈ 3500 to 3200 Ma old tonalitic to trondhjemitic gneisses in the AGC of northeast Swaziland. In central Swaziland zircons from the oldest tonalitic gneiss of the AGC have an age of 3560 Ma whereas younger additions of granitoid, which also intrude the Dwalile greenstone enclave in the AGC, record ages of ≈ 3450–3460 Ma. Intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology. Chemical and whole-rock isotopic data are consistent with derivation of some AGC gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors, but this is not generally true for the entire AGC. We suggest that at least some gneisses resulted from intracrustal melting, perhaps during crustal thickening events as recorded in granulite parageneses within the AGC. Documentation of 3.5 Ga or older granitoid crust in all major regions of the AGC now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the Barberton greenstone succession was deposited. The combined geochronologic and structural/metamorphic data for the AGC and BGB reveal a complex history of growth and deformation for the eastern Kaapvaal craton that lasted for at least 600 Ma from 3.64 to about 3 Ga ago. The magmatic and tectonic evolution of the AGC was broadly similar to that of the BGB and suggests that early Archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels. Our age data and the structural/metamorphic evolution of the eastern Kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening, uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening.

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites in Swaziland, southern Africa

Late Archaean cratonization in the Kaapvaal craton of southern Africa was associated with voluminous granitoid emplacement, and we report on the Rb-Sr isotopic systematics of the Lochiel suite and associated tin-bearing pegmatites in northwestern Swaziland, one of the largest composite batholiths of this region. Thirty-nine samples of the various granitoid phases of the Lochiel batholith and related pegmatites were collected within the “tin-belt” of Swaziland and that extends in a SE direction from Makwane beacon (South African border) to Nyonyane hill some 20 km SW of Mbabane. Rb-Sr whole-rock dating yielded an isochron age of 2979 ± 33 M.a. for the Granodiorite phase (GD) with an initial 87Sr/ 86Sr ratio of 0.7017 ± 0.0005 while the Alkali-Feldspar Granite (AFG), that intrudes the GD, has an apparent age of 3000±125 M.a. and a 87Sr/ 86Sr initial ratio of 0.7047 ± 0.0103. Two pegmatite suites from widely separated localities have virtually identical isotopic systematics and, if regressed together, define an age of 2613 ± 35 M.a. The age of ca. 3000 M.a. for the GD and AFG is in good agreement with published ages for the Lochiel Batholith from other localities. The previously held contention that the various phases of the Lochiel Batholith are indistinguishable with respect to their Sr isotopes is supported by our new data. The low initial ratios for both granitoid suites makes it unlikely that the Lochiel batholith was produced by remelting of the 3.5-3.6 G.a. old Ancient Gneiss Complex, and we favour an underplating model with a significant juvenile magma component. The high Sr initial ratio of the pegmatite phase is in line with reported initial ratios of younger granitoids in Swaziland and suggests that these rocks are remelting products of older sialic crust.

Single zircon dating constraining the maximum age of the Barberton Greenstone Belt, southern Africa

One of the central issues in early Precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high‐grade gneiss terranes. The crucial question, in most cases, is which of the two is older. We report zircon ages, using the new single grain evaporation technique, from a metaquartzite and a felsic volcanic flow of the Onverwacht Group in the Barberton greenstone belt (BGB), South Africa and Swaziland, that constrain the age of this basal part of the greenstone sequence between 3451 ± 15 and 3438 ± 6 Ma. This is almost 100 Ma younger than previous age estimates assigned to the Onverwacht and agrees well with recent Sm‐Nd, Pb‐Pb and zircon ion‐microprobe dating. Tonalitic orthogneisses of the Ancient Gneiss Complex (AGC) adjacent to the BGB have ages up to 3644 ± 4 Ma and are thus demonstrably older than the greenstones; also, they cannot be derived from them by partial melting as has been proposed previously. This age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the Onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the BGB and suggest that continental crust was either below or nearby.

Combined use of single zircon ages and Sm-Nd isotopes in the analysis of early archaean crustal growth processes: The Ancient Gneiss Complex of Swaziland, Southern Africa

Combined use of single zircon ages and Sm-Nd isotopes in the analysis of early archaean crustal growth processes: The Ancient Gneiss Complex of Swaziland, Southern Africa

Ion microprobe ages of zircons from early Archaean granite pebbles and greywacke, barberton greenstone belt, Southern Africa

We report single-grain and grain-domain U-Pb zircon ages for three granitoid clasts from the Moodies conglomerate and for detrital zircons from a Fig Tree greywacke from clastic sediments in the early Archaean Barberton greenstone belt of southern Africa. The crystallization ages obtained for the Moodies clasts vary between 3570 ± 6 Ma and 3518 ± 11 Ma, while the Fig Tree zircons show a surprisingly uniform source terrain age of 3453 ± 9 Ma that may date the stratigraphically underlying Onverwacht Group felsic metavolcanics. The discordancy patterns are ascribed to a combination of variable Pb loss at unspecified times in the past and new zircon growth and/or recrystallization < 3300 Ma ago. The SHRIMP data record details of isotopic variation, even within single zircons, that result in precise definition of upper concordia intercept ages. Our results suggest an age range between ∼ 3460 and ∼ 3570 Ma for the source region of the Barberton belt clastic metasediments, presumably the Ancient Gneiss Complex of Swaziland and felsic volcanics of the Overwacht Group. This source included geochemically evolved granitoids that predate the oldest greenstones by some 100 Ma.

Multiple zircon growth within early Archaean tonalitic gneiss from the Ancient Gneiss Complex, Swaziland

U Pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at3644 ± 4Ma(2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at3504 ± 6Ma and3433 ± 8Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of younger zircon of different colour and texture, dated at2986 ± 20Ma and2867 ± 30Ma. Euhedral, finely-zoned whole grains having the 2986 Ma age are present also, evidently contributed by very thin felsic veins associated with the nearby Lochiel granite. The age of3644 ± 4Ma combined with precise zircon U Pb dating of volcanics from the Onverwacht Group reported elsewhere demonstrates that at least part of the Ancient Gneiss Complex is older than the Barberton Greenstone Belt.

Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa

The upper, dominantly terrigenous sedimentary successions of the Early Archean Barberton Sequence, Kaapvaal Province, record an evolution similar to that of Phanerozoic foredeeps and may represent the oldest known foredeep sequence, rather than a passive continental-margin sequence as previously proposed. Pre-foredeep ocean crust, the 3.5 Ga-old Onverwacht Group, consists of bimodal volcanics with intercalated shallow-water sediments. Gravity modeling and boundary shear zones indicate that deformed Onverwacht crust was obducted northward onto sialic continental crust immediately before, during or after burial by the synorogenic terrigenous clastic fill. Foredeep basin deepening due to crustal loading from the south is recorded by basinal iron-formation, chert, and mudstone of the basal Fig Tree Group which accumulated paraconformably on the Onverwacht. Overlying sediments of the Fig Tree Group consist of ~ 2 km of submarine-fan greywackes and mudstones. Shoaling of the basin is indicated by the transition to the overlying braided-alluvial and shallow-marine sediments of the ca. 3.3 Ga-old Moodies Group (~ 3 km thick). Changing REE patterns and relative abundances of Ni, Cr, Sr, Th and Ur in the sediments of the Fig Tree and Moodies Groups indicate a significant ultramafic-mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution. A similar trend is reflected in sandstone petrography, in which the ratio of extrusive to intrusive rock fragments decreases upwards in the Fig Tree Group whereas the Moodies Group sediments are enriched in quartz and potassium feldspar. A persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of Phanerozoic recycled orogens. The distribution of facies in the Fig Tree and Moodies Groups, and paleocurrent, petrographic and geochemical data, indicate progressive unroofing of a southerly source terrane consisting of the Onverwacht volcanics and intercalated sediments, and the 3.5-3.2 Ga-old Ancient Gneiss Complex (AGC). In addition, older components of the Fig Tree Group were recycled into overlying stratigraphie units. The AGC consists of mantle-derived 3.5 Ga-old bimodal metavolcanics and metasediments, a tonalite-gneiss batholith, metabasite dikes, and meta-anorthosites. Its structures record progressive northwestward uplift, beginning around 3.4 Ga ago. Intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large-scale northwestward simple shear, followed by imbricate faulting and pseudotachylite formation. By 3.3 Ga ago the Fig Tree and Moodies Groups had also shortened, with similar kinematics to those of the AGC, by (partly syndepositional) folding and volume loss during the formation of transecting cleavage. Further folding and slide formation followed during subsolidus diapiric rise of the initially tabular Kaap Valley tonalite by 3.2 Ga and emplacement of trondhjemite plutons from 3.3 to 3.0 Ga ago.

Deformation of the Assegaai supracrustals and adjoining granitoids, Transvaal, South Africa

The structures in Archaean quartzites of the Assegaai greenstone belt in the Transvaal of South Africa can be interpreted in terms of three major deformation events. Earliest D 1, involved thrusting along brittle ramp and flat crush zones. Rising temperatures then led to the mobilization of SiO 2 and the development of syn- D 1 stylolites and omnidirectional quartz veins in the quartzites. Increasingly ductile behaviour results in some of the early D 1 crush zones being overprinted by late D 1 mylonitic shear zones while eastward-verging recumbent folds formed about N-S axes in the layered quartzites. The Assegaai supracrustals were then separated from a contiguous basement of Ancient Gneiss Complex (AGC) by the intrusion of a granitoid sheet about 3 km thick along their sheared subhorizontal contact either late in D 1 or in the interval between D 1 and D 2. The supracrustals were then refolded about steep NNE-trending axial surfaces by concentric F 2 folds on a variety of scales. Such folds were the second major structures to affect the supracrustals but the first to deform the granitoid sheet. The older of two suites of quartzo-feldspathic veins in the AGC were probably generated penecomtemporaneously with the syn- D 1 quartz veins in ther Assegaai quartzites. This interpretation can be tested by removing the intense D 2 constriction (and 30% volume increase) recorded by a later network of syn-granitoid quartzofeldspathic veins in the AGC from the D 1 + D 2 flattening recorded by the older veins. This procedure can be carried out by geometric vector subtraction on a Hsü diagram. The result reveals that the D 1 strains in the Assegaai supracrustals and the then contiguous AGC were identical penetrative subhorizontal flows prior to the intrusion of the intervening granitoid. The last significant strain in the region is represented by a brittle NNE-trending strike-slip fault and, locally in the quartzites, disharmonic conjugate folds.

Petrography and geochemistry of granitoid and metamorphite pebbles from the early Archaean Moodies Group, Barberton Mountainland/South Africa

Pebbles of potassic granitoids and metamorphites constitute up to 5% of the basal conglomerate of the Moodies Group in a ratio of 2 : 1. The granitoid pebbles frequently show micrographic quartz–feldspar intergrowth, whereas the metamorphites—of a modal composition similar to that of the granitoids—are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors. The granitoids show high K 2O, Sr, K 2O/Na 2O, and K/Rb, small enrichment of light REE, large negative Eu-anomalies, and slightly depleted and fractionated heavy REE. Compared to the granitoids the metamorphites show higher Fe 2O 3, TiO 2, and Cr concentrations, greater enrichment of light REE, and also large negative Eu-anomalies. There is little similarity between the Moodies pebbles and the majority of the rocks of the Ancient Gneiss Complex of Swaziland (AGC). There is only some similarity of the REE distribution patterns between the pebbles and the Mkhondo Metamorphic Suite, possibly an areally restricted phase of the AGC. The geochemical data, and especially the large negative Eu-anomalies suggest that the Moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed. The granites crystallized at depths of < 7 km from magmas with low H 2O-pressures in a rather thick sialic crust. It appears possible that the pre-Moodies granitoids originated through partial melting of low-Al 2O 3 siliceous gneisses of the AGC. A chronologic connection of the formation of the granitoids with the late Onverwacht Group volcanicity is possible.

Geochemical investigation of Archaean Bimodal and Dwalile metamorphic suites, Ancient Gneiss Complex, Swaziland

The bimodal suite (BMS) comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites. Based on geochemical parameters three main groups of siliceous gneiss are recognized: (i) SiO2 14%, and fractionated light rare-earth element (REE) and flat heavy REE patterns; (ii) SiO2 and Al2O3 contents similar to (i) but with strongly fractionated REE patterns with steep heavy REE slopes; (iii) SiO2 > 73%, Al2O3 < 14%, Zr ∼ 500 ppm and high contents of total REE having fractionated light REE and flat heavy REE patterns with large negative Eu anomalies. The interlayered amphibolites have major element abundances similar to those of basaltic komatiites, Mg-tholeiites and Fe-rich tholeiites. The former have gently sloping REE patterns, whereas the Mg-tholeiites have non-uniform REE patterns ranging from flat (∼ 10 times chondrite) to strongly light REE-enriched. The Fe-rich amphibolites have flat REE patterns at 20–30 times chondrite. The Dwalile metamorphic suite, which is preserved in the keels of synforms within the BMS, includes peridotitic komatiites that have depleted light REE patterns similar to those of compositionally similar volcanics in the Onverwacht Group, Barberton, basaltic komatiites and tholeiites. The basaltic komatiites have REE patterns parallel to those of the BMS basaltic komatiites but with lower total REE contents. The Dwalile tholeiites have flat REE patterns. The basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source. The Fe-rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation.

Sm–Nd age and isotopic systematics of the bimodal suite, ancient gneiss complex, Swaziland

Studies of the development and stabilization of the Archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units (greenstone belts) and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust. At the heart of this interest are the questions of the processes responsible for crust formation in the Archaean and whether or not the various units of an Archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials. One area where this controversy has been particularly pronounced is the Archaean crustal section of south-west Africa1–6. The oldest rocks in the Kaapvaal craton consist of the Onverwacht Group of mafic to ultramafic metavolcanics of the Barberton greenstone belt and a grey-gneiss complex termed the ancient gneiss complex (AGC) of Swaziland. We report here the results of a whole-rock Sm–Nd isotopic study of the AGC and the implications these data may have for crustal evolution in the Kaapvaal craton.

Rb–Sr age and source of the Bimodal Suite of the Ancient Gneiss Complex, Swaziland

The Ancient Gneiss Complex (AGO) in Swaziland1–5 is a complexly deformed suite of gneisses that may be divided into three major units5: (1) the Biomodal Suite (BMS) of inter-layered siliceous low-K leucocratic orthogneisses and amphibolites; (2) homogeneous tonalitic gneiss (the ∼ 3,320 Myr old Tsawela gneiss)6,7; and (3) supracrustal rocks constituting two main lithologic sequences—the Mkhondo Valley1,5 and Dwalile (ref. 8 and M.P.A.J. and A.C.W. unpublished data) metamorphic suites. The Tsawela gneiss is intrusive into both the BMS and the Dwalile metamorphic suite8. Relating the BMS to the greenstone sequence of the Barberton Mountain Land (the Onverwacht Group of the Swaziland Supergroup) has caused much controversy. It has been thought1 that the BMS predates the Onverwacht Group and may represent a basement to the latter. Another view9,10 is that all of the leucocratic gneisses of the AGC are younger than the Onverwacht Group and were derived from an unrelated mantle source. The amphibolite and metasedimentary rocks in the AGC are xenoliths of the Swaziland Supergroup. This controversy has remained unsolved due to the lack both of unambiguous contact relationships between the AGC and the Swaziland Supergroup, and of reliable radiometric data on the emplacement age of either unit. At the only known locality where these two units are juxtaposed, the contact is technically deformed. Early radiometric age data6,11–20 have given results that are either too imprecise or reflect subsequent metamorphism7. Recently, however, a precise age of 3,510±60 Myr (2σ) has been obtained by the Sm–Nd technique for extrusion of the volcanic rocks of the Komati Formation of the lower Onverwacht Group21. The results are reported here of Rb–Sr whole rock and biotite–whole rock isotopic analyses of samples of the leucocratic orthogneisses of the BMS. Results are also presented of initial 87Sr/86Sr ratio studies of samples of amphibolite from the Komati Formation. The relationship of the AGC to the Swaziland Supergroup is re-examined. All Rb–Sr ages mentioned are calculated using 1.42×10−11 yr−1 as the decay constant of 87Rb.

Discussion on “the geochemical nature of the archean ancient gneiss complex and granodiorite suite, Swaziland: A preliminary study” by D.R. Hunter, F. Barker and H.T. Millard Jr.

Discussion on “the geochemical nature of the archean ancient gneiss complex and granodiorite suite, Swaziland: A preliminary study” by D.R. Hunter, F. Barker and H.T. Millard Jr.

The geochemical nature of the Archean Ancient Gneiss Complex and Granodiorite Suite, Swaziland: a preliminary study

Abstract The Ancient Gneiss Complex (AGC) of Swaziland, an Archean gray gneiss complex, lies southeast and south of the Barberton greenstone belt and includes the most structurally complex and highly metamorphosed portions of the eastern Kaapvaal craton. The AGC is not precisely dated but apparently is older than 3.4 Ga. The AGC consists of three major units: (a) a bimodal suite of closely interlayered siliceous, low-K gneisses and metabasalt; (b) homogeneous tonalite gneiss; and (c) interlayered siliceous microcline gneiss, metabasalt, and minor metasedimentary rocks — termed the metamorphite suite. A geologically younger gabbro-diorite-tonalite-trondhjemite suite, the Granodiorite Suite, is spatially associated with the AGC and intrusive into it. The bimodal suite consists largely of two types of low-K siliceous gneiss: one has SiO2 14%, low Rb/Sr ratios, and depleted heavy rare earth elements (REE's); the other has SiO2 > 75%, Al2O3 The siliceous gneisses of the metamorphic suite show low Al2O, K2O/Na2O ratios of about 1, high Rb/Sr ratios, moderate REE abundances and negative Eu anomalies. K/Rb ratios of siliceous gneisses of the bimodal suite are very low (∼130); of the tonalitic gneiss, low (∼225); of the siliceous gneiss of the metamorphite suite, moderate (∼300); and of the Granodiorite Suite, high (∼400). Rocks of the AGC differ geochemically in several ways from the siliceous volcanic and hypabyssal rocks of the Upper Onverwacht Group and from the diapirs of tonalite and trondhjemite that intrude the Swaziland Group.

Crustal development in the Kaapvaal craton, I. The Archaean

Abstract The relationship in time and space of the elements comprising the greenstone—granitie terrane in the eastern Transvaal and Swaziland is discussed. On the evidence derived from structural analysis, metamorphic style, geochemistry, and geophysics it is concluded that sialic crust (now represented by the Ancient Gneiss Complex in Swaziland) pre-dates the Swaziland Sequence. It is postulated that the sialic crust formed as a result of partial and total melting of hydrous basaltic lithosphere under tectonically metastable conditions. Limited sedimentation and volcanism in small basins on this early crust took place during periods of quiescence, following which deformation resulted in the tectonic interslicing of the early sialic crust and the sedimentary—volcanic sequences that were metamorphosed at high temperatures and low pressure (Abukuma-type), and included limited partial melting. The protocontinental crust so formed was distended along linear zones overlying sites of mantle upwelling. Rifting resulted from the distension and was accompanied by intense volcanism typical of greenstone belts. Following mantle withdrawal sagging was initiated in the linear zone leading to sedimentation that was initially of turbidite type. As greater stability was achieved, the style of sedimentation changed and cratonic-type, Moodies Group sediments were deposited. The cyclic nature of the volcanism and sedimentation is considered to be a response to, and a reflection of, the degree of distension and of the vertical adjustments along the bounding faults. Diapiric rise of tonalitic magma produced as a result of partial melting of the early sialic crust mixing with mantle material caused the deformation of the original linear geometry. Continued depression of the amphibolite facies of the sialic crust into the zone of partial melting gave rise to potassic granitic magma that spread at higher crustal levels at interfaces of low free energy to form hood-like sheets of granite flanking the original linear rift. It is concluded that the eastern Transvaal and Swaziland area attained a crustal thickness of ± 25 km prior to 3.0 b.y.