Gary R. Byerly

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.

Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine 207Pb/206Pb and UPb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of ∼ 3445–3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 ± 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 ± 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherty metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 ± 3 Ma. The basal Fig Tree Group has units as old as 3259 ± 3 Ma, and upper units in the Fig Tree are as young as 3225 ± 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20–40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.

THE INFLUENCE OF ALTERATION ON THE TRACE-ELEMENT AND ND ISOTOPIC COMPOSITIONS OF KOMATIITES

Abstract To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵNd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵNd is far greater ( −6.2 to +8.8). The variable initial ϵNd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵNd of + 2.3.

The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga

Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform 207Pb/206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵNd(t) values between + 1.1 and − 1.1 and Nd TDM model ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.

Geological and Geochemical Record of 3400-Million-Year-Old Terrestrial Meteorite Impacts

Beds of sand-sized spherules in the 3400-million-year-old Fig Tree Group, Barberton Greenstone belt, South Africa, formed by the fall of quenched liquid silicate droplets into a range of shallow-to deep-water depositional environments. The regional extent of the layers, their compositional complexity, and lack of included volcanic debris suggest that they are not products of volcanic activity. The layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions. Geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt, komatiite, and CI carbonaceous chondrite. The spherules are thought to be the products of large meteorite impacts on the Archean earth.

Spherule Beds 3.47-3.24 Billion Years Old in the Barberton Greenstone Belt, South Africa: A Record of Large Meteorite Impacts and Their Influence on Early Crustal and Biological Evolution

Four layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approximately 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record.

Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type

Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extra-terrestrial components in beds S2, S3, and S4 are estimated to be approx. l%, 50% - 60%, and 15% - 30%, respectively. These beds are probably the distal, and possibly global, ejecta from major large-body impacts. These impacts were probably much larger than the Cretaceous-Tertiary event, and all occurred over an interval of approx. 20 m.y., implying an impactor flux at 3.2 Ga that was more than an order of magnitude greater than the present flux.

Early Archean silicate spherules of probable impact origin, South Africa and Western Australia

Layers of sand-sized silicate spherules in 3.2 to 3.5 Ga Archean greenstone belts in South Africa and Western Australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences. The layers and particles appear to be unrelated to volcanic activity, and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts. If so, these spherules are the oldest known terrestrial impact products. If these objects are representative of a larger population of early Archean impact features, it may be possible to directly estimate the meteorite flux on the early Archean earth.

Spinel from Archean impact spherules

Abstract Large meteorite impacts are recorded in distal fall deposits preserved in two early Archean greenstone belt sequences (3.2–3.5 Ga). Mapping, stratigraphie, and geochemistry studies have been used to identify at least four impact layers in the Barberton greenstone belt of southern Africa and one in the Pilbara greenstone belt of Western Australia. Spherules that represent impact melt droplets distinguish these impact layers from associated tuffs. Spinel with unusual compositional traits occurs within spherules in two of these impact layers. Spinel is the only preserved primary phase, and is found within up to 10% of the spherules in the best preserved layers. Some spherules contain up to 40% spinel, which is often the only primary phase that crystallized within the spherules. Spinel morphologies include dendrites and skeletal octahedra, typical of those found in high temperature melts. Spinel compositions are similar to those of Cenozoic impacts, including very high Ni and ferric iron contents, but the Archean spinel is distinctive in being composed dominantly of the chalcophile elements Fe, Ni, Cr, and V. This spinel is not metamorphic or metasomatic in origin. Closely associated komatiites in the underlying Onverwacht Group, both fresh and highly altered, have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel. Characteristics of spinel preserved in these Archean impact spherules suggest a rather complex series of steps following a large bolide impact. Initial high temperature reduction produced melt droplets, some of which were immiscible silicate-metal/sulfide. Density differences allowed separation of some of these immiscible melts, which subsequently evolved independently and formed spherules with highly variable chalcophile element contents. Oxidation during atmospheric reentry increased Ni Fe and Fe +3 Fe +2 in the chalcophile-siderophile melts. Archean impact spinel has higher Ni Fe than Cenozoic impact spinel, though distinctly lower Fe +3 Fe +2 , which probably reflects a substantially less oxidizing atmosphere during the Archean. The Archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric H 2 O H 2 . Bolide size is estimated to be 24 km in diameter based on the average diameter of 0.85 mm for the spinel-bearing spherules. This estimate is consistent with the observed Ir fluence in spinel-bearing impact layers of approximately 5.3 mg/cm2, which would be produced by a 30 km diameter chondritic bolide.

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

Publisher Summary This chapter presents an overview of the geology of the Barberton Greenstone belt and vicinity. Rocks in the 3.55 to 3.22 Ga Barberton Granite Greenstone Terrain (BGGT), South Africa and Swaziland, represent one of the oldest, well-preserved pieces of continental crust on the Earth. Together with similar rocks of nearly identical ages in the Pil-bara Craton of Western Australia, rocks of the BGGT have provided most of the direct geologic evidence on the nature and evolution of the pre-3.0 Ga Earth, its crust, surface environment, ocean, atmosphere, and biota. The BGB includes volcanic, sedimentary, and shallow intrusive rocks ranging in age from >3547 to