Komati Formation Research Articles

The intensity of the geomagnetic field at 3.5 Ga: paleointensity results from the Komati Formation, Barberton Mountain Land, South Africa

Paleointensity measurements have been carried out on 3.5 Ga samples from the Komati Formation type locality using both the Thellier and Van Zijl methods. These samples contain a single steeply-directed negative TRM component acquired during metamorphism of the Komati lavas. Thellier experiments yielded values ranging from 12 to 37 μT but an average paleofield intensity for the four best determinations is 20 ± 3 μT. A slightly lower average paleointensity of 15 ± 3 μT was obtained using Van Zijl experiments. Preheating was used to chemically stabilize seven samples used in Van Zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 ± 2 μT. A single basaltic komatiite sample gave a nearly ideal Van Zijl plot indicating about 21 μT, nearly the same paleointensity as the peridotitic komatiite samples even though its NRM intensity was several orders of magnitude lower. Since the Komati characteristic remanence was acquired during a slow cooling, the data must be reduced by a factor of 1.55 to account for the difference between laboratory and natural cooling rates. Calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep Komati characteristic remanence then gives value of 5 μT for the intensity of the geomagnetic field at 3.5 Ga, lower than the present value of about 30 μT.

Evidence for an Early Archean Geomagnetic Field: A paleomagnetic study of the Komati Formation, Barberton Greenstone Belt, South Africa

3.5 Ga old peridotitic komatiites of the Komati formation, Barberton greenstone belt, South Africa have a characteristic NRM (natural remanent magnetization) that is stable during alternating field and thermal demagnetization. The thermal nature of this remanence is demonstrated by successful Thellier‐type paleo‐intensity determinations on some samples in the temperature range 350 to 575°C. The characteristic NRM is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470±20 Ma. The mean direction of magnetization after cleaning is D=326° I=81° (k=60, α95=8°, N= 7 sites), with a paleopole at 11°S, 21°E (δp=δm=15°). The paleopole falls in southern Africa; ancestral southern Africa was therefore in polar latitudes around 3.5 Ga, the same position it occupied in the late Archean and early Proterozoic. Paleofield intensities are about one‐half present‐day intensities for polar latitudes. A vigorous earth's magnetic field was already in existence in the early Archean, implying that the core had a suitable size, composition and convective flow to support dynamo action.

Geochemistry of some ultramafic komatiite lava flows from the Barberton Mountain Land, South Africa

Thin ultramafic flows from the Komati Formation in the Barberton Greenstone Belt show striking textural variations similar to those found in some ultramafic flows from Munro Township, Ontario and Western Australia. The spinifex textured A2 zone has been divided into an A2r zone consisting of short randomly oriented olivine blades with vesicles in the interblade areas and an A2p zone made up of booklets of olivine blades forming a ‘thermotactic’ spinifex texture. The composition of the A2r zone is considered to be representative of the initial ultramafic liquid, while the variation in the composition of the A2p zone reflects the evolving residual liquid composition as the flow crystallized. Detailed modelling of major and trace element data obtained from a 3.1 m thick flow demonstrates that olivine fractionation and accumulation after extrusion probably generated the observed compositional range (25.7–31.7 wt.% MgO). The Ni content of olivine cores analysed from the B zone of this flow are consistent with these crystals having existed in equilibrium with a liquid composition represented by the random spinifex textured A2r zone. A DCr (Cr % in olivine/Cr % in ‘liquid’) of 0.5 found for these olivine crystals suggests that the high Cr content of olivines from Archaean ultramafic lavas is due to the very high Cr content of the liquids from which they crystallized. The textural and chemical data are combined to develop a cooling and crystallization model for this flow.

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.

Reassessment of part of the Barberton type area, South Africa

New mapping and chemical data are presented for parts of the type area for the Onverwacht Group in the Barberton greenstone belt. The structure and stratigraphy may be more complex than in the scheme suggested for this area on which the “Barberton Model” has been based (Viljoen and Viljoen, 1969a). A 1-km-thick sequence of basaltic komatiite has been identified above the Middle Marker in the Hooggenoeg Formation. The komatiites are identical to those in the upper part of the Komati Formation and it appears that the broad-scale alternation of komatiitic and tholeiitic basalts found in other greenstone terrains also occurs in the Barberton sequence. Genetic cyclicity and the significance of the Middle Marker appear to have been overemphasised in previous discussions of the development of the Hooggenoeg Formation. Two possible unconformities are discussed, together with the strike faulting previously recognised between the Theespruit and Komati Formations. Strike faulting may also occur elsewhere in the column. There is structural evidence that may suggest hitherto undocumented large-scale isoclinal folding in the Theespruit Formation and in the upper part of the Hooggenoeg Formation. The possibility that the Theespruit Formation deformation may predate Komati Formation deposition is discussed. There is a need for further detailed structural mapping and overall reassessment of parts of the Onverwacht Group, particularly where it is best developed in the southern part of the Barberton greenstone belt.

Major, minor, and trace element compositions of peridotitic and basaltic komatiites from the precambrian crust of Southern Africa

Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earth's crust (∼3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue.