Archean Environment: the habitat of early life

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Geodynamic and Metabolic Cycles of the Young Earth. Abstracts EGU 2006

 

Paleoproterozoic marine anoxia post-dating the "Great Oxidation Event"

M. Bau (1), P. Dulski (2)

(1) International University Bremen, Geosciences & Astrophysics, PO Box 750561, 28725 Bremen, Germany, (2) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

Several lines of evidence from indicators such as mass-independent sulphur isotope fractionation , Fe-Ti relationships in paleosols, and sediment mineralogy, for example, suggest that atmospheric oxygen levels increased significantly between about 2.4 Ga and 2.2 Ga ago. This is often referred to as the “Great Oxidation Event”.We here focus on Rare Earths and Yttrium (REY) data from 2.6 to 2.2 Ga old iron- and manganese formation , limestone and dolomite from the Transvaal Supergroup, South Africa . The Transvaal Supergroup contains abundant marine chemical sediments that are devoid of detrital aluminosilicates, have experienced only little post-depositional overprint, and hence have recorded the chemical evolution of contemporaneous (shallow) seawater. Regardless of the specific lithology, all marine chemical sediments discussed here display the REY distribution pattern typical of the non-redox-sensitive REY in modern seawater, i.e. the y show enrichment of HREE over LREE, positive anomalies of La and Gd, and super- chondritic Y/Ho ratios. However, the size of the positive Eu anomaly that is indicative of the presence of a high-temperature hydrothermal component in Early Precambrian seawater decreases as the sediments get younger. We emphasize that even the iron- and manganese-formation in the Hotazel Formation does not show any positive Eu anomaly, suggesting that high-temperature hydrothermal REY input had become insignificant. The most interesting feature of the REY distribution is the abundance of Ce relative to its REE neighbours. Cerium anomalies result from the oxidation of Ce(III) to Ce(IV), the subsequent fixation of the latter in compounds with low solubility, and the eventual decoupling of REY(III) and Ce(IV). Hence, the presence of a Ce anomaly in a sediment is evidence for oxic conditions in the Earth’s (near) surface environment. The first chemical sediments in the Transvaal Supergroup that show Ce anomalies are the manganese-formation in the Hotazel Formation (note that the Hotazel iron-formation does not display Ce anomalies). In marked contrast, the slightly younger Mooidraai limestones and dolomites lack any Ce anomaly . Considering that the Mn-rich sediments from the Hotazel Formation show Ce anomalies (as expected from a Mn-oxide-rich sediment), the absence of Ce anomalies from the younger Mooidraai carbonates indicates the re-establishment of anoxic conditions in the Paleoproterozoic Mooidraai Sea after the postulated Great Oxidation Event. This suggests that the ultimate oxygenation of the Earth’s atmospherehydrosphere system in the Paleoproterozoic involved a transitional period characterized by transient oxygen-rich environments that became more and more abundant until fully oxic conditions were established in the Late Paleoproterozoic.

 

Archaean microbial evolution: The age of Rubisco

E. G. Nisbet, N.V. Grassineau
Dept. of Geology, Royal Holloway Univ. of London , Egham , TW20 0EX , UK
e.nisbet@gl.rhul.ac.uk

Archaean microbial evolution can be mapped out by its isotopic consequences. Rubisco ( ribulose bisphosphate carboxylase/oxygenase) governs most biological extraction of carbon from air and hence rubisco controls are important in determining the isotopic partitioning of carbon and the atmospheric ratio of carbon dioxide to oxygen. The early record is poor. Isotopic signatures in reduced carbon in rocks from Barberton South Africa (3.5 Ga) and Isua, Greenland , (3.8 Ga) imply the presence of organisms using Rubisco II and Rubisco-like protein (RLP). These organisms probably included sulphur-processing bacteria and methanogenic archaea.

By mid-late Archaean the record is better. Carbon and sulphur isotopes from rocks in Steep Rock Ontario imply that fractionation by Rubisco I is at least 3.0 Ga old. This strongly suggests that cyanobacteria were present. By 2.7 Ga ago, the extremely wellpreserved rocks of the Manjeri and Cheshire Formations, Belingwe belt, Zimbabwe , show that a complex and sophisticated microbial biosphere existed.

Major later branches in Rubisco phylogeny may record Proterozoic snowball events. After the snowball 2. 3 Ga ago Rubisco compensation controls may have maintained the strong atmospheric disproportion between O 2 and CO 2 .

 

Rare earth elements and Nd isotope systematics in 2.9 Ga -old chemical sediments from the Kaapvaal Craton, South Africa : constraining solute sources in Archean seawater

B. Alexander (1) , M. Bau (1), P. Andersson (2)

(1) International University Bremen , Bremen , Germany , (2) Swedish Museum for Natural History, Stockholm , Sweden

The Pongola Supergroup and the Pietersburg Group are contemporaneous marine sediment packages which formed on the margin of the Kaapvaal craton _ 2.9 Ga ago. Siliclastic lithologies dominate the Pongola sequence, yet marine chemical sediments such as banded iron formations ( IFs) are present, and along with the contemporaneousWitwatersrand Supergroup , these iron formations represent the oldest documented Lake-Superior type IFs. Deposition of the Pongola IFs should therefore contrast with the production of iron formation within the Pietersburg Group, which constitutes the Pietersburg Greenstone Belt (PGB), and represents Algoma type IF depositional environments.

Both the Pongola IF and the Pietersburg IF were subjected to low grade greenschist facies regional metamorphism.

The Pongola IFs are pure chemical sediments, as indicated by very low concentrations of incompatible elements (e.g., Na, K, Rb, Zr, Cs, Hf, and Th). The marine origin of these samples is supported by super- chondritic Y/Ho ratios (average Y/Ho = 42), and shale-normalized rare earths and yttrium distributions (REY SN ) in the Pongola IFs exhibit positive La SN,Gd SN, and Y SNanomalies, which are consistent with typical marine waters throughout the Archean and Proterozoic. Associated Pongola shales are generally geochemically evolved for Mesoarchean fine-grained clastic sediments, and have been interpreted as having provenances within 3.2-3.0 Ga granites which were similar in composition to Phanerozoic igneous intrusives.

The Pietersburg IFs display incompatible element concentrations that likely reflect varying proportions of clastic detritus, with Zr between 4.3 – 60ppm and Th between 0.13 – 1.2 ppm. The purest Pietersburg samples (i.e., low amounts of Zr, Hf, Th, etc.) possess shale- normalizedpositive La SN,Gd SN, and Y SNanomalies indicative of marine chemical sediments, and exhibit REY distributions that are similar to older Isua and younger Kuruman IFs. The Pongola IFs are generally depleted in heavy rare earth elements (HREE) and exhibit Sm/Yb _ 2.0 compared to the Pietersburg (1.0), Isua (0.9), and Kuruman (0.7) IFs. The similarities between the REY distributions of the Pietersburg and the Isua and Kuruman IFs, when compared to the Pongola IFs, suggests that Archean seawater influencing depositional environments on the Kaapvaal craton 3.0Ga was heterogeneous with respect to trace element composition. Such heterogeneity affecting coeval IF deposition on the Kaapvaal craton 2.9Ga could result from variability in solute sources and/or differences in depositional environment (e.g., Superior vs. Algoma type IFs).

Neodymium isotope systematics are used to potentially identify sediment and solute sources within the shales and IFs of the Pongola Supergroup. The " Nd ( 2.9 Ga) for the Pongola shales ranges from –2.7 to –4.2, whereas " Nd (2.9 Ga) values for the IFs bracket the shale values (range –1.9 to –10.9). The similarity in " Nd (t) values for the shales and IF samples suggests that mantle-derived Nd was not a significant source of REE within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples (and some shale samples) indicates that high- T hydrothermal input did contribute to their REE distributions. The above observations suggest that continentally derived sources for solutes dominated the major and trace element budgets of shallow marine chemical sediments during deposition of the Pongola Supergroup .

 

Modern stromatolites from Lagoa Vermelha, Brazil: An analogue for benthos microbial associations on the early Earth

Crisogono Vasconcelos (1), Pieter T. Visscher(2), Rolf J. Warthmann(1) and Judith A. McKenzie( 1)

(1) Geological Institute, ETH Zentrum, CH 8092 Zurich , Switzerland (2) Department of Marine Sciences, UConn, Groton , CT 06340

Stromatolites are laminated organo-sedimentary structures shaped by microbial activity. These laminated structures represent possibly the oldest record of life and provide the main source of information on the evolution of the early benthos, which has inhabited the shallow seafloor since the Archean. Recent studies conducted on modern Bahamian stromatolites have shown that lamination structures are a product of symbiotic processes in well-developed microcosms. Here, we report on the first recognized occurrence of microbiolite stromatolitic structures, associated with Ca-dolomite, growing in a hypersaline coastal lagoon, Lagoa Vermelha, near Rio de Janeiro , Brazil . Mineralogical and geochemical characteristics of these living stromatolites indicate the presence of a diverse symbiotic association resulting in calicification. The balance between precipitation and dissolution is controlled by biogeochemical gradients within the uppermost living microbial mat, as well as the environmental conditions, such as high salinity.

SEM and geomicrobiological studies provide information about the biomineralization processes and associated metabolic mechanisms, such as photosynthesis, aerobic respiration, sulfate reduction, methanogenesis, sulfide oxidation and fermentation. Despite the high diversity detected in the microbial mat, the stable carbon isotope data measured in the biominerals only reflect contributions from photosynthesis and sulfate reduction . These modern examples reveal the existence of very complex microcosms involved in the formation of the biomineral laminations leading to lithification. These communities may be representative of early benthos associations involved in Archean stromatolite formation.

 

The antiquity of autotrophic metabolism and the biogeochemical carbon cycle: evidence from sedimentary carbon isotopes from the 3.52 Ga Coonterunah Group, Australia

R. Buick, J.P. Harnmeijer

Department of Earth & Space Sciences and Astrobiology Program, University of Washington , Seattle , Washington , USA (buick@ess.washington.edu)

The 3.52 Ga old Coonterunah Group of northwestern Australia contains the oldest known low-grade metasedimentary rocks. Thus, it can potentially yield the most ancient, minimally modified, carbon isotopic data pertinent to the early evolution of life . Laminated sedimentary carbonates have _ 13 C carb values of -1 %¸ to -4 %¸ (mean = - 2.5 % ¸) with _ 18 O PDB values of -7 %¸ to -20 %¸ (mean = -16.2 %¸) . Kerogenous magnetitic cherts have a wide range of _ 13 C org values from -5 %¸ to -47 %¸ (mean = - 24.0 % ¸) , but most values lie close to a major mode of -25.4 %¸ . The mean isotopic difference between bulk reduced (major mode) and oxidized carbon species ( _ 13 C) is thus 22.9 %¸ , very similar to that within individual carbonate samples (mean = 23.4 % ¸) . The slight depletion in _ 13 C carb resembles that in other Archean sedimentary carbonates associated with banded iron formations and probably results from precipitation in a stratified ocean where deeper environments derive some dissolved inorganic carbon from remineralized organic debris. The spread in _ 13 C org values probably partly results from incipient metamorphic resetting, as these rocks were subjected to mid-greenschist to lowermost- amphibolite facies conditions at which isotopic re-equilibration starts to occur. After accounting for these effects, _ 13 C falls in the range typical of biological autotrophic fractionation and is dissimilar to any plausible abiotic processes. As these are the oldest rocks from which a sedimentary _ 13 C can be obtained, the results show that autotrophic, presumably photosynthetic, life was already well established in marine environments in the first billion years of Earth history.

 

Xe isotopes in Hadean zircons: constraining the Pu/U ratio of the early Earth

A. Busfield (1), S. Crowther (1), J. Gilmour (1), G. Turner (1), S. Mojzsis (2), M. Harrison (3)

(1) SEAES, University of Manchester , Oxford Road, Manchester , M13 9PL , UK (2) Dept. Geological Sciences, University of Colorado, Boulder, CO 80309-0399, US (3) RSES, Australian National University , Canberra , ACT 0200, Australia (a.busfield@manchester.ac.uk)

Models of the origin and evolution of Earth, volatile transport in the mantle and development of the atmosphere require knowledge of noble gas compositions in the past.

One important parameter for determining this is the initial Pu/U ratio. 244 Pu (t 1 / 2 = 82 Ma) is now effectively extinct in the solar system. Both 244 Pu and 238 U decay through spontaneous fission to 131 , 132 , 134 , 136 Xe. Modelling of the evolution of fissiogenic Xe ratios with time shows that plutogenic Xe will dominate at timescales > 4.4 Ga, and uranogenic Xe from 3.8 Ga to the present. In theory the Pu/U ratio can be directly determined from measurement of xenon isotopes in samples from this era.

Detrital Hadean Jack Hills zircons have Pb-Pb ages up to 4.4 Ga and provide a potential window to the geochemistry of the very early Earth. Measurement of Xe isotopes in these minute zircons requires a very sensitive technique and we have recently shown evidence for in situ decay of 244 Pu in individual zircons (1). Variations in the inferred ( Pu/U ) 0 of specific grains are expected to occur either as a result of degassing of Xe, or igneous fractionation of Pu and U. We have neutron-irradiated a suite of zircons, allowing the simultaneous determination of the apparent Pu/U ratio and U- Xe age of each sample. Quantification of any degassing event experienced by the zircon is therefore possible . We show that the apparent ( Pu/U ) 0 varies up to a maximum of 0.011 in grains with concordant Pb-Pb ages, but that some have suffered significant Xe loss.

Igneous fractionation may have played a role in the recorded Pu/U value of specific grains and we intend to investigate this by examining the REE patterns of each grain.

References: (1) Turner et al. (2004) Science, 306, 89-91.

 

The Habitat of Early Life: Solar X-ray and UV Radiation at Earth’s Surface 4-3.5 Billion Years Ago

I. Cnossen (1,3), F. Favata (1), J. Sanz-Forcada (1), O. Witasse (1), T. Zegers (1,2)

(1) ESA-ESTEC, Noordwijk, The Netherlands (2) Utrecht University , The Netherlands (3) Now at the Dept. of Physics and Astronomy, University of Leicester , United Kingdom (icnossen@yahoo.com / Fax: +44(0)116 252 3555)

The conditions prevailing on Earth at the time that life formed must have played a crucial role in the processes that led to the origin of life during the Archean. The solar X-ray and UV radiation (0.1-320 nm) received at Earth’s surface was an important aspect , and depends on two main variables: the radiation emitted by the young Sun and the absorption by Earth’s early atmosphere. The spectrum emitted by the Sun when life formed, between 4 and 3.5 Ga, was modeled here using a solar-like star that has the same age now as the Sun had 4-3.5 Ga. Atmospheric absorption was calculated with the Beer-Lambert law, assuming several density profiles for the atmosphere of the Archean Earth. It was found that almost all radiation with a wavelength shorter than 200 nm is absorbed effectively in all cases, even by very tenuous atmospheres.

Longer wavelength radiation is progressively less well absorbed, and its absorption is more sensitive to atmospheric composition. Minor atmospheric components, such as methane , ozone, water vapor, etc., have only negligible effects, but changes in CO 2 concentration can cause large differences in surface flux. In all cases the amount of radiation in the 200-300 nm wavelength range reaching the surface of the Archean Earth is several orders of magnitude larger than what is received at present. This means that any form of life that might have been present at Earth’s surface 4-3.5 Ga, must have been exposed to much higher levels of damaging radiation than at present.

 

Geodynamics and the Archean carbon cycle

S. Franck , C. Bounama and W. von Bloh

Potsdam Institute for Climate Impact Research, ( franck@pik-potsdam.de)

We present a minimal model for the global carbon cycle of the Earth containing the reservoirs mantle, ocean floor, continental crust, continental biosphere, the kerogen, as well as the aggregated reservoir ocean and atmosphere. This model is coupled to a parameterised mantle convection model for describing the thermal and degassing history of the Earth. We take into account lower continental area and higher heat flow and spreading rates in the past. In this study the evolution of the mean global surface temperature , the biomass, and reservoir sizes are investigated.We find that the parameterisation of the hydrothermal flux and the evolution of the ocean pH in the past has a strong influence on the atmospheric carbon reservoir and surface temperature. The different parameterisations give a rather hot as well as a freezing climate on the early Earth ( Hadean and early Archaean).

 

Micro-facies and origin of some Archaean Cherts (Pilbara, Australia)

B. Orberger1, V. Rouchon1, F. Westall, F.2, S. T. de Vries3, D. L. Pinti4; C. Wagner5, R. Wirth6, J.P. Gallien 7), K. Hashizume8

Laboratoire IDES, UMR 8148 (CNRS-UPS)f, Université Paris Sud XI, Bât. 504, 91405 Orsay Cedex, France, e-mail: orberger@geol.psud.fr; 2) Centre de Biophysique Moléculaire, CNRS, 45071 Orléans cedex 2, France ; 3) Faculty of Geosciences, Utrecht University , 3584 CD Utrecht , The Netherlands ; 4) GEOTOP-UQAM-McGill, Montréal , QC , Canada H3C 3P8 ; 5) Laboratoire de Pétrologie- PMMP, CNRS-UMR 7160, Université Paris VI, 75251 Paris cedex 05, France; 6) GeoForschungsZentrum Potsdam, Sektion 4.1 4, 14482 Potsdam, Germany ; 7) Laboratoire Pierre Süe, CEA-CNRS, Saclay , France ; 8) Department of Earth & Space Science, Graduate School of Science, Osaka University , Osaka 560-0043, Japan

Four chert samples (Marble Bar, Towers Formation; North Pole Dome, Dresser Formation; Apex Basalt, Apex Formation and Kittys Gap, Panorama Formation) were studied for micro- and nano-mineralogy and geochemistry to determine their protoliths and to provide insights into the physico-chemical and biological conditions of their depositional environments. The Marble Bar chert was formed at the interface with a basaltic rock by hydrothermal fluids, having leached major and trace elements from the basalt and silicified the protolith under reducing conditions. Stromatolite like micro iron oxide-bands are composed of magnetite and carbonates and co-precipitated REE, Cr, Ni, Pd and Au under reducing and alkaline conditions. Relict fluids precipitated quartz intergrown with K-feldspars. Later oxidizing fluids replaced the precursor minerals by Fe- Mn oxyhydroxides, partly of vermicular micro-textures. At nanoscale, filaments composed of hematite host carbon nodules, revealing several thousands of ppm of N and C, and C/N ratios similar to those observed in organic matter from marine sediments . The two black cherts from Dresser and Apex formations, sampled from hydrothermal dykes, had a black shale precursor, as suggested by their REE, trace metal characteristics and organic matter presence. They were entrained into the dykes by silicifying fluids. The North Pole chert from the chert-barite unit, was formed under reducing and alkaline conditions, indicated by clusters of bacteriomorphous nano-sulfide structures on precursor calcite, web-like Fe- sulfides intergrown with sphalerite, As-pyrite and vaesite. The Apex Basalt chert was formed under oxidizing conditions, as indicated by the same bacteriomorphous clusters, but composed of Fe-oxides and showing a negative Ce anomaly. The black and white laminated Kittys Gap chert originated in a shallow marine to subaerial environment under slightly oxidizing (Ti-oxides and negative Ce-anomaly) and acid conditions (K- phyllosilicates), mainly by diagenetic silicification of a rhyodacitic volcaniclastic rock. Microbial mats and colonies developed on the sediment surfaces around precursor-K-feldspars, Ti-bearing biotites and amphiboles. Elevated Cu and Zn contents in the black laminae point to a limited influence from hydrothermal fluids.

 

Extreme upper mantle depletion in the Archean: evidence from Hf-Nd isotope compositions of Neoarchean peridotites, N China craton

C. Münker (1,2), A. Polat (3), T. Kusky (4), J. Li (5)

(1) Mineralogisch-Petrologisches Institut, Universität Bonn, Germany, (2) Institut für Mineralogie , Universität Münster , Germany , (3) Dept. of Earth Sciences, University of Windsor, Windsor , Canada , (4) Dept. of Earth and Atmospheric Sciences, Saint Louis University , USA , (5) Dept. of Geology, Peking University , China ( muenker@uni-bonn.de / FAX: ++49 228 73 2763)

In conventional view, there is no clear evidence for Hadean crust-mantle depletion preserved on Earth. This view has been challenged by a recently reported spread of initial Hf isotope values in Hadean and early Archean zircons from western Australia . The spread in initial Hf isotope values suggests that early crust-mantle differentiation occurred (1) very early and (2) at a larger scale. So far, direct isotope evidence from Archean mantle rocks was scarce due to the lack of preserved ophiolite sequences. Here we present Hf-Nd isotope and high precision HFSE concentration data for ca. 2.55 Ga old peridotites from the Zunhua belt within the central orogenic belt of the North China craton. These peridotites include chromite and clinopyroxene bearing harzburgites and lherzolites, displaying whole-rock Mg numbers of 83-87. Together with LREE enrichment and negative Nb anomalies, these features indicate a suprasubduction origin of the Zunhua peridotites.

Initial epsilon Hf values for whole rock peridotite samples range from +7.9 to +10.4 and overlap to within 1 epsilon unit with values obtained from clinopyroxene separates. In Lu- Hf isochron space, the samples yield an age of 2528 ± 130 Ma (2 sigma). Initial epsilon Nd values obtained for whole rock and clinopyroxene separates display a larger scatter from -1.5 to +6.0. Together with trace element systematics, the observed decoupling of Nd from Hf isotope values suggests that significant amounts of the Nd was added by slab derived components, whereas the Hf isotope compositions reflect those of the pristine mantle wedge. At 2.55 Ga, the initial epsilon Hf values for the Zunhua peridotites plot well above the depleted mantle curve, suggesting an extremely depleted mantle composition. Even the most radiogenic epsilon Nd values lie above values predicted from currently used mantle growth curves, suggesting a depleted endmember in the sources of the subduction components. Altogether, the Hf-Nd isotope relationships indicate the presence of highly depleted mantle domains early in the Archean.

Measured Nb/Ta and Zr/Hf ratios in the Zunhua peridotites range from 11.6 to 15.6 and 30.6 to 36.3, respectively, overlapping with values for the present day silicate Earth (ca. 14 and 34, respectively). These observations indicate that the silicate Earth has not been depleted in Nb since the end of the Archean, confirming earlier models that post-Archean subduction processes had a negligible impact on the Nb-Ta budget of the Earth’s upper mantle.

 

Delineating subduction polarity based on different PT-evolutions of basement gneisses in the Meso-Archaean Barberton Mountain Land, South Africa.

A. Dziggel (1), A.F.M. Kisters (2)

(1) Institute of Mineralogy and Economic Geology, RWTH Aachen , Wüllnerstr. 2, 52062 Aachen , Germany , (2) Department of Geology, University of Stellenbosch , Private Bag X1, Matieland 7602, South Africa

The existence and modalities of plate-tectonic processes during the formation of the Early Earth is one of the central controversies in Archaean geology. The ca. 3500-3200 Ma Barberton greenstone belt and its surrounding granitoid-gneiss terrain in South Africa are one of the best studied Archaean terrains. Particularly the unique degree of preservation of the rocks affords us with an opportunity to study and semiquantitatively describe the geodynamic processes responsible for the formation of this Meso-Archaean continental nucleus. It is now widely accepted that the granitoid-greenstone terrain formed during the collision of a Northern and Southern terrane in an arc-trench setting at ca. 3230 Ma. Although structural styles and sedimentary environments seem to be in accordance with uniformitarian tectonic models, the location and geometry of the subduction zone remain elusive. Here, we present evidence, though circumstantial, that may be taken to indicate the presence of the subduction zone and the direction of subduction. On both sides of the belt, the sheared granite greenstone contacts are characterized by condensed metamorphic gradients. These marginal zones have been interpreted to represent an extensional detachment along which high-grade metamorphic basement gneisses were exhumed and juxtaposed against the low-grade supracrustal belt during the orogenic collapse of the overthickened orogen [1]. In basement gneisses of the southern terrane, the peak assemblages in greenstone enclaves record high-P, intermediate-T metamorphism of up to 650-700°C and 8-12 kbar, indicating burial of the continental rocks to depths of 35-45 km along a low apparent geothermal gradient of ca. 20 °C/km [2,3]. In contrast, basement rocks exposed along the northern margin of the greenstone belt experienced high-T, intermediate pressure metamorphism (ca. 600-700°C and 5 ± 1 kbar), implying a substantially higher geothermal gradient of close to 40°C/km. This northern margin is also characterized by voluminous synkinematic granitoid intrusions. Similar to basement rocks in the south, these mid- crustal rocks underwent a clockwise retrograde PT-evolution during one progressive exhumation event that is characterized by a near isothermal decompression to conditions of ca . 475-620 °C and 1-3 kbar. Both the presence of 3230 Ma old synkinematic granitoids in the Northern terrane, together with the higher geothermal gradients, probably as a result of advective heating of the gneisses by the granitoids , suggest that the northern margin of the greenstone belt represented the upper plate during north to north-west-directed subduction of the downgoing southern plate .

[1] Kisters, A.F.M., Stevens, G., Dziggel, A., Armstrong, R.A. (2003). Extensional detachment faulting at the base of the Barberton greenstone belt: evidence for a 3.2 Ga orogenic collapse. Precambrian Research, 127, 355-378.

[2] Dziggel, A., Stevens, G., Poujol, M., Anhaeusser, C.R., Armstrong, R.A. (2002). Metamorphism of the granite-greenstone terrane to the south of the Barberton greenstone belt , South Africa : an insight into the tectono-thermal evolution of the “lower” portions of the Onverwacht Group. Precambrian Research, 114, 221-247.

[3] Diener, J.F.A., Stevens, G., Kisters, A.F.M., & Poujol, M. (2005). Metamorphism and exhumation of the basal parts of the Barberton greenstone belt, South Africa : Constraining the rates of Mesoarchaean tectonism. Precambrian Research, 143, 87- 112.