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Greenstone Belt Studies
Members of our group have been involved in
Greenstone Belt studies over the last 30 years.
Some of this work has been highlighted in a
recent book on Greenstone Belts. |
Greenstone Belt
Maarten
J de Wit Department
of Geological Sciences Rondebosch,
7701, and Lewis
D Ashwal Department
of Geology CLAREDON
PRESS = 1997 |
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This
book comprises a collation of papers on Archean processes and objective
information on cratons and greenstone belts world wide,
by over 100 experts.
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Much of our own work focuses
on South African greenstone belts. In particular we have ongoing projects in
the |
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Komatii Formation
Recently Dr Jesse Dann (now at MIT) has remapped the circa 3.475
Ga Komatii Formation, the type sections for Komatiites discovered by Morris
and Richard Viljoen in the 1960s. Dann's map will be published at
1:5000 in the South African Journal of Geology, early 2000, and will also be
available in digital format. |
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This 5-year project has yielded a substantial number of new ideas
about the origin and evolution of the Komatii Formation and komatiites in
general; one of these imples that the Barberton komatiites may have been
derived from a hydrous magma (4-6% H2O); another suggests that
many of the komatiites in Barberton were intruded at the base of a ~8km thick
pile of tholeiitic lavas. (see section on Archean Mantle and
Komatiites). Much of this research is still ongoing and is spearheaded
by Tim Grove and Jesse Dann (both MIT). |
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Sections of the |
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The Hooggenoeg Formation and the Other work in progress takes a new look at the Hooggenoeg
Formation and the The focus of the new research is on the geometry and
architecture of the early sedimentary basins and the structural setting in
relation to syn-sedimentary deformation. The Early Archean basins of
the Duffer Subgroup in the Pilbara and of the Hooggenoeg Formation of the
Barberton greenstone belt in South Africa have been selected for this study
since they are of almost identical age (3.45- 3.47 Ga). It has recently
shown that this sequence may have been contiguous. (Tanja Zegers et al.,
: Vaalbara, Earth’s oldest
assembled continent? A
combined structural, geochronological and paleomagnetic test. Terra
Nova 10, 250-259), when the KaapVAAL and PilBARA cratons may have been part
of Earth’s oldest supercontinent, Vaalbara. (A term coined by Eric
Cheeney). Both sedimentologically and structurally, the Pilbara and
Kaapvaal Cratons show a high degree of similarity. However, until our
recent studies in the Hooggenoeg Formation, a basic difference between the
areas was the apparent absence of an extensional deformation phase in the |
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Basin analysis (Wouter
Nijman, Ponpe De Boer, Sjouke de Vries: Structural geology and tectonics ( Volcanology /petrology /geochemistry (Tim Grove and Jesse Dann: MIT; Van Bergen:
UU) Fluid inclusions (Cornel
De Ronde: Geol. Surv. New.Zealand) Shallow burial metamorphism (Kisch: Ben Gurion Univ. Israel) Geophysical crustal modeling (Arie Van den Berg and Vlaar: Theoret.
Geophysics UU) Basin modelling (Sjierd
Cloetingh: VU Amsterdam) Geochronology (Richard
Armstrong: ANU, Geol. Surv.WA, Paleomagnetism (Car
Langereis and Tanja Zegers: UU) Comparative Recent studies, Comparison with Archean basins in the Zimbabwean
greenstone belts (Jan Dirks and Jelsma:
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Earth’s earliest sedimentary
basins: Based
on the above preliminary results, we have clubbed together with Wouter Nijman
and some of his UU colleagues re-examine the early sedimentary sequence of
the Earth's Earliest Sedimentary Basins-
A
project proposal by Wouter Nijman ( |
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Early life studies, and hydrosphere-atmosphere-biosphere
interactions. Together
with Frances Westall, (paleobiology group Johnson Space Centre, NASA) we have
focused on some of the earliest cherts of the We
are continuing this work, with an aim to better catagorise the fossils and
their physical/chemical environments. One of the sites we are studying
in detail is the Buck Ridge Chert – here Sjoukje de Vries and Francis
Westall are working in close co-operation. Very
recently, we have found that the cherts and iron formation in this section of
the greenstone belt may have been subaerially exposed. Yet these
sections indicate early oxidising conditions. We are now investigating
the possible inter-relationships between cyanobacterial activity and these
(local?) oxidising conditions in greater detail. |
Archean paleomagnetism
As
part of testing continental-block movements in the Archean, and the possible
accretion of small lithospheric platelets to form the Kaapvaal craton (de
Wit, 1996, On Archean granites, greenstone and tectonics, Precambrian
Research, Vol.91), a pilot paleomagnetic study was started in 1996.
First measurements were made during a sabbatical of M de Wit at the
paleomagnetic lab of the One
interesting aspect of this work has been a confirmation of the early work of
Hale and Durlop, 1984) which suggests that the lower section of the Barberton
Greenstone belt was generated in a polar region. Recently we have
discovered drop-stone structures in the Buck Ridge Chert. Could these
be ices related? Could it be that there was already a cryosphere in the
Archean? These
and other interesting aspects of our greenstone belts studies will continue
to focus on reconstructing Earth’s surface conditions and processes as
for back in time as we can hope to go using directobservational
evidence. We would welcome exchange of ideas on these topics of our
studies. |
