Field methods in Archaean Field methods in Archaean geology: observations from geology: observations from

southern Africasouthern Africa

Axel HofmannAxel Hofmann

School of Geological Sciences, University of School of Geological Sciences, University of KwaZuluKwaZulu--Natal, Durban, South AfricaNatal, Durban, South Africa

Archaean litho-tectonic units

• granitoid-greenstone terrains• high-grade gneiss terrains• cratonic rift and cover successions

Valley et al. 2006

Geological map for southern Africa with main lithostratigraphic units

Kaapvaal Craton

Zimbabwe Craton

H. Jelsma

Archaean provincesArchaean provinces

•• granitoidgranitoid--greenstone terrainsgreenstone terrains Belingwe

Barberton

•• highhigh--grade gneiss grade gneiss terrainsterrains

Limpopo Belt

AncientGneissComplex

Archaean provincesArchaean provinces

•• cratonic rift and cratonic rift and cover successionscover successions

Witwatersrand/PongolaSupergroups

Archaean provincesArchaean provinces

Manjeri Formation

Cratonic rift and cover successionsCratonic rift and cover successions

• ≤ greenschist grade, relatively undeformed• unconformably overlie basement rocks• rift-related volcano-sedimentary

successions• epicontinental, passive-margin to foreland

basin sedimentary successions

Frimmel, 2005

Pongola SupergroupGold, 2007

Granite

Quartzarenite

Basalt

Schopf, 2006

Nsuze Group, White Umfolozi Inlier

subtidal

intertidal

supratidal

West Rand GroupBasement

NsuzeMozaan

LST

LST

TST

TST

NsuzeMozaan

LST

LST

TST

TST

CentralRand

WestRand

MozaanGroup

NzuseBeukes and Cairncross, 1991

Young et al., 1998

Granitoid-greenstone terrains• greenstone belts

wrapped around or intruded by granitoids

• typically at greenschist to amphibolite facies metamorphic grade

• characterised by a large percentage of submarine mafic volcanic rocks

Ridley et al. 1997

Palaeoarchaean GBs (e.g. Barberton GB)

Barberton GBKromberg Fm (3.35 Ga)

Neoarchaean GBse.g. Belingwe GB

Belingwe GBZeederbergs Fm (2.70 Ga)

Younging indicators

cher

t H3c

alter

ed b

asalt

basa

ltHooggenoeg Formation, 3.47 Ga

Barberton GB

Toggekry Formation, 3.53 Ga

Nondweni GB

Metamorphism• mainly greenschist to

amphibolite facies

• one or more regional metamorphic events

• local contact metamorphism

• seafloor alteration in pre-3.0 Ga belts

Wilkins, 1997

Structure

• Strata are steeply dipping and commonly tightly to isoclinally folded due to polyphase deformation. – granitoid doming/diapirism– folding and thrusting– transcurrent shearing

Jackson et al. 1987

Structure

C. Anhaeusser

S2

S3

Clemens et al. 2006

Campbell and Pitfield, 1994

Midlands GB

Granitoid composition

• multiphase, composite granitoid batholiths, plutons and stocks

• tonalitic to granitic in composition

• pre-, syn- or post-tectonic

van Kranendonk 2004

C. Anhaeusser

Granitoid composition

pre-tectonic syn-tectonic post-tectonic

C. Anhaeusser

pre-/ syn-tectonic pluton

C. Anhaeusser

post-tectonic pluton

Steynsdorp pluton, 3.51 Ga Theespruit Fm, 3.54 Ga

Granitoid-greenstone contact relationships

• intrusive contact• unconformable contact• tectonic contact

C. Lana

Greenstones and basement unconformities

Ridley et al. 1997

Bleeker 200?

Belingwe Greenstone Belt, ZimbabweYellowknife Supergroup, Canada

Belingwe GB

basement gneiss Manjeri Fm

Belingwe GB: Manjeri – Reliance contact

Kusky and Vearncombe 1997

Greenstones and thrusts

• correct identification and interpretation of thrust planes is fundamental for an understanding of the tectonic evolution of greenstone belts

Kusky and Vearncombe 1997

Stowe, 1968

Shurugwi greenstone belt, Zimbabwe

de Wit 2004

Terabayashi et al. 2003Hickman and van Kranendonk 2004

High-grade gneiss terrains

• granitoid gneisses intrusive into or tectonically interleaved with supracrustal rocks

• upper amphibolite- to granulite-facies metamorphic grade

• complex, polyphase deformation and high-grade metamorphism

High-grade gneiss terrains

• A geological history is inferred from establishing a record of depositional, structural and metamorphic events on the basis of various structural, mineralogical and cross-cutting relationships.

• Of importance is the correct identification of supracrustal gneisses and their protoliths. This may be tricky, as intensely deformed and metamorphosed metasomatised volcanic rocks may resemble sedimentary rocks.

Passchier et al. 1992

Nutman et al. 1984, in Nisbet, 1987 Myers, 2001

Protolith identification, Isua greenstone belt

pillow basalt

carbonate alteration in pillow basalt

calc-silicate gneiss

pillow basalt

pillow basalt, silicified

quartz-muscovite schist

carbonated lapillistone

ultramafic lapillistone

marble

komatiite

silicified komatiite

banded fuchsitic quartzite