Evolution of continental lithosphere:Archean – Proterozoic transition at the

SW margin of the Kaapvaal Craton

David Bell1,2, Ute Weckmann3, Phil Janney2, Maarten de Wit1

1AEON, 2Arizona State University, 3GFZ

Outline• Continental evolution and the lithosphere• Kaapvaal Archean mantle lithosphere• Comparison of Archean and Proterozoic mantle

compositions• Implications for petrogenetic and tectonic formation

conditions of Proterozoic mantle• Seismic and thermal structure: Steady state vs.

dynamic interpretations• Kimberlite eruption ages and magmatic reactivation• New MT results• Comparisons with seismic structure, heat flow, and

possible modern analog tectonic setting

Age distribution of preserved continental crust

Large continental land masses emerged at ~3.0 –2.5 Ga

Cratonal sedimentary

Before ~ 3 Ga either not much continental crust was created, or most of it was destroyed

Condie, 2000

Chemical composition of continental crust changed

Cratonal sedimentary basins begin to develop

Episodic crustal growth and preservation

Condie, 2000

Continental mantle roots

Oceanic mantle Lithosphere

Oceanic mantle Lithosphere Subcontinental

lithospheric mantle root

Continental crust

MOHO

Oceanic crustOceanic crust

Oceanic mantle

Lithosphere

Oceanic mantle

Lithosphere

Mg# = 93LAB

Convecting mantle

Mg# = 89

Mg# = 93

The preservation of Archean continental crust appears to depend on the presence of lithospheric mantle roots

Thermal structure of continental lithosphere

Lithospheric architecture of southern Africa

Schmitz & Bowring 2003

Figure 2a: Jelsma et al., Preferential distribution....

kimberlites

~2.7

~1.8~1.0

Carlson et al., 2005

Age of continental mantle≈

Age of continental crust

Composition ofArchean SCLM

Griffin et al. 2003

Highly melt-depleted

Many samples are also Si-richfor their Mg content

Kaapvaal Craton mantle composition

90

92

94

96

9810

0Mg/

(Mg+

Fe)

Off craton xenolithsAbyssalKaapvaal low-TKimberley low-TWiedemannfjordNorway WGRIzu Bonin ForearcIwanai-dake

ol opx

Felsic melt or fluid addition

Melting with fluid

84

86

88

90

0.60.81.01.21.41.61.82.02.2

(Mg+Fe)/Si - atomic

100M

g/(M

g+F

e)

Iwanai-dakeBFT olivine-rich

melting

Melt addition at the base of Archean mantle

Mafic melt reaction:adds Fe, removes opx

Chemical and thermal structure of Archean continental lithosphere

Origin of Proterozoic mantle lithosphere

• Archean mantle has multiple metasomatic episodes

• Could Proterozoic mantle have formed by melt enrichment of Archean mantle (Le Roux et al. 2007; Griffin et al. 2008)

• Single or multiple stage history ?

• What was/were the tectonic setting(s) ?

Archean vs. Proterozoic

Mg#(=100Mg/[Mg+Fe])

Janney et al 2010

Proterozoic lithosphere compositions

Most samples appears to have various amounts of felsic melt reactionOnly a few have evidence of mafic melt reaction

KaapvaalSeismicStructure

Image courtesy ofM. Fouch

Kaapvaal Seismic StructureJames et al. 2001; Carlson et al. 2005

Os-isotope model ages of SA Proterozoic mantle lithosphere

Janney et al.2010

90

100

100M

g/(M

g+Fe

)Kimberlite clinopyroxene megacrystsMonastery

Orapa

Leicester

Kimberley

Jagersfontein

Pofaddder

80

25 30 35 40 45 50 55

100M

g/(M

g+Fe

)

100Ca/(Ca+Mg)

10

20

30

40

50

60

70

80

400 600 800 1000 1200 1400 1600

T (oC)- BKN90

P (k

bar)

- B

KN

90

Archean craton kimberlites

Proterozoic domain kimberlites

180 km

140 km

AverageMantleAdiabat

o

Steady state vs.

dynamic Interpretation ofxenolith PT data

10

20

30

40

50

60

70

80

400 600 800 1000 1200 1400 1600

T (oC)- BKN90

P (

kbar

) -

BK

N90

Archean craton kimberlites

Proterozoic domain kimberlites

AverageMantleAdiabat

0

50

100

150

200

250

300

-500 -300 -100 100 300 500km

Proterozoic Archean

0

50

100

150

200

250

300

-500 -300 -100 100 300 500

km

Proterozoic Archean

1000 Ma

300 Ma

Thermal evolutionof Proterozoic

lithosphere

0

50

100

150

200

250

300

-500 -300 -100 100 300 500

km

Proterozoic Archean

100 Ma

Off-craton peridotites - Gibeon

10

20

30

40

50

60

70

80

400 600 800 1000 1200 1400 1600

T (deg. C) - BKN90

P (

kbar

) -

BK

N90

Frank Smith Mine

Kimberley

Jagersfontein

Lesotho

Finsch

Gibeon (Namibia)

Namaqua kimberlites and orangeites: 150 - 180 Ma

0

10

20

30

40

50

60

70

80

400 600 800 1000 1200 1400 1600

T (deg. C) - BKN90

P (

kbar

) -

BK

N90

Kaapvaal Craton

East Griqualand (S of craton)

Melton Wold (SW of craton)

Namaqua (Karoo) kimberlites: 70-100 Ma

0

10

20

30

40

50

60

70

80

400 600 800 1000 1200 1400 1600

T (deg. C) - BKN90

P (k

bar)

- B

KN

90

Kaapvaal Craton

70-100 Ma Karoo kimberlites

150-180 Ma Karoo orangeite

East Griqualand (150 - 180 Ma)

T (deg. C) - BKN90

Longitude vs. AgeMesozoic kimberlites and related rocks

150

200

250 OrangeitesTransitional orangeitesKimberlites on cratonKimberlites off cratonMelilititesCarbonatites Karoo

Etendeka

Group II

Anomalous

0

50

100

10 20 30 40Longitude (E)

Age

(M

a)

EtendekaGroup I

Anomalous(off craton I)

AnomalousAges (I & II)

Archean –Proterozoic boundary

region

Schmitz & Bowring 2004

Tectonic boundariesfrom Schmitz and Bowring 2004

-30

-29

-28

MKT

SDF

KWAWBP

JW

MW

Hol

Kheis

KaapvaalCraton

Surface-wave velocity

-32

-31

20 21 22 23 24

Other kimberlites

Phillipstown 1

Hanover 1

Britstown 1

Carnarvon

Witberg Gp 1

Prieska-Vosburg 2

Meltonwold 2

MtP

Aar

SIH

DFT

MWD

GSF

SKP

AKS

BeyPam

WKL

HebRJA

Lus

Cor

KPT

Swp

MKT

Uin

Bot

Bbg

Lov

Surface-wave velocitymodel for 100 km depthfromChevrot and Zhao 2006

-8% < Vs < +8%

Heat flow in Archean and Proterozoic lithosphere

Nyblade and Pollack 1993

A possible modern analog tectonic setting

for the Kaapvaal –Namaqua transition

Fichtner, De Wit& Van Bergen2010

Conclusions• Proterozoic mantle lithosphere in southern Africa has a multistage origin

beginning in Palaeoproterozoic• Compositional trends are similar to Archean lithosphere suggesting

subduction zone origins or modification, with no strong evidence for conversion of Archean mantle by mafic melt percolation

• Thrusting of younger crust over Archean basement consistent with Precambrian continental heat flow observations in several locations worldwide.

• Similar to modern subduction of Australian continent beneath Banda Arc.• Similar to modern subduction of Australian continent beneath Banda Arc.• Kimberlite eruption patterns perturbed near A-P boundary faults,

suggesting preferential ascent in these regions• Conductive zones may reflect metasomatic fluid/kimberlite ascent:

evidence for highly evolved compositions in these regions, rich in hydrous minerals and oxides

• More pervasive volatile-rich melt percolation in true “off craton” mantle could contribute to overall lower resistivity of Namaqua mantle

• Consistent with regional westward-migrating heating pulse in lower crust and mantle lithosphere in early Cretaceous