evolution of continental...
TRANSCRIPT
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