partial melting of amphibolites and the genesis of archaean ttg (and some geodynamical implications)...
TRANSCRIPT
Partial melting of amphibolites and the
genesis of Archaean TTG(and some geodynamical implications)
Jean-François Moyenand
Gary Stevens
Stellenbosch University, South Africa
TTG are...• Orthogneisses• Tonalites, Trondhjemites & Granodiorites
(Na-rich series)• Fractionnated REE, etc.• Largely homogeneous throughout the
Archaean• Originated by partial melting of amphibolites
(hydrated basalts), in garnet stability field
Trace elements features of Archaean TTGs
Nb-Ta anomaly
Sr contents
Y & HREEdepletion
Les « gneiss gris »
Minéralogie
Eléments majeurs
REE
Conditions for making TTGs
Experimental melts
In Garnet stability field (Gt in residue)
Melting of hydrous basalt
KD
Gt/melt= 10 - 20
(other minerals ≤ 1)
Yb
Geodynamic site ?
Thick (oceanic or continental) crust(e.g. Oceanic plateau)
Subduction
Intermediate cases:• Shallow subduction
(± underplating)• Stacked oceanic crust
Gt-in
Gt-in
Gt-in
Gt-in
Gt-in
Partial melting of amphibolites
15-20 « modern » studies(1990-2000)
+ Phase diagrams (1970-80)
114 exp. fluid present or saturated
209 exp. « dehydration melting »
Goal of the study
• Review and compilation of published data on experimental melting
• Elaboration of a global model for amphibolite melting
• Implications for trace element contents
• Geological/geodynamical consequences
Review and compilation of published work
• Starting materials
• Solidus position & melt productivity
• Mineral stability fields
(Moyen & Stevens, subm. to AGU monographs)
Starting materials
Fluids and melting
• Fluid-saturated (free fluid phase)
• Fluid-present (yielded by breakdown of hydrous minerals in the near sub-solidus), limited availability
• Fluid-absent (dehydration melting)
• Dry
Fluid saturated
Dehydration melting
Fluid-present
Experimental solidus position
Melt productivity: dehydration melting
Melt productivity: water saturated
(+ Qz)
Melt productivity: fluid-present
(- Qz)
Mineral stability limits
Control on amphibole stability
Control on plagioclase stability
Mineralogical models
CaO 11.0 10.0 9.0
Na2O 2.2 2.8 3.3
K2O 0.1 0.5 1.0
TiO2 1.2 2.1 0.8
Amp. Comp.Ti-rich
High Mg#
Si poor
Int.
Ti-poor
Low Mg#
Si rich
KoB ThB AB
Quartz 0 1 10
Plagioclase 25 40 54
Amphibole 75 59 36
Amp:Plag 3:1 3:2 2:3
Mineralogical models
KoBKoB ThBKoB ThB AB
Composition of experimental melts
Very unlikely for amphibolite melting!
Na2O contents in experimental melts
K2O
Major elemen
ts
A linear model, of the form
C/C0 = a F + b
Modelled melts
Model vs. TTGs
Preliminary conclusions (1)
• K2O content depends on the source. Only relatively K-poor sources (< 0.7 %) make TTGs … but really depleted sources won’t.
• This means that K-rich amphibolites can indeed melt into granites (Sisson et al., 2005)
• With appropriate sources, tonalites & trondjhemites occur for F = 20-40 % (900-1100 °C)
Model for trace element
Cl = C0
F + D (1 - F)
Experimental data
D = Kdi. Xi
Arbitrary
Litterature
KoBKoB ThBKoB ThB AB
Trace elements contents of the 3 sources
Melt proportions
KoB ThB AB
Mineral proportions: amphibole and plagioclase
KoB ThB AB
Mineral proportions: garnet
KoB ThB AB
KD
Gt/melt= 10 - 20Yb
Mineral proportions: rutile
KoB ThB AB
KD
Rt/melt= 25 - 150Nb
KD
Rt/melt= 50 - 200Ta
REE contents in (modelled) melts
KoB ThB AB
REE contents in (modelled) melts
KoB ThB AB
REE contents: La/Yb
KoB ThB AB
Y contents
KoB ThB AB
Sr contents and the role of residual plagioclase
(Martin & Moyen, 2001, Geology 30 p 319-322; after Zamora, 2000)
Sr/Y
KoB ThB AB
Nb/Ta
KoB ThB AB
Effect of pressure
TTG composition as a depth indicator
Nb-Ta anomalyand Nb/Ta
Sr contents
Y & HREEdepletion
TTG composition as a depth indicator (cont.)
HREEdepletion
Eu anomaly
Preliminary conclusions (2)
• Appropriate depletion in Y, Yb, etc. requires pressures above ca. 15 kbar (rather than 10 kbar = Gt-in)
• Y, Yb, Sr/Y, Nb/Ta etc. are indicators of melting depth
• Low- and high-pressure TTGs with contrasted signatures?
High P TTGs
Low P TTGs
Not really TTGs
Archaean granulites (and intraplate geotherms)
Subduction of younglithosphere
(5 M
a)
(20
Ma)
(50
Ma)
Subduction of oldlithosphere
Tonalites & trondhjemites
(F = 20-40 %)Appropriate trace elts. signature
High Sr, La/Yb, Nb/TaLow Y, Yb
Low Sr, La/Yb, Nb/TaHigh Y, Yb
TTG genesis in P-T space
A regional example
• Barberton, South Africa
• 3.5 to 3.2 greenstone belt and gneisses
Swaziland
R.S.A
.
20 km
Crust accretion around BSB3600-3500 Ma
Steynsdorp pluton
3509 ± 7 Ma
Ngwane gneisses (Swaziland)
3490 ± 3 to 3644 ± 2 Ma
Lower Onverwacht groupca. 3500 Ma
Dwalile Suite greenstone remnantsCa. 3500 Ma ?
Swaziland
R.S.A
.
20 km
Crust accretion around BSB3450 Ma
Stolzburg, Theespruit, etc. plutons
3443 ± 4 to 3460 ± 5 Ma
Tsawela gneisses (Swaziland)
3458 ± 6 to 3437 ± 6 Ma
Upper Onverwacht groupca. 3400 Ma
Swaziland
R.S.A
.
20 km
Crust accretion around BSB3220 Ma
Kaap Valley, Neelshoogte, Badplaas, etc. plutons
ca. 3220 Ma
Usutu granodiorite (Swaziland)
3231 ± 4 to 3216 ± 3 Ma
Fig Tree and Moodies groupsca. 3200 Ma
Dalmein plutonCa 3220 Ma
Geochemistry:3600-3500 Ma
Steynsdorp plutonSteynsdorp pluton
Ngwane gneissesNgwane gneisses
Geochemistry:3450 Ma event
Stolzburg & Theespruit plutonsStolzburg & Theespruit plutons
Tsawela gneissesTsawela gneisses
Geochemistry:3220 Ma event
Kaap Valley, Nelshoogte Kaap Valley, Nelshoogte & Badplaas plutons& Badplaas plutons
TTG evolution around Barberton Greenstone Belt
3.6 – 3.4 Ga
3.4 – 3.2 Ga
Amphibolites with HP relicts
Preliminary conclusions (3)
• TTGs in Barberton record progressively deeper sources
• This is consistent with progressive steepening or onset of subduction, and could witness the progressive accretion of a continental nucleus and its early growth
• At 3.2 Ga (true subduction established), the geothermal gradient recorded in some metamorphic rocks is consistent with the gradient corresponding to TTG genesis
Secular/Geodynamical implications
Progressively cooler gradients ?
Early ArchaeanLate ArchaeanModern
Geodynamical implicationsSteepening/onset of subduction ?
Preliminary conclusions (4)
• Secular chemical evolution of TTGs reflects increasing melting depth and increasing interactions with the mantle
• This is consistent with a subduction origin for TTGs
• Secular cooling of the Earth makes the melting deeper and deeper along the subducted slab, allowing more and more interactions with the mantle
• Alternately, this could witness progressive onset of subduction
Conclusions
• TTGs are diverse, and their chemistry reflects the depth of melting; melting occurred mostly at 15-20 kbar, but can have occurred anywhere between 10-12 and 30 kbar.
• Most TTGs are probably originated in subductions, and interacted with the mantle to some degree
• The changes in TTG compositions can probably be correlated with changes in tectonic styles –either in terms of secular evolution, or in one single area
The Sand River GneissesCa. 3.1 Ga TTG gneisses in Messina area,Limpopo Belt, South Africa(R. White, Melbourne, for scale)