partial melting of amphibolites and the genesis of archaean ttg (and some geodynamical implications)...
TRANSCRIPT
![Page 1: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/1.jpg)
Partial melting of amphibolites and the
genesis of Archaean TTG(and some geodynamical implications)
Jean-François Moyenand
Gary Stevens
Stellenbosch University, South Africa
![Page 2: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/2.jpg)
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
![Page 3: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/3.jpg)
Trace elements features of Archaean TTGs
Nb-Ta anomaly
Sr contents
Y & HREEdepletion
![Page 4: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/4.jpg)
![Page 5: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/5.jpg)
Les « gneiss gris »
![Page 6: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/6.jpg)
Minéralogie
![Page 7: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/7.jpg)
Eléments majeurs
![Page 8: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/8.jpg)
REE
![Page 9: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/9.jpg)
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
![Page 10: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/10.jpg)
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
![Page 11: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/11.jpg)
Partial melting of amphibolites
15-20 « modern » studies(1990-2000)
+ Phase diagrams (1970-80)
114 exp. fluid present or saturated
209 exp. « dehydration melting »
![Page 12: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/12.jpg)
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
![Page 13: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/13.jpg)
Review and compilation of published work
• Starting materials
• Solidus position & melt productivity
• Mineral stability fields
(Moyen & Stevens, subm. to AGU monographs)
![Page 14: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/14.jpg)
Starting materials
![Page 15: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/15.jpg)
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
![Page 16: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/16.jpg)
Fluid saturated
![Page 17: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/17.jpg)
Dehydration melting
![Page 18: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/18.jpg)
Fluid-present
![Page 19: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/19.jpg)
Experimental solidus position
![Page 20: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/20.jpg)
Melt productivity: dehydration melting
![Page 21: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/21.jpg)
Melt productivity: water saturated
(+ Qz)
![Page 22: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/22.jpg)
Melt productivity: fluid-present
(- Qz)
![Page 23: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/23.jpg)
Mineral stability limits
![Page 24: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/24.jpg)
Control on amphibole stability
![Page 25: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/25.jpg)
Control on plagioclase stability
![Page 26: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/26.jpg)
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
![Page 27: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/27.jpg)
Mineralogical models
KoBKoB ThBKoB ThB AB
![Page 28: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/28.jpg)
Composition of experimental melts
![Page 29: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/29.jpg)
Very unlikely for amphibolite melting!
Na2O contents in experimental melts
![Page 30: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/30.jpg)
K2O
![Page 31: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/31.jpg)
Major elemen
ts
A linear model, of the form
C/C0 = a F + b
![Page 32: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/32.jpg)
Modelled melts
![Page 33: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/33.jpg)
Model vs. TTGs
![Page 34: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/34.jpg)
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)
![Page 35: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/35.jpg)
Model for trace element
Cl = C0
F + D (1 - F)
Experimental data
D = Kdi. Xi
Arbitrary
Litterature
![Page 36: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/36.jpg)
KoBKoB ThBKoB ThB AB
Trace elements contents of the 3 sources
![Page 37: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/37.jpg)
Melt proportions
KoB ThB AB
![Page 38: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/38.jpg)
Mineral proportions: amphibole and plagioclase
KoB ThB AB
![Page 39: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/39.jpg)
Mineral proportions: garnet
KoB ThB AB
KD
Gt/melt= 10 - 20Yb
![Page 40: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/40.jpg)
Mineral proportions: rutile
KoB ThB AB
KD
Rt/melt= 25 - 150Nb
KD
Rt/melt= 50 - 200Ta
![Page 41: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/41.jpg)
REE contents in (modelled) melts
KoB ThB AB
![Page 42: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/42.jpg)
REE contents in (modelled) melts
KoB ThB AB
![Page 43: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/43.jpg)
REE contents: La/Yb
KoB ThB AB
![Page 44: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/44.jpg)
Y contents
KoB ThB AB
![Page 45: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/45.jpg)
Sr contents and the role of residual plagioclase
(Martin & Moyen, 2001, Geology 30 p 319-322; after Zamora, 2000)
![Page 46: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/46.jpg)
Sr/Y
KoB ThB AB
![Page 47: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/47.jpg)
Nb/Ta
KoB ThB AB
![Page 48: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/48.jpg)
Effect of pressure
![Page 49: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/49.jpg)
![Page 50: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/50.jpg)
![Page 51: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/51.jpg)
TTG composition as a depth indicator
Nb-Ta anomalyand Nb/Ta
Sr contents
Y & HREEdepletion
![Page 52: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/52.jpg)
TTG composition as a depth indicator (cont.)
HREEdepletion
Eu anomaly
![Page 53: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/53.jpg)
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?
![Page 54: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/54.jpg)
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
![Page 55: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/55.jpg)
A regional example
• Barberton, South Africa
• 3.5 to 3.2 greenstone belt and gneisses
![Page 56: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/56.jpg)
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 ?
![Page 57: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/57.jpg)
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
![Page 58: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/58.jpg)
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
![Page 59: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/59.jpg)
Geochemistry:3600-3500 Ma
Steynsdorp plutonSteynsdorp pluton
Ngwane gneissesNgwane gneisses
![Page 60: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/60.jpg)
Geochemistry:3450 Ma event
Stolzburg & Theespruit plutonsStolzburg & Theespruit plutons
Tsawela gneissesTsawela gneisses
![Page 61: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/61.jpg)
Geochemistry:3220 Ma event
Kaap Valley, Nelshoogte Kaap Valley, Nelshoogte & Badplaas plutons& Badplaas plutons
![Page 62: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/62.jpg)
TTG evolution around Barberton Greenstone Belt
3.6 – 3.4 Ga
3.4 – 3.2 Ga
![Page 63: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/63.jpg)
Amphibolites with HP relicts
![Page 64: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/64.jpg)
![Page 65: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/65.jpg)
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
![Page 66: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/66.jpg)
Secular/Geodynamical implications
Progressively cooler gradients ?
Early ArchaeanLate ArchaeanModern
![Page 67: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/67.jpg)
Geodynamical implicationsSteepening/onset of subduction ?
![Page 68: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/68.jpg)
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
![Page 69: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/69.jpg)
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
![Page 70: Partial melting of amphibolites and the genesis of Archaean TTG (and some geodynamical implications) Jean-François Moyen and Gary Stevens Stellenbosch](https://reader035.vdocument.in/reader035/viewer/2022062221/56649f575503460f94c7b65d/html5/thumbnails/70.jpg)
The Sand River GneissesCa. 3.1 Ga TTG gneisses in Messina area,Limpopo Belt, South Africa(R. White, Melbourne, for scale)