Transcript Experimental melts

Partial melting of amphibolites
and the genesis of Archaean
TTG
(and some geodynamical implications)
Jean-François Moyen
and
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 & HREE
depletion
Les « gneiss gris »
Minéralogie
Eléments majeurs
REE
Conditions for making
TTGs
Experimental melts
KD
Gt/melt
Yb
= 10 - 20
(other minerals ≤ 1)
Melting of hydrous
basalt
In Garnet stability
field (Gt in residue)
Geodynamic
site ? Subduction
Gt-in
Gt-in
Intermediate cases:
• Shallow subduction
(± underplating)
• Stacked oceanic crust
Gt-in
Gt-in
Thick (oceanic or continental) crust
(e.g. Oceanic plateau)
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: fluidpresent
(- Qz)
Mineral stability limits
Control on amphibole
stability
Control on plagioclase
stability
Mineralogical models
KoB
ThB
AB
0
1
10
Plagioclase
25
40
54
Amphibole
75
59
36
Amp:Plag
CaO
3:1
11.0
3:2
10.0
2:3
9.0
Na2O
2.2
2.8
3.3
K 2O
0.1
0.5
1.0
TiO2
1.2
2.1
0.8
Int.
Ti-poor
Low Mg#
Si rich
Quartz
Amp. Comp.
Ti-rich
High Mg#
Si poor
Mineralogical models
KoB
ThB
AB
Composition of
experimental melts
Na2O contents in
experimental melts
Very unlikely for amphibolite melting!
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 % (9001100 °C)
Model for trace element
Arbitrary
Cl =
C0
F + D (1 - F)
Experimental data
D = S Kdi. Xi
Litterature
Trace elements contents of
the 3 sources
KoB
ThB
AB
Melt proportions
KoB
ThB
AB
Mineral proportions:
amphibole and plagioclase
KoB
ThB
AB
Mineral proportions: garnet
KoB
ThB
AB
KD
Gt/melt
Yb
= 10 - 20
Mineral proportions: rutile
KoB
ThB
AB
KD
KD
Rt/melt
Nb
Rt/melt
Ta
= 25 - 150
= 50 - 200
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 anomaly
and Nb/Ta
Sr
contents
Y & HREE
depletion
TTG composition as a depth
indicator (cont.)
Eu anomaly
HREE
depletion
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?
TTG genesis in P-T space
Subduction of old
lithosphere
Subduction of young
lithosphere
High Sr, La/Yb,
Nb/Ta
Tonalites
Low Y, Yb
Appropriate trace
elts. signature
&
trondhjemites
(F = 20-40 %)
High P TTGs
Low Sr, La/Yb, Nb/Ta
High Y, Yb
Low P TTGs
Not really TTGs
Archaean granulites (and intraplate geotherms)
A regional example
• Barberton, South Africa
• 3.5 to 3.2 greenstone belt and
gneisses
Crust accretion around BSB
3600-3500 Ma
Lower Onverwacht group
ca. 3500 Ma
Steynsdorp pluton
3509 ± 7 Ma
Ngwane gneisses
(Swaziland)
3490 ± 3 to 3644 ± 2 Ma
20 km
Dwalile Suite greenstone
remnants
Ca. 3500 Ma ?
Crust accretion around BSB
3450 Ma
Upper Onverwacht group
ca. 3400 Ma
Stolzburg, Theespruit, etc.
plutons
3443 ± 4 to 3460 ± 5 Ma
20 km
Tsawela gneisses
(Swaziland)
3458 ± 6 to 3437 ± 6 Ma
Crust accretion around BSB
3220 Ma
Fig Tree and Moodies groups
ca. 3200 Ma
Kaap Valley, Neelshoogte,
Badplaas, etc. plutons
ca. 3220 Ma
Dalmein pluton
Ca 3220 Ma
Usutu granodiorite
(Swaziland)
20 km
3231 ± 4 to 3216 ± 3 Ma
Geochemistry:
3600-3500 Ma
Steynsdorp pluton
Ngwane gneisses
Geochemistry:
3450 Ma event
Stolzburg & Theespruit plutons
Tsawela gneisses
Geochemistry:
3220 Ma event
Kaap Valley, Nelshoogte
& 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 ?
LateModern
Archaean
Early
Archaean
Geodynamical implications
Steepening/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 Gneisses
Ca. 3.1 Ga TTG gneisses in Messina area,
Limpopo Belt, South Africa
(R. White, Melbourne, for scale)