Using trace elements to define the PT conditions of TTG genesis.
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Transcript of Using trace elements to define the PT conditions of TTG genesis.
Using trace elements to define the PT conditions of TTG
genesis.
Jean-François MoyenGary Stevens
University of StellenboschSouth Africa
Key ideas
• At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions
• P-T conditions of amphibolites melting exert a strong control on the major and trace elements composition of TTG liquids
• “True” TTG signature requires high pressure (15-20 kbar)
• However, both low and high pressure TTGs can be identified
1 If there is fractionnation, the same logics could also apply using the most primitive liquid – it requires a little more work to determine its composition
TTG are...• Orthogneisses
• Tonalites, Trondhjemites & Granodiorites
(Na-rich series)
• Fractionnated REE, high La/Yb and Sr/Y, etc.
• Largely homogeneous throughout the Archaean
Stolzburg pluton (Barberton, South Africa -- 3.45 Ga)
Sand River GneissesCa. 3.1 Ga TTG gneisses in Messina area,Limpopo Belt, South Africa
TTG “gneisses” or “plutons” ?
It is difficult to constrain good petrogenetical studies on complex, multiply injected and molten, possibly tectonic assemblages!
Conditions for making TTGs
In Garnet stability field (Gt in residue)
Melting of hydrous basalt
Gt-inGt-in
Little or nodirect tectonic meaning!
Using published experimental data
• Review and compilation of published data on experimental melting( > 320 runs described in 15-
20 studies)
• Building of a global model for amphibolite melting
• Implications for trace element contents
(Moyen & Stevens, AGU monographs 164 pp.149-175)
Melt composition
Compilation of experimental data
Interpolated “maps” of modal composition
“Maps” of trace elements composition
Cl = C0
F + D (1 - F)
Sr/Y
NB- 1.Similar maps can be established for all elements or ratios, e.g. La/Yb2. Actually different models were built for dirrerent types of amphibolites
Effect of pressure
TTG composition as a depth indicator
Nb-Ta anomalyand Nb/Ta
Sr contents
Y & HREEdepletion
HREEdepletion
Eu anomaly
• 900-1000 °C = trondhjemite and tonalite composition
• 15-20 kbar = proper trace elements signature
• Typical geotherm 15-20 °C/km(Comparable to Barberton HP amphibolites)
TTGs and TTGs?
Stolzburg (3.44 Ga)
Steynsdorp (3.56 Ga)
Theespruit (3.44 Ga)
Kenogamissi (2.74-2.71 Ga)
Stolzburg (3.44 Ga)
Steynsdorp (3.56 Ga)
Theespruit (3.44 Ga)
Kenogamissi (2.74-2.71 Ga)
SubductionLow Y Yb etc. trondhjemites
Intraplate (plume/orogenic collapse/etc.)(relatively) high Y Yb etc. tonalites - diorites
Lower Y YbHigher Sr/Y La/YbEtc. Tdj.
Ton.
Grd.
Dio.
Key ideas
• At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions
• P-T conditions of amphibolites melting exert a strong control on the major and trace elements composition of TTG liquids
• “True” TTG signature requires high pressure (15-20 kbar)
• However, both low and high pressure TTGs can be identified
1 If there is fractionnation, the same logics could also apply using the most primitive liquid – it requires a little more work to determine its composition