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Using trace elements to define the PT conditions of TTG genesis. Jean-François Moyen Gary Stevens University of Stellenbosch South Africa. Key ideas. At least part of the geochemistry of TTG plutons can be interpreted as reflecting melting conditions

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using trace elements to define the pt conditions of ttg genesis

Using trace elements to define the PT conditions of TTG genesis.

Jean-François Moyen

Gary Stevens

University of Stellenbosch

South Africa

key ideas
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
TTG are...
  • Orthogneisses
  • Tonalites, Trondhjemites & Granodiorites

(Na-rich series)

  • Fractionnated REE, high La/Yb and Sr/Y, etc.
  • Largely homogeneous throughout the Archaean
slide4

TTG

“gneisses” or “plutons” ?

It is difficult to constrain good petrogenetical studies on complex, multiply injected and molten, possibly tectonic assemblages!

Sand River Gneisses

Ca. 3.1 Ga TTG gneisses in Messina area,

Limpopo Belt, South Africa

Stolzburg pluton (Barberton, South Africa -- 3.45 Ga)

conditions for making ttgs

Gt-in

Melting of hydrous basalt

In Garnet stability field (Gt in residue)

Gt-in

Conditions for making TTGs

Little or no

direct tectonic meaning!

using published experimental data
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)

slide8

C0

Cl

=

F + D (1 - F)

Compilation of experimental data

Interpolated “maps” of modal composition

“Maps” of trace elements composition

slide9
Sr/Y

NB- 1.Similar maps can be established for all elements or ratios, e.g. La/Yb

2. Actually different models were built for dirrerent types of amphibolites

ttg composition as a depth indicator
TTG composition as a depth indicator

Sr contents

Nb-Ta anomaly

and Nb/Ta

Y & HREE

depletion

slide12

Eu anomaly

HREE

depletion

slide13
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

Kenogamissi (2.74-2.71 Ga)

Stolzburg (3.44 Ga)

Theespruit (3.44 Ga)

Steynsdorp (3.56 Ga)

TTGs and TTGs?
slide15

Kenogamissi (2.74-2.71 Ga)

Stolzburg (3.44 Ga)

Theespruit (3.44 Ga)

Steynsdorp (3.56 Ga)

slide16

Lower Y Yb

Higher Sr/Y La/Yb

Etc.

Tdj.

Subduction

Low Y Yb etc. trondhjemites

Ton.

Dio.

Intraplate (plume/orogenic collapse/etc.)

(relatively) high Y Yb etc. tonalites - diorites

Grd.

key ideas1
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

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