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