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 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 TTGs, as I ’ll show in the next few slides, are sodic granitoids, defining a sodic series of differenciation. They also have a very typical REE pattern, with characteristic depletions in HREE. It is now commonly accepted that they are generated by partial melting of hydrated basalts (amphibolites), within garnet stability field. However, details of their petrogenesis are still debatted; in particular, it is unclear whether it is, or not, possible to make TTGs through metling of eclogite, or if you need to be within amphibolite facies.

“gneisses” or “plutons” ? 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)

However, the most common component of the grey gneisses is relatively constant

Conditions for making TTGs Melting of hydrous basalt In Garnet stability field (Gt in residue) TTGs, as I ’ll show in the next few slides, are sodic granitoids, defining a sodic series of differenciation. They also have a very typical REE pattern, with characteristic depletions in HREE. It is now commonly accepted that they are generated by partial melting of hydrated basalts (amphibolites), within garnet stability field. However, details of their petrogenesis are still debatted; in particular, it is unclear whether it is, or not, possible to make TTGs through metling of eclogite, or if you need to be within amphibolite facies. Gt-in Little or no direct tectonic meaning! Gt-in

Stolzburg pluton (3.44 Ga, Barberton) No clear fractionation trend Apparently “compatible” Na (prob. a melting reaction Ab + Amp = Melt + Grt)

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

C0 Cl = F + D (1 - F) Compilation of experimental data Interpolated “maps” of modal composition Cl = C0 F + D (1 - F) “Maps” of trace elements composition

Major elements

REE contents: La/Yb KoB ThB AB

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

Effect of pressure

TTG composition as a depth indicator Sr contents Nb-Ta anomaly and Nb/Ta Y & HREE depletion

Eu anomaly HREE depletion

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) Steynsdorp (3.56 Ga) Theespruit (3.44 Ga)

Stolzburg (3.44 Ga) Steynsdorp (3.56 Ga) Theespruit (3.44 Ga) Kenogamissi (2.74-2.71 Ga)

Low Y Yb etc. trondhjemites 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 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