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 In Garnet stability field (Gt in residue) Melting of hydrous basalt KDKD Gt/melt = (other minerals ≤ 1) Yb

Geodynamic site ? Thick (oceanic or continental) crust (e.g. Oceanic plateau) Subduction Intermediate cases: Shallow subduction (± underplating) Stacked oceanic crust Gt-in

Partial melting of amphibolites « modern » studies ( ) + Phase diagrams ( ) 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

Types de fusion ? 1) Fusion en présence d ’eau Minéraux + H 2 O -> Liquide Exple : Qz+KF+Pg+H 2 O -> Liq Pg+Qz+Amp+H 2 O -> Liq + ? 2) Fusion-déshydratation (« dehydration melting ») Minéraux hydratés -> Minéraux anhydres + H 2 0 Minéraux + H 2 O -> Liquide Exple : Bt+Pg+Qz -> Liq + (Opx+Crd)/Gt Pg+Amp -> Liq + Opx/Gt Min. hydratés -> Min. anhydres + liquide

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: fluid- present (- Qz)

Mineral stability limits

Control on amphibole stability

Control on plagioclase stability

Mineralogical models CaO Na 2 O K2OK2O TiO Amp. Comp. Ti-rich High Mg# Si poor Int. Ti-poor Low Mg# Si rich KoBThBAB Quartz 0110 Plagioclase Amphibole Amp:Plag 3:13:22:3

Mineralogical models KoB ThBKoBThBAB

Composition of experimental melts

Very unlikely for amphibolite melting! Na 2 O contents in experimental melts

K2OK2O

Major elemen ts A linear model, of the form C/C 0 = 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 = % ( °C)

Model for trace element ClCl = C0C0 F + D (1 - F) Experimental data D =  Kd i. X i Arbitrary Litterature

KoB ThBKoBThBAB Trace elements contents of the 3 sources

Melt proportions KoBThBAB

Mineral proportions: amphibole and plagioclase KoBThBAB

Mineral proportions: garnet KoBThBAB KDKD Gt/melt = Yb

Mineral proportions: rutile KoBThBAB KDKD Rt/melt = Nb KDKD Rt/melt = Ta

REE contents in (modelled) melts KoBThBAB

REE contents in (modelled) melts KoBThBAB

REE contents: La/Yb KoBThBAB

Y contents KoBThBAB

Sr contents and the role of residual plagioclase (Martin & Moyen, 2001, Geology 30 p ; after Zamora, 2000)

Sr/Y KoBThBAB

Nb/Ta KoBThBAB

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.) HREE depletion Eu anomaly

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?

High P TTGs Low P TTGs Not really TTGs Archaean granulites (and intraplate geotherms) Subduction of young lithosphere (5 Ma) (20 Ma) (50 Ma) Subduction of old lithosphere Tonalites & trondhjemites (F = %) Appropriate trace elts. signature High Sr, La/Yb, Nb/Ta Low Y, Yb Low Sr, La/Yb, Nb/Ta High Y, Yb TTG genesis in P-T space

A regional example Barberton, South Africa 3.5 to 3.2 greenstone belt and gneisses

Swaziland R.S.A. 20 km Crust accretion around BSB Ma Steynsdorp pluton 3509 ± 7 Ma Ngwane gneisses (Swaziland) 3490 ± 3 to 3644 ± 2 Ma Lower Onverwacht group ca Ma Dwalile Suite greenstone remnants Ca Ma ?

Swaziland R.S.A. 20 km Crust accretion around BSB 3450 Ma Stolzburg, Theespruit, etc. plutons 3443 ± 4 to 3460 ± 5 Ma Tsawela gneisses (Swaziland) 3458 ± 6 to 3437 ± 6 Ma Upper Onverwacht group ca Ma

Swaziland R.S.A. 20 km Crust accretion around BSB 3220 Ma Kaap Valley, Neelshoogte, Badplaas, etc. plutons ca Ma Usutu granodiorite (Swaziland) 3231 ± 4 to 3216 ± 3 Ma Fig Tree and Moodies groups ca Ma Dalmein pluton Ca 3220 Ma

Crust accretion around BSB 3100 Ma & younger Swaziland R.S.A. 20 km GMS suite: Piggs Peak, Heerenveen & Mpuluzi batholiths Boesmanskop syenite ca Ma Ushuswana complex 2900 Ma & K granites Ma

Geochemistry: 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 ? Early Archaean Late Archaean Modern

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 kbar, but can have occurred anywhere between 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)