Archaean magmatism
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Archaean magmatism. NB- Arch ea n (US spelling) or Arch aea n (UK spelling). Why?. Somehow different from modern magmas Interesting to test our understanding of petrogenetic processes Not that rare, and good South African examples (Barberton) Economic interest

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Archaean magmatism

Archaean magmatism

NB- Archean (US spelling) or Archaean (UK spelling)

Archaean magmatism

  • Somehow different from modern magmas

    • Interesting to test our understanding of petrogenetic processes

  • Not that rare, and good South African examples (Barberton)

  • Economic interest

    • Gold (large part of world’s gold + secondary deposits)

    • PGE bearing sulphides

    • Nickel

  • Department’s research interests

Two characteristic rock types
Two characteristic rock types

  • Komatiites = ultra-mafic, Mg-rich lavas

  • TTGs = Tonalites, Trondhejmites & Granodiorites

  • Link with Archaean geodynamic style?

Archaean magmatism

75 % of the crust was formed at ca. 2.5 Ga

The Archaean is a major crust-forming period

Archaean magmatism

Earth’s heat production

►A 2- to 4-fold decrease from the Archaean to now

Effects of higher archaean heat production
Effects of higher Archaean heat production?

  • Shape of convection

  • Partitioning of heat flux

  • Effects on the continents thermal structure and behaviour

  • Petrogenesis?

Shape of convection
Shape of convection ?

(Ra > 105)

(Ra = 103 - 104)

Ra = function of many things, including DT (or heat production)

Archaean dome and keel patterns
Archaean dome-and-keel patterns

Vertical tectonics


Zimbabwe (2.7 Ga)

Pilbara (3.5 Ga)

Bimodal archaean terranes
Bimodal Archaean terranes

  • Greenstone belts (commonly dominated by greenschist facies amphibolites)

    • Mafic and ultramafic (= komatiites) lavas

      • Some intermediate lavas (andesites)

    • Detrical sediments

      • Some chemical sediments (BIFs) or biogenic formations (stromatholites)

  • Gneissic « basement » or plutons

  • Late plutons

Archaean magmatism

2.9 – 2.7 Ga granites

3.1 Ga granites

& syenites


Fig Tree


Ca. 3.2 Ga TTG

Ca. 3.4 Ga TTG

« Ancient gneisses »

(3.6 – 3.4 Ga)

1 komatiites
1. Komatiites

Viljoen, M. J. and R. P. Viljoen (1969). "The geology and geochemistry of the lower ultramafic unit of the Onverwacht group and a proposed new class of igneousrocks."Geological Society of South Africa Special Publication 2: 55-86.

A truly South-African rock type!

Onverwacht group bgb
Onverwacht group, BGB

The original komatiites in Komatii formation (~1.5 km from type locality)

Archaean magmatism

Archaean magmatism

Subdivision of komatiite flows (Arndt et al. 1977)

Polysutured top

Random spinifex

Orientated spinifex

parallel blades of spinifex

solid subhedral olivine


Basal chill, polysutured

Chilled brecciated top
Chilled/brecciated top

Subaquatic emplacement

Spinifex textured layer s
Spinifex textured layer(s)

  • Random spinifex

  • Orientated spinifex

  • Plate spinifex

Spinifex grass, Western Australia (Barnes 1990)

Origin of komatiites
Origin of komatiites

  • High Mg contents require high degree of mantle melting (40-60 %)

  • This implies very high temperatures and fast rise

What are the implications of komatiites
What are the implications of komatiites?

  • Probably formed in hot-spot like situations (difficult to arrive to > 1600° else)

  • Even though, this is hotted than modern hotspots

  • At least some parts of the Earth were very hot

  • At least part of the GSB formed from hotspots (intraplate situation)

Archaean magmatism

Archaean magmatism

2 ttg
2. TTG Ga)

  • Archaean TTG (Tonalite, Trondhjemites and Granodiorites)

  • ≈ grey gneisses (although in details, some TTGs are not grey gneisses and some grey gneisses are not TTG…)

Archaean grey gneisses
Archaean grey gneisses Ga)

Some relatively simple orthogneisses

Stolzburg pluton (Barberton, 3.45 Ga)

Archaean magmatism

The Sand River Gneisses Ga)

Ca. 3.1 Ga TTG gneisses in Messina area,

Limpopo Belt, South Africa

(R. White, Melbourne, for scale)

Mineralogy relatively constant

Major elements
Major elements relatively constant

Archaean magmatism
REE relatively constant

Archaean magmatism

Nb-Ta anomaly relatively constant

Sr contents



Experimental studies
Experimental studies relatively constant

Archaean magmatism

Conditions for making ttgs

Melting of hydrous basalt appropriate to generate TTG-like sodic melts



= 10 - 20


In Garnet stability field (Gt in residue)

(other minerals ≤ 1)

Conditions for making TTGs

Experimental melts

Archaean magmatism
NB appropriate to generate TTG-like sodic melts

  • Some people propose that TTGs can be formed by hornblende dominated FC of andesites

  • Not impossible (at least in theory) but..

    • Where are the cumulates?

    • High viscosity of felsic melts

    • Lack of andesitic plutonic terms associated with TTGs

  • Regarded as unlikely to impossible by maybe 80-90% of the petrologists

Ttg are
TTG are... appropriate to generate TTG-like sodic melts

  • 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

Garnet stability in mafic rocks

Gt/melt appropriate to generate TTG-like sodic melts


= 10 - 20


(other minerals ≤ 1)

Garnet stability in mafic rocks

  • From a dozen of experimental studies

  • Well-constrained grt-in line at about 10-12 kbar

From chemistry to geodynamic
From chemistry to geodynamic appropriate to generate TTG-like sodic melts

  • TTGs = partial melts of amphibolites in garnet stability field

  • Does this tell something about geodynamic conditions?

Geodynamic site
Geodynamic site ? appropriate to generate TTG-like sodic melts




  • Intermediate cases:

  • Shallow subduction

  • (± underplating)

  • Stacked oceanic crust



Thick (oceanic or continental) crust

(e.g. Oceanic plateau)


Ttgs in a plate model
TTGs in a « plate » model appropriate to generate TTG-like sodic melts

Ttgs in a non plate model
TTGs in a « non plate » model appropriate to generate TTG-like sodic melts

Some lines of research
Some lines of research appropriate to generate TTG-like sodic melts

  • TTG and adakites

  • Secular evolution of TTGs

  • TTGs and partial melting of amphibolites

  • Diversity and components of the « grey gneisses »

  • « Sanukitoids » etc.

You’re now entering the field of active research and controversies!

Ttgs and adakites
TTGs and adakites appropriate to generate TTG-like sodic melts

  • Are TTGs and adakites similar?

Yes !

No !

That’s the stuff active scientific research is made of …

Are ttgs and adakite similar
Are TTGs and adakite similar? appropriate to generate TTG-like sodic melts

  • If they are: Adakites can be used as an indicator of the site of TTG formation, but…

    • Are the adakites formed as slab melts

    • .. Or as melts of underplated basalts (Cordilera Blanca)?

  • If they are not: they still are rather similar, so what the… ?

Archaean magmatism

Secular evolution of Mg# in TTG appropriate to generate TTG-like sodic melts

  • Fractional crystallization reduces Mg#

  • For each period the higher Mg# represents TTG parental magma

  • From 4.0 to 2.5 Ga Mg# regularly increased in TTG parental magmas

Archaean magmatism

  • SiO2 decreases inTTG in course of time

  • Adakites have exactly the same evolution pattern as (young) TTG

  • For the same SiO2, experimental melts are systematically MgO poorer than TTG

Our conclusions
Our conclusions appropriate to generate TTG-like sodic melts

  • Relatively young TTGs are similar to adakites

  • Both are different from melts from amphibolites (higher Mg etc.)

  • We propose that this corresponds to interactions with the mantle

  • … which can be achieved only in subduction (slab melting) situation – both for young TTGs and adakites

NB- This is just our interpretation – it is challenged

Martin & Moyen 2002

Archaean magmatism

INTERPRETATION appropriate to generate TTG-like sodic melts




High heat production High geothermal gradients Shallow depth slab melting

Thin overlying mantle No or few magma/mantle interactions Low Mg-Ni-Cr TTG

Lower heat production Lower geothermal gradients Deep slab melting

Thick overlying mantle important magma/mantle interactions High-Mg-Ni-Cr TTG

Low heat production Low geothermal gradients No slab but mantle wedge melting

Sanukitoids geographic repartition
Sanukitoids: geographic repartition appropriate to generate TTG-like sodic melts

Sanukitoids petrography
Sanukitoids: petrography appropriate to generate TTG-like sodic melts

Diorites, monzodiorites and granodiorites

Lots of microgranular mafic enclaves

Qz + Pg + KF + Bt + Hb ± Cpx

Ap + Ilm + Sph + Zn

Sanukitoids geochemistry
Sanukitoids: geochemistry appropriate to generate TTG-like sodic melts

Making sanukitoids
Making sanukitoids appropriate to generate TTG-like sodic melts

Archaean magmatism

Archaean magmatism