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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)


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

    • 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?






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

The Archaean is a major crust-forming period


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

(“sagduction”)

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


2.9 – 2.7 Ga granites

3.1 Ga granites

& syenites

Moodies

Fig Tree

Onverwacht

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)




Subdivision of komatiite flows (Arndt et al. 1977)

Polysutured top

Random spinifex

Orientated spinifex

parallel blades of spinifex

solid subhedral olivine

B4

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)






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)



The Sand River Gneisses Ga)

Ca. 3.1 Ga TTG gneisses in Messina area,

Limpopo Belt, South Africa

(R. White, Melbourne, for scale)



Mineralogy
Mineralogy relatively constant


Major elements
Major elements relatively constant


REE relatively constant


Nb-Ta anomaly relatively constant

Sr contents

Y & HREE

depletion


Experimental studies
Experimental studies relatively constant



Conditions for making ttgs

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

Gt/melt

KD

= 10 - 20

Yb

In Garnet stability field (Gt in residue)

(other minerals ≤ 1)

Conditions for making TTGs

Experimental melts


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

KD

= 10 - 20

Yb

(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

Gt-in

Subduction

Gt-in

  • Intermediate cases:

  • Shallow subduction

  • (± underplating)

  • Stacked oceanic crust

Gt-in

Gt-in

Thick (oceanic or continental) crust

(e.g. Oceanic plateau)

Gt-in


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… ?


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


  • 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


INTERPRETATION appropriate to generate TTG-like sodic melts

EARLY ARCHAEAN

LATE ARCHAEAN/ADAKITES

TODAY

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