Archaean magmatism

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Archaean magmatism
NB- Archean (US spelling) or Archaean (UK
spelling)
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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 worlds gold secondary
  deposits)
• PGE bearing sulphides
• Nickel
• Departments research interests

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Barberton gold fields
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Two characteristic rock types

• Komatiites ultra-mafic, Mg-rich lavas
• TTGs Tonalites, Trondhejmites Granodiorites
• Link with Archaean geodynamic style?

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The Archaean
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Oldest crustal remnants
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Jack Hill (Australia) zircon 4.404 ? 0.008 Ga
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The oceanic crust is young
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75 of the crust was formed at ca. 2.5 Ga The
Archaean is a major crust-forming period
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The average continental crust
C.C. is roughly andesitic
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Continental crust
Ca. 30 km
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The continental crust

• has the composition of magmatic rocks, and is
  largely made of magmatic rocks
• formed mostly in the Archaean
• ? We have to study the geology of Archaean
  continental crust.

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Earths heat production
?A 2- to 4-fold decrease from the Archaean to now
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Effects of higher Archaean heat production?

• Shape of convection
• Partitioning of heat flux
• Effects on the continents thermal structure and
  behaviour
• Petrogenesis?

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Shape of convection ?
(Ra gt 105)
(Ra 103 - 104)
Ra function of many things, including DT (or
heat production)
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Archaean dome-and-keel patterns
Vertical tectonics (sagduction)
Zimbabwe (2.7 Ga)
Pilbara (3.5 Ga)
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Superior Province
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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

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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)
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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 igneous rocks." Geological Society of
  South Africa Special Publication 2 55-86.

A truly South-African rock type!
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Onverwacht group, BGB
The original komatiites in Komatii formation
(1.5 km from type locality)
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Komatiites composition
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• Structure of komatiites flows
• Origin of komatiites
• Komatiites and the Archaean mantle

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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
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Chilled/brecciated top

• Subaquatic emplacement

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Spinifex textured layer(s)

• Random spinifex
• Orientated spinifex
• Plate spinifex

Spinifex grass, Western Australia (Barnes 1990)
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Random spinifex
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Orientated spinifex
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Plate spinifex
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Polyedral olivine
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Origin of komatiites

• High Mg contents require high degree of mantle
  melting (40-60 )
• This implies very high temperatures and fast rise

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What are the implications of komatiites?

• Probably formed in hot-spot like situations
  (difficult to arrive to gt 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)

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Komatiites and the history of the Archaean mantle
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3 groups of komatiites, from the shape of their
HREE pattern (or Gd/Yb ratios) Role of garnet
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• Correlation with Al (and also Ca)
• Al depleted (grp II) vs. Al-undepleted (grp. III)
• Only grp I komatiites exist in the late Archaean

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• Early differenciation of the Earth mantle
  (completed at 3.80 Ga)
• Deep origin of Late-Archaean komatiites (or
  locally non-differenciated bits of mantle?)
• Maybe due to a cooler Earth, hot temperatures
  found only very deep?

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2. TTG

• Archaean TTG (Tonalite, Trondhjemites and
  Granodiorites)
• grey gneisses (although in details, some TTGs
  are not grey gneisses and some grey gneisses are
  not TTG)

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Archaean grey gneisses
Some relatively simple orthogneisses
Stolzburg pluton (Barberton, 3.45 Ga)
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Commonly complex, migmatitic, polydeformed
orthogneisses
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The Sand River Gneisses Ca. 3.1 Ga TTG gneisses
in Messina area, Limpopo Belt, South Africa (R.
White, Melbourne, for scale)
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• However, the most common component of the grey
  gneisses is relatively constant

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Mineralogy
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Major elements
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REE
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Nb-Ta anomaly
Sr contents
Y HREE depletion
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Experimental studies
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• Partial melting of amphibolites ( metabasalts)
  is appropriate to generate TTG-like sodic melts
• Melting reactions of the form
• Amp Plag M Opx Ilm
• Amp Plag M Grt Ilm
• (Incongruent melting / amphibole dehydration
  melting)

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Conditions for making TTGs

Experimental melts
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NB

• 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

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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

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Garnet stability in mafic rocks

• From a dozen of experimental studies
• Well-constrained grt-in line at about 10-12 kbar

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From chemistry to geodynamic

• TTGs partial melts of amphibolites in garnet
  stability field
• Does this tell something about geodynamic
  conditions?

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Geodynamic site ?
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
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TTGs in a  plate  model
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TTGs in a  non plate  model
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Some lines of research

• TTG and adakites
• Secular evolution of TTGs
• TTGs and partial melting of amphibolites
• Diversity and components of the  grey gneisses 
•  Sanukitoids  etc.

Youre now entering the field of active research
and controversies!
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TTGs and adakites

• Are TTGs and adakites similar?

Yes !
No !
Thats the stuff active scientific research is
made of
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Are TTGs and adakite similar?

• 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 ?

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Secular evolution of Mg in TTG

• 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

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• MgO increases inTTG in course of time

• 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

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Our conclusions

• 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
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INTERPRETATION
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
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Sanukitoids geographic repartition
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Sanukitoids petrography
Diorites, monzodiorites and granodiorites
Lots of microgranular mafic enclaves
Qz Pg KF Bt Hb Cpx
Ap Ilm Sph Zn
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Sanukitoids geochemistry
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Making sanukitoids
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• Sanukitoids also suggest interactions between the
  mantle and TTG (or TTG like) melts
• Again, this is more consistent with slab melting
• .. At least at the end of the Archaean

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• As usual, the answer is certainly somewhere in
  between the extremes!
• Some TTGs are probably slab melts maybe not all
• Some TTGs certainly formed in subduction zones
  (and therefore subduction zones existed quite
  early) but probably not all, nor everywhere