Archaean magmatism

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					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
Barberton gold fields
  Two characteristic rock types
• Komatiites = ultra-mafic, Mg-rich lavas
• TTGs = Tonalites, Trondhejmites &
  Granodiorites

• Link with Archaean geodynamic style?
The Archaean
Oldest crustal remnants
Jack Hill (Australia) zircon = 4.404  0.008 Ga
The oceanic crust is young…
 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?
• Shape of convection
• Partitioning of heat flux
• Effects on the continents thermal structure
  and behaviour

• Petrogenesis?
       Shape of convection ?




(Ra = 103 - 104)                         (Ra > 105)
   Ra = function of many things, including DT (or heat production)
                                 Archaean
                               dome-and-keel
                                 patterns
                                Vertical tectonics
                                (“sagduction”)




Zimbabwe (2.7 Ga)




            Pilbara (3.5 Ga)
Superior
Province
   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
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!
               Onverwacht group, BGB
The original komatiites in Komatii formation (~1.5 km from type locality)
         Komatiites composition

Two types Al-undepleted, Al-depleted
       Al-undepleted     Al-depleted
      Baby-Belleterre Norway
SiO2 48.20           46.11
TiO2    0.34         0.77
Al2O3 8.58            5.47
FeO 10.96            11.00
MnO      0.17         0.17
MgO     23.94         25.01
CaO     7.08         9.24
Na2O 0.60             0.07
K2O     0.05         0.01
P2O5 0.02             0.03
• Structure of komatiites flows
• Origin of komatiites
• Komatiites and the Archaean mantle
Subdivision of komatiite flows (Arndt et al. 1977)
     A1           Polysutured top


     A2            Random spinifex




     A3           Orientated spinifex

     B1             parallel blades of spinifex



                   solid subhedral olivine
     B2



     B4           Basal chill, polysutured
   Chilled/brecciated top

                            A1

                            A2



                            A3
                            B1



                            B2




Subaquatic emplacement
     Spinifex textured layer(s)
A1       • Random spinifex
A2
         • Orientated spinifex
A3
         • Plate spinifex
B1



B2




               Spinifex grass, Western Australia (Barnes 1990)
     Random spinifex
A1

A2



A3
B1



B2
     Orientated spinifex
A1

A2



A3
B1



B2
Plate spinifex
   A1

   A2



   A3
   B1



   B2
     Polyedral olivine
A1

A2



A3
B1



B2
                 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?
• 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)
Komatiites and the history of the
       Archaean mantle
3 groups of komatiites,
from the shape of their
HREE pattern (or Gd/Yb
ratios)


Role of garnet
• Correlation with Al (and also Ca)
   – Al depleted (grp II) vs. Al-undepleted (grp. III)
• Only grp I komatiites exist in the late Archaean
1. Early differenciation of the Earth mantle (completed at 3.80
   Ga)
2. 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?
                  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…)
            Archaean grey gneisses
Some relatively simple orthogneisses




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


                   Sr
                contents




                       Y & HREE
                       depletion
Experimental studies
• 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)
    Conditions for making TTGs
Experimental melts
                           Gt/melt
                      KD   Yb        = 10 - 20

                        (other minerals ≤ 1)




Melting of hydrous   In Garnet stability
basalt               field (Gt in residue)
                      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
            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
Garnet stability in mafic rocks
                  • From a dozen of
                    experimental
                    studies
                  • Well-
                    constrained grt-
                    in line at about
                    10-12 kbar
                          Gt/melt
                     KD   Yb    = 10 - 20

                      (other minerals ≤ 1)
 From chemistry to geodynamic
• TTGs = partial melts of amphibolites in
  garnet stability field
• Does this tell something about
  geodynamic conditions?
  Geodynamic
     site ? Subduction                  Gt-in




                                       Gt-in



        Intermediate cases:
        • Shallow subduction
               (± underplating)          Gt-in
        • Stacked oceanic crust


                                       Gt-in


Thick (oceanic or continental) crust
(e.g. Oceanic plateau)                  Gt-in
TTGs in a « plate » model
TTGs in a « non plate » model
      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.

    You’re now entering the field of active research and controversies!
                       TTGs and adakites
  • Are TTGs and adakites similar?




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

                                                          LATE
                                                    EARLY ARCHAEAN
                                                         TODAY
                                                    ARCHAEAN/ADAKITES




High heat production  High geothermal gradients  Shallow depth slab melting
Lower heat production  Lower geothermal gradients  Deep slab melting
Thin heat production  No or few magma/mantle  No slab but High-Mg-Ni-Cr TTG
Low overlying mantle Low geothermal gradients interactions  Low Mg-Ni-Cr TTG
Thick overlying mantle important magma/mantle interactions mantle wedge melting
Sanukitoids: geographic repartition
     Sanukitoids: petrography

Diorites, monzodiorites and
granodiorites

Lots of microgranular
mafic enclaves

Qz + Pg + KF + Bt + Hb ± Cpx

Ap + Ilm + Sph + Zn
Sanukitoids: geochemistry
Making sanukitoids
• 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
• 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

				
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posted:10/12/2011
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