The Archean

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The Archean Proterozoic

• 4.0 Ga to 543 Ma

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Growth of Continental Crust

• There is some considerable debate regarding the
  rate at which continental crust has been added to
  the earth.

Slow-growth models Rapid growth models Pulse
Intermediate growth models
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Onset of Plate Tectonics?
There is actually considerable debate about when
modern-style plate tectonics began on the
planet. Through secular cooling and decay of
long and short lived radioisotopes, the Earths
thermal regime should be decreasing through
time. The early hot earth may have inhibited
subduction of oceanic crust (i.e. the convection
and heat in the mantle would have stopped
subduction. Average age of subducted crust today
is 125 Ma. Contrary View The early oceanic
crust was of a different (denser) composition and
therefore able to subduct because of its negative
buoyancy.
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In terms of the thermal budget for the Earth, the
overall heat budget from the decay of radioactive
elements in the Late Neoproterozoic is not
significantly different than today.
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Komatiites

• Komatiites are very high temperature lavas with
  MgO contents up to 33. Uprise from great depths
  is the easiest way to achieve such high liquid
  temperatures. Now it used to be thought that such
  high temperatures (gt1600C) could only be
  achieved in the mantle of the early Earth
  (Archaean).

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Spinnefex fabric in Komatiite lavas-Africa
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Cretaceous Komatiites

• It was thought that komatiites could not form in
  younger times, but they were discovered on the
  island of Gorgona (near Colombia).
• Some newer ideas are that komatiites are formed
  from deep mantle melts (where temps are higher).
• The Archean still had high temps and it is
  possible that a Komatiitic ocean crust could
  subduct.

8
Crustal Motion

• There is clear evidence that continental drift
  did occur in the Archean.
• Paleomagnetic studies on Archean-age rocks show
  that the crustal blocks did move, but continental
  drift is not the same thing as plate tectonics.

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Ophiolites

• Ophiolites-Pieces of oceanic plate that have been
  thrusted (obducted) onto the edge of continental
  plates.

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How do we know?
Pillow Basalts Marine Sediments
Feeder Dikes
Massive Gabbro
Layered Gabbro- Chemical equivalent of basalt.
Metamorphic sole (base) and peridotite
(Mg-silicate rock.
Fault block melange
Marine Sediments on Continental block
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Franciscan melange
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Peridotite
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Layered Gabbro in Oman
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Massive Gabbro
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Sheeted Dyke Complex
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Pillow Basalts
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Ocean Floor Sediments
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Ophiolites indicate Subduction of Oceanic Crust.

• Eldredge Moores (1994) suggested that ophiolites
  became common around 1000 Ma and this marked the
  onset of modern-style plate tectonics.
• A dismembered ophiolite was found in Dongwanzi
  China by Tim Kusky (2002).
• The ophiolite is 2.5 Ga and provides evidence
  that modern-style plate tectonics was already
  underway at 2.5 Ga.
• Others argue that the time interval from 1.9-2.1
  Ga marked the onset of modern-style plate
  tectonics.

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Are we arguing about silly things?

• Possibly. However, we use modern plate tectonic
  models to help us identify regions of mineral
  wealth.
• What is it that we know positively?
• Continental crust had formed in the Archean
• Continental crust was moving in the Archean
• Mafic crust in the Archean was formed in a hotter
  environment
• Most of that mafic crust is gone (subduction).

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Types of Archean Continental Crust

• Granite-Greenstone Belts
• Tonalite-Trondhjemite Gneisses (TTGs)

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Tonalites-Trondhjemites

• Tonalite is an igneous, plutonic (intrusive)
  rock, of felsic composition, with phaneritic
  texture. Mineral assembly is composed typically
  of plagioclase, more than 20 of quartz and rare
  alkali feldspar. Amphiboles and pyroxenes are
  common accessory minerals. Trondhjemite is a
  variety of tonalite where biotite is the only
  mafic mineral.
• Both are associated with subduction and the
  formation of island arcs.

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Granite-Greenstone Belts
Granites are an intrusive felsic rock that forms
at temperatures around 800 C. Greenstones are
low-grade metamorphosed basalts. Their
association in the Archean is quite strange. They
are likely tectonic slices (fault-bounded)