Transcript The Archean - Gondwana Research

The Archean & Proterozoic
4.0 Ga to 543 Ma
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
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 Earth’s 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.
Time Period of Interest
-11
Mantle Heat Production (x 10 W/kg)
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.
2
1
0
0
Total
235
U
40
238
K
U
232
1
2
Age (Ga)
3
4
Th
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 (>1600°C) could
only be achieved in the mantle of the early Earth
(Archaean).
“Spinnefex” fabric in Komatiite lavas-Africa
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.
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.
Ophiolites
• Ophiolites-Pieces of oceanic plate that
have been thrusted (obducted) onto the
edge of continental plates.
How do we know?
Pillow Basalts & Marine
Sediments
Feeder Dikes
Massive Gabbro
Layered GabbroChemical equivalent of
basalt.
Metamorphic sole
(base) and peridotite
(Mg-silicate rock.
Fault block melange
Marine Sediments on
Continental block
Franciscan melange
Peridotite
Layered Gabbro in Oman
Massive Gabbro
Sheeted Dyke Complex
Pillow Basalts
Ocean Floor Sediments
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.
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?
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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).
Types of Archean Continental Crust
• Granite-Greenstone Belts
• Tonalite-Trondhjemite Gneisses (TTG’s)
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.
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 (faultbounded)