Transcript 2 Precambrian Geology

The Precambrian Record
Key Events of Precambrian time
Acasta Gneiss is dated at
3.96 bya. It is near Yellowknife Lake , NWT Canada
Zircons possibly a bit older in Australia
Precambrian
•4.6 billion years to, say, 548 or 544 million years (depending on method).
•Represents 88% of all of the history of the earth.
•Referred to as the Cryptozoic Eon.
–“hidden life”
(no more BIFs)
(prokaryotes)
Early Hadean Highlights 1
• Earth formed about 4.6 billion years ago from
coalescing interstellar dust.
• Earth was bombarded by large
planetesimals adding to earth’s mass (adds
heat)
• Hot spinning pre-earth mass melted, caused
differentiation of materials according to density.
• Distinct earth layers begin to form
– Dense iron and nickel migrate to center (core)
– silicate material moves out to mantle
Early Hadean Highlights 2
• Huge impact from a Mars-sized
planetessimal created the moon.
– Caused earth to spin faster.
– Possible Tilt change
– Moon controls earth’s spin and creates tidal
forces.
– Moon’s orbit at an angle to planets around
Sun
– Earth got most of the core – outer part molten.
Earth rotates. We have magnetic field and,
therefore, an atmosphere
Moon Origin hypotheses -1
Speed and approach angle unlikely
Moon Origin hypotheses - 2
Chemical composition of the Moon suggests
that it could not have co-formed with the earth.
Moon Origin hypotheses - 3
Precambrian
Early Atmosphere
•First earth atmosphere H He lost to solar wind. No magnetic field
•Early permanent earth atmosphere mostly Nitrogen (inert) and CO2
Post-differentiation start of liquid core dynamo
•Liquid water is required to remove CO2 from atmosphere.
–Mars is too cold to have liquid water.
–Venus is too hot to have liquid water.
–So both have CO2 atmospheres.
•On Earth, most of the world’s CO2 was converted to O2 by photosynthesis.
•Enough by 2.0 bya to sustain life.
•CO2 is locked up in life, limestones, dolomites.
Mars
Venus
Earth
Early Permanent Atmosphere
• Gasses from cooling magmas formed early
atmosphere mostly N2, CO2, with CH4, H2O
•Early earth not conducive to modern oxygen
breathing organisms: too much solar UV gets
through atmosphere.
• Little oxygen occurred in the atmosphere until
the evolution of photosynthetic organisms
(Eubacteria) 3.5 billion years ago. Fully
oxygenated about 1.9 billion years ago.
Precambrian
Early Oceans from 4 bya
OCEANS
•Much water vapor from volcanic degassing.
•Salt in oceans is derived from weathering and
carried to the oceans by rivers.
•Blood of most animals has chemistry of seawater.
•Part of the earth’s water probably came from comets.
–Comets are literally large dirty snowballs.
–Provide fresh water.
Archean To Proterozoic Sedimentary Rocks
• Archean
•4 bya: mostly deep water clastic deposits such as mudstones and muddy
sandstones.
–high concentration of eroded volcanic minerals (Sandstones called Graywackes).
• 3 bya: absence of shallow water shelf carbonates.
–increasing chert.
– low oxygen levels, free iron was much more common in the Archean.
–Iron formed “chemical sinks” that consumed much of the early planetary oxygen.
–Formed banded ironstones, commonly with interbedded chert.
•Proterozoic – 2 bya Carbonates* become important
*Franklin Marble Field Trip 1
- Non-marine sediments turn red – iron is oxidized by the oxygen in AIR
Precambrian Hadean
Formation of Continents
•Early earth surface was magma sea, gradually cooled to form the
crust.
•Continents did not always exist but grew from the chemical
differentiation of early, mafic magmas in the young hot earth.
Floating “Volcanic Islands” of less dense higher silica magmas.
Precambrian Hadean and Archean
Formation of Felsic Islands
Convection was fast due high temperatures – ultramafic magma
• Partial Melting of base of Ultramafic Islands, OR
• Fractional Crystallization of Mafic Magmas THEN
• Once both mafic and felsic rocks (with different densities) exist,
subduction under protocontinents possible.
• Water squeezed from subducted ocean materials partially
melts mantle
• Volcanic arcs add land to protocontinents.
•Increasing amounts of Felsic continental material
•
First continental crust
At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite”
First
Komatiite partially melts, Basalt gets
to surface, piles up. The stack sinks,
partially melts when pressure high
enough. Fractionation makes
increasingly silica-rich magmas
Then:
Water out
Density differences allow subduction
of mafic rocks. Further partial melting
and fractionation makes higher silica
melt that won’t subduct
Archean: Growth of the early continents
Magmatism from Subduction Zones causes thickening
Growth of the early continents
Island Arcs and other terranes accrete as
intervening ocean crust is subducted
Little Archean ocean crust survives: most subducted
But silica-rich continental crust too buoyant to subduct.
Growth of the early continents
Sediments extend continental materials seaward
Quartz sand
becomes SS
or quartzite,
too buoyant
to subduct
Growth of the early continents
•Continent-Continent collisions result in larger continents
•Again, not very big in Archean; convection cells too small
Archean-Age Surface Rocks
Precambrian
Early Continents (Cratons) Archean
•Archean cratons consist of regions of light-colored felsic rock (granulite gneisses)
• surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks).
–Pilbara Shield, Australia
–Canadian Shield
–South African Shield.
Chlorite is the Low T
metamorphic product of
clay muds.
Mafic Greenstone
Belts
Felsic Islands
40km
Archean Crustal Provinces were once separated
Canadian Shield assembled from small cratons
Discussion: Where to
look for diamonds
Intensely folded rocks where cratons
were later sutured together in
Early Proterozoic
Longest: Trans-Hudson Orogen
Granulite gneiss and greenstone
Canadian Shield
Exposed by Pleistocene glaciers
Stratigraphic Sequence of a Greenstone belt
Banded Iron Formations
Younger lavas richer in silica
Increasingly Silica-rich extrusives,
some rhyolites with granites
below them.
Komatiites form at very high temps. They
are absent later as Earth cooled
DEMO: Banded Iron Sample
Archean Formation of greenstone belts
•Early continents formed by collision of felsic proto-continents.
•Greenstone belts represent volcanic rocks and sediments that accumulated
along and above subduction zones and then were sutured to the protocontinents
during collisions.
•Protocontinents small, rapid convection breaks them up
Proterozoic Tectonics:
The Wilson Cycle
• Proterozoic – Convection Slows
• Rift Phase
– Coarse border, valley and lava rocks in normal faulted
basins
• Drift Phase
– Passive margin sediments
• Collision Phase
– Subduction of ocean floor, island arcs form
– Then collision
Crustal provinces: Proterozoic Tectonics
Slave Craton
Rift and Drift
Followed by
Wopmay
Orogen:
remnants of
old
collisional
mountains
Intensely folded rocks where cratons
were sutured together in
Early Proterozoic
Wilson Cycle 1&2 Rift & Drift
Coronation Supergroup
1. Rift 2. Passive Margin sediments
Much later stuff
1. Rift Valley
Proterozoic 2 bya as Slave craton pulled apart
Near-collision phase of the
Wilson Cycle in the Wopmay
Orogen
3. End of Wilson cycle in the
Wopmay Orogeny
Coronation Supergroup thrust faulted eastward over Slave Craton
Note the vertical exaggeration
Key Events of Precambrian time
Proterozoic Assembly of Laurentia
• Trans-Hudson Orogen mostly 2.5 - 2 bya
– Superior, Wyoming, Hearne plates sutured
– Mountain range now eroded away
• Greenland, N. Gr. Brit., Scandinavia by 1.8 bya
• Continued accretion 1.8-1.6 bya of island arcs.
Most of S. US “Mazatzal Province”
• Last piece “Grenville Orogeny” 1.3-1 bya
• Assembly of Rodinia by about 750 mya
Proterozoic Oxygen - Rich Atmosphere
• Eubacteria are photosynthetic
2 bya formed stromatolites along shores
• Free oxygen in atmosphere
• Band Iron Formations (common 3.8 – 2 bya) become
rare, probably depended on disappearing conditions
• 2 bya Redbeds begin forming when iron in freshwater
sediment is exposed to abundant atmosphere oxygen
• Oxygen in atmosphere irradiated - Ozone layer forms,
protecting shallow water and land life forms from UV
First Redbeds
Key Events of Precambrian time
Final Assembly of Rodinia
Grenville Orogeny 1.3 – 1.0 BYA
• Eastern US Grenville collided with west
coast of S.America (maybe)
• We saw this Field Trip 1, Volcanic Arc at Park n Ride
• Southwest US collided w/ Antarctica
– Grenville Orogeny continues in Antarctica
• South collided with Africa
• Rifted apart by about 600 mya, after time
of “Snowball Earth”
Growth of Laurentia
Grenville: Shallow Water sandstones
(lots of graywacke), mudstones and
carbonates subjected to high-grade
metamorphism and igneous intrusion
Some workers think our
local Grenville Collider
was NW S. America
Proterozoic Rifting
• Grenville Time Rifting 1.3 – 1 bya
• Kansas to Ontario to Ohio
• Rift Valley sediments and lavas 15 km
(9 miles) thick!
• Rich in Copper, as are the rift valley
sediments here.
• Why?
Midcontinent rift
1500 km long, exposed near L. Superior
Key Events of Precambrian time
Plenty of highlands,
equator to poles
Grenville
Orogen
What Plate Tectonic
conditions favor
glaciation?
Snowball Earth
• Rodinia: abundant basalts with easily weathered
Ca feldspars. Ocean gets Ca+ + . CO2 tied up in
extensive limestones. Less greenhouse effect.
Atmosphere can’t trap heat – Earth gets colder
• Grenville Orogeny left extensive highlands
– From high latitudes to equator
• About 635 mya glacial deposits found in low
latitudes and elevations
• Huge Ice sheet reflects solar radiation “Albedo”
• Some workers believe oceans froze
Stable isotopes of C and O
d13C and d18O :3 - 4 Proterozoic Glaciations
Earth surface became cold enough to produce glaciations and ice ages
G - Glaciation
BIF - Banded Iron Formation
Cambrian
Snow-ball
Earth
Break up of Rodinia
• Hypothesis: Ice an insulator, heat builds
up
• Heavy volcanic activity poured CO2 into
atmosphere – greenhouse effect
• Warming melted snowball earth