Transcript The Precambrian: Hadean, Archean and Proterozoic

THE PRECAMBRIAN
HADEAN
ARCHEAN
PROTEROZOIC
The Big Bang
• Event that occurred approximately 13.7
BILLION years ago
• All the mass and energy concentrated at a
point
• The universe began expanding and
continues to expand
• After 1 million years matter began to cool
enough to form atoms- Hydrogen- the
building block of stars
Galaxies and Stars
• Galaxy- huge rotating aggregation of stars,
dust, gas held together by gravity
• Earth, the sun and our solar system is part
of the Milky Way
• Stars are massive spheres of
incandescent gases (hydrogen and
helium)
The Solar System
• Our solar system is located away from the
galaxy’s center
• Our sun and the planets originated from a solar
nebula that had been enriched with heavy
elements from nearby super novae (Stellar
Synthesis)
• Solar system is approximately 5 Billion years old
• Composition is 75% hydrogen, 23% helium and
2% other materials
Formation of a
Protostar
Center
contracts
Center
continues to
heat up
Protostar
radiates more
heat
Fusion
begins in the
stars core
Shockwaves
radiate
outward
releasing
material
Material
coalesces
into planets,
moons or
comets
Other
material is
ejected to the
periphery
Our Solar
System
4 inner
planets
(terrestrial)
4 outer
planets
(gaseous)
Solar nebula
photographed
by Hubble
Structure of the Earth
•
•
•
•
Solid Inner Core
Outer Liquid Core
Lower Mantle
Upper Mantle
– Asthenosphere
– Brittle Mantle
• Lithosphere
• Crust
Structure of the Earth
• Refraction of Seismic
Waves
• Changes in Velocity
due to density
A Magma Ocean
• Lunar evidence
– Textures, Uniform Composition, Age
– Crystallization of well-mixed magma ocean
produces uniform layered crust
• Terrestrial Magma Ocean
– Existence of large amount initial heat
– Outer part of Earth melt during accretion
– Depth estimates 100 to >1000 Km
– Ultramafic (high Fe & Mg)
– Crystallization complete in 100 my
Composition of the Early Crust
•
•
•
•
Ultramafic
High Fe & Mg
Komatiites: volcanic, extrusive rocks
Rapid break-up and recycling of crust
– Due to vigorous convection
– Impacts
• Existence of Plate Tectonics
Solidifying Basalt- Hawaii
The Earth
•
•
Core is composed of mixtures or alloys of iron
(pressure is more than a million times that at the
surface and temperature is estimated to be at
4000°C); has a solid inner core and a liquid outer
core (earth's magnetic field may be produced by
the motion of the liquid material in the iron-rich
outer core)
Layer outside the earth's core is the mantle; it is
solid but very hot, near the melting point of rocks,
so it flows almost like a liquid, though much
slower; it is 70% of the earth's volume
The Earth
• Outermost layer is the crust; it is extremely thin
(is thinner under the oceans than under the
continents)
– Oceanic crust is made of basalt (low in silica and
high in iron and magnesium) and has a higher density
compared to continental crust, which is made of
granite (high content of aluminum and magnesium
silicate with quartz and feldspar) and has a lower
density
– Thus, continents lie above sea level and oceanic
crust lies below sea level because of density
differences
Hotspots and Flood Basalts
The Lithosphere and Mantle
Reservoirs
• Archean
– hotter Earth >> thinner lithosphere
– steeper geothermal gradient >> Komatiites
(require higher temperatures)
– lithospheric plates smaller and less stable
• Mantle Reservoirs
– Between 4 by and 2 by mantle separated
into reservoirs which have remained
homogeneous since formation
The Origin of the Crust
• Age based on lunar rocks and
meteorites 4.4 to 4.5 by
• Archean rocks of Canada’s Slave
Province 3962 +/- 3 my based on zircon
mineral crystals
• Hadean rocks of Australia’s Pilbara
region 4400 my based on zircon
• Recycled due to rapid convection
Composition of the Early Crust
• Composition largely Speculative
• Oldest lunar crustal rocks- representation of
early earth
• Granitic?
– Too buoyant, resists subduction, no evidence
• Lunar Highlands (4.4 bybp)
– Fractional crystallization of basaltic magma
– Gabbros and anorthosites, rich in mafics
minerals
• Komatiites and Basalts
Anorthosite vs Komatiite
• Anorthosite
– dry magma forms crust (moon)
– wet magma plagioclase sinks and does not form
crust (earth)
• Komatiite or Basalt
– abundant in Archean terranes
– high density and convective drag forces makes for
easy recycling
– formed as localized islands
Lower Crust
• Metamorphism
– low grade for at shallow depths (<15 km)
– medium grade (15-25 km)
– high grade (25-80 km)
• Granulites
• Rapid tectonic uplift <50 my
• Continuous convergence leads to
crustal thickening, erosion and uplift will
eventually expose high grade rocks at
the surface
The First Continents
• Continental crust resists recycling due to
buoyancy
• Produced by partial melting of oceanic crust
in subduction zones
• Tonalites- abundant plagioclase, quartz, high
in Ca,Na, Al
• Accretion of small islands into bigger
continents
• Oldest remnants 3.8 to 4.0 by
– detrital zircons 4.2-4.4 by
• < 500km diameter
• Tonalites and granodiorites
Early Continental Crust
• Amitsoq
Gneiss
• Isua
Greenland
• 4.0-3.8 by
Growth of Crust- Mechanism
• Vertical growth- thickening
– underplating (intrusion of magma to lower
crust)
• Lateral growth
– rifting
• High grade granulites (representing 3540 km) on the surface underlain by
normal 40km thick crust implies
– underplating kept pace with uplift and
erosion
Mechanisms Continental
Growth
•
•
•
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Magma addition in arcs
Terrane accretion
Continental collision
Welding of marginal sediments
Mechanism for Continental Growth
• (a) Magma addition
in arcs
• (b) Seaward
migration of ocean
plate
• (c) Terrane accretion
through suturing
• (d) Continental
collision’
• (e) Welding of
marginal sediments
Continental Growth Rates
• Rapid early growth
– recycling not feasible
• Linear growth
• Episodic growth
vol
time
– 2.7by, 2.0 by, 1.0 by correspond to major
orogenic episodes in North America
Crustal Provinces
• Large segments >107 km2
• Provinces are identified by geologic history
and isotopic dates
– most gneisses and granites
• Recognized several large Precambrian
provinces in North America >2.5 by
– Nain; Rae; Slave; Hearne; Wyoming: Superior
• 5 provinces <2.5 by but >0.9 by
– Wopmay; Yavapai-Mazatzal; Trans-Hudson;
Mid-Continent; Grenville
Precambrian Provinces of N. Am.
The Assembling of North
America
• Collision and suturing of provinces to
make a continent
• Assembly of Archean plates took only 10
my
• 50% Late Archean (2.5-3.0 by)
• 30% Early Proterozoic (1.6-2.0 by)
• <10% Mid to late Proterozoic (0.9-1.2 by)
• <10% Phanerozoic (<544 my)
Cratons
•
•
•
•
Stable part of continent
Oldest part of continents
Composed of Shield and Platform
All continents contain at least one
cratonic mass
• Small <500km across
• Time of formation varies
– 50-100 my after major orogenies
– uplift 200-400 my and erosion
– deposition of sediments (Platform)
North
American
Cratonshield, and
platform
The Hadean Eon
• No direct record of the first 800 my
• Formation of core 4.4 to 4.5 by
• Creation of Magma Ocean and cooling
of a komatiite crust
• Mosaic of small rapidly moving plates
• Recycling of crust at subduction zones
• Partial melting of crust gives rise to
tonalite magmas
Hadean Crust
• (a) 4.6 to 4.3
by; rapid
recycling of an
unstable crust
• (b) 4.3 to 3.8
by; the
formation of
continental
islands
The Archean
&
The Proterozoic
• Subdivisions of the
Precambrian
• Major Events
– Origin of the Earth
– Major outgassing
development of internal
structure
– Origin of Life
– BIFs
– Kenoran Orogeny
– Red Beds
– Glaciations
– Grenville Orogeny
Precambrian Basement
• Igneous & Metamorphic Rocks
• Association of rocks based on Superposition and
Cross-cutting relationships
• Divided into ARCHEAN and PROTEROZOIC
– Archean 3.96 b.y. to 2.5 b.y
– Proterozoic 2.5 b.y to 0.544 b.y.
– Hadean No record >3.96 b.y.
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Differentiation of Earth
No free Oxygen
Rich in CO2 & H2O vapor
Meteoric impacts for 100 m.y.
No evidence on Earth, But evidence on Moon & Mars
Shields
• Geologic Stable Regions- Every continent has 1 or
more
• Canadian Shield, center of North America around
Hudson Bay
• Exposed by Pleistocene Glaciation
• Surrounded by Platforms
– Thin Blankets of Sedimentary Rocks
• Shield + Platform = CRATON
Canadian Shield
• 11 Provinces
– Superior, Wyoming, Slave, Nam, Hearne, Rae &
Grenville, Wopmay,
• Based on Faults and Folds
• Based on Age of Rocks
• Boundaries marked by
– Truncations in Structural Lineations
– Bands of severely deformed rocks
• Suture zones consolidated by 1.9 b.y
Precambrian Provinces of N. Am.
Archean Rocks
• The Granitoid- Greenstone Association
– Broad basins, subsiding, subaqueous volcanics
– Shallow water deposition > stromatolites
– Greenstone Belts
•
•
•
•
folded and metamorphosed
linear to irregular-shaped successions
chlorite, amphiboles, pillow basalts
some chert, BIFs, komatiites, felsic and intermediate
volcanics, greywackes
– Granitoid Gneiss
• intrusive granitic rocks > metamorphosed to gneiss
GREENSTONE BELTS
Greenstone Showing Well Developed Pillow Structures
Greenstone Belts of the Superior Province
Greenstone Belt: Barberton
Mountain Land, South Africa
• Riches of Greenstone
Belts
• Copper, Zinc, Silver
and Gold
• Witswatersrand:
Placer Gold Deposit
Plate Tectonic Model for the Development of Greenstone Belts
and Growth of Continental Crust
Origin of Atmosphere
• Atmosphere evolved in 4 steps:
– primordial gases, later lost from sun's
radiation
– exhalations from the molten surface (volcanic
venting); bombardment from icy comets
– steady additions of carbon dioxide, water
vapor, carbon monoxide, nitrogen, hydrogen,
hydrogen chloride, ammonia, and methane
from volcanic activity
– addition of oxygen by plant/bacterial life
ATMOSPHERE
• Present Composition
– 78% Nitrogen; 21% Oxygen; trace amounts of CO2, Argon,
ect.
• Atmosphere Unique Among Other Planets
– Venus & Mars CO2 Gaseous planets H, He, CH4
– Pressure in Venus 100x Earth on Mars 1/100
– Surface Temperature 450-500oC Venus; -130-25oC Mars
• Atmospheric Gases Controlled by volcanoes and
interactions between gases and the solid Earth &
Oceans as well as biotic component
• Ozone (O3): produced by photochemical Rx absorbs
harmful UV radiation
The Origin of the Atmosphere
• Primary Gases from Accretion
– rich in H, He, CH4
• Secondary Atmosphere
– Degassing of Earth by volcanic activity
• large number of volcanoes/volcanic rks
• Rich in Argon-40 (99.6%) as compared to Sun
(0.01%)
– 40Ar product of radioactive decay of 40K
Was there a Primary
Atmosphere?
• No evidence that one existed or if it did it was gone
soon after planetary accretion
• Primary Atmosphere then disappeared early as a
consequence of:
– Solar wind
– Formation of moon
Secondary Atmosphere
• Degassing- liberates CO2 and H2O vapor
– Outgassing of water occurred within first 1by
– volcanism
• Gases in near surface reservoirs are identical to volcanic
gases
– weathering
• Terrestrial atmosphere rich in CO2 and H2O by
4by
N2 retained in atmosphere, H2O vapor lost by
condensation to ocean; CO2 combined with Ca &
Mg to form carbonate Rks; H2 lost to space
Oxygen in the Atmosphere
• Earth only planet in solar system with oxygen
thus only planet able to sustain higher forms of
life
• Oxygen produced by
– Photosynthesis- algae and plants
– Photolysis-fragmentation of water molecules into
Hydrogen and Oxygen
• Oxygen consumed by
– Respiration
– Decay
– Weathering (chemical oxidation)
Oxygen in the Primitive
Atmosphere
• Photosynthesis NOT important prior to
advent of microorganisms; only after 3.5 by
• Controlled by rate of Photolysis which was
controlled by the outgassing of water from
volcanoes, the rate of hydrogen escaping to
space and the losses from weathering
Geologic Indicators of Atmospheric
Oxygen Levels
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•
Banded Iron Formations (BIFs)
Redbeds, Sulfates and Uraninite
Paleosols
Biological Indicators
Banded Iron Formations
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Sedimentary Rocks >15% iron
Hematite interbedded with chert
Chemically precipitated on the seafloor
Present from 3.8 to 0.8 by
During Late Archean and Early Proterozoic
dissolved iron entered the ocean from weathering
and submarine volcanic activity and reacted with
dissolved oxygen to form hematite and magnetite
• Only after most BIFs precipitated did oxygen
escape from ocean to atmosphere ~1.7 by
BIFs
Redbeds, Sulfates and Detrital
Uraninite & Pyrite
• Redbeds- sandstones and shales w/ iron
oxides require enough oxygen to oxidized.
– Absent from geologic record until 2.4 by and
only abundant after 1.5 by
• Sulfates (gypsum and anhydrite) require
free oxygen; not present in geologic record
until 2 by
• Uraninite (uranium mineral) & Pyrite
unstable under oxidizing conditions; present
in rocks 2.3 to 2.8 by, none younger
PALEOSOLS
• Ancient weathering profiles or soils
• Contain information about atmosphere
• Iron is not fully oxidized in weathering
deposits >2 by
• Only oxidized in granites but not in basalts
since basalts contain more Fe
Biological Indicators
• Archean and Early Proterozoic cells were
primitive (prokaryotic) unicellular organisms
• Developed in oxygen-free environments
• Advanced cells requiring free-oxygen do not
appear in the record until 1.5by at the time
oxygen levels had to be 1% of present levels
• Appearance of simple multicellular organisms at
about 700 my required 7% O2
Formation of the Oceans:
Prevailing Theory
• The major trapped volatile was water (H2O). Others
included nitrogen (N2), the most abundant gas in the
atmosphere, carbon dioxide (CO2), and hydrochloric
acid (HCl), which was the source of the chloride in sea
salt (mostly NaCl).
• The volatiles were probably released early in the Earth's
history, when it melted and segregated into the core,
mantle, and crust. This segregation occurred because
of differences in density, the crust being the "lightest"
material.
• Volcanoes have released additional volatiles throughout
the Earth's history, but probably more during the early
years when the Earth was hotter.
• Probably, the oceans formed as soon as the Earth
cooled enough for water to become liquid, about 4 billion
years ago. The oldest rocks on the earth's surface today
are 3.8 billion years old.
Outgassing
Oceans are byproducts of
heating and differentiation:
as earth warmed and
partially melted, water
locked in the minerals as
hydrogen and oxygen was
released and carried to the
surface by volcanic venting
activity
Archean Life
• Organic compounds and macrofossils or
microstructures
• Carbon isotopes (C-13 to C-12) in kerogen similar
to modern organisms; 3.8 by
– Isua, Greenland
• Rodshaped and filamentous structures, spheroidal
bodies common in Archean cherts from 3.6 by
– Warrawoona Group, Pilbara Region Australia
– Fig Tree Group, Barberton, South Africa
• Stromatolites: laminated domed-shaped mounds
deposited by cyanobacteria 3.6 by, Pilbara region
of Australia
Fig. 9.8f
Proterozoic
• Rocks widespread on stable cratons covered by
younger sedimentary and volcanic rocks
• 60% of present continental crust evolved into stable
craton by 2.4 by
• Plate tectonics well established by 2 by
• Canadian Shield: Huronian Supergroup (2.4-2.3 by)
–
–
–
–
–
12 km thick sequence of clastic sediments (sandstones)
Nearshore marine to fluvial origin
lower section >> detrital uraninite (anoxic)
upper section >> red beds (oxygenated)
Gowganda Fm. tillites & varves >> glaciation
Proterozoic Orogenies
• Wopmay 2.1 by- suture of Slave Plate and
Bear Plates
• Accretion in southwestern US by a series
of arcs between 1.8 and 1.2 by, starting
with the Cheyene suture
• Mazatzal Orogeny 1.4-1.3 by
• Animikie Group
• western shores of Lake
Superior
• BIFs record presence of free
oxygen
• some deposits over 1000 m
thick and over 100 km in
extent
• Gunflint Chert contains a
series of interesting
assemblage of
cyanobacteria
BIF upper peninsula MI
BIF Wadi Kareim, Egypt
• Keweenawan Sequence
• clean quartz sandstone & conglomerates & basaltic
volcanics
• lava flows are several km thick and contain copper
• Rift zones associated with volcanism developed 1.2 to
1.0 by
The Grand Canyon
• Older lower units
Vishnu Schist
– metamorphosed sed rks
and gneisses
– intensely folded and
intruded by granites
emplaced during the
Mazatzal Orogeny (1.41.3 by)
• Younger Precambrian
rocks >> Grand Canyon
Supergroup
– clastic rks; sandstones,
conglomerates, shales
– accumulated in a trough
– Chuar Gr. Contains algal
spheres
Major Glaciations
• Glaciations
240 my period
of glaciation evidence
in UT, NV, w. Canada
AK, Greenland, S. Am
Scandinavia, Africa
Gowganda Fm
Witwatersrand Fm
2.8 by
Precambrian Mineral Deposits
• Magmatic Deposits
– Bushveld Complex; fractional crystallization of
basalts and komatiites
• Platinum, chromium
• Massive Sulfide Deposits (Keweenawan)
• BIFs (Animikie)
• Placer Gold
– Witwatersrand South Africa, ancient river
channels, conglomerates and sandstones
• Diamonds
– kimberlites, alkali-rich ultramafic rocks formed in
the upper mantle, 200km deep, South Africa
ProtoPangea
• Grenville Orogeny ~1.0 by
• Consolidated continents to form the
supercontinent RODINIA
• The great ocean that surrounded Rodinia
was MIROVIA
• Break up between 700-600 my
The
Neoproterozoic
supercontinent
Rodinia as it
began to break
apart.
(After Hoffman, P. F. 1991.
Science 252: 1409-1412.)
Proterozoic Life
• By 2 by unicellular
organisms widespread
• 1.9 by Gunflint fauna
– thread bacteria &
cyanobacteria
A: Eoasterion
B: Eophaera
C: Animikiea
D: Kakabekia
• Abundant stromatolites
– reached peak diversity 750
my
• By 1.8 by evolution of
eukaryotic cells
– Acritarchs, unicellular,
spherical microfossils,
planktic, photosynthetic,
common in rocks <1.5by
Organisms from the Gunflint
Chert
• Advent of the
Metazoans
• Metazoans
Ediacaran Fauna
– multicellular,
differentiated cells with
tissues and organs
– established by 1by
• Ediacaran Fauna
– Australia
– Pound Quartzite
– 31 species; soft bodied
Spriggina floundersi
segmented worm
• annelids, cnidarians,
arthropods, echinoderms
– Late Proterozoic(550my)
Kimberella- mollusc-like
Cloudina, the
earliest
known
calcium
carbonate
shell-bearing
fossils.